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How did we get here the roots and impacts of the climate crisis.

People’s heavy reliance on fossil fuels and the cutting down of carbon-storing forests have transformed global climate.

For more than a century, researchers have honed their methods for measuring the impacts of human actions on Earth's atmosphere.

Sam Falconer

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By Alexandra Witze

March 10, 2022 at 11:00 am

Even in a world increasingly battered by weather extremes, the summer 2021 heat wave in the Pacific Northwest stood out. For several days in late June, cities such as Vancouver, Portland and Seattle baked in record temperatures that killed hundreds of people. On June 29, Lytton, a village in British Columbia, set an all-time heat record for Canada, at 121° Fahrenheit (49.6° Celsius); the next day, the village was incinerated by a wildfire.

Within a week, an international group of scientists had analyzed this extreme heat and concluded it would have been virtually impossible without climate change caused by humans. The planet’s average surface temperature has risen by at least 1.1 degrees Celsius since preindustrial levels of 1850–1900. The reason: People are loading the atmosphere with heat-trapping gases produced during the burning of fossil fuels, such as coal and gas, and from cutting down forests.

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A little over 1 degree of warming may not sound like a lot. But it has already been enough to fundamentally transform how energy flows around the planet. The pace of change is accelerating, and the consequences are everywhere. Ice sheets in Greenland and Antarctica are melting, raising sea levels and flooding low-lying island nations and coastal cities. Drought is parching farmlands and the rivers that feed them. Wildfires are raging. Rains are becoming more intense, and weather patterns are shifting .

The roots of understanding this climate emergency trace back more than a century and a half. But it wasn’t until the 1950s that scientists began the detailed measurements of atmospheric carbon dioxide that would prove how much carbon is pouring from human activities. Beginning in the 1960s, researchers started developing comprehensive computer models that now illuminate the severity of the changes ahead.

Today we know that climate change and its consequences are real, and we are responsible. The emissions that people have been putting into the air for centuries — the emissions that made long-distance travel, economic growth and our material lives possible — have put us squarely on a warming trajectory . Only drastic cuts in carbon emissions, backed by collective global will, can make a significant difference.

“What’s happening to the planet is not routine,” says Ralph Keeling, a geochemist at the Scripps Institution of Oceanography in La Jolla, Calif. “We’re in a planetary crisis.”

Setting the stage

One day in the 1850s, Eunice Newton Foote, an amateur scientist and a women’s rights activist living in upstate New York, put two glass jars in sunlight. One contained regular air — a mix of nitrogen, oxygen and other gases including carbon dioxide — while the other contained just carbon dioxide. Both had thermometers in them. As the sun’s rays beat down, Foote observed that the jar of CO 2 alone heated up more quickly, and was slower to cool down, than the one containing plain air.

The results prompted Foote to muse on the relationship between CO 2 , the planet and heat. “An atmosphere of that gas would give to our earth a high temperature,” she wrote in an 1856 paper summarizing her findings .

Three years later, working independently and apparently unaware of Foote’s discovery, Irish physicist John Tyndall showed the same basic idea in more detail. With a set of pipes and devices to study the transmission of heat, he found that CO 2 gas, as well as water vapor, absorbed more heat than air alone. He argued that such gases would trap heat in Earth’s atmosphere, much as panes of glass trap heat in a greenhouse, and thus modulate climate.

Today Tyndall is widely credited with the discovery of how what we now call greenhouse gases heat the planet, earning him a prominent place in the history of climate science. Foote faded into relative obscurity — partly because of her gender, partly because her measurements were less sensitive. Yet their findings helped kick off broader scientific exploration of how the composition of gases in Earth’s atmosphere affects global temperatures.

Heat-trapping gases 

In 1859, John Tyndall used this apparatus to study how various gases trap heat. He sent infrared radiation through a tube filled with gas and measured the resulting temperature changes. Carbon dioxide and water vapor, he showed, absorb more heat than air does.

Carbon floods in

Humans began substantially affecting the atmosphere around the turn of the 19th century, when the Industrial Revolution took off in Britain. Factories burned tons of coal; fueled by fossil fuels, the steam engine revolutionized transportation and other industries. Since then, fossil fuels including oil and natural gas have been harnessed to drive a global economy. All these activities belch gases into the air.

Yet Swedish physical chemist Svante Arrhenius wasn’t worried about the Industrial Revolution when he began thinking in the late 1800s about changes in atmospheric CO 2 levels. He was instead curious about ice ages — including whether a decrease in volcanic eruptions, which can put carbon dioxide into the atmosphere, would lead to a future ice age. Bored and lonely in the wake of a divorce, Arrhenius set himself to months of laborious calculations involving moisture and heat transport in the atmosphere at different zones of latitude. In 1896, he reported that halving the amount of CO 2 in the atmosphere could indeed bring about an ice age — and that doubling CO 2 would raise global temperatures by around 5 to 6 degrees C.

It was a remarkably prescient finding for work that, out of necessity, had simplified Earth’s complex climate system down to just a few variables. But Arrhenius’ findings didn’t gain much traction with other scientists at the time. The climate system seemed too large, complex and inert to change in any meaningful way on a timescale that would be relevant to human society. Geologic evidence showed, for instance, that ice ages took thousands of years to start and end. What was there to worry about?

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One researcher, though, thought the idea was worth pursuing. Guy Stewart Callendar, a British engineer and amateur meteorologist, had tallied weather records over time, obsessively enough to determine that average temperatures were increasing at 147 weather stations around the globe. In a 1938 paper in a Royal Meteorological Society journal, he linked this temperature rise to the burning of fossil fuels . Callendar estimated that fossil fuel burning had put around 150 billion metric tons of CO 2 into the atmosphere since the late 19th century.

Like many of his day, Callendar didn’t see global warming as a problem. Extra CO 2 would surely stimulate plants to grow and allow crops to be farmed in new regions. “In any case the return of the deadly glaciers should be delayed indefinitely,” he wrote. But his work revived discussions tracing back to Tyndall and Arrhenius about how the planetary system responds to changing levels of gases in the atmosphere. And it began steering the conversation toward how human activities might drive those changes.

When World War II broke out the following year, the global conflict redrew the landscape for scientific research. Hugely important wartime technologies, such as radar and the atomic bomb, set the stage for “big science” studies that brought nations together to tackle high-stakes questions of global reach. And that allowed modern climate science to emerge.

The Keeling curve

One major effort was the International Geophysical Year, or IGY, an 18-month push in 1957–1958 that involved a wide array of scientific field campaigns including exploration in the Arctic and Antarctica. Climate change wasn’t a high research priority during the IGY, but some scientists in California, led by Roger Revelle of the Scripps Institution of Oceanography, used the funding influx to begin a project they’d long wanted to do. The goal was to measure CO 2 levels at different locations around the world, accurately and consistently.

The job fell to geochemist Charles David Keeling, who put ultraprecise CO 2 monitors in Antarctica and on the Hawaiian volcano of Mauna Loa. Funds soon ran out to maintain the Antarctic record, but the Mauna Loa measurements continued. Thus was born one of the most iconic datasets in all of science — the “Keeling curve,” which tracks the rise of atmospheric CO 2 .

When Keeling began his measurements in 1958, CO 2 made up 315 parts per million of the global atmosphere. Within just a few years it became clear that the number was increasing year by year. Because plants take up CO 2 as they grow in spring and summer and release it as they decompose in fall and winter, CO 2 concentrations rose and fell each year in a sawtooth pattern. But superimposed on that pattern was a steady march upward.

“The graph got flashed all over the place — it was just such a striking image,” says Ralph Keeling, who is Keeling’s son. Over the years, as the curve marched higher, “it had a really important role historically in waking people up to the problem of climate change.” The Keeling curve has been featured in countless earth science textbooks, congressional hearings and in Al Gore’s 2006 documentary on climate change, An Inconvenient Truth .

Each year the curve keeps going up: In 2016, it passed 400 ppm of CO 2 in the atmosphere as measured during its typical annual minimum in September. Today it is at 413 ppm. (Before the Industrial Revolution, CO 2 levels in the atmosphere had been stable for centuries at around 280 ppm.)

Around the time that Keeling’s measurements were kicking off, Revelle also helped develop an important argument that the CO 2 from human activities was building up in Earth’s atmosphere. In 1957, he and Hans Suess, also at Scripps at the time, published a paper that traced the flow of radioactive carbon through the oceans and the atmosphere . They showed that the oceans were not capable of taking up as much CO 2 as previously thought; the implication was that much of the gas must be going into the atmosphere instead.

Steady rise 

Known as the Keeling curve, this chart shows the rise in CO 2 levels as measured at the Mauna Loa Observatory in Hawaii due to human activities. The visible sawtooth pattern is due to seasonal plant growth: Plants take up CO 2   in the growing seasons, then release it as they decompose in fall and winter.

Monthly average CO 2 concentrations at Mauna Loa Observatory

“Human beings are now carrying out a large-scale geophysical experiment of a kind that could not have happened in the past nor be reproduced in the future,” Revelle and Suess wrote in the paper. It’s one of the most famous sentences in earth science history.

Here was the insight underlying modern climate science: Atmospheric carbon dioxide is increasing, and humans are causing the buildup. Revelle and Suess became the final piece in a puzzle dating back to Svante Arrhenius and John Tyndall. “I tell my students that to understand the basics of climate change, you need to have the cutting-edge science of the 1860s, the cutting-edge math of the 1890s and the cutting-edge chemistry of the 1950s,” says Joshua Howe, an environmental historian at Reed College in Portland, Ore.

Evidence piles up

Observational data collected throughout the second half of the 20th century helped researchers gradually build their understanding of how human activities were transforming the planet.

Ice cores pulled from ice sheets, such as that atop Greenland, offer some of the most telling insights for understanding past climate change. Each year, snow falls atop the ice and compresses into a fresh layer of ice representing climate conditions at the time it formed. The abundance of certain forms, or isotopes, of oxygen and hydrogen in the ice allows scientists to calculate the temperature at which it formed, and air bubbles trapped within the ice reveal how much carbon dioxide and other greenhouse gases were in the atmosphere at that time. So drilling down into an ice sheet is like reading the pages of a history book that go back in time the deeper you go.

Scientists began reading these pages in the early 1960s, using ice cores drilled at a U.S. military base in northwest Greenland . Contrary to expectations that past climates were stable, the cores hinted that abrupt climate shifts had happened over the last 100,000 years. By 1979, an international group of researchers was pulling another deep ice core from a second location in Greenland — and it, too, showed that abrupt climate change had occurred in the past. In the late 1980s and early 1990s, a pair of European- and U.S.-led drilling projects retrieved even deeper cores from near the top of the ice sheet, pushing the record of past temperatures back a quarter of a million years.

Together with other sources of information, such as sediment cores drilled from the seafloor and molecules preserved in ancient rocks, the ice cores allowed scientists to reconstruct past temperature changes in extraordinary detail. Many of those changes happened alarmingly fast. For instance, the climate in Greenland warmed abruptly more than 20 times in the last 80,000 years , with the changes occurring in a matter of decades. More recently, a cold spell that set in around 13,000 years ago suddenly came to an end around 11,500 years ago — and temperatures in Greenland rose 10 degrees C in a decade.

Evidence for such dramatic climate shifts laid to rest any lingering ideas that global climate change would be slow and unlikely to occur on a timescale that humans should worry about. “It’s an important reminder of how ‘tippy’ things can be,” says Jessica Tierney, a paleoclimatologist at the University of Arizona in Tucson.

More evidence of global change came from Earth-observing satellites, which brought a new planet-wide perspective on global warming beginning in the 1960s. From their viewpoint in the sky, satellites have measured the rise in global sea level — currently 3.4 millimeters per year and accelerating, as warming water expands and as ice sheets melt — as well as the rapid decline in ice left floating on the Arctic Ocean each summer at the end of the melt season. Gravity-sensing satellites have “weighed” the Antarctic and Greenlandic ice sheets from above since 2002, reporting that more than 400 billion metric tons of ice are lost each year.

Temperature observations taken at weather stations around the world also confirm that we are living in the hottest years on record. The 10 warmest years since record keeping began in 1880 have all occurred since 2005 . And nine of those 10 have come since 2010.

Worrisome predictions

By the 1960s, there was no denying that the planet was warming. But understanding the consequences of those changes — including the threat to human health and well-being — would require more than observational data. Looking to the future depended on computer simulations: complex calculations of how energy flows through the planetary system.

A first step in building such climate models was to connect everyday observations of weather to the concept of forecasting future climate. During World War I, British mathematician Lewis Fry Richardson imagined tens of thousands of meteorologists, each calculating conditions for a small part of the atmosphere but collectively piecing together a global forecast.

But it wasn’t until after World War II that computational power turned Richardson’s dream into reality. In the wake of the Allied victory, which relied on accurate weather forecasts for everything from planning D-Day to figuring out when and where to drop the atomic bombs, leading U.S. mathematicians acquired funding from the federal government to improve predictions. In 1950, a team led by Jule Charney, a meteorologist at the Institute for Advanced Study in Princeton, N.J., used the ENIAC, the first U.S. programmable, electronic computer, to produce the first computer-driven regional weather forecast . The forecasting was slow and rudimentary, but it built on Richardson’s ideas of dividing the atmosphere into squares, or cells, and computing the weather for each of those. The work set the stage for decades of climate modeling to follow.

By 1956, Norman Phillips, a member of Charney’s team, had produced the world’s first general circulation model, which captured how energy flows between the oceans, atmosphere and land. The field of climate modeling was born.

The work was basic at first because early computers simply didn’t have much computational power to simulate all aspects of the planetary system.

An important breakthrough came in 1967, when meteorologists Syukuro Manabe and Richard Wetherald — both at the Geophysical Fluid Dynamics Laboratory in Princeton, a lab born from Charney’s group — published a paper in the Journal of the Atmospheric Sciences that modeled connections between Earth’s surface and atmosphere and calculated how changes in CO 2 would affect the planet’s temperature. Manabe and Wetherald were the first to build a computer model that captured the relevant processes that drive climate , and to accurately simulate how the Earth responds to those processes.

The rise of climate modeling allowed scientists to more accurately envision the impacts of global warming. In 1979, Charney and other experts met in Woods Hole, Mass., to try to put together a scientific consensus on what increasing levels of CO 2 would mean for the planet. The resulting “Charney report” concluded that rising CO 2 in the atmosphere would lead to additional and significant climate change.

In the decades since, climate modeling has gotten increasingly sophisticated . And as climate science firmed up, climate change became a political issue.

The hockey stick 

This famous graph, produced by scientist Michael Mann and colleagues, and then reproduced in a 2001 report by the Intergovernmental Panel on Climate Change, dramatically captures temperature change over time. Climate change skeptics made it the center of an all-out attack on climate science.

The rising public awareness of climate change, and battles over what to do about it, emerged alongside awareness of other environmental issues in the 1960s and ’70s. Rachel Carson’s 1962 book Silent Spring , which condemned the pesticide DDT for its ecological impacts, catalyzed environmental activism in the United States and led to the first Earth Day in 1970.

In 1974, scientists discovered another major global environmental threat — the Antarctic ozone hole, which had some important parallels to and differences from the climate change story. Chemists Mario Molina and F. Sherwood Rowland, of the University of California, Irvine, reported that chlorofluorocarbon chemicals, used in products such as spray cans and refrigerants, caused a chain of reactions that gnawed away at the atmosphere’s protective ozone layer . The resulting ozone hole, which forms over Antarctica every spring, allows more ultraviolet radiation from the sun to make it through Earth’s atmosphere and reach the surface, where it can cause skin cancer and eye damage.

Governments worked under the auspices of the United Nations to craft the 1987 Montreal Protocol, which strictly limited the manufacture of chlorofluorocarbons . In the years following, the ozone hole began to heal. But fighting climate change is proving to be far more challenging. Transforming entire energy sectors to reduce or eliminate carbon emissions is much more difficult than replacing a set of industrial chemicals.

In 1980, though, researchers took an important step toward banding together to synthesize the scientific understanding of climate change and bring it to the attention of international policy makers. It started at a small scientific conference in Villach, Austria, on the seriousness of climate change. On the train ride home from the meeting, Swedish meteorologist Bert Bolin talked with other participants about how a broader, deeper and more international analysis was needed. In 1988, a United Nations body called the Intergovernmental Panel on Climate Change, the IPCC, was born. Bolin was its first chairperson.

The IPCC became a highly influential and unique body. It performs no original scientific research; instead, it synthesizes and summarizes the vast literature of climate science for policy makers to consider — primarily through massive reports issued every couple of years. The first IPCC report, in 1990 , predicted that the planet’s global mean temperature would rise more quickly in the following century than at any point in the last 10,000 years, due to increasing greenhouse gases in the atmosphere.

IPCC reports have played a key role in providing scientific information for nations discussing how to stabilize greenhouse gas concentrations. This process started with the Rio Earth Summit in 1992 , which resulted in the U.N. Framework Convention on Climate Change. Annual U.N. meetings to tackle climate change led to the first international commitments to reduce emissions, the Kyoto Protocol of 1997 . Under it, developed countries committed to reduce emissions of CO 2 and other greenhouse gases. By 2007, the IPCC declared the reality of climate warming is “unequivocal.” The group received the Nobel Peace Prize that year, along with Al Gore, for their work on climate change.

The IPCC process ensured that policy makers had the best science at hand when they came to the table to discuss cutting emissions. Of course, nations did not have to abide by that science — and they often didn’t. Throughout the 2000s and 2010s, international climate meetings discussed less hard-core science and more issues of equity. Countries such as China and India pointed out that they needed energy to develop their economies and that nations responsible for the bulk of emissions through history, such as the United States, needed to lead the way in cutting greenhouse gases.

Meanwhile, residents of some of the most vulnerable nations, such as low-lying islands that are threatened by sea level rise, gained visibility and clout at international negotiating forums. “The issues around equity have always been very uniquely challenging in this collective action problem,” says Rachel Cleetus, a climate policy expert with the Union of Concerned Scientists in Cambridge, Mass.

By 2015, the world’s nations had made some progress on the emissions cuts laid out in the Kyoto Protocol, but it was still not enough to achieve substantial global reductions. That year, a key U.N. climate conference in Paris produced an international agreement to try to limit global warming to 2 degrees C, and preferably 1.5 degrees C , above preindustrial levels.

Every country has its own approach to the challenge of addressing climate change. In the United States, which gets approximately 80 percent of its energy from fossil fuels, sophisticated efforts to downplay and critique the science led to major delays in climate action. For decades, U.S. fossil fuel companies such as ExxonMobil worked to influence politicians to take as little action on emissions reductions as possible.

Biggest footprint 

These 20 nations have emitted the largest cumulative amounts of carbon dioxide since 1850. Emissions are shown in billions of metric tons and are broken down into subtotals from fossil fuel use and cement manufacturing (blue) and land use and forestry (green).

Total carbon dioxide emissions by country, 1850–2021 

Such tactics undoubtedly succeeded in feeding politicians’ delay on climate action in the United States, most of it from Republicans. President George W. Bush withdrew the country from the Kyoto Protocol in 2001 ; Donald Trump similarly rejected the Paris accord in 2017 . As late as 2015, the chair of the Senate’s environment committee, James Inhofe of Oklahoma, brought a snowball into Congress on a cold winter’s day to argue that human-caused global warming is a “hoax.”

In Australia, a similar mix of right-wing denialism and fossil fuel interests has kept climate change commitments in flux, as prime ministers are voted in and out over fierce debates about how the nation should act on climate.

Yet other nations have moved forward. Some European countries such as Germany aggressively pursued renewable energies, including wind and solar, while activists such as Swedish teenager Greta Thunberg — the vanguard of a youth-action movement — pressured their governments for more.

In recent years, the developing economies of China and India have taken center stage in discussions about climate action. China, which is now the world’s largest carbon emitter, declared several moderate steps in 2021 to reduce emissions, including that it would stop building coal-burning power plants overseas. India announced it would aim for net-zero emissions by 2070, the first time it has set a date for this goal.

Yet such pledges continue to be criticized. At the 2021 U.N. Climate Change Conference in Glasgow, Scotland, India was globally criticized for not committing to a complete phaseout of coal — although the two top emitters, China and the United States, have not themselves committed to phasing out coal. “There is no equity in this,” says Aayushi Awasthy, an energy economist at the University of East Anglia in England.

Past and future 

Various scenarios for how greenhouse gas emissions might change going forward help scientists predict future climate change. This graph shows the simulated historical temperature trend along with future projections of rising temperatures based on five scenarios from the Intergovernmental Panel on Climate Change. Temperature change is the difference from the 1850–1900 average.

Historical and projected global temperature change

Facing the future

In many cases, changes are coming faster than scientists had envisioned a few decades ago. The oceans are becoming more acidic as they absorb CO 2 , harming tiny marine organisms that build protective calcium carbonate shells and are the base of the marine food web. Warmer waters are bleaching coral reefs. Higher temperatures are driving animal and plant species into areas in which they previously did not live, increasing the risk of extinction for many.

No place on the planet is unaffected. In many areas, higher temperatures have led to major droughts, which dry out vegetation and provide additional fuel for wildfires such as those that have devastated Australia , the Mediterranean and western North America in recent years.

Then there’s the Arctic, where temperatures are rising at more than twice the global average and communities are at the forefront of change. Permafrost is thawing, destabilizing buildings, pipelines and roads. Caribou and reindeer herders worry about the increased risk of parasites for the health of their animals. With less sea ice available to buffer the coast from storm erosion, the Inupiat village of Shishmaref, Alaska, risks crumbling into the sea . It will need to move from its sand-barrier island to the mainland.

“We know these changes are happening and that the Titanic is sinking,” says Louise Farquharson, a geomorphologist at the University of Alaska Fairbanks who monitors permafrost and coastal change around Alaska. All around the planet, those who depend on intact ecosystems for their survival face the greatest threat from climate change. And those with the least resources to adapt to climate change are the ones who feel it first.

“We are going to warm,” says Claudia Tebaldi, a climate scientist at Lawrence Berkeley National Laboratory in California. “There is no question about it. The only thing that we can hope to do is to warm a little more slowly.”

That’s one reason why the IPCC report released in 2021 focuses on anticipated levels of global warming . There is a big difference between the planet warming 1.5 degrees versus 2 degrees or 2.5 degrees. Each fraction of a degree of warming increases the risk of extreme events such as heat waves and heavy rains, leading to greater global devastation.

The future rests on how much nations are willing to commit to cutting emissions and whether they will stick to those commitments. It’s a geopolitical balancing act the likes of which the world has never seen.

Science can and must play a role going forward. Improved climate models will illuminate what changes are expected at the regional scale, helping officials prepare. Governments and industry have crucial parts to play as well. They can invest in technologies, such as carbon sequestration, to help decarbonize the economy and shift society toward more renewable sources of energy.

Huge questions remain. Do voters have the will to demand significant energy transitions from their governments? How can business and military leaders play a bigger role in driving climate action? What should be the role of low-carbon energy sources that come with downsides, such as nuclear energy? How can developing nations achieve a better standard of living for their people while not becoming big greenhouse gas emitters? How can we keep the most vulnerable from being disproportionately harmed during extreme events, and incorporate environmental and social justice into our future?

These questions become more pressing each year, as carbon dioxideaccumulates in our atmosphere. The planet is now at higher levels of CO 2 than at any time in the last 3 million years.

At the U.N. climate meeting in Glasgow in 2021, diplomats from around the world agreed to work more urgently to shift away from using fossil fuels. They did not, however, adopt targets strict enough to keep the world below a warming of 1.5 degrees.

It’s been well over a century since chemist Svante Arrhenius recognized the consequences of putting extra carbon dioxide into the atmosphere. Yet the world has not pulled together to avoid the most dangerous consequences of climate change.

Time is running out.

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Open Access

Peer-reviewed

Research Article

How relevant is climate change research for climate change policy? An empirical analysis based on Overton data

Roles Conceptualization, Methodology, Writing – original draft, Writing – review & editing

* E-mail: [email protected]

Affiliations Science Policy and Strategy Department, Administrative Headquarters of the Max Planck Society, Munich, Germany, Max Planck Institute for Solid State Research, Stuttgart, Germany

Roles Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Writing – original draft, Writing – review & editing

Affiliation Max Planck Institute for Solid State Research, Stuttgart, Germany

Roles Data curation, Formal analysis, Investigation, Methodology, Visualization, Writing – original draft, Writing – review & editing

Affiliation SciTech Strategies, Inc., Albuquerque, NM, United States of America

Roles Conceptualization, Writing – original draft, Writing – review & editing

Roles Conceptualization, Supervision, Writing – original draft, Writing – review & editing

Affiliation Mercator Research Institute on Global Commons and Climate Change (MCC), Berlin, Germany

• Lutz Bornmann, 
• Robin Haunschild, 
• Kevin Boyack, 
• Werner Marx, 
• Jan C. Minx

• Published: September 22, 2022
• https://doi.org/10.1371/journal.pone.0274693
• Reader Comments

Climate change is an ongoing topic in nearly all areas of society since many years. A discussion of climate change without referring to scientific results is not imaginable. This is especially the case for policies since action on the macro scale is required to avoid costly consequences for society. In this study, we deal with the question of how research on climate change and policy are connected. In 2019, the new Overton database of policy documents was released including links to research papers that are cited by policy documents. The use of results and recommendations from research on climate change might be reflected in citations of scientific papers in policy documents. Although we suspect a lot of uncertainty related to the coverage of policy documents in Overton, there seems to be an impact of international climate policy cycles on policy document publication. We observe local peaks in climate policy documents around major decisions in international climate diplomacy. Our results point out that IGOs and think tanks–with a focus on climate change–have published more climate change policy documents than expected. We found that climate change papers that are cited in climate change policy documents received significantly more citations on average than climate change papers that are not cited in these documents. Both areas of society (science and policy) focus on similar climate change research fields: biology, earth sciences, engineering, and disease sciences. Based on these and other empirical results in this study, we propose a simple model of policy impact considering a chain of different document types: The chain starts with scientific assessment reports (systematic reviews) that lead via science communication documents (policy briefs, policy reports or plain language summaries) and government reports to legislative documents.

Citation: Bornmann L, Haunschild R, Boyack K, Marx W, Minx JC (2022) How relevant is climate change research for climate change policy? An empirical analysis based on Overton data. PLoS ONE 17(9): e0274693. https://doi.org/10.1371/journal.pone.0274693

Editor: Alberto Baccini, University of Siena, Italy, ITALY

Received: March 21, 2022; Accepted: September 1, 2022; Published: September 22, 2022

Copyright: © 2022 Bornmann et al. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: The data underlying the results presented in the study are available from https://doi.org/10.17617/3.DUY0LD .

Funding: The author(s) received no specific funding for this work.

Competing interests: The authors have declared that no competing interests exist.

Introduction

People have long believed that nature is so vast and powerful that mankind has not the potential for any major and lasting effect on the earth’s climatic system. One century ago, Arrhenius [ 1 ], one of the discoverers of the greenhouse effect, even welcomed a hotter climate for Northern Europe. According to Weart [ 2 ], the World Climate Conference in Geneva in 1979 and the reports of the US National Academy of Sciences (NAS) and the US Environmental Protection Agency (EPA) in 1983 are important milestones at the beginning of the climate debate, particularly beyond the scientific community.

In the 1960s many experts assumed that swings of the global mean temperature take tens of thousands of years; in the 1970s, they assumed thousands of years. Meanwhile, ice core data from the last Glacial Period show that abrupt global warming is possible and can happen within a few decades or even within a few years as a climate shock [see 3 , climate change beyond 2100, irreversibility and abrupt changes]. In the 1980s, climate change was no longer a theoretical problem. It was widely agreed among experts that global warming could be a concrete threat. A growing number of well-respected climate researchers (like Roger Revelle, Stephen Schneider, James Hansen, Bert Bolin) were deeply concerned and pointed out that the earth was getting noticeably warmer. A series of meetings of meteorologists held in Villach, Austria, led to a growing conviction that global warming may not be a problem of the far future but might become serious within the scientists’ own lifetimes. Subsequently, scientists took an active stance and prompted governments to act soon, because the rate and degree of future warming could be influenced by governmental policy [see 2 , breaking into policy].

The year 1988 marked an important turning point for climate science and policy. Supported by governments around the globe, the Intergovernmental Panel on Climate Change (IPCC) was founded under the roof of the World Meteorological Organization (WMO) and the United Nations Environment Program (UNEP) as a unique science-policy interface. The panel, i.e., participating governments, tasked a set of elected scientists to assess the state of climate science in dedicated reports, i.e., to review and synthesize scientific information relevant to understanding the scientific basis of climate change and of its risk, its environmental, political, and economic impacts and possible response options (see https://www.ipcc.ch/reports/ ). The latest report is from 2021 [ 4 ].

These assessments follow strict principles and procedures (see https://www.ipcc.ch/site/assets/uploads/2018/09/ipcc-principles.pdf and https://www.ipcc.ch/site/assets/uploads/2018/09/ipcc-principles-appendix-a-final.pdf ) to ensure policy relevance without being policy prescriptive. Hundreds of scientists and other experts contribute to the assessment in diverse author teams from a wide range of disciplines including climate physics, engineering, economics, geography, political science, psychology, sociology or urban science and from different world regions to ensure balanced findings. Review is another critical element of IPCC reports. Authors have to respond to tens of thousands of submitted comments by experts and governments in two rounds of review. Important for the dignity of IPCC assessments in the political sphere is the formal acceptance of the reports by the 195 member countries and the line-by-line approval of the summary for policymakers [ 5 – 8 ].

IPCC has been designed and used as the prime scientific input to international climate diplomacy under the United Nations Framework Convention on Climate Change–and as such contributed to international climate agreements–most importantly, the Kyoto Protocol and the Paris Agreement. Meanwhile, climate policy has become an integral part of most national policy programs. These programs include political actions that governments take to achieve the goal of limiting climate change and its consequences [see 9 ].

In its summary for policymakers, the Climate change 2014 synthesis report [ 3 ] states that “human influence on the climate system is clear, and recent anthropogenic emissions of greenhouse gases are the highest in history. Recent climate changes have had widespread impacts on human and natural systems” (p. 2). A recent study found that detectable and attributable climate impacts are documented in tens of thousands of scientific studies affecting 80% of the world’s land area, where 85% of the world population resides [ 10 ]. As such, it is unsurprising that the topic of climate change has become a hot topic in political and public debates and now features widely on political agendas across many different fields.

In this study, we deal with the question of how research on climate change and policy are connected. According to Yin, Gao [ 11 ], the systematic understanding of the connection between science and policy is still limited, since reliable data are missing on a global scale. In 2019, however, the new Overton database of policy documents was released including links to research papers that are cited by policy documents. Yang, Huang [ 9 ] define policy documents in this context as “‘carriers’ of policies … [that] provide a channel through which policy science researchers can study the main contents of policies, policymaking processes and policy instruments”. Using Overton data, Yin, Gao [ 11 ] analyzed the connection between science and policy with respect to COVID-19. They found that “many policy documents in the COVID-19 pandemic substantially access recent, peer-reviewed, and high-impact science. And policy documents that cite science are especially highly cited within the policy domain. At the same time, there is a heterogeneity in the use of science across policy-making institutions. The tendency for policy documents to cite science appears mostly concentrated within intergovernmental organizations (IGOs), such as the World Health Organization (WHO), and much less so in national governments, which consume science largely indirectly through the IGOs” (p. 128).

Impact measurement of scientific papers on the policy area is part of a new branch in scientometrics: measurement of societal impact [ 12 ]. Whereas science impact measurements of papers were restricted to citation analyses (using Web of Science, WoS, or Scopus data) until recently, societal impact measurements are focused on impact analyses of papers on other parts of society than science [ 13 ]. One part of the society is of special interest in this respect: the policy area. The policy area is permanently required to find answers on certain societal demands (such as COVID-19 or climate change). Since science permanently produces research results that can (and should) be used in the response to these demands, it is interesting to know, whether and to what extent this happens. Fang, Dudek [ 14 ] defines the term ‘policy impact’ in this respect as impact that “tells the story of how research outputs provide concrete evidence to support policy-making processes, which can be reflected by the references to research outputs in policy documents”. The use of research findings in the policy-making process is denoted as evidence-based policy-making [ 15 ] or science-based policy-making [ 16 ]. OPENing UP [ 17 ] regards “informing policy and influencing decisions … as one of the most notable effects of scientific research” (p. 24).

Overview of studies on policy impact

The overview of studies dealing with the use of scientific information/publications in policy making by Vilkins and Grant [ 18 ] reveals that a number of studies exists that are based on interviews and surveys (with policymakers). These studies show, e.g., that the use of scientific publications in policy documents seems to depend on organizational culture and perspectives towards their use. Furthermore, some policy areas (such as information technology) use scientific information more frequently than others (e.g., immigration or justice). The use of scientific information in policy might be distinguished according to three stylized purposes: “‘instrumental’ use is direct and measurable for policy; ‘conceptual’ use … [is] indirect but rather affects thinking over a longer period of time; ‘symbolic’ use is when specific findings are selected for rhetorical or political argument” [ 18 ]. Sources of scientific information preferred by policymakers are the internet, meetings, and emailing colleagues. Yang, Huang [ 9 ] reviewed some studies that have analyzed networks of policymaking institutions to gain insights into their relationships. These studies focused on policymaking organizations’ networks, public service organizations’ networks, and policy collaboration networks.

In the area of altmetrics research, a recent overview of studies on measuring policy impact using altmetric data can be found in Fang, Dudek [ 14 ] and Yang, Huang [ 9 ]. A number of studies has used policy impact data from Altmetric ( https://www.altmetric.com ) or PlumX ( https://plumanalytics.com ) [see 19 , 20 ]. Very recent studies used Overton data [e.g., 11]. In the following, we summarize some of these policy impact studies chronologically. One of the first studies in this new altmetrics area was published by Bornmann, Haunschild [ 21 ] using an extensive publication set of climate change papers. The authors were interested in the question of how intensively policy documents have cited science publications. Although climate change is an ongoing policy topic worldwide, they found that only 1.2% out of 191,276 papers on climate change in the dataset have at least one policy citation (using data from Altmetric). The results of Bornmann, Haunschild [ 21 ] revealed that review papers were more frequently cited in policy documents than articles. In order to investigate whether the percentage of 1.2% can be thought of as high or low, two of the authors investigated the percentage of papers indexed in the WoS that are mentioned in policy-related documents [ 22 ]. They found that less than 0.5% are mentioned at least once. Thus, the results show that although only 1.2% of climate change papers were relevant for policy documents, this percentage is substantially higher than the percentage among all papers from the database.

Vilkins and Grant [ 18 ] did not use data from Altmetric or PlumX for their empirical study, but used publications from policy-focused Australian Government departments. The authors were interested in the research and reference practices of Australian policymakers. The study is based on 4,649 cited references in 80 government publications from eight departments. They found that mostly peer-reviewed journal articles, federal government reports, and Australian business information have been cited. The study also revealed “a possible increased chance for academic research to be cited if it was open access. Despite criticisms of citation analysis, at least in the field of research utilisation we cannot solely rely on interview or survey data, as cited evidence use differs from reported evidence use” [ 18 ].

Tattersall and Carroll [ 23 ] used Altmetric policy documents data to investigate policy impact of papers published by authors at the University of Sheffield. They found that 0.65% of the papers were cited by at least one policy document. This percentage is slightly higher than that mentioned by Haunschild and Bornmann [ 22 ] for the WoS database. The field-specific policy-impact analysis revealed that “the research topics with the greatest policy impact are medicine, dentistry, and health, followed by social science and pure science” [ 23 ]. In a more recent study, Yang, Huang [ 9 ] used the Chinese database iPolicy that includes policy documents issued by the Chinese government since 1949. The authors used the data to construct networks of policy-making ministries and government departments. They were interested in identifying core policymakers in China and possible changes of their positions in the networks. Yang, Huang [ 9 ] present 15 ministries in China with the highest eigenvector centrality as core government ministries in the policy networks.

Fang, Costas [ 24 ] focused on hot research topics reflected by citations in policy documents (using Altmetric.com data). The study is based on more than 10 million WoS papers published in various disciplines. The authors identified the hot topics in various broad disciplines. For example, they found that infectious diseases were typically of concern to policy-makers, but also topics that focus on industry and finance as well as child and education. In addition, “potential health-threatening environment problems (e.g., ‘ambient air pollution’, ‘environmental tobacco smoke’, ‘climate change’, etc.) drew high levels of attention from policy-makers too” [ 24 ].

Hicks and Isett [ 25 ] published a case study that investigated the policy impact of papers published in the area of quantitative studies of science. The authors speculated that many papers in this area have limited policy impact, but some papers such as the papers selected for their case study received a lot of policy impact. Hicks and Isett [ 25 ] explain in detail the policy impact of the selected papers. For example, the authors selected the well-known study by Mansfield [ 26 ], Mansfield [ 27 ] that estimated the social rate of return to public research spending. Hicks and Isett [ 25 ] describe the diverse policy impact reached by this paper using several sources.

In the most recent study, Pinheiro, Vignola-Gagné [ 28 ] used publication data from Framework Programmes (FPs) for Research and Technological Development. The authors investigated the relationship of cross-disciplinarity on the paper level and policy impact measured by policy citation data from the Overton database. Pinheiro, Vignola-Gagné [ 28 ] conclude as follows: “Our approach enables testing in a general way the assumption underlying many funding programs, namely that cross-disciplinary research will increase the policy relevance of research outcomes. Findings suggest that research assessments could benefit from measuring uptake in policy-related literature, following additional characterization of the Overton database; of the science-policy interactions it captures; and of the contribution of these interactions within the larger policymaking process” (p. 616).

Dataset used

For many years, policy documents’ and policy citations’ data were aggregated only by the companies Altmetric and PlumX. Recently, however, the Overton database (see https://www.overton.io ) was launched with the goal of becoming the largest database of policy documents and citations [ 29 ]. In Overton, policy documents are defined “very broadly as documents written primarily for or by policymakers” (see http://help.overton.io/en/articles/3823271-what-s-your-definition-of-a-policy-document ). Overton includes documents from governments, think tanks (i.e., research institutions that perform research and advocacy in climate change), non-governmental organizations (NGOs) and intergovernmental organizations (IGOs, i.e., organizations that are composed of states) (see http://help.overton.io/en/articles/5062448-which-publications-does-overton-collect ). The database includes not only various bibliographic information on policy documents (e.g., title and appearance), but also the citation links that exist between policy and science as well as among the policy documents in the database themselves. The citation relations are identified by Overton by using text-mining methods. According to Yin, Gao [ 11 ], the Overton database “includes all major economies and large population centers, with a notable exception of mainland China” (p. 128). The database is updated on a weekly basis. In December 2020, the database includes 799,716 policy documents with citation relations to either other policy documents or scientific papers in 66 different languages from 168 countries (including the European Union and IGOs) and more than 1250 different policy sources.

Yin, Gao [ 11 ] studied the reliability of the science-policy citations in the Overton database, by comparing them with the citation links provided by the Microsoft Academic Graph database (see https://academic.microsoft.com/home ). The results show that “although the two datasets are collected for different purposes using different approaches and technologies, the measurements carried out independently across the two datasets show remarkable consistencies” (p. SI). Since the results by Yin, Gao [ 11 ] confirm the reliability of the Overton data, we decided to use the data for the current study on climate change. Overton provided a snapshot (dated December 04, 2020) of their database to some of us (LB and RH). This snapshot has been imported into a local PostgreSQL database at the Max Planck Institute for Solid State Research (Stuttgart, Germany). After an analysis of publication dates of policy documents and consultation with Euan Adie (Overton), we excluded the policy documents with the publication dates ‘1970-01-01’, ‘1970-01-02’, and ‘2002-07-01’ from our analysis because they were confirmed as ‘dummy’ publication dates by Euan Adie or contained many policy documents published later than the specified date (see https://help.overton.io/article/why-am-i-seeing-unknown-date-instead-of-a-publication-date ). We used PostgreSQL and R [ 30 ] commands including the R package ‘tidyverse’ [ 31 ] for data analysis.

We searched in the fields ‘title’, ‘translated title’, and ‘snippet’ for climate-change-related terms in the Overton snapshot. We searched for ‘climate change’ and ‘global warming’ (note that both terms were truncated on both sides and a single arbitrary character was allowed instead of the white space between the words) to cover the bulk of policy documents that are related to climate change. The search strategy is based on keyword analyses in connection with search queries of previous climate change related papers [ 21 , 22 ]. We found 10,846 policy documents that met the climate change search criteria out of 799,716 policy documents with any citation relation to a scientific paper or another policy document.

The Overton database includes links to scientific publications via digital object identifiers (DOIs)–“scholarly” references in Overton must have a DOI. There are 8,533,973 citation relations from 492,958 policy documents to 3,242,626 scientific papers. We used the SciTech Strategies’ in-house version of Scopus containing 52.04 million items indexed as of May 2020 and published between 1996 and 2019 as a database for scientific papers. 76.7% of these items have a DOI. We were able to match 2,071,085 DOIs cited in Overton to Scopus papers. Thus, nearly 4.98% of Scopus items with a DOI have been cited by policy documents indexed in the Overton database. This is substantially higher than the 1.12% mentioned in Fang, Costas [ 24 ].

We used the journal metric CiteScore to measure the citation impact of journals [ 32 ]. It is the mean number of citations for papers published in a journal. For the current study, CiteScore values were downloaded from https://www.scopus.com/sources.uri on November 10, 2020. The most current CiteScore values from 2019 were used for our analyses.

Policy documents

This study is based on 10,846 climate change policy documents covered in the Overton database. This corresponds to 1.36% of all policy documents in the database. Fig 1 shows the distribution of the climate change policy documents across publication years. For a better interpretation of this distribution, we also included distributions for all policy documents in the Overton database and the papers on climate change in the Scopus database. The comparison of climate change with all policy documents reveals that the climate change policy documents reached a plateau in 2015 whereas all policy documents steadily increased until 2018. Since the scientific paper distribution also shows a steadily increasing trend, it seems that the discussion of climate change in the policy area reached its maximum several years ago (at least temporarily).

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Policy documents can be published by various types of institutions. Based on the classification of these institution types used in Overton, Fig 2 shows the percentage of policy documents published by think tanks, governments, and IGOs. The comparison of climate change policy documents with all policy documents in Fig 2 reveals that climate change documents were published by think tanks and IGOs at higher than expected rates given their overall share of policy documents; fewer climate change documents were published by governments than expected.

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This substantially lower share of climate documents issued by governments could be a reflection of their hesitance in dealing with the problem of climate change as documented in continued emissions growth [ 33 – 35 ] as well as the gap between long-term ambition and short-term actions [ 36 , 37 ]. NGOs and IGOs might be particularly active in the field of climate change. IGOs, for example, may consider climate change as a problem of international coordination in nature.

Fig 3 analyzes sectors publishing policy documents in more detail by considering single institutions. The figure shows the relationship for single institutions between number of policy documents and number of climate change policy documents. On the one hand, the results reveal those institutions (with high output) that are focused on climate change and those institutions that deal with climate change besides other topics. For example, due to its focus on a sector that is highly vulnerable to climate change, documents by the Food and Agriculture Organization (FAO) of the United Nations cover frequently the topic of climate change (please see the interactive version of Fig 3 ).

https://doi.org/10.1371/journal.pone.0274693.g003

This is different in the field of health. Policy documents by the World Health Organization often do not cover climate change, even though this is starting to change now. This corresponds to the comparatively small share of publications in the field of medicine related to climate change research [ 38 ]–even though there is a sizable and fast-growing number of research papers on climate and health in absolute terms [ 39 ]. On the other hand, the colors of the institutional dots in Fig 3 point out the relatively high number of think tanks and IGOs with a focus on climate change–of which some like the Global Warming Policy Foundation are alleged to focus on global warming misinformation and ‘climate sceptic’ contents ( https://www.desmog.com/climate-disinformation-database/ ).

Papers cited in policy documents

In this section, we additionally consider the literature cited by climate change policy documents. We would like to know, for example, (1) whether these documents focus on recently published or older science literature and (2) the research institutions that seem to be very important for the policy area (since they were frequently cited). Fig 4 shows the document types of the publications cited by climate change policy documents. In order to facilitate the interpretation of the results, the results for all policy documents have been added. We have aggregated “article in progress” with “article”. The type “other” contains empty document type entries, “abstract”, and “missing”. The results in the figure show that most policy documents reference “articles”, followed by “reviews” and “conference papers”. The other document types play a minor role. The referencing behavior seems rather similar in policy documents in general and in policy documents that are related to climate change.

https://doi.org/10.1371/journal.pone.0274693.g004

Yin, Gao [ 11 ] found that “the COVID-19 policy frontier appears to be deeply grounded in extremely recent, peer-reviewed scientific insights” (p. 129). We expect there to be a similarly short time lag for climate change research on the one hand; but we can imagine a “classics” effect that certain foundational papers are referred to over and over again on the other hand (some of the policy documents might actually reiterate outdated findings/outliers as well). For scientific papers that cite other scientific papers, the results indicate a “classics” effect: If we look at cited references in papers, the average reference age is 13.1 years for all items in Scopus from 1996 to 2019. However, on average, climate change papers (published between 2010 and 2019) cite other scientific papers that are on average 9.7 years old. In this study, we also investigated the time between appearance of the policy document and its cited scientific papers. This difference is on average 5.8 years for climate change policy documents and 6.7 years for all policy documents. Both differences are significantly shorter than the average references ages in scientific papers and correspond to the results by Yin, Gao [ 11 ].

Fig 5 shows the proportions of accumulated citations of scientific papers in climate change policy documents over time. These proportions are compared with the proportions in all policy documents. We expected that climate change policy documents cite more recently published papers than other policy documents because of the great societal relevance of the topic. The results in Fig 5 show that this is indeed the case: The distribution for climate change policy documents increases faster than the distribution that refers to all policy documents. Yin, Gao [ 11 ] found a similar result for COVID-19 policy documents–another topic with high societal relevance.

The publication year differences are the time between publication year of the policy document and publication year of the scientific paper.

https://doi.org/10.1371/journal.pone.0274693.g005

We expected that policy documents preferentially cite papers published in reputable journals. The most valuable papers can be expected to be published in these journals. The results by Yin, Gao [ 11 ] show, for example, that “COVID-19 policy documents disproportionately reference peer-reviewed insights, drawing especially heavily on top medical journals, both general (such as Lancet) and specialized (such as Clinical Infectious Diseases)” (p. 129). In this study, we used CiteScore as the indicator for measuring reputation. Fig 6 shows the correlation between number of policy document citations received by papers in various scientific journals and the CiteScore of these journals. With a Spearman rank correlation coefficient of 0.24 (on the journal level), the relationship between journal reputation and policy citations is quite low.

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One reason for the low correlation might be that Citescore values at the top of the distribution are very spread out. If one were to use journal ranks rather than using Citescore, the coefficient would likely be much higher. In fact, this is the argument made in Fig 7 . We found that scientific literature cited in policy documents is frequently published in high-impact journals: 69.31% of the papers with at least one policy citation were published in first-quartile journals. Thus, one can expect that policy citations of scientific papers correlate with citations of these papers in the scientific literature.

In the first journal quartile, e.g., are those journals that belong to the 25% of the journals with the highest CiteScore in their subject areas. For about 7% of the journals, a CiteScore was not available.

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The results by Yin, Gao [ 11 ] for COVID-19 policy documents show that “the coronavirus research used by policy-makers aligns with what scientists heavily engage with themselves” (p. 129). In this study, the Spearman rank correlation coefficient between Scopus citations and policy citations of papers (n = 2,071,085) that were cited by policy documents at least once is 0.16. The correlation coefficient is slightly higher (0.20) between Scopus citations and policy citations of papers (n = 102,372) that were cited by climate change policy documents at least once. However, climate change papers that are cited in climate change policy documents received significantly more citations (between 3.3 and 5.6 times) on average than climate change papers that are not cited in these documents (see Fig 8 ).

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Fig 9 includes the journal perspective to show the correlation between the number of climate change policy document citations and Scopus citations. The Spearman rank correlation between both citation counts is high at 0.81. The results in the figure point out that some journals receive more policy citations than can be expected based on science citations such as Climatic Change and Nature Climate Change . These climate change specific journals have emerged more recently. We speculate that the scientific communities of some highly specialized research topics are comparatively small, thereby limiting the mean number of citations per paper. Nature and Science papers received many citations in both areas of science and policy.

The size of the circles reflects the CiteScore of the journals (Spearman rank correlation = 0.81; an interactive version can be viewed at: https://s.gwdg.de/4weLvb ).

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The journal analyses in the previous figures could not reveal the field-specific orientation of the papers cited in climate change policy documents. The journals that are labeled in the figures are mostly multi-disciplinary journals such as Science or Nature or are directly related to climate change. In order to explore the fields in which papers cited in climate change policy documents were published, we produced so called overlay maps that are presented in Fig 10 . The overlay maps were created using the global mapping process outlined in Boyack and Klavans [ 40 ]. Here, clustering was done on 46.14 million Scopus-indexed documents (1996–2019) and 27.23 million non-indexed documents cited at least twice with over 1.1 billion citation links using the Leiden algorithm [ 41 ]. Graph layout was then done on the resulting 104,677 clusters using OpenOrd/DrL [ 42 ] and cluster-level relatedness based on the bm25 text relevance measure, which has been shown to produce better clustering than a simple tf-idf measure [ 43 – 45 ].

The maps include (1) all papers, (2) climate change papers, (3) climate change papers with at least one policy citation, (4) all papers in Scopus with at least one policy citation.

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Over 11% of Scopus-indexed documents were not included in clusters or the map because they had no references and were not cited. Each cluster is represented as a dot on the map and was assigned to its dominant field (and colored) using the journal-to-field assignments from the UCSD map of science [ 46 ]. Clusters with similar topical content are close to each other on the map. Aggregations of clusters can be perceived as discipline-level structures; local areas that contain clusters of many colors are multidisciplinary. Although dot sizes for overlays are based on the number of documents matching overlay criteria, the intent is to provide a qualitative (gestalt) visual view of the data, e.g. to show where result sets are concentrated or if they are evenly spread throughout the map.

Fig 10 shows four maps for comparison: (1) All papers from Scopus, (2) Climate change papers in total, (3) Climate change papers with at least one policy citation, and (4) Papers with at least one policy citation. Comparing map (2) with map (3), for example, one can see that there are areas with climate change papers (such as computer science, pink in map 2) that are not well cited by climate change policy documents–there is far less pink in map 3 than in map 2.

Similar to all papers from the Scopus database shown in map (1) of Fig 10 , papers with at least one policy citation extend across all scientific fields [see map (4) of Fig 10 ]. However, some major fields appear less pronounced in map 4: in particular chemistry, physics, computer sciences, and engineering. Biology, disease sciences, and health sciences are accentuated, indicating that in general these fields are more policy relevant. The fields of climate change papers in map 2 of Fig 10 are concentrated in biology, earth sciences, engineering, disease sciences, and physics (less pronounced). Climate change papers with at least one policy citation [see map 3 of Fig 10 ] show a field-specific pattern similar to the overall climate change policy papers in map 2. It seems that politics does not have a specific field, but reflects the field-specific orientation of climate change research.

For COVID-19 research, Yin, Gao [ 11 ] investigated the temporal shift of the literature cited in policy documents concerning the field-specific distribution (compared to the whole policy literature). Their results reveal “a clear shift from drawing primarily on the biomedical literature to citing economics, society, and other fields of study, which is consistent with overall shifts in policy focus” [ 11 ]. In this study, we also investigated whether there is a field-specific shift using the 27 high-level ASJC journal categories. Fig 11 shows the field-specific orientation of papers (with policy citations) over the entire period (1996–2019). For better readability of the figure, we used the top 10 ASJCs of both sets of papers (Scopus papers with policy citations and Scopus papers that were cited by climate change policy documents) and obtained twelve ASJCs as common top 10 ASJCs (the interactive version of the figure shows the same analysis with all 27 ASJCs). Fig 11 demonstrates that there are some subtle shifts but the early years (2000–2010) suffer from small number effects relative to the most recent decade. Climate change policy documents cite different fields than the whole. The large shifts shown in Yin, Gao [ 11 ] aren’t seen here, but COVID-19 is a rather unique situation where social concerns followed after the medical ones on a short time scale.

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Scientific institutions and policy sources involved in political climate change discussions

In the final section of the empirical results, we focus on the scientific institutions and policy sources that are involved in the political climate change discussions. We are interested in the policy sources that are very active in political climate change discussions (and decisions) and science institutions that provide research results as inputs for the discussions. Table 1 shows the policy sources with the highest number of climate change policy documents. The table also reveals the number of scientific papers cited by these institutions and the number of climate change papers (the number in brackets is the number of policy documents citing the climate change papers). The results show that Publications Office of the European Union and World Bank are the institutions with the most climate change policy documents. According to Euan Adie (founder and director of Overton) the Publications Office of the European Union is a special case as it aggregates documents from many different EU agencies. Cross-regional institutions such as European Union and World Bank are best-suited for dealing with global issues and thus are focused on major problems such as global warming.

The table also reveals the number of scientific papers cited by these institutions and the number of climate change papers (the number in brackets is the number of policy documents citing the climate change papers).

https://doi.org/10.1371/journal.pone.0274693.t001

Table 2 focuses on policy sources that are rooted in climate change research. The results in the table reveal that IPCC is the source that referenced the largest number of papers. Considering the large amount of scientific information collected and presented in the various IPCC reports over many years, this is not surprising as the assessment of the scientific literature on climate change is its core mandate.

The table shows policy sources that cited more than 4.000 papers.

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We differentiated the results in Table 2 further by specifically looking at government, IGO, and think tank sources: We show policy sources in Table 3 that cite science for governments, IGOs, and think tanks. Yin, Gao [ 11 ] reveal the results of similar analyses based on COVID-19 datasets. The results show that governments and IGOs are of similar importance, both with regard to the overall number of policy documents and climate change related policy documents. The top ranked think tanks produced about half of the overall number of policy documents compared to the top ranked governmental organizations and IGOs. Their share of climate change research related documents is roughly the same.

The table differentiates between all documents of the sources citing these papers and documents focussing on climate change.

https://doi.org/10.1371/journal.pone.0274693.t003

Table 4 is related to the cited institution side of the science-policy link: Which science institutions received the most citations from policy documents? The table presents reputable institutions of climate change research or research units located at universities, with the University of East Anglia with its long-lasting tradition in climate change research and meteorology at the top.

The table includes all institutions with more than 2000 papers cited.

https://doi.org/10.1371/journal.pone.0274693.t004

It is noteworthy that throughout Tables 2 to 4 , we find institutions that are alleged to focus on climate misinformation according to the Climate Disinformation Database ( https://www.desmog.com/climate-disinformation-database/ ) like the Heartland Institute, the Foundation for Economic Education, the Heritage Foundation, and Acton Institute; those are very active publishers of policy documents. The Acton Institute also features among the most prolific think thanks publishing policy documents related to climate change. In the overall climate change dataset, we found 17 policy organizations that are listed in the Climate Disinformation Database. The organizations produced 99 policy documents (that cited any Scopus paper) within our dataset; these documents cited 6507 Scopus papers. That is 1.4% of the policy documents and 4.1% of the cited Scopus papers in our dataset.

The use of results and recommendations from research on climate change might be reflected in citations of scientific papers in policy documents. Studies analyzing the impact of research on policy belong to the area of societal impact measurements in scientometrics [ 13 ]. According to Vilkins and Grant [ 18 ], “capturing this impact on policy has significant potential benefits, including showing the impact of research on real-world settings, and building a better case for support for researchers and institutions or even broader research directions” (p. 1682). For Yin, Gao [ 11 ] policy-science citations may occur “for different reasons … including (i) instrumental uses (knowledge directly applied to solve problems); (ii) conceptual uses (research influences or informs the way policymakers think); (iii) tactical uses (citing research to support or challenge an idea) among others, suggesting the need to understand the semantics of the policy science citations” (p. SI).

This study focusses on the connection of climate change research and policy. The study is based on data from the (new) Overton database including policy documents (10,846 climate change policy documents covered in the database) and their citations of scientific publications. With this study, we followed other studies using Overton data investigating links between policy and research (e.g., on COVID-19). Although the Overton database captures a large collection of policy documents, potential biases in coverage and data sample cannot be excluded [ 11 ]. For example, the Overton providers will not have access to many governmental archives, and if they have access, it will be restricted to only a part of the existing documents. Other shortcomings of Overton are mentioned by Yang, Huang [ 9 ]: “the metadata of such policy documents cannot reveal the semantic information contained in the policy process. At the same time, some policy documents have unstructured features, so attribute identification and labeling may be required”.

Overton uses a very broad definition of policy documents, i.e., “documents primarily written by and for policy makers”. The idea behind this is to cover not only text that documents the policy or legislation itself in the corpus, but also documents that were written to inform or influence decisions. Our analyses do not distinguish between those two fundamentally different classes of policy documents. Documents written for policymakers are often written with the purpose to inform or influence documents authored by policymakers and are as such fundamentally different from documents authored by scientists. Moreover, under this wide umbrella definition there are very different types of documents: scientific assessments by the scientific community, legislations, policy reports by IGOs and NGOs, policy briefs, speeches etc.

The different nature of these documents explains some of the results here. For example, it is the main purpose of scientific assessments as those by IPCC to assess the state of knowledge in climate change research and inform international climate diplomacy and national climate policy with robust evidence. In nature, these assessments are comprehensive reviews of the literature with tens of thousands of references. On the other hand, policy briefs are designed for communications and often deliberately strip out literature sources. The policy impact analysis in this study, therefore to some extent simply highlights different policy document types. Any interpretation of policy impact of research can only be undertaken based on such an important caveat.

In this study, we empirically targeted several aspects of the connection between climate change research and policy. Focusing on the time trend of this connection reveals that the discussion of climate change in policy seems to have had its peak some years ago. Although we suspect a lot of uncertainty related to the coverage of policy documents in Overton, there seems to be an impact of international climate policy cycles on policy document publication. We observe local peaks in climate policy documents around major decisions in international climate diplomacy. For example, we observe temporal peaks in policy documents around the failed Copenhagen Summit in 2009 and the Paris Agreement; there is a growth in policy documents from IPCC’s Fifth Assessment in 2013/2014 with a peak in 2015 when the Paris Agreement was made. IPCC reports might play a particular role as they are usually released 2–3 years ahead of major international climate diplomacy events and could trigger substantial co-publication activities. In 2023, the first Global Stocktake on progress with the Paris Agreement is scheduled with IPCC AR6 being released during 2021 and 2022. We might thus expect to see increases in climate change policy documents and citations to the scientific literature in the 2–3 years following.

Various types of institutions publish policy documents. Our results point out that IGOs and think tanks–with a focus on climate change–have published more climate change policy documents than expected given their overall share of policy documents (this result may be partly driven by the biased coverage of the Overton database). The policy documents published by the different types of institutions have especially cited more recent publications. Since climate change is of great societal relevance worldwide, research activities are on a high level (compared to other topics) that can be picked up in a timely manner by the policy area. Although one might expect that policy and science impact correlate (what is relevant for the scientific discourse might be equally relevant for the policy discourse), we found the opposite: The correlation between policy citations and science citations and the correlation between policy citations and the impact factor of the journals publishing the papers are both low. Thus, it seems that both areas of society (science and policy) focus on different papers from climate change research. If the scientific discourse and the policy discourse are scarcely related in terms of citation counts, one might expect that they focus on different fields. Our results reveal, however, that this is not the case: Climate change papers with at least one policy citation are concentrated on similar fields as all climate change papers (biology, earth sciences, engineering, and disease sciences). Since field differences scarcely exist between both publication sets of interest, it would be interesting to explore in future studies how the differences can be characterized by other means.

What are the policy sources that are very active in the political climate change discourse and which scientific institutions provide the necessary scientific information? Our results show that the Publication Offices of the European Union, World Health Organization, and World Bank have published the most climate change policy documents. Since climate change is a worldwide problem and demand, it comes as no surprise that these cross-regional institutions have the highest publication output. The relevant science institutions for policy sources are mostly institutions with high reputation in science–this might be in contrast to the low correlation between science and policy citations on the single paper level. On the institutional level, policy sources seem to trust scientific institutions being renowned for reputable research on climate change (e.g., the University of East Anglia).

In this study, we found that some research outcomes seem to be more relevant for the scientific discourse and some outcomes that seem to be more relevant for the policy area. This discrepancy has been found also in other studies. One reason for the differences might be barriers to academic outcomes from policy institutions such as access to climate change publications [ 18 ]. Another reason might be missing summaries of research results that are understandable for people outside academia. Bornmann and Marx [ 47 ] recommend therefore that researchers should write assessment reports (such as the IPCC) summarizing “the status of the research on a certain subject … Societal impact is given when the content of a report is addressed outside of science (in a government document, for example)” (p. 211).

Our analyses revealed the challenges in measuring policy impact via citation patterns. In fact, the closer a document is related to actual decision-making the fewer citations it may contain. For example, scientific assessments of the literature contain large numbers of citations, but they are not directly used in policy-making. Instead they are further built upon and “translated” in policy briefs, policy reports, briefing notes or ministerial expertise. The final political decision–usually a legal text–usually does not contain any citations. As we move towards real decisions it therefore gets increasingly challenging to measure impact in this way. Future work may therefore be organized around a simple model of policy impact considering a chain of different document types. Scientific assessment reports, systematic reviews or meta-analyses–as recommended by Bornmann and Marx [ 47 ]–may be the starting point as rigorous syntheses of the available summaries. Next might be science communication documents such as policy briefs, policy reports or plain language summaries. Government reports might be compiled to directly inform particular decisions and, finally, legislative documents cover the policies themselves. In this context, Isett and Hicks [ 48 ] speak about knowledge intermediaries in document chains. Future research could attempt measuring the impact on policy along such a document chain. As citations would be expected to fade away as you move down the chain, it will become increasingly relevant to use text mining or other methods from natural language processing (e.g., text similarity approaches; argumentation mining) to measure impact.

Finally, as primary studies are very dependent on their specific research design, data and methods applied, there is a widespread argument that policy should be informed by the most robust scientific evidence and as such be built from secondary research (reviews) whenever possible [ 49 ]. Therefore, future scientometric research may explore to what extent primary and secondary research is used in policy documents and how this varies across different sectors.

Acknowledgments

The bibliometric data used in this paper are from an in-house database developed and maintained by SciTech Strategies, Inc. derived from Scopus, prepared by Elsevier BV (Amsterdam, The Netherlands). The policy document data were shared with us by Overton on December 04, 2020.

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There is unequivocal evidence that Earth is warming at an unprecedented rate. Human activity is the principal cause.

• While Earth’s climate has changed throughout its history , the current warming is happening at a rate not seen in the past 10,000 years.
• According to the Intergovernmental Panel on Climate Change ( IPCC ), "Since systematic scientific assessments began in the 1970s, the influence of human activity on the warming of the climate system has evolved from theory to established fact." 1
• Scientific information taken from natural sources (such as ice cores, rocks, and tree rings) and from modern equipment (like satellites and instruments) all show the signs of a changing climate.
• From global temperature rise to melting ice sheets, the evidence of a warming planet abounds.

The rate of change since the mid-20th century is unprecedented over millennia.

Earth's climate has changed throughout history. Just in the last 800,000 years, there have been eight cycles of ice ages and warmer periods, with the end of the last ice age about 11,700 years ago marking the beginning of the modern climate era — and of human civilization. Most of these climate changes are attributed to very small variations in Earth’s orbit that change the amount of solar energy our planet receives.

The current warming trend is different because it is clearly the result of human activities since the mid-1800s, and is proceeding at a rate not seen over many recent millennia. 1 It is undeniable that human activities have produced the atmospheric gases that have trapped more of the Sun’s energy in the Earth system. This extra energy has warmed the atmosphere, ocean, and land, and widespread and rapid changes in the atmosphere, ocean, cryosphere, and biosphere have occurred.

Earth-orbiting satellites and new technologies have helped scientists see the big picture, collecting many different types of information about our planet and its climate all over the world. These data, collected over many years, reveal the signs and patterns of a changing climate.

Scientists demonstrated the heat-trapping nature of carbon dioxide and other gases in the mid-19th century. 2 Many of the science instruments NASA uses to study our climate focus on how these gases affect the movement of infrared radiation through the atmosphere. From the measured impacts of increases in these gases, there is no question that increased greenhouse gas levels warm Earth in response.

Scientific evidence for warming of the climate system is unequivocal.

Intergovernmental Panel on Climate Change

Ice cores drawn from Greenland, Antarctica, and tropical mountain glaciers show that Earth’s climate responds to changes in greenhouse gas levels. Ancient evidence can also be found in tree rings, ocean sediments, coral reefs, and layers of sedimentary rocks. This ancient, or paleoclimate, evidence reveals that current warming is occurring roughly 10 times faster than the average rate of warming after an ice age. Carbon dioxide from human activities is increasing about 250 times faster than it did from natural sources after the last Ice Age. 3

The Evidence for Rapid Climate Change Is Compelling:

Global Temperature Is Rising

The planet's average surface temperature has risen about 2 degrees Fahrenheit (1 degrees Celsius) since the late 19th century, a change driven largely by increased carbon dioxide emissions into the atmosphere and other human activities. 4 Most of the warming occurred in the past 40 years, with the seven most recent years being the warmest. The years 2016 and 2020 are tied for the warmest year on record. 5 Image credit: Ashwin Kumar, Creative Commons Attribution-Share Alike 2.0 Generic.

The Ocean Is Getting Warmer

The ocean has absorbed much of this increased heat, with the top 100 meters (about 328 feet) of ocean showing warming of 0.67 degrees Fahrenheit (0.33 degrees Celsius) since 1969. 6 Earth stores 90% of the extra energy in the ocean. Image credit: Kelsey Roberts/USGS

The Ice Sheets Are Shrinking

The Greenland and Antarctic ice sheets have decreased in mass. Data from NASA's Gravity Recovery and Climate Experiment show Greenland lost an average of 279 billion tons of ice per year between 1993 and 2019, while Antarctica lost about 148 billion tons of ice per year. 7 Image: The Antarctic Peninsula, Credit: NASA

Glaciers Are Retreating

Glaciers are retreating almost everywhere around the world — including in the Alps, Himalayas, Andes, Rockies, Alaska, and Africa. 8 Image: Miles Glacier, Alaska Image credit: NASA

Snow Cover Is Decreasing

Satellite observations reveal that the amount of spring snow cover in the Northern Hemisphere has decreased over the past five decades and the snow is melting earlier. 9 Image credit: NASA/JPL-Caltech

Sea Level Is Rising

Global sea level rose about 8 inches (20 centimeters) in the last century. The rate in the last two decades, however, is nearly double that of the last century and accelerating slightly every year. 10 Image credit: U.S. Army Corps of Engineers Norfolk District

Arctic Sea Ice Is Declining

Both the extent and thickness of Arctic sea ice has declined rapidly over the last several decades. 11 Credit: NASA's Scientific Visualization Studio

Extreme Events Are Increasing in Frequency

The number of record high temperature events in the United States has been increasing, while the number of record low temperature events has been decreasing, since 1950. The U.S. has also witnessed increasing numbers of intense rainfall events. 12 Image credit: Régine Fabri,  CC BY-SA 4.0 , via Wikimedia Commons

Ocean Acidification Is Increasing

Since the beginning of the Industrial Revolution, the acidity of surface ocean waters has increased by about 30%. 13 , 14 This increase is due to humans emitting more carbon dioxide into the atmosphere and hence more being absorbed into the ocean. The ocean has absorbed between 20% and 30% of total anthropogenic carbon dioxide emissions in recent decades (7.2 to 10.8 billion metric tons per year). 1 5 , 16 Image credit: NOAA

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2. In 1824, Joseph Fourier calculated that an Earth-sized planet, at our distance from the Sun, ought to be much colder. He suggested something in the atmosphere must be acting like an insulating blanket. In 1856, Eunice Foote discovered that blanket, showing that carbon dioxide and water vapor in Earth's atmosphere trap escaping infrared (heat) radiation. In the 1860s, physicist John Tyndall recognized Earth's natural greenhouse effect and suggested that slight changes in the atmospheric composition could bring about climatic variations. In 1896, a seminal paper by Swedish scientist Svante Arrhenius first predicted that changes in atmospheric carbon dioxide levels could substantially alter the surface temperature through the greenhouse effect. In 1938, Guy Callendar connected carbon dioxide increases in Earth’s atmosphere to global warming. In 1941, Milutin Milankovic linked ice ages to Earth’s orbital characteristics. Gilbert Plass formulated the Carbon Dioxide Theory of Climate Change in 1956.

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7. I. Velicogna, Yara Mohajerani, A. Geruo, F. Landerer, J. Mouginot, B. Noel, E. Rignot, T. Sutterly, M. van den Broeke, M. Wessem, D. Wiese, "Continuity of Ice Sheet Mass Loss in Greenland and Antarctica From the GRACE and GRACE Follow-On Missions." Geophysical Research Letters 47, Issue 8 (28 April 2020): e2020GL087291. https://doi.org/10.1029/2020GL087291.

8. National Snow and Ice Data Center World Glacier Monitoring Service

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10. R.S. Nerem, B.D. Beckley, J. T. Fasullo, B.D. Hamlington, D. Masters, and G.T. Mitchum, "Climate-change–driven accelerated sea-level rise detected in the altimeter era." PNAS 15, no. 9 (12 Feb. 2018): 2022-2025. https://doi.org/10.1073/pnas.1717312115.

11. https://nsidc.org/cryosphere/sotc/sea_ice.html Pan-Arctic Ice Ocean Modeling and Assimilation System (PIOMAS, Zhang and Rothrock, 2003) http://psc.apl.washington.edu/research/projects/arctic-sea-ice-volume-anomaly/ http://psc.apl.uw.edu/research/projects/projections-of-an-ice-diminished-arctic-ocean/

12. USGCRP, 2017: Climate Science Special Report: Fourth National Climate Assessment, Volume I [Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart, and T.K. Maycock (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, 470 pp, https://doi.org/10.7930/j0j964j6 .

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14. http://www.pmel.noaa.gov/co2/story/Ocean+Acidification

15. C.L. Sabine, et al., “The Oceanic Sink for Anthropogenic CO2.” Science 305 (16 July 2004): 367-371. https://doi.org/10.1126/science.1097403.

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Header image shows clouds imitating mountains as the sun sets after midnight as seen from Denali's backcountry Unit 13 on June 14, 2019. Credit: NPS/Emily Mesner Image credit in list of evidence: Ashwin Kumar, Creative Commons Attribution-Share Alike 2.0 Generic.

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‘Garbage Lasagna’: Dumps Are a Big Driver of Warming, Study Says

Decades of buried trash is releasing methane, a powerful greenhouse gas, at higher rates than previously estimated, the researchers said.

By Hiroko Tabuchi

They’re vast expanses that can be as big as towns: open landfills where household waste ends up, whether it’s vegetable scraps or old appliances.

These landfills also belch methane, a powerful, planet-warming gas, on average at almost three times the rate reported to federal regulators, according to a study published Thursday in the journal Science.

The study measured methane emissions at roughly 20 percent of 1,200 or so large, operating landfills in the United States. It adds to a growing body of evidence that landfills are a significant driver of climate change, said Riley Duren, founder of the public-private partnership Carbon Mapper, who took part in the study.

“We’ve largely been in the dark, as a society, about actual emissions from landfills,” said Mr. Duren, a former NASA engineer and scientist. “This study pinpoints the gaps.”

Methane emissions from oil and gas production , as well as from livestock, have come under increasing scrutiny in recent years. Like carbon dioxide, the main greenhouse gas that’s warming the world, methane acts like a blanket in the sky, trapping the sun’s heat.

And though methane lasts for a shorter time in the atmosphere than carbon dioxide, it is more potent. Its warming effect is more than 80 times as powerful as the same amount of carbon dioxide over a 20-year period.

The Environmental Protection Agency estimates that landfills are the third largest source of human-caused methane emissions in the United States, emitting as much greenhouse gas as 23 million gasoline cars driven for a year.

But those estimates have been largely based on computer modeling, rather than direct measurements. A big reason: It can be difficult and even dangerous for workers with methane “sniffers” to measure emissions on-site, walking up steep slopes or near active dump sites.

Organic waste like food scraps can emit copious amounts of methane when they decompose under conditions lacking oxygen, which can happen deep in landfills. Composting, on the other hand, generally doesn’t produce methane, which is why experts say it can be effective in reducing methane emissions.

For the new study, scientists gathered data from airplane flyovers using a technology called imaging spectrometers designed to measure concentrations of methane in the air. Between 2018 and 2022, they flew planes over 250 sites across 18 states, about 20 percent of the nation’s open landfills.

At more than half the landfills they surveyed, researchers detected emissions hot spots, or sizable methane plumes that sometimes lasted months or years. That suggested something had gone awry at the site, like a big leak of trapped methane from layers of long-buried, decomposing trash, the researchers said.

“You can sometimes get decades of trash that’s sitting under the landfill,” said Daniel H. Cusworth, a climate scientist at Carbon Mapper and the University of Arizona, who led the study. “We call it a garbage lasagna.”

Many landfills are fitted with specialized wells and pipes that collect the methane gas that seeps out of rotting garbage in order to either burn it off or sometimes to use it to generate electricity or heat. But those wells and pipes can leak.

The researchers said pinpointing leaks doesn’t just help scientists get a better picture of emissions, it also helps landfill operators fix leaks.

Overseas, the picture can be less clear, particularly in countries where landfills aren’t strictly regulated. Previous surveys using satellite technology have estimated that globally, landfill methane makes up nearly 20 percent of human-linked methane emissions.

“The waste sector clearly is going to be a critical part of society’s ambition to slash methane emissions,” said Mr. Duren of Carbon Mapper. “We’re not going to meet the global methane pledge targets just by slashing oil and gas emissions.”

A growing constellation of methane-detecting satellites could provide a fuller picture. Last month, another nonprofit, the Environmental Defense Fund, launched MethaneSat , a satellite dedicated to tracking methane emissions around the world.

Carbon Mapper, with partners including NASA’s Jet Propulsion Laboratory, Rocky Mountain Institute, and the University of Arizona, intends to launch the first of its own methane-tracking satellites later this year.

Hiroko Tabuchi covers the intersection of business and climate for The Times. She has been a journalist for more than 20 years in Tokyo and New York. More about Hiroko Tabuchi

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Climate is sometimes mistaken for weather. But climate is different from weather because it is measured over a long period of time, whereas weather can change from day to day, or from year to year. The climate of an area includes seasonal temperature and rainfall averages, and wind patterns. Different places have different climates. A desert, for example, is referred to as an arid climate because little water falls, as rain or snow, during the year. Other types of climate include tropical climates, which are hot and humid , and temperate climates, which have warm summers and cooler winters.

Climate change is the long-term alteration of temperature and typical weather patterns in a place. Climate change could refer to a particular location or the planet as a whole. Climate change may cause weather patterns to be less predictable. These unexpected weather patterns can make it difficult to maintain and grow crops in regions that rely on farming because expected temperature and rainfall levels can no longer be relied on. Climate change has also been connected with other damaging weather events such as more frequent and more intense hurricanes, floods, downpours, and winter storms.

In polar regions, the warming global temperatures associated with climate change have meant ice sheets and glaciers are melting at an accelerated rate from season to season. This contributes to sea levels rising in different regions of the planet. Together with expanding ocean waters due to rising temperatures, the resulting rise in sea level has begun to damage coastlines as a result of increased flooding and erosion.

The cause of current climate change is largely human activity, like burning fossil fuels , like natural gas, oil, and coal. Burning these materials releases what are called greenhouse gases into Earth’s atmosphere . There, these gases trap heat from the sun’s rays inside the atmosphere causing Earth’s average temperature to rise. This rise in the planet's temperature is called global warming. The warming of the planet impacts local and regional climates. Throughout Earth's history, climate has continually changed. When occuring naturally, this is a slow process that has taken place over hundreds and thousands of years. The human influenced climate change that is happening now is occuring at a much faster rate.

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Article Contents

Introduction, the analytical framework: linking climate change, vulnerability, and conflict, methodology: a systematic review, pathways between climate change and violent conflict in the mena region, evaluating the “pathways” framework in the mena region.

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Climate Change and Violent Conflict in the Middle East and North Africa

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Kyungmee Kim, Tània Ferré Garcia, Climate Change and Violent Conflict in the Middle East and North Africa, International Studies Review , Volume 25, Issue 4, December 2023, viad053, https://doi.org/10.1093/isr/viad053

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Previous research has demonstrated that climate change can escalate the risks for violent conflict through various pathways. Existing evidence suggests that contextual factors, such as migration and livelihood options, governance arrangements, and existing conflict dynamics, can influence the pathways through which climate change leads to conflict. This important insight leads to an inquiry to identify sets of conditions and processes that make climate-related violent conflict more likely. In this analytic essay, we conduct a systematic review of scholarly literature published during the period 1989–2022 and explore the climate-conflict pathways in the Middle East and North Africa (MENA) region. Through the systematic review of forty-one peer-reviewed publications in English, we identify that society’s ability to cope with the changing climate and extreme weather events is influenced by a range of factors, including preceding government policies that led to the mismanagement of land and water and existing conflict dynamics in the MENA region. Empirical research to unpack the complex and diverse relationship between the climate shocks and violent conflict in the MENA region needs advancing. Several avenues for future research are highlighted such as more studies on North Africa and the Gulf region, with focus on the implications of floods and heatwaves, and exploring climate implications on non-agriculture sectors including the critical oil sector.

Investigaciones previas que han demostrado que el cambio climático puede llegar a aumentar la probabilidad del riesgo de conflictos violentos a través de diversos mecanismos. Las pruebas existentes sugieren que los factores contextuales, tales como la migración y las opciones de medios de subsistencia, los acuerdos de gobernanza y la dinámica de conflicto existente, pueden influir en las vías a través de las cuales el cambio climático conduce a los conflictos. Esta percepción motiva una investigación con el objetivo de identificar una serie de condiciones y procesos que hacen que incrementan la probabilidad de conflictos violentos relacionados con el clima. En este ensayo analítico, llevamos a cabo una revisión sistemática de la literatura académica publicada durante el período entre 1989 y 2022. El estudio explora las vías de conflicto climático en la región de Oriente Medio y el Norte de África (MENA, por sus siglas en inglés). A través de la revisión sistemática de 41 publicaciones en inglés revisadas por expertos, fenómenos meteorológicos extremos está influenciada por una serie de factores, que incluyen tanto las políticas gubernamentales precedentes que condujeron a la mala gestión de la tierra y el agua como la dinámica de conflicto existente en la región MENA. Es esencial avanzar en la investigación empírica para poder desentrañar la compleja y diversa relación existente entre las perturbaciones climáticas y los conflictos violentos en la región de Oriente Medio y el Norte de África. Destacamos varias vías de investigación futura, como la realización de un mayor número estudios sobre el norte de África y la región del Golfo, con un enfoque en las implicaciones de las inundaciones y las olas de calor, así como la exploración de las implicaciones climáticas en los sectores no agrícolas, incluido el sector petrolero, de crítica importancia.

Des travaux de recherche antérieurs ont montré que le changement climatique pouvait aggraver les risques de conflits violents de bien des façons. Les éléments probants existants indiquent que les facteurs contextuels, comme les possibilités d'immigration et de moyens de subsistance, les arrangements gouvernementaux et les dynamiques de conflit existantes, peuvent avoir une incidence sur les mécanismes par lesquels le changement climatique peut créer des conflits. Cette information importante nous pousse à chercher les ensembles de conditions et de processus qui augmentent la probabilité des conflits violents en lien avec le climat. Dans cet article analytique, nous conduisons un examen systématique de la littérature académique publiée entre 1989 et 2022 pour nous intéresser aux liens entre climat et conflits dans la région du Moyen-Orient et de l'Afrique du Nord (MENA). En examinant de façon systématique 41 publications en anglais vérifiées par des pairs, nous remarquons que la capacité d'une société à gérer l’évolution du climat et les phénomènes météorologiques extrêmes est liée à un éventail de facteurs, y compris les politiques précédentes du gouvernement qui ont engendré une mauvaise gestion des terres et de l'eau et les dynamiques de conflit existantes dans la région MENA. La recherche empirique pour décortiquer la relation complexe et plurielle entre les crises climatiques et les conflits violents dans la région MENA doit avancer. Plusieurs pistes de recherches ultérieures sont présentées, comme davantage d’études dans la région de l'Afrique du Nord et du Golfe, en se concentrant plus particulièrement sur les implications des inondations et des vagues de chaleur, et l'analyse des conséquences climatiques sur les secteurs hors agriculture, notamment le secteur décisif du pétrole.

Climate change contributes to conflict risk and undermines livelihoods and human security. The impact of climate change overburdens countries in demanding security environments and exacerbates political instability, which may lead to violent conflict. Researchers have sought to explain the relationship between climate change and violent conflict and climate change as a growing factor for security risks ( Gleditsch 2012 ; Meierding 2013 ; Sakaguchi, Varughese, and Auld 2017 ; Ide 2018 ; Van Baalen and Mobjörk 2018 ). There is a greater consensus that climate change has an impact on human security and sustaining peace ( Abrahams 2020 ; Black et al. 2022 ; Morales-Muñoz et al. 2022 ). The evidence has been gathered on the physical changes in diverse livelihood systems and human migration and the negative effects on human adaptation capacities ( IPCC 2022 ). The debate may have to move on from whether climate change has been the primary cause of a war or not ( Verhoeven 2011 ; e.g., Selby et al. 2017 ). Our understanding of what context climate change matters for conflict and security and how relevant factors play out in local contexts should be based on comprehensive and systematic research that considers various scales, time periods, and localities.

Moreover, existing evidence suggests that climate-related security risks are context specific, and there are multiple pathways by which climate change influences the onsets and patterns of armed conflict ( Brzoska and Fröhlich 2016 ; Mobjörk, Krampe, and Tarif 2020 ). The “climate insecurity pathway” framework assumes that climate change may not be the only contributor to violent conflict but also other factors leading to insecurity such as internal and international migration, livelihood options, and governance arrangements ( Van Baalen and Mobjörk 2018 ). Existing conflict dynamics and security environments can exacerbate climate-related security risks. This analytic essay contributes to the debate on how climate change affects the risk of violent conflict by conducting a systematic review of the literature directly or indirectly linking climate change of violent conflict focusing on the Middle East and North Africa (MENA), a region that has been severely impacted by both. 1 By conducting a systematic literature review, we are particularly interested in synthesizing existing evidence to better understand the climate-conflict links in the MENA region. We included forty-one peer-reviewed articles published between 1989 and 2022 in the analysis. Based on the review, we conclude that the relationship between climate change and violent conflict is predominantly indirect and diverse, highlighting the need to avoid oversimplified assumptions. Climate change’s contribution to conflict risk in the MENA region is further mediated by political economy, institutional weaknesses, elite competition, and existing socio-political relations. A careful examination of evidence is crucial for comprehensive climate security discussions in general and policy considerations for the MENA region. The following systematic review of literature showcases the linkages between climate exposure and various sources of vulnerability in the MENA region.

Climate Exposure and Social Vulnerability in the MENA Region

The MENA region is facing major security challenges from its vulnerability to climate change and violent conflict. The region is the world’s most water-stressed region, hosting thirteen of the world’s twenty most water-stressed countries, with currently over 82 percent of its terrain covered in desert ( Sieghart and Betre 2018 ). Indeed, water rationing and the limitation of water supplies are already a reality in parts of Algeria, Lebanon, Iraq, Palestine, and Jordan ( Sowers, Vengosh, and Weinthal 2011 ). Recent climate science predicts an average global warming of 1.5°C under the business-as-usual scenario, while in the MENA region, it is expected to increase up to 4°C ( Gaub and Lienard 2021 ). Furthermore, the level of mean precipitation is also expected to decrease in the region ( Zittis , et al. 2020 ). By the end of the century, about half of the MENA population could be annually exposed to super- and ultra-extreme heatwaves ( Zittis et al. 2021 ). In essence, the region is likely to become drier and experience extremely high temperatures, followed by extreme and chronic water shortages becoming more frequent.

Many countries in the MENA region are vulnerable to the effects of climate change due to their weak adaptive capacity ( Sowers et al. 2011 ; Namdar, Karami, and Keshavarz 2021 ). The adaptive capacity to climate change varies across the MENA region. While oil-exporting Gulf states have the financial resources for investments in water desalination and wastewater technologies, others suffer from a lack of financial resources and water conservation policies ( Sowers et al. 2011 ). The adverse effect of climate change on agricultural productivity is likely to affect the livelihood conditions of rural populations and may contribute to rural-to-urban migration in some cases ( Waha et al. 2017 ). Changes in precipitation and extreme weather events can reduce the region’s agriculture yields, as up to 70 percent of the crops are rain-fed ( Waha et al. 2017 ). Climate change impacts present a threat to food security in the MENA region and exacerbate the vulnerability to global food price volatility, including Egypt and Lebanon. Countries with a high level of imported grain dependency witness significant inflations in cereal prices that can be a source of political instability ( Tanchum 2021 ). Food price volatility has contributed to political stability in the past, especially during the Arab Spring, and the combined effect of reduced water discharge with the demographic trend of the youth bulge could present a challenge to the political stability of a region ( Borghesi and Ticci 2019 ).

Over the past decade, several of the world’s deadliest conflicts flared up in the MENA region, particularly in Syria, Yemen, Iraq, and Turkey ( Palik et al. 2020 ). The intractable conflict between Israel and Palestine has caused immense human suffering and disrupted regional stability. These conflicts are linked to long-running inequalities and grievances and economic and political instability, which make conflict resolution exceptionally challenging. Deterioration of the physical environment and land degradation further exacerbate risks of communal conflict and political instability in the future. Violent conflict, on the other hand, has been destructive to the adjoining environment. For instance, the effect of intense armed conflict has been significant in Syria’s already declining land and water resources ( Mohamed, Anders, and Schneider 2020 ). Environmental degradation leading to water and food insecurity has adversely affected the livelihoods of the population.

The linkages between conflict and the environment are an integral component that constitutes peace and security in the MENA region. The arid natural environment of the region and the changing climate are part of consideration when analyzing conflict in the region ( Smith and Krampe 2019 ). This article focuses on the MENA region and analyzes the role of climate-related environmental factors in violent conflict by drawing evidence from existing research. This systematic review provides an overview of conditions and processes in the climate-conflict nexus. The findings demonstrate that indirect pathways between climate change and violent conflict that are found in other regions such as East Africa, South Asia, and Southeast Asia, and West Africa are also applicable to the MENA region. In addition, downstream impacts of water development projects such as dams and irrigation projects in transboundary river basins, weaponization of water by armed groups, and the government’s mismanagement of water and land have particularly affected vulnerability to climate change in the MENA region. Climate change exacerbates water scarcity in the MENA region, which in turn can incentivize policies such as unilaterally building water storages and weaponization of water as an instrument for leverage during armed conflicts. These MENA region-specific dimensions of climate-conflict pathways appear to be influenced by the region’s internal politics, relations between neighboring countries, and conflict dynamics.

The article is organized in the following order. We present the analytical framework of a set of pathways that connects climate change and violent conflict and then an outline of the methodology for a systematic review, which includes the operationalization of the variables and the sampling strategy. This is followed by the description of the methodology for conducting a systematic review. The review of literature is organized into four categories that are specified in the analytical framework, and then a synthesized analysis is detailed. Finally, we conclude by summarizing policy and research relevant implications from the finding in the MENA regional contexts with a set of recommendations.

The climate-conflict nexus is complex. Climate change has implications for various forms of interstate and intrastate conflict, including communal violence, insurgencies, mass civil resistance campaigns, protests, and interpersonal disputes ( Hendrix et al. 2023 ). Specific contexts of environment, socio-political systems, and pre-existing conflict matter when examining the connection between climate-related environmental changes and conflict. The analytical framework is based on a premise that the relationship between climate change and conflict is mediated by social, political, and ecological vulnerability ( Daoudy 2021 ). When climate impacts contribute to social outcomes such as deteriorating livelihood conditions, migration, escalation of armed groups’ tactics, and elite capture, risks of violent conflict can increase. The following outlines four “pathways” between climate change and conflict ( Figure 1 ).

A framework of climate insecurity pathways

The deterioration of livelihood conditions is a centerpiece in linking environmental changes and violent conflict. Climate-exposed sectors such as agriculture, forestry, fishery, energy, and tourism are highly likely to suffer from economic damages from climate change ( IPCC 2022 , SPM-11). Consequently, people whose livelihoods are dependent on the natural environment are subjected to additional economic burdens due to the changing climate or climate shocks. Extreme weather events such as droughts, heatwaves, sandstorms, flooding, and long-term changes in the environment can affect the income from the aforementioned sectors ( IPCC 2022 , SPM-11). Populations with low adaptive capacity including marginalized groups are disproportionately affected and vulnerable to short-term economic damages related to climate change ( IPCC 2022 , SPM-8). Demographic changes may accelerate the deterioration of livelihood conditions. Population growth in the MENA region has been rapid from 105 million in 1960 to 486 million in 2021 ( World Bank 2022 ), which means more land and water are required for livelihoods. Climate change can worsen coastal erosion and decline tin he productivity of coastal plains in Israel and Morocco, which are important for food production. Sea-level rise has negative impacts on deltas, coastal plains, and human settlements, and tourism and industrial activities are also expected to decline due to heatwaves and worsening water shortages ( Sowers et al. 2011 ).

Existing studies focus on various socio-economic outcomes of climate and environmental changes and their implications on conflict mobilization. Agriculture, fisheries, and livestock sectors are particularly susceptible to the loss of income due to climate shocks such as prolonged droughts ( von Uexkull 2014 ; Schmidt and Pearson 2016 ). Loss of income due to the deterioration of livelihood conditions can lead individuals to seek alternative sources of livelihood, and some may turn to illicit activities, including joining non-state armed groups ( Barnett and Adger 2007 , 644; Seter 2016 , 5).

Another category of social outcomes includes changes in migration and mobility patterns. Migration is one of the climate adaptation strategies, and subsequent socioeconomic and political impacts of migration can be linked to conflict. Declining livelihood conditions can trigger rural-to-urban migration in search for alternative livelihoods ( Rüttinger et al. 2015 , 27). Long-term climate change and weather shocks may accelerate environmental degradation and declining livelihood conditions. The increased migration flow accelerates urbanization and creates instability in hosting cities with inadequate infrastructure for public services ( Balsari, Dresser, and Leaning 2020 ).

Changing migratory patterns of pastoralist or agropastoral groups, influenced by the availability of grazing land and water, can be linked to clashes with other communities ( Abroulaye et al. 2015 ; Mohammed Ali 2019 ). Violent communal clashes and livestock raiding, which have become increasingly lethal, are linked to intensified competition over scarce resources for pastoralist populations ( Detges 2014 ). For instance, farmer-herder conflicts in the Sahel region have become increasingly lethal during recent decades, especially in areas with a higher population and livestock density.

Previous research also focuses on the role of elites who have leveraged social outcomes of climate change for their benefit. Here, elite actors include traditional elites, privileged groups with economic and political power, and even armed group leaders. More frequent and intense climate-related extreme weather events can provide additional opportunities for local elites to capture resources. When climate-induced disasters such as droughts and floods cause humanitarian crises, their basic needs and post-disaster reconstruction would bring in additional resources to the disaster-hit regions, which can be exploited by local elites. Humanitarian aid delivery often needs to cooperate with local elites, whose influence over the aid provision can further strengthen the client-patronage relationship, which is a source of tension ( Uson 2017 ). Elite capture of resources, particularly land, is likely to generate strains within and between communities ( Zaman 1991 ). Local grievances over land rights can be exploited in intercommunal conflict or national conflicts ( Chavunduka and Bromley 2011 ). National elites can exploit local grievances of a population segment that are closely related to climate change. Inadequate government responses to Cyclone Bhola in 1970 led to a devastating human toll in the Bay of Bengal and contributed to the rise of the independence movement, which subsequently led to the secession of Bangladesh ( Busby 2022 , 181).

Changing environmental conditions by climate change may influence armed group tactics and behaviors. Armed groups have utilized the local grievances for a recruitment drive for the youth ( Benjaminsen and Ba 2019 ). Climate change also affects the way of wars are to be fought. In warm climates, prolonged and unpredictable rainy seasons can alter the fighting season and patterns. Due to the reduced water availability in some areas, the strategic importance of water access points and infrastructure may have become more salient. Armed groups can escalate the conflict by weaponizing water by flooding farmland and cities or depriving the population of water ( King 2015 ). Amid droughts and unreliable rainfalls, armed groups may consider water weaponization as a more effective tactic in order to influence and control communities already experiencing water scarcity.

The analytical framework of climate-conflict pathways is applied to analyze findings from existing research relevant to the MENA region. The following details a method of a systematic review of the literature.

This paper leverages from existing evidence by conducting a systematic review of existing studies. Systematic review method has been extensively employed in examining the linkage between climate change and violent conflict ( Ide 2018 ; Nordqvist and Krampe 2018 ; Van Baalen and Mobjörk 2018 ; Tarif 2022 ). Systematic reviews differ from a traditional sense of literature review in a way that it is “focused” and “systematic”; it zooms on a specific research question; and is based on pre-established sets of principles for literature selection. Systematic and focused nature of the review is helpful to “locate previous research, select relevant literature, evaluate contributions and analyses, and synthesize data” ( Denyer and Tranfield 2009 , 671). This approach is particularly useful to yield new insights and provide clarification on frequently debated issues ( Dacombe 2018 , 155). In addition, the method is a highly relevant policy tool that promotes evidence-based policymaking.

We have used the following set of principles for locating, selecting, and evaluating the literature. A Boolean search string containing keywords was composed with keywords from climate change and violent conflict. 2 Search words for climate-related environmental conditions include terms related to the effects of extreme weather events or long-term environmental changes on nature-based livelihoods and water and food insecurity, involuntary displacement, which are adopted from previous research done in a similar scope ( Nordqvist and Krampe 2018 ; Van Baalen and Mobjörk 2018 ; Tarif 2022 ). Several social outcomes are theorized as consequences of climate change such as internal and cross-border migration and elite exploitation of changing environmental conditions. In the paper, violent conflict is defined as the situation when one or more actors engaged in violence against hostile groups due to incompatibilities. This broad definition allows include interstate wars, terrorism to communal clashes involving violence. The definition does not include protests and non-violent actions, which are a crucial class of social phenomena leading to political instability and violence. We paid attention to this element in the analysis but excluded studies exclusively focusing on non-violent conflict (e.g., Ide et al. 2021 ). We used specific keywords relevant to conflict actors and types of conflict in the MENA region.

The Boolean search string was used in searching the abstracts of existing studies in English published during 1989–2020 from Web of Science, a major database of scholarly literature. From the search results, we read the abstracts and selected items with relevance to the relationship between climate-related environmental changes and conflict. The initial screening found 141 articles, which then were reviewed manually for their relevance to the inquiry (see the Online Appendix). In the screening process, we excluded a number of studies that focused on the impact of armed conflict on the environment and studies that did not explicitly focus on violent conflict. Similarly, studies that do not explicitly focus on climate change as in long-term climate trends, climate hazards, and weather events were excluded. Another set of articles that were removed from the list were commentaries and reviews that were not based on either qualitative or quantitative empirical material. While all the selected articles either have at least one country in the MENA region or adapt a regional focus on the MENA, the specific definition of these regions varies. In our literature review, we adhere to a specific list of countries that we recognize as part of the region. 3 After the screening, we manually searched the bibliographies of the selected articles and included eleven relevant articles. In total, forty-one articles are reviewed with a focus on a set of categories stemmed from the analytical framework for explaining the relationship between climate-related environmental change and violent conflict ( Figure 2 ).

Peer-reviewed articles reviewed

The geographical focus of the reviewed studies demonstrates that much of the scholarship focuses on Syria and Iraq. In contrast, North African countries and Gulf countries have received relatively limited attention ( Figure 3 ). The high number of research works focusing on Syria can be explained by the high profile of the contested linkage between climate change and the Syrian civil war. While media narratives have regarded Syria as a prime example of an armed conflict fuelled by climate change and several prominent public figures have publicized it as an illustration of the nexus, it is worth noting that scholarly research has presented differing perspectives on the direct causative role of climate change in conflict escalation ( Miller 2015 ; “Climate Wars - Syria” with Thomas Friedman 2017 ; VICE 2017 ).

The distribution of geographical focus of the reviewed studies

Source: a map drawn by authors.

In this section, we discuss existing explanations from previous research that connect climate-related environmental changes and violent conflict in the MENA region. The linkages between the environmental changes related to climate change and violent conflict constitute a complex chain of events (e.g., Gleditsch 1998 ). Most empirical research contributes to examine parts of the chain under specific temporal and spatial scopes, and this is one reason why it is important to consider the broader implication of each piece of evidence, which then can contribute to the better understanding of the climate-conflict pathways as a larger phenomenon. For clarity and focus, we organized a set of findings from previous studies under four pre-determined analytical categories: worsening livelihood conditions, migration and mobility, armed groups, and elite exploitation. As explained earlier, these categories are not mutually exclusive; rather, explanations under different categories are interlinked and can mutually reinforce each other in different stages of mobilization and conflict.

Direct Link between Climate Change and Violent Conflict

Scholars have examined whether climate impacts such as warmer temperatures and precipitation anomalies are statistically correlated to violent conflict, and several studies have focused on specific countries within the MENA region ( Feizi, Janatabadi, and Torshizi 2019 ; Döring 2020 ; Helman and Zaitchik 2020 ; Helman, Zaitchik, and Funk 2020 ; Sofuoglu and Ay 2020 ; Linke and Ruether 2021 ). Findings from existing research on the direct impact of climate-related factors on violent conflict and political instability suggest that the relationship is not always linear and varied in specific country contexts ( Helman and Zaitchik 2020 ; Helman et al. 2020 ). Water scarcity, for instance, is not only associated with increased communal conflict but also cooperation ( Döring 2020 ). Warming did not unitarily increase or decrease conflict risk—warmer temperatures increased risks of violence in Africa but decreased in the Middle East, and warming did not have a linear effect but had a greater effect on conflict risk in warmer regions ( Helman et al. 2020 ). Increased temperatures and rainfall anomalies are positively associated with political instability in the MENA region ( Helman and Zaitchik 2020 ; Sofuoglu and Ay 2020 ). These findings caution against generalized or simplistic assumptions about the relationship between climate change and violent conflict.

Studies have found an insignificant relationship between water scarcity and violent conflict. Precipitation levels and droughts do not have a direct impact on communal violence in a model including the Middle East and Africa ( Döring 2020 ). The same study also found that communal conflict is more likely to occur in areas with lower rainfalls and limited groundwater availability. Groundwater is less affected by short-term droughts, but prolonged droughts and unsustainable extraction can lead to groundwater shortages, which is the case in northern Syria ( Kelley et al. 2015 ) and Yemen ( Weiss 2015 ). Rainfall variability does not seem to have significantly affected the intensity of civil war violence during the 2011–2019 Syrian civil war ( Linke and Ruether 2021 ). The discussion on climate change’s impact on armed group tactics and behavior is followed in the later part of the paper.

Droughts and water scarcity seem to be a source of social disputes and non-violent conflict ( Feizi et al. 2019 ; Bijani et al. 2020 ; Ide et al. 2021 ). Whether the tension over water scarcity escalates to non-violent conflict or not seems to be contingent on the pre-existing negative socio-political relationships between groups and the types of political systems ( Ide et al. 2021 ). In Iran, irregular rainfalls and water scarcity at the local level are linked to interpersonal conflict and communal tensions and can degrade state legitimacy and contribute to political instability ( Feizi et al. 2019 ; Bijani et al. 2020 ).

Evidence from existing studies on the direct climate-conflict link also alludes to the need to further explore the mechanisms between physical environmental changes and social outcomes. Both large- N and small- N studies can contribute to the understanding of the underlying mechanisms or indirect pathways connecting climate change and conflict. The following sections discuss livelihoods, migration, inadequate management, and armed group behaviors as the pathways between climate-related environmental changes and violent conflict.

Deteriorating Livelihood Conditions

Several studies evaluating the worsening livelihood mechanism in the MENA region focus on the relationship between droughts’ impacts on agriculture and conflict. Severe and frequent droughts due to climate change may affect the region’s food security and livelihoods. In the MENA countries, agriculture, fisheries, and livestock accounts for roughly 15 percent of the total population’s livelihood ( World Bank 2023 ). Agriculture dependency is one of the best predictors of violent conflict ( von Uexkull et al. 2016 ). Indeed, evidence from a study focusing on the MENA region and Africa shows a consistent result that conflict risk is higher in areas where the population depends on agriculture for their livelihoods ( Helman and Zaitchik 2020 ).

Droughts’ impact on agriculture is an important area of research in the implications of the changing climate on the deterioration of livelihood conditions. During the last three decades, droughts in the MENA region have become more frequent and severe. Three out of four most severe multi-year droughts in the Fertile Crescent region referring to parts of Iraq, Syria, Lebanon, Palestine, Israel, Jordan, and Egypt occurred during 1990–2015 ( Kelley et al. 2015 , 3243). The sub-region has historically witnessed periodic droughts; therefore, their agricultural systems are to a degree adaptive to drought conditions and low rainfalls. More frequent and intensifying droughts and drying conditions may jeopardize the population’s adaptive capacity, leading to far-reaching and consequential disruptions in societies. In particular, the 2007–2008 drought severely affected the agricultural production in the Fertile Crescent region. Annual wheat production in Iraq during 2008–2009 declined by 35 percent ( Selby 2019 , 264). Jordan and the West Bank in Palestine also experienced a reduction in agricultural production after the 2007–2008 drought ( Feitelson and Tubi 2017 ). However, none of these countries experienced the same extent of “shock” as in Syria, whose effects some refer to as the “collapse” of the agricultural sector. The 2007–2008 drought is considered “the worst drought in the instrumental record, causing widespread crop failure” and decimation of livestock populations in northeast Syria ( Kelley et al. 2015 , 3241).

A dozen of the reviewed authors have probed the linkage between the 2007 and 2008 multi-year droughts and their impacts on agricultural and livestock production and the Syrian conflict using quantitative and qualitative methods ( De Châtel 2014 ; Gleick 2014 ; Kelley et al. 2015 ; Eklund and Thompson 2017 ; Selby et al. 2017 ; Ide 2018 ; Karnieli et al. 2019 ; Ash and Obradovich 2020 ; Daoudy 2020a , 2021 ; Eklund et al. 2022 ). These reviewed research works have disagreed on what extent the drought’s contribution to the sharp decline in agricultural production and rural livelihood in Syria. Kelley et al. (2015 ) is one of the major empirical studies that argues for the linkage between the multi-year drought and the political instability, which argument is similar to Gleick (2014 ). Other studies have refuted the causal linkage between the drought and the Syrian civil war, but their core reasons for arguing against it have varied.

Several authors point out that the impact of climate shocks on livelihoods is mediated by water governance decisions. This argument downplays the role of climate change as the main driver of livelihood deterioration rather than a contributing factor. Despite being affected by similar rainfall deficits during 2007–2008, farmers in northern Syria generally experienced far worse consequences in productivity compared to northwest Iraq and southeast Turkey ( Eklund and Thompson 2017 ). Turkey’s substantial investment in water infrastructure and placing policies for better management during the 1990s and 2000s seem to have reduced their vulnerability to droughts ( Kelley et al. 2015 ; Eklund and Thompson 2017 ). On the contrary, the Syrian regime’s agricultural expansion policy, unsustainable groundwater use, and economic policy have exacerbated the population’s drought vulnerability. Agricultural expansion schemes in Syria more than doubled the irrigated area from 650,000 ha in 1985 to 1.4 million ha in 2005, driven by “a vision of development through agrarian modernization” ( Selby 2019 , 268). The policy overlooked physical limitations of groundwater resources by over-extracting water from aquifers at a rate of 300 percent or more than the basin’s yield and depleting aquifers prior to the 2007–2008 drought ( Selby 2019 , 266). Groundwater depletion in the region has a major effect on drought vulnerability because groundwater is an important source of water during low rainfall years ( Kelley et al. 2015 ).

Weiss (2015 ) makes a similar observation in Yemen, indicating that governance issues are mainly responsible for groundwater depletion in the country rather than climate-related environmental changes. Factors related to agrarian political economy and governance capacities further affect the vulnerability. The government’s capacity to deal with environmental changes and their impact on local economies and livelihoods is pointed out to be a key mediating factor in the linkage between climate change and violent conflict. The issues related to mismanagement and elite exploitation of climate change are further discussed in the later section of the article.

A few studies found differing climate impacts based on gender and ethnicity. Vulnerability to climate change varies between communities and countries, and intersectional identities of the affected people such as gender, age, and ethnicity influence their capacity to adapt to climate change and resilience ( Thomas et al. 2019 ). Evidence from Iran shows how women are forced to carry the “double burden” of doing off-farm work activities such as weeding or thinning cotton for minimal wages, in addition to the regular household and on-farm tasks ( Keshavarz, Karami, and Vanclay 2013 ). In Syria, the mechanization of agriculture has led to a significant loss of rural employment and disproportionately affected women ( Selby 2019 , 267). The disproportionate effect on women is related to structural gender inequality restricting women’s economic opportunities and wealth accumulation ( Selby 2019 ). This finding aligns with previous literature linking gender and climate change indicating that women are often worse affected by climate impacts due to restrictive norms and rights ( Denton 2002 ). In Israel, pastoralists are often disadvantaged due to the Israeli state’s resource allocation policies prioritizing farmers. In the northern Negev region, the state’s land appropriation disproportionately affected agri-pastoralist Bedouin tribes during the early 1900s. This has led to higher vulnerability of the Bedouins during droughts ( Tubi and Feitelson 2016 ). A similar pattern of marginalization is found in Hasakah, a region in northern Syria, where the state turned open range lands into farmlands ( Selby 2019 ). The findings on differing vulnerability and impacts on livelihoods are based on a handful of studies, and intersectional approaches are generally absent in most studies reviewed in the analytic essay.

Changes in Migration and Mobility Patterns

Migration represents a critical adaptation strategy for populations affected by climate-induced environmental changes. Existing research examines various linkages between climate-induced environmental changes and migration in the MENA region. The main discussions are related to the contribution of climate shocks in internal and international migration and migration as a source of political instability and conflict. Existing evidence in the reviewed studies does not fully confirm that climate shocks and changing climate conditions are the primary drivers of internal or international migration. The link between displacement and violent conflict seems to be contested as well.

One of the predominant narratives links climate, migration, and insecurity theorizes worsening of livelihood conditions due to climate change has led to distressed migration of rural population to urban or peri-urban areas, which can contribute to greater political instability ( Gleick 2014 ; Kelley et al. 2015 ; Feitelson and Tubi 2017 ; Ash and Obradovich 2020 ). This argument gained prominence after out-migration from drought-affected regions in northern Syria in 2008 and the agricultural sector collapse in 2010 preceded the 2011 uprising.

Several studies focus on empirically examining the migration patterns after the 2007–2008 droughts in the Levant ( De Châtel 2014 ; Gleick 2014 ; Kelley et al. 2015 ; Ash and Obradovich 2020 ). There seems to be a wide-ranging estimation of the scale of internal migration in Syria during this time ( Ide 2018 ). While acknowledging the multiple factors contributing to migration, researchers have debated on the number of displaced people in northern Syria and Iraq amid the 2007–2008 drought. While Gleick (2014 , 334) and  Kelley et al . ( 2015 , 3241–2) estimate ∼1.5 million people to be internally displaced, others suggest 40–60,000 households or ∼ 300,000 displaced people ( Selby et al. 2017 , 254). Several methods are employed in estimating drought-induced migration. For instance, Ash and Obradovich (2020 ) used nightlight intensity as a proxy measure for population change, which seemed to detect the changes in population in drought-affected regions. Satellite imagery can be analyzed for measuring agricultural land use, which can be a proxy indicator for out-migration ( Eklund et al. 2022 ). Others relied on official statistics and survey data, which are based on a combination of census, fieldwork, and expert assessment (e.g., OCHA 2009 ). Nightlight intensity and satellite imagery are effective measurements of population changes, but remote sensing data provide little context about who moved, to where, and why. Fieldwork-based studies such as De Châtel (2014 ) provide insights into the socio-economic circumstances of migrants and their political orientation. A UN rapid assessment report is based on various UN-led field reports and assessments during 2006–2008 and supplies valuable on-the-ground information including changing migration patterns, children’s school enrollment, and water availability ( OCHA 2009 ). The evidence indicates that migration after the drought was indeed significant, although we cannot exactly say the scale of it. The question is whether these migrants play a role in the subsequent uprising and civil war.

Critics of this narrative argue that the Syrian uprising emerged due to political discontent, economic recession, youth unemployment, discrimination, and injustice, not because of the mass climate migrants ( De Châtel 2014 ; Selby et al. 2017 ; Daoudy 2020a ). Eklund et al. (2022 ) suggest migration triggered by the 2007–2008 droughts did not play a significant role in the uprising because migrants were likely to have returned as early as 2010 based on the satellite images showing full recovery of agricultural activities in drought-affected areas ( Eklund et al. 2022 ). Rural-to-urban migration in the MENA region is rather influenced by pre-existing socio-economic conditions and political decisions. For example, in Syria, the introduction of neoliberal agrarian policies by the government generated a significant degree of insecurity in the rural populations and prompted rural-to-urban migration ( De Châtel 2014 ; Selby 2019 ). And region’s demographic trend has a much greater and long-lasting impact on the pressure in urban areas. For instance, the urban population in Syria is estimated to have grown from 8.9 million in 2002 to 13.8 million in 2010, and most migrants lived in informal settlements with poor infrastructure and no jobs ( Kelley et al. 2015 ).

The narratives on climate change and migration in the MENA region in existing literature reflect how countries perceive climate-induced migration as a source of conflict and insecurity. Jordan, for instance, fears the influx of migration from the MENA region, mostly Palestine, Iraq, and Syria, would worsen the country’s water scarcity and thus security ( Weinthal, Zawahri, and Sowers 2015 ). Fears of “climate refugees” from Africa have shaped Israel’s discriminatory discourses and practices against African refugees and Bedouin communities inside the country ( Weinthal et al. 2015 ). Media reports have suggested that climate shocks in the MENA regions, where asylum seekers and irregular migrants originated from, have affected their decision to migrate ( O'Hagan 2015 ). More than 2.2 million migrants without legal permits have amassed at EU external borders during 2009–2017, and most migrants during this period were from the MENA region ( Cottier and Salehyan 2021 , 2).

Findings from existing research refute the idea of climate shocks would trigger refugee flows from the MENA region. Climate shocks and precipitation deficits are not linked to the increase of out-migration from the MENA region to Europe ( Abel et al. 2019 ; Cottier and Salehyan 2021 ). Severe droughts and drier weather conditions in the MENA region are associated with the reduced migration flow to Europe, which is contradictory from the popular media narrative about “climate refugees” ( Cottier and Salehyan 2021 ). This finding alone suggests that migration can be an “investment,” because the extra income generated from additional rain reduces financial barriers to emigrating ( Cottier and Salehyan 2021 , 6). The correlation between rainfall variability and asylum-seeking flows has been found during 2010–2012 when the Arab Spring swept a dozen MENA countries but not during other periods between 2006 and 2015 ( Abel et al. 2019 ). This finding demonstrates that the impact of climate change on generating asylum-seeking flows seems to be conditional on the origin country’s political stability.

Armed Group’s Tactical Considerations

Existing research specifically focusing on how climate change affects armed groups’ tactics is sparse in the MENA region (exception of Linke and Ruether 2021 ), but several research works demonstrate that armed groups may escalate their tactics due to the increased environmental stress on water and agricultural land. Changing climate conditions and weather shocks adversely affect water availability for agriculture. This trend underscores the notion that the strategic importance of controlling water and water infrastructure could emerge as an effective instrument for exerting pressure to local populations in times of armed conflicts. Previous research supplies evidence on how water is weaponized by armed groups, which is a case of escalation of tactics ( Grech-Madin 2020 ). Water weaponization is defined as the “intentional or unintentional damage or destruction of (sensitive) components of the water infrastructure like dams, treatment plants, pumping stations, piping and canal systems, sewage plants, reservoirs, wells, etc” ( von Lossow 2016 , 84).

Water has been used as both a target and a weapon by state and non-state actors. Existing studies focus on how non-state armed groups and government militaries have strategically attacked or captured water and other environmental infrastructure ( King 2015 ; von Lossow 2016 ; Sowers, Weinthal, and Zawahri 2017 ; Gleick 2019 ; Daoudy 2020b ). Water scarcity in the region is an incentivizing factor for government troops and armed groups to use water to incur damage to the local population. Attacks on water pipes, sanitation and desalination plants, water treatment, pumping and distribution facilities, and dams have occurred in Syria, Libya, and Yemen during civil wars. Targeting of water infrastructure also occurs in protracted conflict situations such as the Israel and Palestine conflict when Israel was accused of attacking wells in Gaza City ( von Lossow 2016 , 84). Particular attention has been drawn to rebel groups’ ability to use water for strategic but as well psychological terrorism ( King 2015 ).

The weaponization of water is not limited to targeting water infrastructure during wartime. Increasing water scarcity and the importance of water access influence the strategic calculation by armed groups on when and where they would deploy violence ( King 2015 ). Non-state armed groups such as the Islamic State in Syria and Iraq are known to have fought over the control of water infrastructure in the Euphrates and Tigris Rivers as part of their expansion strategy ( von Lossow 2016 ). Armed groups fight more intensely during the growing season, which is linked to tax revenue from agricultural harvest and control of the population who rely on farming ( Linke and Ruether 2021 , 116).

Armed groups can also use water as a tool of governance. By providing water and electricity to the local population, the Islamic State achieved ideological credibility as well as legitimacy over the local population, which was a core component of the IS claim of statehood ( King 2015 ; von Lossow 2016 ). Supplying water is a crucial governance function, so armed groups can obstruct water infrastructure to damage the conflict party’s control and legitimacy.

Elite Exploitation

Previous research demonstrates how elite exploitation is linked to protests and violent conflict by focusing on corruption, elite capture of disaster relief, and elite bias in the MENA region. Political patronage and ethnic, tribal, and religious networks for political mobilization shape elite behavior in the region. Political patronage is not unique to the MENA region, but clientelism explains the viability of political networks of some political elites in the MENA region who maintained power through providing resources and preferential treatment in return for votes, loyalty, and compliance ( Herb 1999 ; Haddad 2012 ). Social fabrics of the MENA are woven with diverse ethnic, tribal, and religious groups, and these minorities have also been part of political cleavage structures ( Belge and Karakoç 2015 ). Political mobilization along ethnic, tribal, and religious lines has been effective in the contexts when these identities are contested ( Yiftachel 1996 ). In the following, three main findings from existing research are outlined.

Climate change may increase opportunities for elites to appropriate humanitarian aid for their benefit, and elite exploitation can worsen the conflict risk amid climate-induced disasters and environmental scarcities. The risk of politicization of humanitarian and development aid has been extensively studied ( Doocy and Lyles 2018 ; Alqatabry and Butcher 2020 ). In situations of climate-induced disasters, local and central elites can have a significant influence on the planning and distribution of humanitarian aid. Political elites can be biased in their relationship with local elites, and this elite bias can have implications for local-level politics ( Brosché and Elfversson 2012 ). After the 2007–2008 drought in Syria, the Assad government directed the UN-led relief efforts to almost entirely focus on the Arab district of Al-Shaddadi, although the Kurdish communities were equally or worse affected ( Selby 2019 , 270). Unequal aid distribution can increase intercommunal tensions during droughts. State intervention can reduce the risk of conflict amid climate-related natural disasters. Tubi and Feitelson (2016 ) demonstrate how proactive relief provisions during droughts have reduced communal violence between Bedouin herders and Jewish farmers in Israel. The findings from Tubi and Feitelson (2016 ) confirm that the state’s capacity to adapt and absorb shocks remains essential for the inhabitants’ perceived marginal benefits and the opportunity cost of conflict ( Post et al. 2016 ).

Powerful elites compete over acquiring land and water resources from weak and vulnerable groups. Mismanagement and corruption in the public sector are other factors that affect the population’s access to water and basic services, which are simultaneously hampered by climate change ( Kim and Swain 2017 ). In Yemen, most communal conflict occurs over water and land when tribal elites compete with one another ( Weiss 2015 ). In southern Iraq, a large volume of water is illegally diverted for commercial farms owned by elites, which worsens water scarcity ( Mason 2022 ). Donor-funded projects for repairing Basra’s aging water infrastructure after the 2003 invasion, worth 2 billion USD over nearly two decades, were succumbed to widespread corruption ( Mason 2022 ). Bureaucratic procedures endow opportunities for officials to extort bribes such as well-licensing in Syria and water development project licensing in Lebanon ( De Châtel 2014 ; Mason and Khawlie 2016 ). In Syria, the government’s requirement to annually renew well licences was an opportunity for security personnel and local officials to collect bribes ( De Châtel 2014 , 12). Protestors in Dara’a, Syria initially demanded to end corruption in the water sector ( De Châtel 2014 ). In Iraq, the epidemic of corruption in the water sector endowed youth and urban poor grievances against the state, which led to widespread protests ( Human Rights Watch 2019 ).

Although the MENA region is a climate change hotspot, governance failures, and mismanagement account for declining water access ( Mason and Khawlie 2016 ; Selby et al. 2017 ; Daoudy 2021 ). Elites in the MENA region have leveraged climate change to explain some of the governance failures in the water and agriculture sectors. The Syrian state and security apparatus have exploited the narratives around climate change by portraying Syria as a “naturally water-scarce” country, although the reality on the ground shows a man-made water crisis due to corruption and inefficient management by the government authorities ( De Châtel 2014 , 9). Similarly, the Lebanese government blamed climate change for the reduction of water flow in the Hasbani Basin, while civil society representatives accused the government of “systematically neglecting their concerns” about water access ( Mason and Khawlie 2016 , 1352–3).

Tensions over transboundary water sharing may continue to rise in the MENA region ( Bulloch and Darwish 1993 ; Amery 2002 ). The Euphrates River and Tigris River are important water sources for Turkey, Iraq, Syria, and Iran, and Turkey controls the water flow through the investment in the Southeastern Anatolia Project consisting of twenty-two large reservoirs and nineteen hydroelectric power stations on the upper tributaries of the Euphrates and Tigris Rivers. Karnieli et al. (2019 ) argue that Turkey’s transboundary investment and dam filling to be the primary driver of 2007–2008 droughts in Syria instead of climate change. This might be inconsequential because Turkey released additional water to Syria during the drought (see Kibaroglu and Scheumann 2011 , 297). As long as the downstream countries, Syria, Iraq, and Iran, see their domestic water problems to be attributed to the upstream dams in Turkey (e.g., Al-Muqdadi et al. 2016 ), transboundary rivers can be a source of interstate tension—although it is unlikely to develop into a full-scale armed conflict ( Bencala and Dabelko 2008 ). The impact of climate change in transboundary water governance is still an under-researched area that deserves more attention. Another area that can be a subject for further research is a growing sub-national competition over water such as brewing tension within Iraq due to the Kurdish Regional Government’s dam building plans ( Tinti 2023 ).

Existing evidence demonstrates that climate impacts, particularly droughts and drying trends, contribute to armed conflict in various ways. This section weighs in on the findings from the analysis to evaluate the overall framework of pathways to climate insecurity in the MENA region. The synthesis of findings highlights consensus and disagreement in existing studies and identifies the areas for further research.

Water scarcity in the MENA region is apparent at multiple scales, from domestic to transboundary, and has various implications for social vulnerability and political stability. The region’s water insecurity is as much driven by governance challenges as climatic and environmental trends. Severe droughts in the Levant during 2007–2009 appear to have led to the decline in agricultural production in the affected areas, but the drought vulnerability is mediated by groundwater availability, the viability of irrigation systems, and the capacity of water infrastructure ( Kelley et al. 2015 ). Decades of mismanagement of water resources and institutional failings undermine adaptive capacities in the region, demonstrated in examples from Lebanon, Yemen, Syria, and Iraq ( Weiss 2015 ; Mason and Khawlie 2016 ; Selby 2019 ; Mason 2022 ).

The depletion of groundwater in parts of the MENA region is largely attributed to the government’s unsustainable agricultural and water policies. Groundwater offers an important source of reserve during droughts, and the unsustainable use of groundwater adversely affects farmers’ drought vulnerability. Government subsidies on fuels encouraged farmers to install diesel pumps to use groundwater for irrigation, without consideration for sustainability in Yemen and Syria ( Weiss 2015 ; Selby 2019 ). These governments’ agricultural and economic policies resulted in farmers growing more water-intensive crops such as cotton and citrus fruits, which accelerated groundwater depletion. Political elites used fuel subsidies to ensure support from farmers at the expense of the environment. These unsustainable water and agricultural policies are not technical “mismanagement” but embedded in a much larger political context and ideology ( Daoudy 2021 , 13). Considering political factors in climate vulnerability is an important aspect to understand the climate-conflict nexus in the MENA region.

This analytic essay also looks into the important debate about the contribution of droughts in the Syrian uprising and subsequent civil war. Fourteen out of thirty-nine existing studies focus on the Syrian conflict and examine various linkages between the conflict and climate-related environmental factors. The popular narrative portrays the Syrian civil war as a climate conflict that is triggered by climate-induced agricultural collapse resulting in mass displacement ( Gleick 2014 ; Werrell, Femia, and Sternberg 2015 ). Research refutes this narrative by contesting the empirical foundations. Drought-displaced people in urban or peri-urban areas did not participate in street protests ( De Châtel 2014 ), and a significant proportion of the displaced returned to northern Syria before the revolution began ( Eklund and Thompson 2017 ; Eklund et al. 2022 ). Reviewing the literature demonstrates that attributing the onset of the Syrian civil war solely to climate change lacks empirical substantiation. Nevertheless, climate-related environmental changes, such as falling groundwater levels, have significant impact on natural resources and livelihoods, which can consequently undermine human and environment security.

Internal migration is more prominent than international migration in the research focusing on climate-induced mobility in the MENA region. This is similar to other studies with different regional focus (e.g., Burrows and Kinney 2016 ). The disruption of the rural livelihoods appears to be a strong push factor in Syria, which can be worsened by droughts ( Fröhlich 2016 ). Data on migration seem to be a challenge in unpacking this complex phenomenon. It is challenging to disentangle environmental changes from economic drivers in migration decision-making. Satellite-based data provide reasonable proxy measures for in- and out-migration in locations (e.g., Ash and Obradovich 2020 ), but they do not offer insights on who moved from where to where and why. More studies incorporating qualitative data are needed to further the understanding of climate-induced internal migration.

There is clear evidence that armed groups have escalated their tactics by weaponizing water in the MENA region. Several studies demonstrate how armed groups escalate their tactics by weaponizing water. Such a wartime trend indicates a heightened risk for civilians and long-term consequences by destructing key water infrastructures. This finding is highly policy relevant for strengthening and enforcing international laws for civilian protection during armed conflict (see Grech-Madin 2021 ). In relation to the armed group’s tactics, more research is needed to unpack the role of climate-related environmental factors in the armed group’s recruitment and tactical decisions.

The findings on differing vulnerability and gendered impacts on livelihoods are based on a handful of studies, and intersectional approaches are generally absent in most studies reviewed in the analytic essay. How climate shocks have varying impacts on people based on their gender, age, livelihoods, ethnicity, and combinations of these identities is missing. If marginalization and grievances are key processes of climate-induced conflict, how climate change affects different segments of the population differently needs better understanding.

The relationship between climate change and violent conflict is primarily indirect and varied, cautioning against generalized assumptions. How climate change influences the risk of violent conflict in the MENA region is mediated by political economy, institutional shortcomings, and elite competition. The risk of violent conflict is contingent on pre-existing negative socio-political relationships, types of political systems, and different climate vulnerabilities of various social groups. Gendered climate vulnerabilities need better understanding for establishing the linkage between climate vulnerability and insecurity. Carefully examining existing evidence is important for both over general climate security discussions as well as for the policy discussions on the MENA region, which has remained a focal point of scholarly and policy debates concerning climate security ( Daoudy, Sowers, and Weinthal 2022 , 7).

Disentangling specific climate impacts is also crucial for enhancing government’s climate adaptation and disaster mitigation policies in the MENA region. Civil society representatives from the MENA region have been concerned that states and political elites blame climate change to legitimize inequalities and to devoid accountability ( Selby et al. 2017 ; Kausch 2022 ). As existing research demonstrated, water and food insecurity in the region is driven by a lack of state capacity to properly manage natural resources and the integrity of public institutions in the MENA region.

Future research should pay attention to other types of climate hazards, including floods, heatwaves, and dust storms. Existing research primarily focuses on droughts and precipitation deficits, failing to account for heatwaves and flooding, which also are common in the MENA region. Floods are understudied despite their severe humanitarian impact. For instance, heavy flooding forced more than 84,000 people to displacement in Yemen, 13,000 people in Iran, and 5,000 people in northern Iraq in 2021 ( IDMC 2023 ). How flooding affects livelihood conditions and social vulnerability would be considerably different from droughts. Studies from other regions suggest floods are not associated with communal violence ( Petrova 2022 ). Ultra-heatwaves are likely to worsen without substantial government interventions ( Zittis et al. 2021 ), and their impact on oil exploitation, tourism, and urban areas demands more research. Oil and tourism industries are economic backbones of several MENA countries, and adverse impact on these sectors is likely lead to ripple effects on the society. A decrease in oil production due to extreme heatwaves and dust storms will affect public service provisions by the governments, which can be a source of instability as previous research points out (e.g., Mason 2022 ).

Future research should look at non-violent conflicts, especially protests linked to climate change in the MENA region. There is already a substantial debate on the role of food security in political stability, such as in the Arab Spring ( Werrell and Femia 2013 ; Schilling et al. 2020 ). And few studies focus on under what conditions droughts and floods can lead to non-violent conflicts such as political unrest and protests ( Ide, Kristensen, and Bartusevičius 2021 ; Ide et al. 2021 ). Youth climate activists in the region have demanded their respective governments to take proactive climate actions ( Altaeb 2022 ). Climate change is becoming a politically salient topic, and the MENA region’s civil society has voiced its concerns about the inaction and growing uncertainty about the future. How the region’s climate activism interacts with politics appears to be an important area for future research.

The narrative about climate change and conflict in the MENA region is shaped by both scientific projections but also a “long history of colonial and postcolonial scholarship invoking environmental determinism as an explanation for underdevelopment” ( Daoudy et al. 2022 , 7). This calls for more “open” and critical approaches in researching the climate-conflict nexus in the region. The evidence from existing studies shows that current water and food insecurity in the MENA region are outcomes of domestic politics and institutional shortcomings rather than past climate change. This highlights the importance of governance reforms for enhancing adaptative capacity in the region ( Sowers et al. 2011 ). Improved understanding of how vulnerability to climate change interacts with political systems, institutions, and social relations can inform policy development. This enhanced understanding can equip relevant stakeholders to more effectively anticipate, prevent, and respond to the intricate web of risks entwining climate change and violent conflict, while concurrently enhancing resilience-building efforts.

We adopt SIPRI’s definition of the MENA region, which includes Bahrain, Egypt, Iran, Iraq, Israel, Jordan, Kuwait, Lebanon, Oman, Palestine, Qatar, Saudi Arabia, Syria, Turkey, the United Arab Emirates (UAE), North Yemen (–1990), South Yemen (–1990) and Yemen; (NA) Algeria, Libya, Morocco, and Tunisia. See “Regional coverage,” See SIPRI databases at https://www.sipri.org/databases/regional-coverage .

The search string was the following: AB=((climat* OR "climat* change" OR "climat* variability" OR rainfall OR precipitation OR drought OR "water scarcity" OR "land degradation" OR weather OR disaster OR temperature OR warming OR "sea level rise" OR desertification OR famine OR “soil erosion” OR flood*) AND (conflict OR jihad* OR armed OR insurgen* OR rebel* OR terror* OR violen* OR war) AND ("middle east*" OR “north africa*” OR MENA OR algeria OR bahrain OR egypt OR iran OR Iraq OR israel OR jordan OR kuwait OR lebanon OR libya OR morocco OR oman OR palestin* OR qatar OR “saudi arabia” OR syria OR tunisia OR “united arab emirates” OR yemen OR “western sahara”)).

Here, we use SIPRI’s definition of the MENA region, which includes Bahrain, Egypt, Iran, Iraq, Israel, Jordan, Kuwait, Lebanon, Oman, Palestine, Qatar, Saudi Arabia, Syria, Turkey, the United Arab Emirates (UAE), North Yemen (–1990), South Yemen (–1990) and Yemen; (NA) Algeria, Libya, Morocco, and Tunisia.

Author’s note : This work is supported by funding from the Swedish Ministry for Foreign Affairs as part of SIPRI’s Climate Change and Security Project and the Norwegian Ministry of Foreign Affairs for SIPRI’s Climate-Related Security and Development Risks Project. We would like to thank two anonymous reviewers for their constructive feedback for improving the manuscript. We are indebted to Florian Krampe, Farah Hegazi, and Kheira Tarif for their helpful comments throughout the writing process.

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Climate Change Threatens the Health of Aging Adults, Researchers Say

A series of research articles published in the scientific journal The Gerontologist collectively outline several risks that climate change poses for aging well.

The journal's special issue, titled "Climate Change and Aging," presents multiple links between climate change and human health, particularly for aging adults.

In one of the forum articles included in the special issue, "A Framework for Assessing the Effects of Climate Change on Dementia Risk and Burden," researchers noted the risk of worsening Alzheimer's disease and related dementias (ADRD) because of climate change.

The study's authors showed multiple ways that climate change could directly and indirectly impact people with ADRD, such as how extreme heat and wildfires can impact healthcare infrastructure, housing, community programming and biological processes. They also noted how climate action could improve health outcomes and the aging experience.

"Because the same fossil fuels that are causing climate havoc are also spewing harmful air pollution that harms older people disproportionately, a transition away from these polluting energy sources towards cleaner, healthier options (such as solar power and wind energy) can deliver benefits for our health and the climate," wrote Dr. Vijay Limaye , co-author of the study and director of Applied Research Initiatives, Science Office & International, for the nonprofit Natural Resources Defense Council (NRDC). "But because of the scale of climate pollution that we've already added to the Earth's atmosphere, we must also better prepare our communities for the climate hazards that we will continue to face in future years."

One of the research articles included in the special issue further examined climate impacts on people with dementia , particularly in terms of disaster preparedness for the people living with dementia and their caregivers, highlighting a need for more resources for caregivers to reduce the stress of disaster preparedness for aging populations.

Another forum article, "Age-Friendly and Climate Resilient Communities: A Grey–Green Alliance," noted that existing frameworks to address livability and wellness for aging populations typically don't incorporate climate resilience. So the study authors wrote a framework to complement the World Health Organization's (WHO) Global Network of Age-Friendly Cities and Communities with an emphasis on sustainability.

In one of the research articles, "Population Aging and Heat Exposure in the 21st Century: Which U.S. Regions Are at Greatest Risk and Why?" researchers identified areas of the U.S. where older adults faced the highest risk of extreme heat, including New England, the upper Midwest and rural mountain regions.

Similarly, another research article included in the special issue examined vulnerabilities to extreme heat in Portland, Oregon, and found one area of the city in particular that was the most vulnerable to extreme heat also included a high number of older adults and had the highest concentration of housing with age and income restrictions.

Additional studies in the special issue examined states' climate adaptation plans and how they address aging populations, emotional well-being for people of different ages who have experienced stress related to hurricanes, and a series of interviews between climate activists ages 16 to 76, which revealed compassion across generations rather than blaming or criticizing.

"Our world is experiencing significant alterations and challenges due to climate change," the special issue authors concluded. "This Special Issue of The Gerontologist clarifies that the seismic changes wrought by climate change will similarly alter how we age and study the aging process."

This article originally appeared on EcoWatch and was syndicated by MediaFeed.

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Climate policy conflict in the U.S. states: a critical review and way forward

Joshua a. basseches.

1 University of Michigan, Ann Arbor, USA

Rebecca Bromley-Trujillo

2 Christopher Newport University, Newport News, USA

Maxwell T. Boykoff

3 University of Colorado, Boulder, USA

Trevor Culhane

4 Brown University, Providence, USA

5 Salem State University, Salem, USA

David J. Hess

6 Vanderbilt University, Nashville, USA

7 Massachusetts Institute of Technology, Cambridge, USA

Rachel M. Krause

8 University of Kansas, Lawrence, USA

Harland Prechel

9 Texas A&M University, College Station, USA

J. Timmons Roberts

Jennie c. stephens.

10 Northeastern University, Boston, USA

Many U.S. states have taken significant action on climate change in recent years, demonstrating their commitment despite federal policy gridlock and rollbacks. Yet, there is still much we do not know about the agents, discourses, and strategies of those seeking to delay or obstruct state-level climate action. We first ask,  what are the obstacles to strong and effective climate policy within U.S. states? We review the political structures and interest groups that slow action, and we examine emerging tensions between climate justice and the technocratic and/or market-oriented approaches traditionally taken by many mainstream environmental groups. Second, what are potential solutions for overcoming these obstacles? We suggest strategies for overcoming opposition to climate action that may advance more effective and inclusive state policy, focusing on political strategies, media framing, collaboration, and leveraging the efforts of ambitious local governments.

Introduction

Powerful interests have rebuffed climate policy efforts in the U.S., leading to decades of federal government inaction and heightened attention at the state level, where there has been comparative progress (Rabe 2007 ; Bromley-Trujillo et al. 2016 ). A great deal has been written about this shift to the states, and a robust literature on U.S. climate federalism has emerged (e.g., Karapin 2016 ; Rabe 2011 ; Thomson 2014 ; Woods 2021 ), including the significant climate policy action undertaken by states in the context of federal gridlock and policy rollbacks (Bromley-Trujillo and Holman 2020 ). For example, after President Trump announced U.S. withdrawal from the Paris climate agreement, cities and states formed coalitions with major companies and institutions to proclaim, “We Are Still In” (We are still in 2021 ). Twenty-five governors joined the United States Climate Alliance (USCA), committing their states to the goals of the Paris Agreement (USCA 2019 ).

Although many states have adopted climate policies, there remain significant obstacles to passing strong and effective state-level climate policies rather than merely symbolic policies that set goals without mandates or that do not include penalties for noncompliance (Stokes 2020 ). Even in liberal states without significant fossil fuel production, policy efforts often fail to meet their emission reduction targets (Basseches 2019 ; Culhane et al. 2021 ). While there has been a proliferation of research on state-level climate and energy policy since the mid-2000s, scholarship using politics as an organizing, theoretical frame has only exploded in the last few years, making a synthesis geared toward this question of political obstacles quite timely (Woods 2021 ). This review thus focuses on two core questions:

First, what are the obstacles to adopting robust climate policy within U.S. states? We review the political structures and interest groups that slow or dilute action, and we also examine emerging tensions between climate justice and the more market-oriented approaches traditionally taken by many mainstream environmental groups. Furthermore, we explore the ways that conservative countermovements have shaped public opinion and elite decision-making on climate policy.

Second, what are potential solutions for overcoming these obstacles? Rather than ending with a mere summation and call for more research, we distill some strategies for overcoming opposition to climate action that may advance more effective and inclusive state policy. We suggest strategies to advance ambitious solutions, with a focus on political strategies, media framing, collaboration, and leveraging the efforts of ambitious local governments.

This review is structured in three main sections: (1) an overview of state climate policy efforts, (2) obstacles to robust state-level climate mitigation policy, and (3) solutions to maximize state-level climate policy effectiveness. Although our focus is entirely on the U.S., many of the obstacles and strategies for overcoming them are not unique to the U.S., and this review is likely to be relevant for researchers, policymakers, and advocates in other countries and at other levels of government. We begin with a brief overview of state climate policy efforts before moving to our discussion of obstacles and solutions.

An overview of state climate efforts

The focus of this paper is on climate mitigation policy, which can take many forms including broad-based climate policies, transportation policies, and electricity sector policies that have climate change implications (Grant et al. 2014 ; Bromley-Trujillo and Holman 2020 ). In the U.S., states have led in this area since the early 2000s as detailed in scholarly work (e.g., Rabe 2004 ; Matisoff and Edwards 2014 ; Bromley-Trujillo and Holman 2020 ).

These studies demonstrate a wide range of policy activity that centers on broad-based climate change efforts such as climate action plans, carbon cap-and-trade, and GHG reduction targets, transportation sector policies including low carbon and alternative fuel standards, and electricity sector policies such as renewable portfolio standards, net metering, and decoupling.

While it would be impossible to discuss in detail every policy states have adopted here, we begin by presenting an overview of key policy instruments states have used with an emphasis on the more frequently adopted policies across the aforementioned categories (broad-based climate efforts, transportation sector and electricity sector policies). Table ​ Table1 1 gives a description of state climate policy instruments, as identified by the Center for Climate and Energy Solutions, which emphasize some of the more comprehensive state climate policies to date.

State climate policy innovations

Source: Center for Climate and Energy Solutions

Figure  1 shows the frequency of these policy adoptions by 2021, demonstrating considerable variance in total adoptions.

Key climate policy enactments across states by 2021

These policies are not the only efforts states engage in. For example, when it comes to the electricity sector and energy efficiency, 20 states have enacted a green building standard requiring public buildings to meet LEED or related standards (DSIRE 2021 ; May and Koski 2007 ). Another 15 states have adopted an appliance efficiency standard that goes beyond federal requirements. With regard to transportation, 45 states have adopted some form of incentive for hybrid/electric vehicles to date (Hartman and Shields 2021 ).

Across state legislatures in 2020, policy has centered on environmental justice and equity bills, development of electric vehicle infrastructure, and electrification of the transportation sector through tax incentives (Andersen et al. 2021 ). Despite these significant advances, it is clear that state policy actions are highly variable and currently insufficient to meet U.S. climate mitigation goals. Variability is evident when looking at RPS policies, which have been adopted by 37 states with considerable differences in stringency. For instance, South Carolina has a modest requirement of 2% generation capacity from renewable energy by 2021, compared to California, which requires 100% of electricity from renewable sources by 2045. Moreover, several states have engaged in policy retrenchment in recent years by making reductions to their state RPS targets (e.g., Ohio) or adjusting their net metering programs through phase outs, or the introduction of fees (e.g., Kentucky, Indiana) (Bromley-Trujillo and Holman 2020 ). Absent more consistent and stringent state policy coverage, the U.S. cannot meet climate mitigation objectives, necessitating efforts to reduce obstacles to more robust state climate policy activity.

Obstacles to subnational climate policy

In this section, we discuss the obstacles to more robust and widespread state-level climate policy. We examine four obstacle categories: (1) governance and institutions, (2) media and public opinion, (3) industry and interest group opposition, and (4) divided pro-climate coalitions.

Governance and institutions

Political party governance and institutional arrangements in state government are important obstacles to climate policy action, particularly as environmental issues have become more politically polarized over time (Daniels et al. 2012 ). Democratic control of state governments facilitates climate policy adoption while Republican leadership acts as a veto point for climate legislation, often necessitating a Democrat trifecta to achieve bill passage (Bromley-Trujillo et al. 2016 ; Coley & Hess 2012 ; Trachtman 2020 ). There is also evidence to suggest a “counter-partisan response” at the state level (Miras and Rouse 2021 ); that is, when one party controls the federal government, the opposing party may become emboldened to act at the state-level (Bromley-Trujillo and Holman 2020 ).

State institutional configurations such as legislative professionalism and administrative capacity also play an important role. Legislative professionalism, which refers to variation in time in session, salary, and staff in state legislatures (Squire 2007 ), can play a meaningful role in the quality and quantity of policy adopted by state governments. For climate change, it is particularly important because this issue is technical and complex. Professionalized legislatures tend to be more adept at crafting innovative legislation around complex issues, while refuting anti-climate “model legislation” from groups like the American Legislative Exchange Council (ALEC), a conservative-business alliance known for providing anti-climate legislation for state legislators to formally introduce (Hertel-Fernandez 2014 ; Jansa et al. 2019 ).

Research also shows that the organization of the executive branch has an important effect on policy outcomes (Karapin 2016 ; Raymond 2016 ). To illustrate, Carlson ( 2017 ) demonstrates that administrative/regulatory capacity has been key to California’s climate policy innovation. Meckling and Nahm ( 2018 ) argue that when state legislatures delegate significant policymaking authority to executive branch agencies, the latter tend to be relatively depoliticized and less susceptible to powerful interest groups. However, the success of administrative delegation is contingent on administrative capacity (Meckling and Nahm 2018 ).

Another important institutional consideration is the formal powers afforded to majority party leaders and committee chairs in legislative bodies (e.g., Anzia and Jackman 2013 ; Anderson et al. 2016 ). Formal powers are in part a product of other institutional arrangements, such as the presence or absence of term limits (Carey et al. 2006 ; Mooney 2012 ; Shay 2020 ). Basseches ( 2019 ) shows that the concentration of institutional power in the hands of majority party leadership, even when the majority party is Democratic, facilitates access and influence for business actors while limiting it for environmental groups.

Media and public opinion

Media coverage and public opinion around climate change also present obstacles to robust climate policy in the case where public concern is low (Bromley-Trujillo and Poe 2020 ; Bromley-Trujillo et al. 2019 ) and when media coverage frequency and content fail to raise the issues’ salience (Boykoff et al. 2021 ).

Media representations are powerful conduits of climate science and policy (mis)information. Moreover, media coverage of climate change, which is heavily driven by elite cues, is likely to shape public attitudes (Carmichael and Brulle 2016 ). Research on media portrayals of science-based issues shows that quantity and content of media coverage influences state-level agenda-setting (Bromley-Trujillo and Karch 2019 ). As such, when coverage presents climate science as uncertain, or fails to engage the views of different subgroups (Howarth and Black 2015 ), that coverage can shift climate change off of public and governmental agendas (Boykoff et al. 2021 ).

Public opinion also emerges as a barrier to climate action through influence on state legislative agendas (Bromley-Trujillo et al. 2019 ) and broader public discourse. Despite the scientific consensus on climate change (IPCC 2014 ), public attitudes are highly polarized (Guber 2013 ; McCright and Dunlap 2011 ). Variation in climate attitudes tends to fall in four primary areas: public understanding and awareness, the existence of climate change, issue salience, and public policy (Egan and Mullin 2017 ). On understanding and awareness, a 2020 Yale survey showed that only a slight majority (55%) of the public believes that “most scientists think global warming is happening,” which does not reflect the current scientific consensus (Leiserowitz et al. 2020 ; Egan and Mullin 2017 ). Furthermore, while a large majority of the public (72%) say climate change is happening, only a smaller majority (57%) indicate that it is human-caused (Marlon et al. 2020 ).

With respect to issue salience (i.e., the level of importance placed on climate change), U.S. residents have historically seen climate change as a low governmental priority (McCarthy 2016 ), especially compared to the populaces of other countries (Egan and Mullin 2017 ). Attitudes toward specific climate policies are mixed, and sensitive to question wording. Support tends to be high for renewable energy investment and broad climate policy pronouncements (Bowman et al. 2016 ; Stoutenborough et al. 2014 ), but lower for more complex policies and for those imposing costs (Stokes and Warshaw 2017 ).

Partisan differences are also significant barriers to climate policy action. Republicans are more likely to believe that climate change does not exist, is the result of natural processes, or is too costly to address (Hornsey et al. 2016 ). Additionally, factors shown to influence climate attitudes (e.g., extreme weather experience and scientific knowledge) are moderated by partisanship (Shao et al. 2017 ). Direct experience with extreme weather is perceived differently by Republicans, Independents, and Democrats, with Republicans typically understating the seriousness of their experiences, and Independents most sharply swinging with recent weather (Hamilton 2011 ; Hamilton and Stampone 2013 ; Shao et al. 2017 ; Myers et al. 2012 ).

Industry and interest group opposition

A third source of climate policy obstacles are interest groups, including fossil fuel and business lobbies, electric utilities, and a broad conservative countermovement.

Fossil fuel lobbying, corporate political activity, and corporate-state relations

U.S. federalism delegates immense authority to states when it comes to climate and energy policy, and state efforts have expanded in the face of federal inaction (Karapin 2020 ; Thomson 2014 ; Rabe 2011 ). This creates new opportunities for corporations and their lobbyists to influence climate policy. Initially, the increased authority of states prompted researchers to anticipate a “race to the top” with some states setting higher environmental standards (Fiorino 2006 ). However, subsequent research showed that the political economy of the environment often generates a “race to the bottom,” with some states competing for fossil fuel companies to develop their energy resources (Rabe 2007 , 2013 ; Davis 2012 ; Cook 2017 ). Furthermore, after states become dependent on employment and tax revenues from the fossil fuel companies, they tend to make concessions to them. Wingfield and Marcus ( 2007 ) show that many of the states most dependent on fossil fuel industries have among the weakest environmental policies (e.g., Wyoming, Alabama, North Dakota, West Virginia, Louisiana).

The political alignment of subnational states and the fossil fuel sector is also motivated by economic co-dependence between state governments and the fossil fuel sector, resulting in states’ protecting business interests in order to advance the states’ economic growth and development agendas. However, this strategy can create conflict with neighboring states where air quality is adversely affected by high-polluting states. To mediate this conflict between states, the Obama Administration enacted the Cross-State Air Pollution Rule to limit the drift of airborne pollution across state borders. This policy quickly became a contested terrain between states and the federal government over jurisdiction, and it was resolved by the federal government making concessions to high-polluting states (Prechel 2012 ). Economic co-dependence also results in other actions by states that benefit the fossil fuel industry. To illustrate, several Republican lawmakers in Texas recently proposed legislation that threatened to divest the state’s more than $100 billion in retirement funds from banks and asset managers that boycott the fossil fuel sector (Douglas 2021 ).

Further, relaxed antitrust enforcement at the federal level has permitted the emergence of giant fossil fuel corporations (e.g., ExxonMobil, Koch Industries), which have virtually unlimited capital to spend on lobbying, political contributions, and media campaigns to oppose climate legislation. To illustrate, the Koch Brothers spent some of their $80 billion in wealth on an extensive media campaign to discredit scientific research on environmental pollution (Mayer 2017 ). Furthermore, during the 2019–2020 federal election cycle, the Koch Brothers’ Super PAC, Americans for Prosperity Action, spent more than $47.7 million on federal elections in disclosed contributions compared to less than $41.5 million for all contributions by the largest 20 environmental organizations (Open Secrets 2020a , 2020b ). Moreover, historically, Americans for Prosperity Action has spent much more on undisclosed contributions (i.e., dark money), which reached $407 million during the 2012 federal election (Fang 2014 ).

Some of the most active anti-climate policy trade groups include state chapters of the American Petroleum Institute, the Oil Heat Institute, and associations of manufacturers and state Chambers of Commerce. Trade organizations are often dominated by a few of the largest firms, which have key positions on boards of directors, experts to serve on policy-drafting committees, and influence over hiring in state governments. Interviews with Chamber of Commerce representatives and observations of testimony show substantial variation in major industry group positions, though they generally resist new taxes or regulations (Culhane et al. 2021 ).

Despite their massive resources, fossil fuel corporations and trade groups do not have the expertise to address every environmental issue. Thus, many are members of the neoliberal policy organization, ALEC, which is committed to small government and unregulated markets. ALEC is dominated by the largest corporations because it charges high membership dues in exchange for model legislation that it distributes to state lawmakers. ALEC also operates as a networking mechanism that facilitates connections between corporations with shared interests (Prechel 2021a ). For example, Koch Industries created a political coalition with the former Enron Corp. and succeeded in enacting model legislation in twenty-four U.S. states (Hertel-Fernandez 2019 ).

Given that electricity accounts for more than a quarter of U.S. greenhouse gas emissions (U.S. EPA 2018 ), electric utilities are critical actors in state-level climate policymaking (Prechel 2012 ; Basseches 2020 ; Isser 2015 ; Stokes 2020 ). The U.S. electric sector is complex, with variation across states in the degree to which utilities are private corporations (known as “investor-owned utilities”) or customer-owned utilities, which can either be government-owned or electricity cooperatives (Greenberg & McKendry 2021 ). However, most U.S. residents receive electricity from investor-owned utilities (IOUs) rather than from public or cooperative organizations (U.S. Energy Information Administration 2017 ). States vary in the degree to which they undertook efforts to break up vertically integrated utilities and introduce retail competition beginning in the late 1990s (Borenstein and Bushnell 2015 ), and this variation led to differences in how these actors came to view climate policy proposals (Basseches 2020 ).

The technical complexities of utilities’ operations and regulations make the policy area less accessible to many observers, but the scholarship that attends to IOUs' political activities shows them to be among the most politically powerful actors in state-level climate policymaking (e.g., Basseches 2020 ; Culhane et al. 2021 ; Stokes 2020 ). The sources of their influence include monopoly control of electricity distribution, unparalleled technical expertise, their lobbying force, and flexible corporate organization (Basseches 2020 ). The latter has facilitated mergers and acquisitions that have allowed utility parent companies to operate across state lines, despite being mainly regulated at the state level (Hempling 2020 ; Prechel 2021a , b ).

Despite their political power, the degree to which utilities undermine climate policy is unclear. The primary concern of IOUs is to maximize shareholder profits, but because of the manner in which they are regulated, state-level climate and renewable electricity laws do not necessarily contradict this goal (Basseches 2020 ). In fact, Basseches ( 2020 ) finds IOUs have been instrumental actors in supporting ambitious RPS policies in states such as California, Massachusetts, and Oregon. However, other utilities have historically obstructed or slowed climate policy progress, often mobilizing quietly to achieve these objectives. Whether they serve as proponents or obstructionists depends on their fuel mix, the individual state-level policy regime and the particular policy at hand; for example, utilities tend to uniformly oppose solar net metering policies because they threaten their monopoly control of the electric grid (Stokes 2020 ). As Romankiewicz et al. ( 2021 ) find, the largest utilities set renewable portfolio goals but then fail to make the investment decisions necessary to achieve them. They also find that the preexisting portfolios of utilities (prior to the adoption of climate policy) is typically the strongest predictor of future investment decisions.

An important debate has emerged about whether IOUs versus public- or customer-owned utilities are preferable for advancing climate policy (Brown & Hess 2016 ; Homsy 2020 ; Heiman & Soloman 2004 ). From the standpoint of “energy democracy” (Greenberg & McKendry 2021 ), public power is clearly preferable. However, when it comes to renewable portfolios, public power’s track record is less clear (Romankiewicz et al. 2021 ). Although more research is needed to further specify conditions for utilities’ support of effective climate policies, it is clear that utilities are a powerful source of obstruction in many cases. For example, at the enforcement and implementation stages, utilities often dominate public utility/service commission rulings (e.g., Stokes 2020 ).

Conservative countermovement

Many of the aforementioned industry groups have also been central players in a broad countermovement that opposes the scientific community and the climate movement’s push for action (Brulle 2020 ; Dunlap and McCright 2010 ; 2015 ). This countermovement has been a significant contributor to climate policy obstruction (McCright and Dunlap 2003 ). Climate change narratives have frequently been coopted by the fossil fuel sector, conservative politicians and think tanks, media, and interest groups. All of these actors comprise a climate denial movement that, at times, coordinates their efforts.

The beginnings of the climate denial movement emerged in response to the environmental movement’s success in passing major legislation such as the Clean Air Act in the 1960s-1970s. Soon after, the Reagan administration took direct aim at environmental regulations under a neoliberal mantra of free markets. These actions in turn prompted a swift backlash from the environmental movement (Brulle 2020 ). Those opposed to environmental regulations learned an important lesson from this backlash; rather than directly attacking environmental programs, efforts should instead focus on undermining the science that supports such policies (Jacques et al. 2008 ; Michaels 2008 ). The conservative countermovement has constructed three primary narratives about climate change: (1) that it does not exist, (2) that if it does exist, it is not anthropogenic, and is possibly even desirable, and (3) that any efforts to mitigate climate change would harm the economy (Dunlap and McCright 2010 ).

The climate denial movement is financially supported by the fossil fuel industry and other conservative businesses and foundations (McCright and Dunlap 2003 ). These funds flow to conservative think tanks that elevate contrarian scientists casting doubt on the veracity of anthropogenic climate change. Parts of the movement organize campaigns to create uncertainty around climate modeling, methodology, and the integrity of scientists themselves (Hess 2014 ). One of the first such climate denial think tanks was the George C. Marshall Institute (Oreskes and Conway 2008 ). Others include the Cato Institute, the Competitive Enterprise Institute, the Heritage Foundation, and the Heartland Institute. Conservative think tanks and foundations brand themselves as an alternative universe of scientists outside of academia. They publish policy briefs, books, and analyses that question the credibility of climate science (McCright and Dunlap 2015 ).

Although the scientists associated with these think tanks often lack relevant credentials, their findings are amplified by Republican politicians (Dunlap and Jacques 2013 ). Contrarian scientists are disproportionately vocal and present at congressional hearings. Republican politicians typically refer to climate change as a hoax and have invoked cold weather and “Climategate” to signal that the science is corrupt (Jacques et al. 2008 ).

Think tank reports are also amplified by conservative media including radio hosts (Wolcott 2007 ), the Wall Street Journal, Fox News, and columnists such as George Will (Boykoff 2013 ; McCright et al. 2016 ). Media coverage on climate change, in turn, likely influences elected officials (Bromley-Trujillo and Karch 2019 ) and also polarizes public and elite attitudes (Leiserowitz et al. 2020 ; Tesler 2018 ).

Although scholarship often focuses on the climate denial movement’s influence on national politics, the movement is closely linked to efforts to sway state-level politics. The climate denial movement aligns with the State Policy Network, Americans for Prosperity, and ALEC, which often work in concert to stall state-level policy (Hertel-Fernandez 2014 , 2019 ). Conservative foundations (Brulle 2014 ; Farrell 2019 ) as well as personnel links (Farrell 2016 ) connect these organizations in a centralized network.

Divided pro-climate policy coalitions

One obstacle to subnational climate policy that is perhaps less well recognized is the fragmentation of pro-climate policy coalitions. One source of fragmentation is divisions among the different alternative or renewable energy industries, which must operate in a political arena dominated by powerful fossil fuel incumbents (Kelsey and Meckling 2018 ). For example, a study of lobbying and testimony in Massachusetts found that more concentrated renewable energy industries were better able to engage in paid lobbying than dispersed ones (Culhane et al. 2021 ). Relatedly, Si and Stephens ( 2021 ) find disparate participation among solar developers and installers surrounding efforts to target solar installation among low-income households in Massachusetts. The solar industry is more fragmented in small installation firms, whereas the wind industry has higher capital barriers to entry and is consequently concentrated in a few, large firms. Solar firms are further divided between rooftop residential developers and those installing utility-scale projects, and between in-state and out-of-state firms (Stokes 2020 ).

In addition to divisions based on concentration, size, and capacity to influence politics/policy, the renewable energy industries also tend to restrict their participation to issues that affect them most directly. For example, studies in Massachusetts and Rhode Island revealed that solar, wind, and other renewable firms did not show up to testify for legislation (e.g., carbon pricing) that did not target benefits to their economic sector. By contrast, environmentalists testified in large numbers in favor of the full range of climate bills. The picture that emerges is a fragmented renewables sector, with firms only lobbying and testifying for their own, narrow issues and sometimes battling each other over carve-outs for particular technologies in state-level RPS policies (Culhane et al. 2021 ).

Another source of division in pro-climate coalitions is between those who advocate for market-based, technocratic approaches to climate mitigation versus those who advocate for more holistic, climate justice approaches involving large public investments in jobs, infrastructure, equity, and health (Boyle et al. 2021 ). The more holistic approach acknowledges the power of the polluting elite, who have strategically invested for decades in undermining public trust in government and minimizing protections and support for marginalized communities, communities of color, and economically disadvantaged groups who are being disproportionately impacted by climate change and pollution (Stephens 2020 ). To further concentrate their wealth and power, big business has also reduced worker rights and protections, and it has shifted corporate culture to prioritize shareholders instead of workers (Stephens 2020 ). This approach tends to be aligned with progressive-left political coalitions, whereas the technocratic approach has a more moderate political position and tends not to emphasize issues of structural inequality. The structural vulnerabilities and under-investment that has been revealed by the COVID-19 pandemic have strengthened the political appeal of the holistic investment-based climate justice approaches (Boyle et al. 2021 ).

Most adopted and proposed state-level climate policies are based on a narrow, technocratic, carbon-centric model, which misses opportunities to invest in marginalized communities (Galvin and Healy 2020 ). To date, climate policy has been largely designed within the context of “climate isolationism,” which refers to the common framing of climate change as a narrow, isolated, discrete, scientific problem that requires a technological solution (Stephens 2020 ). Decision-makers working through a climate isolationism lens often focus in a technocratic way on achieving carbon reductions while inadvertently dismissing the social justice implications and human dimensions of these measures (Stephens Forthcoming 2021 ). Controversy surrounding California’s cap-and-trade program illustrates the conflict between climate justice and mainstream, technocratic policies (Basseches et al. 2021 ).

Until the Green New Deal framework gained traction on the national stage in 2018 (Galvin and Healy 2020 ), climate policies were often limited to market-based approaches. With more diverse leadership, including women, people of color and Indigenous people, a new approach is emerging that links climate/energy policy with jobs and economic justice, health, food, housing, and transportation. Several states and cities have proposed ambitious Green New Deal policies, such as New York’s Climate Leadership and Community Protection Act (Boyle et al. 2021 ). This approach focuses on justice-oriented policies and direct investments in under-invested in households and communities. For example, climate justice proponents are now pushing for more equitable housing and community development, equitable access to clean and affordable energy, and more inclusive public engagement around climate policy development (Clifton and Kelly 2020 ). An expansion of the “just transition” concept includes worker protections and recognition of fossil-dependent communities and consumers (Healy and Barry 2017 ).

A related division in pro-climate policy coalitions is between actors who advocate for energy-transition policies (including those with a justice orientation) and actors who focus more on opposition to unwanted energy infrastructure and fossil fuel reliance. A review of many different types of state-level climate policies revealed that there are many more policies to advance renewables than there are to end fossil fuel reliance (Burke and Stephens 2017 ). Climate justice activists have thus engaged in multi-year protests targeting fossil fuel infrastructure, advocating for supply-side climate policies such as fracking bans, fossil fuel moratoria, state pension divestment campaigns, and litigation for climate harms (Piggot 2018 ; Healy & Barry 2017 ). Controversy about whether or not institutions and investment portfolios should “divest” from fossil fuels demonstrates this division in pro-climate policy coalitions (Trinks et al 2018 ); many colleges and universities have resisted the urge to divest and have pledged instead to “invest” in renewables (Mikkelson et al., 2021 , Stephens et al 2018 ).

Another important division in pro-climate coalitions is between the labor and environmental wings of progressive coalitions. Since the 1990s, “green jobs” and economic development frames have emerged along with some partnerships between unions and environmentalists (e.g., the United Steelworkers and the Sierra Club in the BlueGreen Alliance, (Hess 2012 ). These partnerships have reduced the longstanding image of environmental policy as a threat to working-class jobs; however, not all unions support the “green jobs” approach, and mistrust and opposition remain. For example, in some states, utilities have worked closely with their own workers and unions to mobilize opposition to energy-transition policies and fossil-fuel opposition (e.g., anti-pipeline mobilizations) by arguing that the opposition policies will harm local economies, taking away jobs. In states with a strong extractive fossil-fuel sector, anti-green labor alliances can also extend beyond utility unions to unions and other workers in the mining, drilling, and processing industries.

Solutions to advancing robust climate policy

Despite the challenges just discussed, there are promising strategies for moving robust state-level climate policies forward that we cover below.

A significant barrier to climate action centers on governance around a highly polarized policy issue. As such, the first set of solutions concerns electoral strategies and working with local governments to move climate mitigation policy forward in the states.

To begin, elections matter, and the need to elect political leaders motivated to address climate mitigation is essential. The impacts of the Sunrise Movement and other progressive groups on U.S. federal and state elections in 2018 and 2020 showed that pro-climate positions and policies can quickly become influential, at least in the Democratic Party (Stuart et al. 2020 ). However, it will take large majorities of climate policy advocates to influence or replace legislative leadership in state governments. Furthermore, Basseches ( 2019 )’s findings suggest that even in states with overwhelming Democratic majorities, strong climate policy can be elusive; elected climate champions must be elevated to positions of institutional power within the majority party caucus (e.g., Speaker of the House, Senate President, etc.).

An improved political strategy is needed, including climate advocates’ engagement in primary (as well as general) elections. Unfortunately, most non-profits working in this area are 501(c)3 organizations, which are constrained from lobbying and endorsing political candidates by U.S. tax laws (IRS 2021 ). Philanthropic foundations and the NGOs they fund tend to be extremely cautious about political action, and this makes many of them less effective (Berry 2003 ). Despite this, these groups fill a special need because they can undertake efforts like distributing questionnaires to candidates, interviewing them for endorsements, electioneering, and forming political action committees.

In the many states where electing climate advocates proves to be difficult, there are also ways to encourage a path to renewable energy as a source of economic development and growth (Carley and Lawrence 2014 ). Policy instruments like clean energy and renewable portfolio standards can be discussed in terms of economic development, which may encourage conservative state governments to act (Carley and Lawrence 2014 ). For instance, Texas was an early adopter of a modest RPS (compared to today’s standards) that yielded significant early gains in wind energy development that also facilitated economic growth (Slattery et al. 2011 ).

Moreover, innovative local governments still have opportunities to act when state leadership chooses not to. Local governments can reduce GHG emissions by adopting policies that promote/require clean and efficient energy use. They can also influence state governments via formal lobbying efforts or, indirectly, by demonstrating innovative approaches that can be scaled-up. In the U.S. and globally, trans-municipal climate and sustainability networks—including C40 Cities, ICLEI, and the Urban Sustainability Directors Network—advance these avenues and have been credited with shaping the landscape around local governments’ climate policy engagement (Acuto 2016 ; Nguyen Long and Krause 2020 ).

Many local governments in the U.S. go beyond federal and state climate change policy (Hughes 2019 ; Krause and Hawkins 2021 ). After then-President Trump announced the U.S.’ withdrawal, over 290 municipalities committed to honor the Paris Climate Agreement (We Are Still In 2021 ), and by 2021, over 150 pledged a transition to 100% renewable energy (Sierra Club 2021 ). Local governments can shape energy use practices within their own operations and often have authority over building codes, public transportation, waste management, and a variety of land use and infrastructure decisions impacting GHG emissions. The aggregate impact of local efforts is potentially large; however, debate persists around the magnitude of their impact, and sustained progress by local governments has been highly uneven (Gurney et al. 2021 ; van der Heijden et al. 2019 ).

Municipalities frequently lobby higher-level governments to pass policies that yield local benefit (Goldstein and You 2017 ). Regarding climate change, three strands of local lobbying efforts are evident. First, municipal lobbying is most often aimed at acquiring money and resources—as illustrated by the coordinated efforts advocating the inclusion of Energy Efficiency and Conservation Block Grants (EECBG) in the 2009 American Reinvestment and Recovery Act (US Conference of Mayors 2014 ). Second, local governments may lobby their state governments for the expanded authority necessary to enact specific portions of their climate action plans (Hughes 2019 ). Finally, local governments and trans-municipal networks can seek to persuade higher levels of government to enact their own climate policies (Lee and Jung 2018 ; Curtis and Acuto 2018 ). For example, cities may ask their states to develop comprehensive energy plans, and organize efforts to sway international bodies to adopt stricter mitigation commitments.

“Leading from below” is a final way that local governments are impacting broader climate policy. Often credited for their innovative climate programming, these efforts are experiments that can be up-scaled and adopted by state governments (Kern 2019 ). However, such innovation can be risky when it occurs in conservative states hostile to climate objectives. In these venues, local-state conflict often plays out via state preemption laws, which revoke local authority to act on certain issues or in certain manners (e.g., fracking restrictions and electricity provider choice) that results in stifling local policy innovation (Riverstone-Newell 2017 ).

Second, media and public attitudes critically shape individual and collective engagement around contemporary climate challenges (Boykoff 2011 ). As such, solutions to climate policy inaction should pursue efforts to influence the media and public opinion landscape.

As indicated previously, media coverage presents an obstacle and an opportunity to motivate climate action. In order to keep climate policy on state-level agendas, there is a need to maintain high levels of climate change media coverage, even as other crises grab headlines. In addition, the content of that coverage is important. Although the frequency of climate change coverage has increased globally, challenges associated with quantity and quality of representations of climate change topics remain (Boykoff et al. 2021 ).

Analyses of media representations demonstrate how media portrayals (quantity and quality) play into climate governance at multiple scales in the U.S. (Brulle et al. 2012 ; Fisher 2013 ). For example, climate change garnered coverage through stories intersecting political , economic , scientific , cultural as well as ecological and meteorological themes, which ultimately influence public and political discourse on the subject (Boykoff et al. 2021 ). Media framing of climate change can also affect attitude change and scholars have considered how climate change communication must be tailored to different audiences to be persuasive. Most prominently among audience segmentation work resides the ‘Global Warming’s Six Americas’ project on climate communication (Leiserowitz et al. 2011 ). Howarth and Black ( 2015 ) note that “the communication of climate change historically has been generic, untailored and untargeted” (p. 506). As such, more effort is needed to carefully frame communications and dialogue that values different perspectives on climate change in order to increase concern and engagement across each of the 50 US states.

In addition to legacy media portrayals, social media platforms play an important role in the public arena (Tandoc and Eng 2017 ; Fownes et al 2018 ). Given the potential for social media to drive mainstream media coverage, savvy climate policy advocates can use social media to generate coverage of climate change and craft a message that can move varying subgroups (Anderson 2017 ).

While media coverage can influence public attitudes, research suggests that attitudes can shift through the following strategies: (1) depoliticizing climate change through alternative issue framing and discussions of policy co-benefits, (2) amplifying current support for climate policies, and (3) raising the salience of climate change through connections with visible climate change impacts.

Although some U.S. residents remain doubtful or dismissive of climate change, research shows that linking the issue to economic development and public health can increase policy support, even among Republicans (Rabe 2004 ; Stokes and Warshaw 2017 ). Moreover, “climate policy bundles” that bring together broader issues, like economic inequality and environmental justice, may increase climate policy support (Bergquist et al. 2020 ).

Though climate policy attitudes vary, several policy options receive considerable public support, including investment in renewable energy, tax rebates, subsidies, and renewable portfolio standards (Stokes and Warshaw 2017 ; Stoutenborough et al. 2014 ). Nevertheless, bipartisan public support for addressing climate change has not always translated into action by elected officials. Politicians (particularly Republicans) and their staff tend to drastically underestimate their constituencies’ support for climate policy (Hertel-Fernandez et al. 2019 ). Consequently, efforts to educate policymakers about existing public support and raise the salience of climate change have the potential to promote policy change.

Despite these opportunities, because climate impacts are presented to the public as complex and abstract, they are perceived to be far away and uncertain, which makes it difficult to raise public awareness (Lubell et al. 2007 ; Boykoff 2019 ). However, as climate impacts become more frequent, it may become easier to raise their salience. Some scholars find that temperature anomalies and extreme weather increase climate concern, though effects are temporal (Borick and Rabe 2014 ; Egan and Mullin 2012 ; Konisky et al. 2016 ); others find no link between the two (Brulle et al. 2012 ; Mildenberger and Leiserowitz 2017 ). Still, as climate impacts become more prevalent, there may be more opportunities for political actors, the media, and interest groups to educate the public on climate risks and to encourage policy action (Howe et al. 2015 ).

Third, as a number of powerful industries and other interest groups have moved to obstruct climate policy, there is a need to either leverage or reduce the power that these groups wield over climate mitigation policy.

To begin, IOUs have enormous political power that can be leveraged to promote ambitious state-level climate policies. In addition, there are pathways available to reduce their power, if they cannot be won over. Basseches ( 2020 ) finds that in states like California and Massachusetts, with restructured electricity sectors in which IOUs no longer own fossil fuel generation, a suite of policies rewarding IOUs financially for promoting energy efficiency can neutralize opposition to economy-wide GHG reduction policies. IOUs have supported ambitious RPS policies in other states, like Oregon, as well (Basseches 2020 ). States where IOUs support climate policy are likely to be “blue states” (Adua and Clark 2021 ), consistent with the literature on the role of partisanship in climate policymaking (e.g., Coley and Hess 2012 ; Fowler and Breen 2013 ; Vasseur 2014 ). Unfortunately, this strategy of leveraging IOUs’ political power does not work for net metering policies, which IOUs oppose, even in the blue states (Hess 2016 ). Still, Smith et al. ( 2021 ) suggest that IOUs’ opposition to net metering can be mitigated by policy designs that give utilities credit toward their RPS requirements when their customers install solar panels.

However, some IOUs continue to obstruct state-level climate policy (Stokes 2020 ). One pathway toward motivating IOUs to change is the use of local-level and private-sector resolutions in support of 100% renewable or clean energy (Greenberg and McKendry 2021 ; Hess and Gentry 2019 ). Another pathway is the growth of community-choice aggregation (CCA) in states where it is authorized (Hess and Lee 2020 ). CCA is easier to achieve than municipalization, which has numerous hurdles (e.g., strong utility resistance, capital cost, and the lack of local expertise). CCA organizations can also opt for high renewable or clean energy mixes that put pressure on utilities to shift their energy mix and long-term goals. Both of these pathways can help to motivate utilities to adopt stronger long-term energy-transition plans. A third pathway is to shift legislative reform to public utilities commissions; when they are not captured by utilities, the commissions can provide a mechanism for stating broad goals and insulating legislators from utility pressure (Brown and Hess 2016 ). Municipal (publicly owned) utilities and CCAs offer an alternative method of aligning utilities with climate policy through local governments and elected officials. Cities are often more aggressive than states, in turn leading to more aggressive action by municipally-owned utilities and CCAs on climate policy.

Another pathway to weakening obstructionist IOUs’ power is to increase coalition-formation among non-IOU interest groups, as Brown and Hess ( 2016 ) found was the key to success in cases in which pro-climate coalitions included not only environmentalists and the renewable energy industry, but also real estate, insurance, or HVAC companies. Finally, given that IOUs are private corporations selling a public service, it may be advantageous—to the degree it’s constitutional—to reduce their access to private politics by, for example, limiting their campaign spending (Brown 2016 ).

Reducing divisions in pro-climate policy coalitions

Reducing divisions in pro-climate policy coalitions requires attention to the different types of divisions that were outlined in Sect. 3.4. One way to reduce intra-industry divisions within the clean or renewable-energy sector is to encourage the development of broader industry associations that link together the disparate, reform-oriented actors (e.g., the solar and wind industries, energy efficiency advocates, and those advocating for Community Choice Aggregation (CCA) (Raymond 2016 ). Although the specific trade associations may continue to pick their battles based on narrower, industry-specific benefits, if they also support broader associations (e.g., green or sustainable business councils in different states), then some of their political resources can be more easily channeled toward broader coalition activity.

A deeper division is between the more technocratic approaches to climate policy and the justice-oriented approaches, which in the U.S. are reflected in tensions between the moderate and progressive wings of the Democratic Party. Moderates in the party, especially in conservative states, might opt to resist the linkage to justice because they are concerned that the justice framing will reduce the likelihood of gaining crucial conservative support in state legislatures. There is a need to think carefully about framing and coalitions that are attuned to the level of government, the issue, and the relative power of different political constituencies. Research on the “red states, green laws” phenomenon (where “red” refers to Republican states) has started to show the types of pro-climate change policies that can gain traction in more conservative locations (Hess et al 2016 ). Pro-business, pro-energy choice, pro-health (clean air), and pro-economic development frames can work well in this context, but the laws can also have justice implications even if they are not highlighted for political purposes. But even in these conservative states, the more justice-oriented frames may be successful in the more progressive and diverse cities (the blue islands in the red seas). Likewise, anti-pipeline and other anti-infrastructure mobilizations have great potential to utilize co-existing frames that can bridge political divisions (e.g., property rights for rural landowners and sovereignty for Indigenous people, health and safety concerns for communities, and ecological preservation for environmentalists and local recreation industries).

There is more research on the approaches to overcoming the labor-environmental divisions in pro-climate coalitions, and a strong working partnership between labor and climate policy advocates is integral to a rapid transformation of the U.S. to a low-carbon economy (Basseches et al. 2021 ; Healy and Barry 2017 ). State-level just transition policies can play a role in broader “build back better” programs in the aftermath of the COVID-19 pandemic.

One example of successful green-jobs legislation at the state level was the 2009 Green Jobs, Green New York law, which directed revenues from the regional cap-and-trade initiative toward job training and energy-efficiency programs for residential and commercial buildings (Lennon 2017 ; Hess 2018 ). These initiatives were part of broader calls for “energy democracy” that included unionized, green jobs (Stephens 2019 ), and they were also the basis for subsequent reform initiatives introduced under the banner of the “Green New Deal” (GND) (Galvin and Healy 2020 ).

The New York State Climate Leadership and Community Protection Act was passed in 2019 after years of grassroots advocacy by NY Renews—a statewide coalition that included labor unions, economic justice advocates, environmental organizations and other progressive groups (Boyle et al. 2021 ). This law set a new benchmark for climate ambition, which includes groundbreaking equity provisions (Senate Assembly 2019 ). State-level GND proposals are emerging as new vehicles for garnering union support for climate policies (Boyle et al. 2021 ).

For example, GND proposals in California and Massachusetts are forging new coalitions among unions, environmentalists, and social and racial justice advocates (Boyle et al. 2021 ).

To maximize the yield of this strategy, the GND movement could engage with electricity unions, one of the most unionized industries in the economy and often a sector of unionized labor that opposes energy-transition policies (Huber 2021 ). Labor, environmental justice, tribal, and community groups need greater involvement in climate-labor policy decision-making, such as the process that led to Colorado’s Office of Just Transition and Washington State’s Initiative 1631. Creating and expanding government rapid response teams in every state will mitigate job displacement and mass layoffs (e.g., the Rapid Response Team in Massachusetts) (Cha et al. 2021 ). Bridge funding will also be necessary for regions where the public sector is affected by the withdrawal of fossil fuel tax revenues (Cha et al. 2021 ).

State-level climate policy has shown great promise in the context of federal obstruction or inaction. Nevertheless, significant obstacles to robust state-level climate policy remain and this review provides a novel synthesis of the literature detailing these barriers. As we note, scholars describe obstacles associated with governance and political institutions, public opinion and media coverage, industry and interest groups, and fragmentation within pro-climate coalitions. What remains less clear from this scholarship is how we can harness this knowledge to formulate solutions to policy obstacles; our primary contribution lies here.

Based on the broad, interdisciplinary literature discussed here, we suggest a series of strategies to move climate change policy forward. The politicization of climate change necessitates bringing other groups into the fold of climate policy support. In addition, there is a need for enhanced coordination among climate policy advocates and potential coalition partners and to support electoral gains for climate policy advocates. To achieve these goals, we suggest the following strategies.

First, climate policy advocates should become more skilled in the game of politics, by employing campaign finance strategies, electoral mobilization, and support for existing elected officials who are sympathetic to climate policy as they seek to gain institutional influence (i.e., ascending to leadership positions, etc.). Climate policy opponents have had a great deal more practice and experience doing this, but there is no reason that proponents cannot learn from them and deploy strategic political operations of their own. A related strategy includes “bottom-up” pressure from local governments and municipalities. Second, climate policy proponents should seek to improve the quality and quantity of media coverage, including by tailoring messages to particular audiences and constituencies and continuously linking climate action to co-benefits.

Third, the political power of IOUs can be leveraged in support of strong climate policy if the right conditions and incentives are put in place so that utilities see opportunities for financial growth as a result of these policies. However, in cases where this is not feasible, efforts should be made to reduce their political power, by empowering municipal utilities and CCAs, by building broad coalitions of non-utility business interests, and, when strategic, by shifting the venue of policymaking between the legislative and executive branches. Finally, divisions within the pro-climate coalition should be reduced. This can be achieved through more inclusive policy design that attends to environmental justice issues as well as by encouraging better coordination among “green business” actors, such as renewable energy firms, energy efficiency consultants, green capital, etc.

Although this review moves the research field toward integrated discussion of climate-policy obstacles and solutions, it also has several limitations that could be the basis for future research. One limitation is that both the problems and solutions have a U.S. focus. Although many countries have undertaken restructuring of their electricity systems, each system is unique, and many still have a larger role for public power than in the U.S. Moreover, the polarized political culture characterized by a climate denial machine and heavy influence by wealthy donors and corporations on political outcomes does not necessarily translate well to other countries. Thus, there is a need for additional comparative research on climate policy obstacles and solutions, which will likely reveal topics that are much more salient in other countries.

Moreover, further work is needed in tailoring these solutions to particular states, considering their distinct partisan tendencies, energy economies, media landscapes and government contexts. Nevertheless, the strategies outlined above should be broadly valuable in reducing state-level climate policy obstacles and ensuring comprehensive progress at the state level despite continued uncertainty regarding federal climate policy. In addition, we have suggested ways of tailoring climate messaging by the media and others to make climate policy action more palatable to Republicans. In the context of energy and climate federalism, the states will likely remain key players in the years to come.

Author contribution

J.B. and R.B. designed the study and oversaw collaboration. All authors contributed to the drafting of this manuscript. J.B. contributed sections on partisan governance/institutions, utilities, and IOUs. R.B. contributed sections on public opinion, conservative countermovements, and the conclusion. H.P. wrote the section on fossil fuel lobbying. M.B. wrote the media section. T.C. and G.H. contributed to the renewable energy cohesion section. G.H. contributed to conservative countermovements. D.Hess contributed to aligning IOUs and green jobs. D.Hsu contributed to the aligning utilities section. J.S and N.H. wrote sections on climate justice and renewable energy fragmentation. R.K. wrote on municipal governments. T.R. contributed to the introduction, renewable energy sector cohesion and political leadership.

Publisher's note

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Contributor Information

Joshua A. Basseches, Email: ude.hcimu@hcessabj .

Rebecca Bromley-Trujillo, Email: [email protected] .

Maxwell T. Boykoff, Email: ude.odaroloc@ffokyob .

Trevor Culhane, Email: ude.nworb@enahluc_rovert .

Galen Hall, Email: ude.nworb@llah_nelag .

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Jennie C. Stephens, Email: [email protected] .

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Scientists warn Australians to prepare for megadroughts lasting more than 20 years

New climate modelling suggests Australians should be preparing for the possibility of megadroughts lasting more than 20 years.

Research from the Australian National University, published in a special edition of the journal Hydrology and Earth System Sciences, has indicated future droughts in Australia could be far worse than anything experienced in recent times — even without factoring in human impacts.

Climate scientist Georgy Falster said while megadroughts occurred naturally, climate change would make them more severe.

"We have this situation where on the one hand, there's the possibility for naturally occurring megadroughts that can last multiple decades and might come along every maybe 150 to 100 years," Dr Falster said.

"But then on the other hand, we found climate change is tending to make droughts longer, particularly in south, western and eastern Australia, and climate change is also making droughts more severe because of the hotter temperature."

She pointed to the recent Tinderbox Drought that occurred in south-east Australia, linked to the Black Summer bushfires, which lasted "only three years".

"So we can imagine droughts that last from anywhere, sort of four times as long as that up to 20 or even 30 years," Dr Falster said.

"They don't happen often, but they can happen and it's very difficult to predict when that might be.

"We should be prepared for one to happen even in the next 10 or so years."

Preparing for long droughts

The research team used 11 different models to look at how Australia's climate has changed over more than 1,000 years, but there was also evidence of past rainfall changes in tree rings to help paint a picture of when megadroughts had occurred.

Dr Falster hoped the research would help farmers and the wider community be prepared for longer and more severe droughts.

"We can reduce the impact of megadroughts by being prepared with things like water management strategies, community support networks and financial support for farmers, environmental management plans, that sort of thing," she said.

"But then to reduce the actual risk of megadroughts and their severity, of course, the only thing that we can do is to rapidly reduce greenhouse gas emissions."

Concern for farmers

Far west NSW grazier Richard Wilson has lived through many droughts on Yalda Downs Station, located 85 kilometres north of White Cliffs, but particularly remembers one from 2016 that lasted four years.

"Everyone found it tough. There's no other way to say it," Mr Wilson said.

"It's on your mind all the time. It's often very hard to talk about it."

He said planning for the inevitable was important.

"There was no warning signs it was going to come. You just have to be prepared for it," Mr Wilson said.

"Set some plans up and you need to be also quite prepared to change those plans as things develop, but think about it well before it happens so that you've got some thought process in place that you're going to be dealing with the lack of feed and water."

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• Published: 21 March 2024

Expert review of the science underlying nature-based climate solutions

• B. Buma   ORCID: orcid.org/0000-0003-2402-7737 1 , 2   na1 ,
• D. R. Gordon   ORCID: orcid.org/0000-0001-6398-2345 1 , 3   na1 ,
• K. M. Kleisner 1 ,
• A. Bartuska 1 , 4 ,
• A. Bidlack 5 ,
• R. DeFries   ORCID: orcid.org/0000-0002-3332-4621 6 ,
• P. Ellis   ORCID: orcid.org/0000-0001-7933-8298 7 ,
• P. Friedlingstein   ORCID: orcid.org/0000-0003-3309-4739 8 , 9 ,
• S. Metzger 10   nAff15   nAff16 ,
• G. Morgan 11 ,
• K. Novick   ORCID: orcid.org/0000-0002-8431-0879 12 ,
• J. N. Sanchirico 13 ,
• J. R. Collins   ORCID: orcid.org/0000-0002-5705-9682 1 , 14 ,
• A. J. Eagle   ORCID: orcid.org/0000-0003-0841-2379 1 ,
• R. Fujita 1 ,
• E. Holst 1 ,
• J. M. Lavallee   ORCID: orcid.org/0000-0002-3028-7087 1 ,
• R. N. Lubowski 1   nAff17 ,
• C. Melikov 1   nAff18 ,
• L. A. Moore   ORCID: orcid.org/0000-0003-0239-6080 1   nAff19 ,
• E. E. Oldfield   ORCID: orcid.org/0000-0002-6181-1267 1 ,
• J. Paltseva 1   nAff20 ,
• A. M. Raffeld   ORCID: orcid.org/0000-0002-5036-6460 1 ,
• N. A. Randazzo 1   nAff21   nAff22 ,
• C. Schneider 1 ,
• N. Uludere Aragon 1   nAff23 &
• S. P. Hamburg 1  

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Viable nature-based climate solutions (NbCS) are needed to achieve climate goals expressed in international agreements like the Paris Accord. Many NbCS pathways have strong scientific foundations and can deliver meaningful climate benefits but effective mitigation is undermined by pathways with less scientific certainty. Here we couple an extensive literature review with an expert elicitation on 43 pathways and find that at present the most used pathways, such as tropical forest conservation, have a solid scientific basis for mitigation. However, the experts suggested that some pathways, many with carbon credit eligibility and market activity, remain uncertain in terms of their climate mitigation efficacy. Sources of uncertainty include incomplete GHG measurement and accounting. We recommend focusing on resolving those uncertainties before broadly scaling implementation of those pathways in quantitative emission or sequestration mitigation plans. If appropriate, those pathways should be supported for their cobenefits, such as biodiversity and food security.

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Nature-based climate solutions (NbCS) are conservation, restoration and improved management strategies (pathways) in natural and working ecosystems with the primary motivation to mitigate GHG emissions and remove CO 2 from the atmosphere 1 (similar to ecosystem-based mitigation 2 ). GHG mitigation through ecosystem stewardship is integral to meeting global climate goals, with the greatest benefit coming from near-term maximization of emission reductions, followed by CO 2 removal 3 . Many countries (for example, Indonesia, China and Colombia) use NbCS to demonstrate progress toward national climate commitments.

The scope of NbCS is narrower than that of nature-based solutions (NbS) which include interventions that prioritize non-climate benefits alongside climate (for example, biodiversity, food provisioning and water quality improvement) 4 . In many cases, GHG mitigation is considered a cobenefit that results from NbS actions focused on these other challenges 2 . In contrast, NbCS are broader than natural climate solutions, which are primarily focused on climate mitigation through conservation, restoration and improved land management, generally not moving ecosystems beyond their unmodified structure, function or composition 5 . NbCS may involve moving systems beyond their original function, for example by cultivating macroalgae in water deeper than their natural habitat.

The promise of NbCS has generated a proliferation of interest in using them in GHG mitigation plans 6 , 7 ; 104 of the 168 signatories to the Paris Accord included nature-based actions as part of their mitigation plans 8 . Success in long-term GHG management requires an accurate accounting of inputs and outputs to the atmosphere at scale, so NbCS credits must have robust, comprehensive and transparent scientific underpinnings 9 . Given the urgency of the climate problem, our goal is to identify NbCS pathways with a sufficient scientific foundation to provide broad confidence in their potential GHG mitigation impact, provide resources for confident implementation and identify priority research areas in more uncertain pathways. Evaluating implementation of mitigation projects is beyond our scope; this effort focuses on understanding the underlying science. The purpose is not evaluating any specific carbon crediting protocol or implementation framework but rather the current state of scientific understanding necessary to provide confidence in any NbCS.

In service of this goal, we first investigated nine biomes (boreal forests, coastal marine (salt marsh, mangrove, seagrass and coral reef), freshwater wetlands, grasslands, open ocean (large marine animal and mesopelagic zone biomass, seabed), peatlands, shrublands, temperate forests and tropical forests) and three cultivation types (agroforestry, croplands and macroalgae aquaculture); these were chosen because of their identified potential scale of global impact. In this context, impact is assessed as net GHG mitigation: the CO 2 sequestered or emissions reduced, for example, discounted by understood simultaneous emissions of other GHG (as when N 2 O is released simultaneously with carbon sequestration in cropland soils). From there, we identified 43 NbCS pathways which have been formally implemented (with or without market action) or informally proposed. We estimated the scale of mitigation impact for each pathway on the basis of this literature and, as a proxy measure of NbCS implementation, determined eligibility and activity under existing carbon crediting protocols. Eligibility means that the pathway is addressed by an existing GHG mitigation protocol; market activity means that credits are actively being bought under those eligibility requirements. We considered pathways across a spectrum from protection to improved management to restoration to manipulated systems, but some boundaries were necessary. We excluded primarily abiotically driven pathways (for example, ocean alkalinity enhancement) or where major land use or land-use trade-offs exist (for example, afforestation) 10 , 11 , 12 . Of the 43 pathways, 79% are at present eligible for carbon crediting (sometimes under several methodologies) and at least 65% of those have been implemented (Supplementary Table 1 ). This review was then appraised by 30 independent scholars (at least three per pathway; a complete review synthesis is given in the Supplementary Data ).

Consolidation of a broad body of scientific knowledge, with inherent variance, requires expert judgement. We used an expert elicitation process 13 , 14 , 15 with ten experts to place each proposed NbCS pathway into one of three readiness categories following their own assessment of the scientific literature, categorized by general sources of potential uncertainty: category 1, sufficient scientific basis to support a high-quality carbon accounting system or to support the development of such a system today; category 2, a >25% chance that focused research and reasonable funding would support development of high-quality carbon accounting (that is, move to category 1) within 5 years; or category 3, a <25% chance of development of high-quality carbon accounting within 5 years (for example, due to measurement challenges, unconstrained leakage, external factors which constrain viability).

If an expert ranked a pathway as category 2, they were also asked to rank general research needs to resolve: leakage/displacement (spillover to other areas), measuring, reporting and verification (the ability to quantify all salient stocks and fluxes), basic mechanisms of action (fundamental science), durability (ability to predict or compensate for uncertainty in timescale of effectiveness due to disturbances, climate change, human activity or other factors), geographic uncertainty (place-to-place variation), scaling potential (ability to estimate impact) and setting of a baseline (ability to estimate additionality over non-action; a counterfactual). To avoid biasing towards a particular a priori framework for evaluation of the scientific literature, reviewers could use their own framework for evaluating the NbCS literature about potential climate impact and so could choose to ignore or add relevant categorizations as well. Any pathway in category 1 would not need fundamental research for implementation; research gaps were considered too extensive for useful guidance on reducing uncertainty in category 3 pathways. Estimates of the global scale of likely potential impact (PgCO 2 e yr −1 ) and cobenefits were also collected from expert elicitors. See Methods and Supplementary Information for the survey instrument.

Four pathways with the highest current carbon market activity and high mitigation potential (tropical and temperate forest conservation and reforestation; Table 1 and Supplementary Data ), were consistently rated as high-confidence pathways in the expert elicitation survey. Other NbCS pathways, especially in the forestry sector, were rated relatively strongly by the experts for both confidence in scientific basis and scale of potential impact, with some spread across the experts (upper right quadrant, Fig. 1 ). Conversely, 13 pathways were consistently marked by experts as currently highly uncertain/low confidence (median score across experts: 2.5–3.0) and placed in category 3 (for example, cropland microbial amendments and coral reef restoration; Supplementary Tables 1 and 2 ). For the full review, including crediting protocols currently used, literature estimates of scale and details of sub-pathways, see Supplementary Data .

Pathways in the upper right quadrant have both high confidence in the scientific foundations and the largest potential scale of global impact; pathways in the lower left have the lowest confidence in our present scientific body of knowledge and an estimated smaller potential scale of impact. Designations of carbon credit eligibility under existing protocols and market activity at the present time are noted. Grassland enhanced mineral weathering (EMW) is not shown (mean category rating 2.9) as no scale of impact was estimated. See Supplementary Table 1 for specific pathway data. Bars represent 20th to 80th percentiles of individual estimates, if there was variability in estimates. A small amount of random noise was added to avoid overlap.

The experts assessed 26 pathways as having average confidence scores between 1.5 and 2.4, suggesting the potential for near-term resolution of uncertainties. This categorization arose from either consensus amongst experts on the uncertain potential (for example, boreal forest reforestation consistently rated category 2, with primary concerns about durability) or because experts disagreed, with some ranking category 1 and others category 3 (for example, pasture management). We note that where expert disagreement exists (seen as the spread of responses in Fig. 1 and Supplementary Table 1 ; also see Data availability for link to original data), this suggests caution against overconfidence in statements about these pathways. These results also suggest that confidence may be increased by targeted research on the identified sources of uncertainty (Supplementary Table 3 ).

Sources of uncertainty

Durability and baseline-setting were rated as high sources of uncertainty across all pathways ranked as category 2 by the experts (mean ratings of 3.6 and 3.4 out of 5, respectively; Supplementary Table 3 ). Understanding of mechanisms and geographic spread had the lowest uncertainty ratings (2.1 and 2.3, respectively), showing confidence in the basic science. Different subsets of pathways had different prioritizations, however, suggesting different research needs: forest-centric pathways were most uncertain in their durability and additionality (3.8 and 3.4, respectively), suggesting concerns about long-term climate and disturbance trajectories. Agricultural and grassland systems, however, had higher uncertainty in measurement methods and additionality (3.9 and 3.5 respectively). Although there were concerns about durability from some experts (for example, due to sea-level rise), some coastal blue carbon pathways such as mangrove restoration (mean category ranking: 1.7 (20th to 80th percentile 1.0–2.0)) have higher confidence than others (for example, seagrass restoration: mean category ranking 2.8, 20th to 80th percentile 2.6–3.0)), which are relatively poorly constrained in terms of net radiative forcing potential despite a potentially large carbon impact (seagrass median: 1.60 PgCO 2 e yr −1 ; see Supplementary Data for more scientific literature estimates).

Scale of impact

For those pathways with lower categorization by the expert elicitation (category 2 or 3) at the present time, scale of global impact is a potential heuristic for prioritizing further research. High variability, often two orders of magnitude, was evident in the mean estimated potential PgCO 2 e yr −1 impacts for the different pathways (Fig. 1 and Supplementary Table 2 ) and the review of the literature found even larger ranges produced by individual studies (Supplementary Data ). A probable cause of this wide range was different constraints on the estimated potential, with some studies focusing on potential maximum impact and others on more constrained realizable impacts. Only avoided loss of tropical forest and cropland biochar amendment were consistently estimated as having the likely potential to mitigate >2 PgCO 2 e yr −1 , although biochar was considered more uncertain by experts due to other factors germane to its overall viability as a climate solution, averaging a categorization of 2.2. The next four highest potential impact pathways, ranging from 1.6 to 1.7 PgCO 2 e yr −1 , spanned the spectrum from high readiness (temperate forest restoration) to moderate (cropland conversion from annual to perennial vegetation and grassland restoration) to low (seagrass restoration, with main uncertainties around scale of potential impact and durability).

There was high variability in the elicitors’ estimated potential scale of impact, even in pathways with strong support, such as tropical forest avoided loss (20th to 80th percentile confidence interval: 1–8 PgCO 2 e yr −1 ), again emphasizing the importance of consistent definitions and constraints on how NbCS are measured, evaluated and then used in broad-scale climate change mitigation planning and budgeting. Generally, as pathway readiness decreased (moving from category 1 to 3), the elicitor-estimated estimates of GHG mitigation potential decreased (Supplementary Fig. 1 ). Note that individual studies from the scientific literature may have higher or lower estimates (Supplementary Data ).

Expert elicitation meta-analyses suggest that 6–12 responses are sufficient for a robust and stable quantification of responses 15 . We tested that assumption via a Monte Carlo-based sensitivity assessment. Readiness categorizations by the ten experts were robust to a Monte Carlo simulation test, where further samples were randomly drawn from the observed distribution of responses: mean difference between the original and the boot-strapped data was 0.02 (s.d. = 0.05) with an absolute difference average of 0.06 (s.d. = 0.06). The maximum difference in readiness categorization means across all pathways was 0.20 (s.d. = 0.20) (Supplementary Table 2 ). The full dataset of responses is available online (see ʻData availabilityʼ).

These results highlight opportunities to accelerate implementation of NbCS in well-supported pathways and identify critical research needs in others (Fig. 1 ). We suggest focusing future efforts on resolving identified uncertainties for pathways at the intersection between moderate average readiness (for example, mean categorizations between ~1.5 and 2.0) and high potential impact (for example, median >0.5 PgCO 2 e yr −1 ; Supplementary Table 1 ): agroforestry, improved tropical and temperate forest management, tropical and boreal peatlands avoided loss and peatland restoration. Many, although not all, experts identified durability and baseline/additionality as key concerns to resolve in those systems; research explicitly targeted at those specific uncertainties (Supplementary Table 3 ) could rapidly improve confidence in those pathways.

We recommend a secondary research focus on the lower ranked (mean category 2.0 to 3.0) pathways with estimated potential impacts >1 PgCO 2 e yr −1 (Supplementary Fig. 2 ). For these pathways, explicit, quantitative incorporation into broad-scale GHG management plans will require further focus on systems-level carbon/GHG understandings to inspire confidence at all stages of action and/or identifying locations likely to support durable GHG mitigation, for example ref. 16 . Examples of this group include avoided loss and degradation of boreal forests (for example, fire, pests and pathogens and albedo 16 ) and effective mesopelagic fishery management, which some individual studies estimate would avoid future reductions of the currently sequestered 1.5–2.0 PgC yr −1 (refs. 17 , 18 ). These pathways may turn out to have higher or lower potential than the expert review suggests, on the basis of individual studies (Supplementary Data ) but strong support will require further, independent verification of that potential.

We note that category 3 rankings by expert elicitation do not necessarily imply non-viability but simply that much more research is needed to confidently incorporate actions into quantitative GHG mitigation plans. We found an unsurprising trend of lower readiness categorization with lower pathway familiarity (Supplementary Fig. 3 ). This correlation may result from two, non-exclusive potential causes: (1) lower elicitor expertise in some pathways (inevitable, although the panel was explicitly chosen for global perspectives, connections and diverse specialties) and (2) an actual lack of scientific evidence in the literature, which leads to that self-reported lack of familiarity, a common finding in the literature review (Supplementary Data ). Both explanations suggest a need to better consolidate, develop and disseminate the science in each pathway for global utility and recognition.

Our focus on GHG-related benefits in no way diminishes the substantial conservation, environmental and social cobenefits of these pathways (Supplementary Table 4 ), which often exceed their perceived climate benefits 1 , 19 , 20 , 21 . Where experts found climate impacts to remain highly uncertain but other NbS benefits are clear (for example, biodiversity and water quality; Supplementary Table 4 ), other incentives or financing mechanisms independent of carbon crediting should be pursued. While the goals here directly relate to using NbCS as a reliably quantifiable part of global climate action planning and thus strong GHG-related scientific foundations, non-climate NbS projects may provide climate benefits that are less well constrained (and thus less useful from a GHG budgeting standpoint) but also valuable. Potential trade-offs, if any, between ecosystem services and management actions, such as biodiversity and positive GHG outcomes, should be explored to ensure the best realization of desired goals 2 .

Finally, our focus in this study was on broad-scale NbCS potential in quantitative mitigation planning because of the principal and necessary role of NbCS in overall global warming targets. We recognize the range of project conditions that may increase, or decrease, the rigour of any pathway outside the global-scale focus here. We did not specifically evaluate the large and increasing number of crediting concepts (by pathway: Supplementary Data ), focusing rather on the underlying scientific body of knowledge within those pathways. Some broad pathways may have better defined sub-pathways within them, with a smaller potential scale of impact but potentially lower uncertainty (for example, macroalgae harvest cycling). Poorly enacted NbCS actions and/or crediting methodologies at project scales may result in loss of benefits even from high-ranking pathways 22 , 23 , 24 and attention to implementation should be paramount. Conversely, strong, careful project-scale methodologies may make lower readiness pathways beneficial for a given site.

Viable NbCS are vital to global climate change mitigation but NbCS pathways that lack strong scientific underpinnings threaten global accounting by potentially overestimating future climate benefits and eroding public trust in rigorous natural solutions. Both the review of the scientific literature and the expert elicitation survey identified high potential ready-to-implement pathways (for example, tropical reforestation), reinforcing present use of NbCS in planning.

However, uncertainty remains about the quantifiable GHG mitigation of some active and nascent NbCS pathways. On the basis of the expert elicitation survey and review of the scientific literature, we are concerned that large-scale implementation of less scientifically well-founded NbCS pathways in mitigation plans may undermine net GHG budget planning; those pathways require more study before they can be confidently promoted at broad scales and life-cycle analyses to integrate system-level emissions when calculating totals. The expert elicitation judgements suggest a precautionary approach to scaling lower confidence pathways until the scientific foundations are strengthened, especially for NbCS pathways with insufficient measurement and monitoring 10 , 24 , 25 or poorly understood or measured net GHG mitigation potentials 16 , 26 , 27 , 28 . While the need to implement more NbCS pathways for reducing GHG emissions and removing carbon from the atmosphere is urgent, advancing the implementation of poorly quantified pathways (in relation to their GHG mitigation efficacy) could give the false impression that they can balance ongoing, fossil emissions, thereby undermining overall support for more viable NbCS pathways. Explicitly targeting research to resolve these uncertainties in the baseline science could greatly bolster confidence in the less-established NbCS pathways, benefiting efforts to reduce GHG concentrations 29 .

The results of this study should inform both market-based mechanisms and non-market approaches to NbCS pathway management. Research and action that elucidates and advances pathways to ensure a solid scientific basis will provide confidence in the foundation for successfully implementing NbCS as a core component of global GHG management.

NbCS pathway selection

We synthesized scientific publications for nine biomes (boreal forests, coastal blue carbon, freshwater wetlands, grasslands, open ocean blue carbon, peatlands, shrublands, temperate forests and tropical forests) and three cultivation types (agroforestry, croplands and macroalgae aquaculture) (hereafter, systems) and the different pathways through which they may be able to remove carbon or reduce GHG emissions. Shrublands and grasslands were considered as independent ecosystems; nonetheless, we acknowledge that there is overlap in the numbers presented here because shrublands are often included with grasslands 5 , 30 , 31 , 32 , 33 .

The 12 systems were chosen because they have each been identified as having potential for emissions reductions or carbon removal at globally relevant scales. Within these systems, we identified 43 pathways which either have carbon credit protocols formally established or informally proposed for review (non-carbon associated credits were not evaluated). We obtained data on carbon crediting protocols from international, national and regional organizations and registries, such as Verra, American Carbon Registry, Climate Action Reserve, Gold Standard, Clean Development Mechanism, FAO and Nori. We also obtained data from the Voluntary Registry Offsets Database developed by the Berkeley Carbon Trading Project and Carbon Direct company 34 . While we found evidence of more Chinese carbon crediting protocols, we were not able to review these because of limited publicly available information. To maintain clarity and avoid misrepresentation, we used the language as written in each protocol. A full list of the organizations and registries for each system can be found in the Supplementary Data .

Literature searches and synthesis

We reviewed scientific literature and reviews (for example, IPCC special reports) to identify studies reporting data on carbon stocks, GHG dynamics and sequestration potential of each system. Peer-reviewed studies and meta-analyses were identified on Scopus, Web of Science and Google Scholar using simple queries combining the specific practice or pathway names or synonyms (for example, no-tillage, soil amendments, reduced stocking rates, improved forest management, avoided forest conversion and degradation, avoided mangrove conversion and degradation) and the following search terms: ‘carbon storage’, ‘carbon stocks’, ‘carbon sequestration’, ‘carbon sequestration potential’, ‘additional carbon storage’, ‘carbon dynamics’, ‘areal extent’ or ‘global’.

The full literature review was conducted between January and October 2021. We solicited an independent, external review of the syntheses (obtaining from at least three external reviewers per natural or working system; see p. 2 of the Supplementary Data ) as a second check against missing key papers or misinterpretation of data. The review was generally completed in March 2022. Data from additional relevant citations were added through October 2022 as they were discovered. For a complete list of all literature cited, see pp. 217–249 of the Supplementary Data .

From candidate papers, the papers were considered if their results/data could be applied to the following central questions:

How much carbon is stored (globally) at present in the system (total and on average per hectare) and what is the confidence?

At the global level, is the system a carbon source or sink at this time? What is the business-as-usual projection for its carbon dynamics?

Is it possible, through active management, to either increase net carbon sequestration in the system or prevent carbon emissions from that system? (Note that other GHG emissions and forcings were included here as well.)

What is the range of estimates for how much extra carbon could be sequestered globally?

How much confidence do we have in the present methods to detect any net increases in carbon sequestration in a system or net changes in areal extent of that?

From each paper, quantitative estimates for the above questions were extracted for each pathway, including any descriptive information/metadata necessary to understand the estimate. In addition, information on sample size, sampling scheme, geographic coverage, timeline of study, timeline of projections (if applicable) and specific study contexts (for example, wind-break agroforestry) were recorded.

We also tracked where the literature identified trade-offs between carbon sequestered or CO 2 emissions reduced and emissions of other GHG (for example, N 2 O or methane) for questions three and five above. For example, wetland restoration can result in increased CO 2 uptake from the atmosphere. However, it can also increase methane and N 2 O emissions to the atmosphere. Experts were asked to consider the uncertainty in assessing net GHG mitigation as they categorized the NbCS pathways.

Inclusion of each pathway in mitigation protocols and the specific carbon registries involved were also identified. These results are reported (grouped or individually as appropriate) in the Supplementary Data , organized by the central questions and including textual information for interpretation. The data and protocol summaries for each of the 12 systems were reviewed by at least three scientists each and accordingly revised.

These summaries were provided to the expert elicitation group as optional background information.

Unit conversions

Since this synthesis draws on literature from several sources that use different methods and units, all carbon measurements were standardized to the International System of Units (SI units). When referring to total stocks for each system, numbers are reported in SI units of elemental carbon (that is, PgC). When referring to mitigation potential, elemental carbon was converted to CO 2 by multiplying by 3.67. Differences in methodology, such as soil sampling depth, make it difficult to standardize across studies. Where applicable, the specific measurement used to develop each stock estimate is reported.

Expert elicitation process

To assess conclusions brought about by the initial review process described above, we conducted an expert elicitation survey to consolidate and add further, independent assessments to the original literature review. The expert elicitation survey design followed best practice recommendations 14 , with a focus on participant selection, explicitly defining uncertainty, minimizing cognitive and overconfidence biases and clarity of focus. Research on expert elicitation suggests that 6–12 responses are sufficient for a stable quantification of responses 15 . We identified >40 potential experts via a broad survey of leading academics, science-oriented NGO and government agency publications and products. These individuals have published on several NbCS pathways or could represent larger research efforts that spanned the NbCS under consideration. Careful attention was paid to the gender and sectoral breakdown of respondents to ensure equitable representation. Of the invitees, ten completed the full elicitation effort. Experts were offered compensation for their time.

Implementation of the expert elicitation process followed the IDEA protocol 15 . Briefly, after a short introductory interview, the survey was sent to the participants. Results were anonymized and standardized (methods below) and a meeting held with the entire group to discuss the initial results and calibrate understanding of questions. The purpose of this meeting was not to develop consensus on a singular answer but to discuss and ensure that all questions are being considered in the same way (for example, clarifying any potentially confusing language, discussing any questions that emerged as part of the process). The experts then revisited their initial rankings to provide final, anonymous rankings which were compiled in the same way. These final rankings are the results presented here and may be the same or different from the initial rankings, which were discarded.

Survey questions

The expert elicitation survey comprised five questions for each pathway. The data were collected via Google Forms and collated anonymously at the level of pathways, with each respondent contributing one datapoint for each pathway. The experts reported their familiarity (or the familiarity of the organization whose work they were representing) with the pathway and other cobenefits for the pathways.

The initial question ranked the NbCS pathway by category, from one to three.

Category 1 was defined as a pathway with sufficient scientific knowledge to support a high-quality carbon accounting system today (for example, meets the scientific criteria identified in the WWF-EDF-Oeko Institut and ICAO TAB) or to support the development of such a system today. The intended interpretation is that sufficient science is available for quantifying and verifying net GHG mitigation. Note that experts were not required to reference any given ‘high-quality’ crediting framework, which were provided only as examples. In other words, the evaluation was not intended to rank a given framework (for example, ref. 35 ) but rather expert confidence in the fundamental scientific understandings that underpin potential for carbon accounting overall. To this end, no categorization of uncertainty was required (reviewers could skip categorizations they felt were not necessary) and space was available to fill in new categories by individual reviewers (if they felt a category was missing or needed). Uncertainties at this category 1 level are deemed ‘acceptable’, for example, not precluding accounting now, although more research may further substantiate high-quality credits.

Category 2 pathways have a good chance (>25%) that with more research and within the next 5 years, the pathway could be developed into a high-quality pathway for carbon accounting and as a nature-based climate solution pathway. For these pathways, further understanding is needed for factors such as baseline processes, long-term stability, unconstrained fluxes, possible leakage or other before labelling as category 1 but the expert is confident that information can be developed, in 5 years or less, with more work. The >25% chance threshold and 5-year timeframe were determined a priori to reflect and identify pathways that experts identified as having the potential to meet the Paris Accord 2030 goal. Other thresholds (for example, longer timeframes) could have been chosen, which would impact the relative distribution of pathways in categories 2 and 3 (for example, a longer timeframe allowed could move some pathways from category 3 into category 2, for some reviewers). We emphasize that category 3 pathways do not necessarily mean non-valuable approaches but longer timeframes required for research than the one set here.

Category 3 responses denoted pathways that the expert thought had little chance (<25%) that with more research and within the next 5 years, this pathway could be developed into a suitable pathway for managing as a natural solutions pathway, either because present evidence already suggests GHG reduction is not likely to be viable, co-emissions or other biophysical feedbacks may offset those gains or because understanding of key factors is lacking and unlikely to be developed within the next 5 years. Notably, the last does not mean that the NbCS pathway is not valid or viable in the long-term, simply that physical and biological understandings are probably not established enough to enable scientific rigorous and valid NbCS activity in the near term.

The second question asked the experts to identify research gaps associated with those that they ranked as category 2 pathways to determine focal areas for further research. The experts were asked to rank concerns about durability (ability to predict or compensate for uncertainty in timescale of effectiveness due to disturbances, climate change, human activity or other factors), geographic uncertainty (place-to-place variation), leakage or displacement (spillover of activities to other areas), measuring, reporting and verification (MRV, referring to the ability to quantify all salient stocks and fluxes to fully assess climate impacts), basic mechanisms of action (fundamental science), scaling potential (ability to estimate potential growth) and setting of a baseline (ability to reasonably quantify additionality over non-action, a counterfactual). Respondents could also enter a different category if desired. For complete definitions of these categories, see the survey instrument ( Supplementary Information ). This question was not asked if the expert ranked the pathway as category 1, as those were deemed acceptable, or for category 3, respecting the substantial uncertainty in that rating. Note that responses were individual and so the same NbCS pathway could receive (for example) several individual category 1 rankings, which would indicate reasonable confidence from those experts, and several category 2 rankings from others, which would indicate that those reviewers have lingering concerns about the scientific basis, along with their rankings of the remaining key uncertainties in those pathways. These are important considerations, as they reflect the diversity of opinions and research priorities; individual responses are publicly available (anonymized: https://doi.org/10.5281/zenodo.7859146 ).

The third question involved quantification of the potential for moving from category 2 to 1 explicitly. Following ref. 14 , the respondents first reported the lowest plausible value for the potential likelihood of movement (representing the lower end of a 95% confidence interval), then the upper likelihood and then their best guess for the median/most likely probability. They were also asked for the odds that their chosen interval contained the true value, which was used to scale responses to standard 80% credible intervals and limit overconfidence bias 13 , 15 . This question was not asked if the expert ranked the pathway as category 3, respecting the substantial uncertainty in that rating.

The fourth question involved the scale of potential impact from the NbCS, given the range of uncertainties associated with effectiveness, area of applicability and other factors. The question followed the same pattern as the third, first asking about lowest, then highest, then best estimate for potential scale of impact (in PgCO 2 e yr −1 ). Experts were again asked to express their confidence in their own range, which was used to scale to a standard 80% credible interval. This estimate represents a consolidation of the best-available science by the reviewers. For a complete review including individual studies and their respective findings, see the Supplementary Data . This question was not asked if the expert ranked the pathway as category 3, respecting the substantial uncertainty in that rating.

Final results

After collection of the final survey responses, results were anonymized and compiled by pathway. For overall visualization and discussion purposes, responses were combined into a mean and 20th to 80th percentile range. The strength of the expert elicitation process lies in the collection of several independent assessments. Those different responses represent real differences in data interpretation and synthesis ascribed by experts. This can have meaningful impacts on decision-making by different individuals and organizations (for example, those that are more optimistic or pessimistic about any given pathway). Therefore, individual anonymous responses were retained by pathway to show the diversity of responses for any given pathway. The experts surveyed, despite their broad range of expertise, ranked themselves as less familiar with category 3 pathways than category 1 or 2 (linear regression, P  < 0.001, F  = 59.6 2, 394 ); this could be because of a lack of appropriate experts—although they represented all principal fields—or simply because the data are limited in those areas.

Sensitivity

To check for robustness against sample size variation, we conducted a Monte Carlo sensitivity analysis of the data on each pathway to generate responses of a further ten hypothetical experts. Briefly, the extra samples were randomly drawn from the observed category ranking mean and standard deviations for each individual pathway and appended to the original list; values <1 or >3 were truncated to those values. This analysis resulted in only minor differences in the mean categorization across all pathways: the mean difference between the original and the boot-strapped data was 0.02 (s.d. = 0.05) with an absolute difference average of 0.06 (s.d. = 0.06). The maximum difference in means across all pathways was 0.20 (s.d. = 0.20) (Supplementary Table 2 ). The results suggest that the response values are stable to additional responses.

All processing was done in R 36 , with packages including fmsb 37 and forcats 38 .

Data availability

Anonymized expert elicitation responses are available on Zenodo 39 : https://doi.org/10.5281/zenodo.7859146 .

Code availability

R code for analysis available on Zenodo 39 : https://doi.org/10.5281/zenodo.7859146 .

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Acknowledgements

This research was supported through gifts to the Environmental Defense Fund from the Bezos Earth Fund, King Philanthropies and Arcadia, a charitable fund of L. Rausing and P. Baldwin. We thank J. Rudek for help assembling the review and 30 experts who reviewed some or all of those data and protocol summaries (Supplementary Data ). S.M. was supported by a cooperative agreement between the National Science Foundation and Battelle that sponsors the National Ecological Observatory Network programme.

Author information

Present address: Department of Atmospheric and Oceanic Sciences, University of Wisconsin-Madison, Madison, WI, USA

Present address: AtmoFacts, Longmont, CO, USA

R. N. Lubowski

Present address: Lombard Odier Investment Managers, New York, NY, USA

Present address: Ecological Carbon Offset Partners LLC, dba EP Carbon, Minneapolis, MN, USA

L. A. Moore

Present address: , San Francisco, CA, USA

J. Paltseva

Present address: ART, Arlington, VA, USA

N. A. Randazzo

Present address: NASA/GSFC, Greenbelt, MD, USA

Present address: University of Maryland, College Park, MD, USA

N. Uludere Aragon

Present address: Numerical Terradynamic Simulation Group, University of Montana, Missoula, MT, USA

These authors contributed equally: B. Buma, D. R. Gordon.

Authors and Affiliations

Environmental Defense Fund, New York, NY, USA

B. Buma, D. R. Gordon, K. M. Kleisner, A. Bartuska, J. R. Collins, A. J. Eagle, R. Fujita, E. Holst, J. M. Lavallee, R. N. Lubowski, C. Melikov, L. A. Moore, E. E. Oldfield, J. Paltseva, A. M. Raffeld, N. A. Randazzo, C. Schneider, N. Uludere Aragon & S. P. Hamburg

Department of Integrative Biology, University of Colorado, Denver, CO, USA

Department of Biology, University of Florida, Gainesville, FL, USA

D. R. Gordon

Resources for the Future, Washington, DC, USA

A. Bartuska

International Arctic Research Center, University of Alaska, Fairbanks, AK, USA

Department of Ecology Evolution and Environmental Biology and the Climate School, Columbia University, New York, NY, USA

The Nature Conservancy, Arlington, VA, USA

Faculty of Environment, Science and Economy, University of Exeter, Exeter, UK

P. Friedlingstein

Laboratoire de Météorologie Dynamique/Institut Pierre-Simon Laplace, CNRS, Ecole Normale Supérieure/Université PSL, Sorbonne Université, Ecole Polytechnique, Palaiseau, France

National Ecological Observatory Network, Battelle, Boulder, CO, USA

Department of Engineering and Public Policy, Carnegie Mellon University, Pittsburgh, PA, USA

O’Neill School of Public and Environmental Affairs, Indiana University, Bloomington, IN, USA

Department of Environmental Science and Policy, University of California, Davis, CA, USA

J. N. Sanchirico

Department of Marine Chemistry & Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA, USA

J. R. Collins

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Contributions

D.R.G. and B.B. conceived of and executed the study design. D.R.G., K.M.K., J.R.C., A.J.E., R.F., E.H., J.M.L., R.N.L., C.M., L.A.M., E.E.O., J.P., A.M.R., N.A.R., C.S. and N.U.A. coordinated and conducted the literature review. G.M. and B.B. primarily designed the survey. A. Bartuska, A. Bidlack, B.B., J.N.S., K.N., P.E., P.F., R.D. and S.M. contributed to the elicitation. B.B. conducted the analysis and coding. S.P.H. coordinated funding. B.B. and D.R.G. were primary writers; all authors were invited to contribute to the initial drafting.

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Correspondence to B. Buma .

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The authors declare no competing interests. In the interest of full transparency, we note that while B.B., D.R.G., K.M.K., A.B., J.R.C., A.J.E., R.F., E.H., J.M.L., R.N.L., C.M., L.A.M., E.E.O., J.P., A.M.R., N.A.R., C.S., N.U.A., S.P.H. and P.E. are employed by organizations that have taken positions on specific NbCS frameworks or carbon crediting pathways (not the focus of this work), none have financial or other competing interest in any of the pathways and all relied on independent science in their contributions to the work.

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Supplementary information

Supplementary information.

Supplementary Tables 1–4, Figs. 1–3 and survey instrument.

Supplementary Data

Literature review and list of reviewers.

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Buma, B., Gordon, D.R., Kleisner, K.M. et al. Expert review of the science underlying nature-based climate solutions. Nat. Clim. Chang. (2024). https://doi.org/10.1038/s41558-024-01960-0

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DOI : https://doi.org/10.1038/s41558-024-01960-0

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