green house effect essay for class 6

Essay on Greenhouse Effect for Students and Children

500 words essay on greenhouse effect.

The past month, July of 2019, has been the hottest month in the records of human history. This means on a global scale, the average climate and temperatures are now seen a steady rise year-on-year. The culprits of this climate change phenomenon are mainly pollution , overpopulation and general disregard for the environment by the human race. However, we can specifically point to two phenomenons that contribute to the rising temperatures – global warming and the greenhouse effect. Let us see more about them in this essay on the greenhouse effect.

The earth’s surface is surrounded by an envelope of the air we call the atmosphere. Gasses in this atmosphere trap the infrared radiation of the sun which generates heat on the surface of the earth. In an ideal scenario, this effect causes the temperature on the earth to be around 15c. And without such a phenomenon life could not sustain on earth.

However, due to rapid industrialization and rising pollution, the emission of greenhouse gases has increased multifold over the last few centuries. This, in turn, causes more radiation to be trapped in the earth’s atmosphere. And as a consequence, the temperature on the surface of the planet steadily rises. This is what we refer to when we talk about the man-made greenhouse effect.

Causes of Greenhouse Effect

As we saw earlier in this essay on the greenhouse effect, the phenomenon itself is naturally occurring and an important one to sustain life on our planet. However, there is an anthropogenic part of this effect. This is caused due to the activities of man.

The most prominent among this is the burning of fossil fuels . Our industries, vehicles, factories, etc are overly reliant on fossil fuels for their energy and power. This has caused an immense increase in emissions of harmful greenhouse gasses such as carbon dioxide, carbon monoxide, sulfides, etc. This has multiplied the greenhouse effect and we have seen a steady rise in surface temperatures.

Other harmful activities such as deforestation, excessive urbanization, harmful agricultural practices, etc. have also led to the release of excess carbon dioxide and made the greenhouse effect more prominent. Another harmful element that causes harm to the environment is CFC (chlorofluorocarbon).

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Some Effects of Greenhouse Effect

Even after overwhelming proof, there are still people who deny the existence of climate change and its devastating pitfalls. However, there are so many effects and pieces of evidence of climate change it is now undeniable. The surface temperature of the planet has risen by 1c since the 19th century. This change is largely due to the increased emissions of carbon dioxide. The most harm has been seen in the past 35 years in particular.

The oceans and the seas have absorbed a lot of this increased heat. The surfaces of these oceans have seen a rise in temperatures of 0.4c. The ice sheets and glaciers are also rapidly shrinking. The rate at which the ice caps melt in Antartica has tripled in the last decade itself. These alarming statistics and facts are proof of the major disaster we face in the form of climate change.

600 Words Essay on Greenhouse Effect

A Greenhouse , as the term suggests, is a structure made of glass which is designed to trap heat inside. Thus, even on cold chilling winter days, there is warmth inside it. Similarly, Earth also traps energy from the Sun and prevents it from escaping back. The greenhouse gases or the molecules present in the atmosphere of the Earth trap the heat of the Sun. This is what we know as the Greenhouse effect.

Greenhouse Gases

These gases or molecules are naturally present in the atmosphere of the Earth. However, they are also released due to human activities. These gases play a vital role in trapping the heat of the Sun and thereby gradually warming the temperature of Earth. The Earth is habitable for humans due to the equilibrium of the energy it receives and the energy that it reflects back to space.

Global Warming and the Greenhouse Effect

The trapping and emission of radiation by the greenhouse gases present in the atmosphere is known as the Greenhouse effect. Without this process, Earth will either be very cold or very hot, which will make life impossible on Earth.

The greenhouse effect is a natural phenomenon. Due to wrong human activities such as clearing forests, burning fossil fuels, releasing industrial gas in the atmosphere, etc., the emission of greenhouse gases is increasing.

Thus, this has, in turn, resulted in global warming . We can see the effects due to these like extreme droughts, floods, hurricanes, landslides, rise in sea levels, etc. Global warming is adversely affecting our biodiversity, ecosystem and the life of the people. Also, the Himalayan glaciers are melting due to this.

There are broadly two causes of the greenhouse effect:

I. Natural Causes

• Some components that are present on the Earth naturally produce greenhouse gases. For example, carbon dioxide is present in the oceans, decaying of plants due to forest fires and the manure of some animals produces methane , and nitrogen oxide is present in water and soil.
• Water Vapour raises the temperature by absorbing energy when there is a rise in the humidity.
• Humans and animals breathe oxygen and release carbon dioxide in the atmosphere.

II. Man-made Causes

• Burning of fossil fuels such as oil and coal emits carbon dioxide in the atmosphere which causes an excessive greenhouse effect. Also, while digging a coal mine or an oil well, methane is released from the Earth, which pollutes it.
• Trees with the help of the process of photosynthesis absorb the carbon dioxide and release oxygen. Due to deforestation the carbon dioxide level is continuously increasing. This is also a major cause of the increase in the greenhouse effect.
• In order to get maximum yield, the farmers use artificial nitrogen in their fields. This releases nitrogen oxide in the atmosphere.
• Industries release harmful gases in the atmosphere like methane, carbon dioxide , and fluorine gas. These also enhance global warming.

All the countries of the world are facing the ill effects of global warming. The Government and non-governmental organizations need to take appropriate and concrete measures to control the emission of toxic greenhouse gases. They need to promote the greater use of renewable energy and forestation. Also, it is the duty of every individual to protect the environment and not use such means that harm the atmosphere. It is the need of the hour to protect our environment else that day is not far away when life on Earth will also become difficult.

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What Is the Greenhouse Effect?

Watch this video to learn about the greenhouse effect! Click here to download this video (1920x1080, 105 MB, video/mp4). Click here to download this video about the greenhouse effect in Spanish (1920x1080, 154 MB, video/mp4).

How does the greenhouse effect work?

As you might expect from the name, the greenhouse effect works … like a greenhouse! A greenhouse is a building with glass walls and a glass roof. Greenhouses are used to grow plants, such as tomatoes and tropical flowers.

A greenhouse stays warm inside, even during the winter. In the daytime, sunlight shines into the greenhouse and warms the plants and air inside. At nighttime, it's colder outside, but the greenhouse stays pretty warm inside. That's because the glass walls of the greenhouse trap the Sun's heat.

A greenhouse captures heat from the Sun during the day. Its glass walls trap the Sun's heat, which keeps plants inside the greenhouse warm — even on cold nights. Credit: NASA/JPL-Caltech

The greenhouse effect works much the same way on Earth. Gases in the atmosphere, such as carbon dioxide , trap heat similar to the glass roof of a greenhouse. These heat-trapping gases are called greenhouse gases .

During the day, the Sun shines through the atmosphere. Earth's surface warms up in the sunlight. At night, Earth's surface cools, releasing heat back into the air. But some of the heat is trapped by the greenhouse gases in the atmosphere. That's what keeps our Earth a warm and cozy 58 degrees Fahrenheit (14 degrees Celsius), on average.

Earth's atmosphere traps some of the Sun's heat, preventing it from escaping back into space at night. Credit: NASA/JPL-Caltech

How are humans impacting the greenhouse effect?

Human activities are changing Earth's natural greenhouse effect. Burning fossil fuels like coal and oil puts more carbon dioxide into our atmosphere.

NASA has observed increases in the amount of carbon dioxide and some other greenhouse gases in our atmosphere. Too much of these greenhouse gases can cause Earth's atmosphere to trap more and more heat. This causes Earth to warm up.

What reduces the greenhouse effect on Earth?

Just like a glass greenhouse, Earth's greenhouse is also full of plants! Plants can help to balance the greenhouse effect on Earth. All plants — from giant trees to tiny phytoplankton in the ocean — take in carbon dioxide and give off oxygen.

The ocean also absorbs a lot of excess carbon dioxide in the air. Unfortunately, the increased carbon dioxide in the ocean changes the water, making it more acidic. This is called ocean acidification .

More acidic water can be harmful to many ocean creatures, such as certain shellfish and coral. Warming oceans — from too many greenhouse gases in the atmosphere — can also be harmful to these organisms. Warmer waters are a main cause of coral bleaching .

This photograph shows a bleached brain coral. A main cause of coral bleaching is warming oceans. Ocean acidification also stresses coral reef communities. Credit: NOAA

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21.1: The Greenhouse Effect and Climate Change

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• Page ID 47008

• Melissa Ha and Rachel Schleiger
• Yuba College & Butte College via ASCCC Open Educational Resources Initiative

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Earth’s Temperature is a Balancing Act

Earth’s temperature depends on the balance between energy entering and leaving the planet. When incoming energy from the sun is absorbed, Earth warms. When the sun’s energy is reflected back into space, Earth avoids warming. When energy is released from Earth into space, the planet cools. Many factors, both natural and human, can cause changes in Earth’s energy balance, including:

• Changes in the greenhouse effect, which affects the amount of heat retained by Earth’s atmosphere;
• Variations in the sun’s energy reaching Earth;
• Changes in the reflectivity of Earth’s atmosphere and surface.

Scientists have pieced together a picture of Earth’s climate, dating back hundreds of thousands of years, by analyzing a number of indirect measures of climate such as ice cores, tree rings, glacier size, pollen counts, and ocean sediments. Scientists have also studied changes in Earth’s orbit around the sun and the activity of the sun itself.

The historical record shows that the climate varies naturally over a wide range of time scales. In general, climate changes prior to the Industrial Revolution in the 1700s can be explained by natural causes, such as changes in solar energy, volcanic eruptions, and natural changes in greenhouse gas (GHG) concentrations. Recent changes in climate , however, cannot be explained by natural causes alone. Research indicates that natural causes are very unlikely to explain most observed warming, especially warming since the mid-20th century. Rather, human activities, especially our combustion of fossil fuels, explains the current warming (figure \(\PageIndex{a}\)). The scientific consensus is clear: through alterations of the carbon cycle, humans are changing the global climate by increasing the effects of something known as the greenhouse effect.

The Greenhouse Effect Causes the Atmosphere to Retain Heat

Gardeners that live in moderate or cool environments use greenhouses because they trap heat and create an environment that is warmer than outside temperatures. This is great for plants that like heat, or are sensitive to cold temperatures, such as tomato and pepper plants. Greenhouses contain glass or plastic that allow visible light from the sun to pass. This light, which is a form of energy, is absorbed by plants, soil, and surfaces and heats them. Some of that heat energy is then radiated outwards in the form of infrared radiation, a different form of energy. Unlike with visible light, the glass of the greenhouse blocks the infrared radiation, thereby trapping the heat energy, causing the temperature within the greenhouse to increase.

The same phenomenon happens inside a car on a sunny day. Have you ever noticed how much hotter a car can get compared to the outside temperature? Light energy from the sun passes through the windows and is absorbed by the surfaces in the car such as seats and the dashboard. Those warm surfaces then radiate infrared radiation, which cannot pass through the glass. This trapped infrared energy causes the air temperatures in the car to increase. This process is commonly known as the greenhouse effect .

The video below made for kids, but provides a clear and simple introduction to the greenhouse effect.

The greenhouse effect also happens with the entire Earth. Of course, our planet is not surrounded by glass windows. Instead, the Earth is wrapped with an atmosphere that contains greenhouse gases (GHGs). Much like the glass in a greenhouse, GHGs allow incoming visible light energy from the sun to pass, but they block infrared radiation that is radiated from the Earth towards space (figure \(\PageIndex{b}\)). In this way, they help trap heat energy that subsequently raises air temperature. Being a greenhouse gas is a physical property of certain types of gases; because of their molecular structure they absorb wavelengths of infrared radiation, but are transparent to visible light. Some notable greenhouse gases are water vapor (H 2 O), carbon dioxide (CO 2 ), and methane (CH 4 ). GHGs act like a blanket, making Earth significantly warmer than it would otherwise be. Scientists estimate that average temperature on Earth would be -18º C without naturally-occurring GHGs.

What is Global Warming?

Global warming refers to the recent and ongoing rise in global average temperature near Earth’s surface. It is caused mostly by increasing concentrations of greenhouse gases in the atmosphere. Global warming is causing climate patterns to change. However, global warming itself represents only one aspect of climate change.

What is Climate Change?

Climate change refers to any significant change in the measures of climate lasting for an extended period of time. In other words, climate change includes major changes in temperature, precipitation, or wind patterns, among other effects, that occur over several decades or longer.

The Main Greenhouse Gasses

The most important GHGs directly emitted by humans include CO 2 and methane. Carbon dioxide  (CO 2 ) is the primary greenhouse gas that is contributing to recent global climate change. CO 2 is a natural component of the carbon cycle, involved in such activities as photosynthesis, respiration, volcanic eruptions, and ocean-atmosphere exchange. Human activities, primarily the burning of fossil fuels and changes in land use, release very large amounts of CO 2 to the atmosphere, causing its concentration in the atmosphere to rise.

Atmospheric CO 2 concentrations have increased by 45% since pre-industrial times, from approximately 280 parts per million (ppm) in the 18th century to 409.8 ppm in 2019 (figure \(\PageIndex{c}\)). The current CO 2 level is higher than it has been in at least 800,000 years, based on evidence from ice cores that preserve ancient atmospheric gases (figure \(\PageIndex{d-f}\)). Human activities currently release over 30 billion tons of CO 2 into the atmosphere every year. While some volcanic eruptions released large quantities of CO 2 in the distant past, the U.S. Geological Survey (USGS) reports that human activities now emit more than 135 times as much CO 2 as volcanoes each year. This human-caused build-up of CO 2 in the atmosphere is like a tub filling with water, where more water flows from the faucet than the drain can take away.

Other Greenhouse Gasses

Although this concentration is far less than that of CO 2 , methane (CH 4 ) is 28 times as potent a greenhouse gas. Methane is produced when bacteria break down organic matter under anaerobic conditions and can be released due to natural or anthropogenic processes. Anaerobic conditions can happen when organic matter is trapped underwater (such as in rice paddies) or in the intestines of herbivores. Anthropogenic causes now account for 60% of total methane release. Examples include agriculture, fossil fuel extraction and transport, mining, landfill use, and burning of forests. Specifically, raising cattle releases methane due to fermentation in their rumens produces methane that is expelled from their GI tract. Methane is more abundant in Earth’s atmosphere now than at any time in at least the past 650,000 years, and CH 4 concentrations increased sharply during most of the 20th century. They are now more than two and-a-half times pre-industrial levels (1.9 ppm), but the rate of increase has slowed considerably in recent decades.

Water vapor is the most abundant greenhouse gas and also the most important in terms of its contribution to the natural greenhouse effect, despite having a short atmospheric lifetime. Some human activities can influence local water vapor levels. However, on a global scale, the concentration of water vapor is controlled by temperature, which influences overall rates of evaporation and precipitation. Therefore, the global concentration of water vapor is not substantially affected by direct human emissions.

Ground-level ozone (O 3 ), which also has a short atmospheric lifetime, is a potent greenhouse gas. Chemical reactions create ozone from emissions of nitrogen oxides and volatile organic compounds from automobiles, power plants, and other industrial and commercial sources in the presence of sunlight (as discussed in section 10.1). In addition to trapping heat, ozone is a pollutant that can cause respiratory health problems and damage crops and ecosystems.

Changes in the Sun’s Energy Affect how Much Energy Reaches Earth

Climate can be influenced by natural changes that affect how much solar energy reaches Earth. These changes include changes within the sun and changes in Earth’s orbit. Changes occurring in the sun itself can affect the intensity of the sunlight that reaches Earth’s surface. The intensity of the sunlight can cause either warming (during periods of stronger solar intensity) or cooling (during periods of weaker solar intensity). The sun follows a natural 11-year cycle of small ups and downs in intensity, but the effect on Earth’s climate is small. Changes in the shape of Earth’s orbit as well as the tilt and position of Earth’s axis can also affect the amount of sunlight reaching Earth’s surface.

Changes in the sun’s intensity have influenced Earth’s climate in the past. For example, the so-called “ Little Ice Age ” between the 17th and 19th centuries may have been partially caused by a low solar activity phase from 1645 to 1715, which coincided with cooler temperatures. The Little Ice Age refers to a slight cooling of North America, Europe, and probably other areas around the globe. Changes in Earth’s orbit have had a big impact on climate over tens of thousands of years. These changes appear to be the primary cause of past cycles of ice ages, in which Earth has experienced long periods of cold temperatures (ice ages), as well as shorter interglacial periods (periods between ice ages) of relatively warmer temperatures.

Changes in solar energy continue to affect climate. However, solar activity has been relatively constant, aside from the 11-year cycle, since the mid-20th century and therefore does not explain the recent warming of Earth. Similarly, changes in the shape of Earth’s orbit as well as the tilt and position of Earth’s axis affect temperature on relatively long timescales (tens of thousands of years), and therefore cannot explain the recent warming.

Changes in Reflectivity Affect How Much Energy Enters Earth’s System

When sunlight energy reaches Earth it can be reflected or absorbed. The amount that is reflected or absorbed depends on Earth’s surface and atmosphere. Light-colored objects and surfaces, like snow and clouds, tend to reflect most sunlight, while darker objects and surfaces, like the ocean and forests, tend to absorb more sunlight. The term albedo refers to the amount of solar radiation reflected from an object or surface, often expressed as a percentage. Earth as a whole has an albedo of about 30%, meaning that 70% of the sunlight that reaches the planet is absorbed. Sunlight that is absorbed warms Earth’s land, water, and atmosphere.

Albedo is also affected by aerosols. Aerosols are small particles or liquid droplets in the atmosphere that can absorb or reflect sunlight. Unlike greenhouse gases (GHGs), the climate effects of aerosols vary depending on what they are made of and where they are emitted. Those aerosols that reflect sunlight, such as particles from volcanic eruptions or sulfur emissions from burning coal, have a cooling effect. Those that absorb sunlight, such as black carbon (a part of soot), have a warming effect.

Natural changes in albedo, like the melting of sea ice or increases in cloud cover, have contributed to climate change in the past, often acting as feedbacks to other processes. Volcanoes have played a noticeable role in climate. Volcanic particles that reach the upper atmosphere can reflect enough sunlight back to space to cool the surface of the planet by a few tenths of a degree for several years. Volcanic particles from a single eruption do not produce long-term change because they remain in the atmosphere for a much shorter time than GHGs.

Human changes in land use and land cover have changed Earth’s albedo. Processes such as deforestation, reforestation, desertification, and urbanization often contribute to changes in climate in the places they occur. These effects may be significant regionally, but are smaller when averaged over the entire globe.

Scientific Consensus: Global Climate Change is Real

The Intergovernmental Panel on Climate Change (IPCC) was created in 1988 by the United Nations Environment Programme and the World Meteorological Organization. It is charged with the task of evaluating and synthesizing the scientific evidence surrounding global climate change. The IPCC uses this information to evaluate current impacts and future risks, in addition to providing policymakers with assessments. These assessments are released about once every every six years. The most recent report, the 5th Assessment, was released in 2013. Hundreds of leading scientists from around the world are chosen to author these reports. Over the history of the IPCC, these scientists have reviewed thousands of peer-reviewed, publicly available studies. The scientific consensus is clear: global climate change is real and humans are very likely the cause for this change.

Additionally, the major scientific agencies of the United States, including the National Aeronautics and Space Administration (NASA) and the National Oceanic and Atmospheric Administration (NOAA), also agree that climate change is occurring and that humans are driving it. In 2010, the US National Research Council concluded that “Climate change is occurring, is very likely caused by human activities, and poses significant risks for a broad range of human and natural systems”. Many independent scientific organizations have released similar statements, both in the United States and abroad. This doesn’t necessarily mean that every scientist sees eye to eye on each component of the climate change problem, but broad agreement exists that climate change is happening and is primarily caused by excess greenhouse gases from human activities. Critics of climate change, driven by ideology instead of evidence, try to suggest to the public that there is no scientific consensus on global climate change. Such an assertion is patently false.

Current Status of Global Climate Change and Future Changes

Greenhouse gas concentrations in the atmosphere will continue to increase unless the billions of tons of anthropogenic emissions each year decrease substantially. Increased concentrations are expected to do the following:

• Increase Earth’s average temperature (figure \(\PageIndex{g}\)),
• Influence the patterns and amounts of precipitation,
• Reduce ice and snow cover, as well as permafrost,
• Raise sea level (figure \(\PageIndex{h}\)),
• Increase the acidity of the oceans.

Figure \(\PageIndex{h}\):  Sea height variation (mm) over time. Sea height has increased about 3.3 millimeters per year on average since 1993. Data is from satellite sea level observations by the NASA Goddard Space Flight Center. Image by NASA (public domain).

These changes will impact our food supply, water resources, infrastructure, ecosystems, and even our own health. The magnitude and rate of future climate change will primarily depend on the following factors:

• The rate at which levels of greenhouse gas concentrations in our atmosphere continue to increase,
• How strongly features of the climate (e.g., temperature, precipitation, and sea level) respond to the expected increase in greenhouse gas concentrations,
• Natural influences on climate (e.g., from volcanic activity and changes in the sun’s intensity) and natural processes within the climate system (e.g., changes in ocean circulation patterns).

Past and Present-day GHG Emissions Will Affect Climate Far into the Future

Many greenhouse gases stay in the atmosphere for long periods of time. As a result, even if emissions stopped increasing, atmospheric greenhouse gas concentrations would continue to remain elevated for hundreds of years. Moreover, if we stabilized concentrations and the composition of today’s atmosphere remained steady (which would require a dramatic reduction in current greenhouse gas emissions), surface air temperatures would continue to warm. This is because the oceans, which store heat, take many decades to fully respond to higher greenhouse gas concentrations. The ocean’s response to higher greenhouse gas concentrations and higher temperatures will continue to impact climate over the next several decades to hundreds of years.

Future Temperature Changes

Climate models project the following key temperature-related changes:

• Average global temperatures are expected to increase by 2°F to 11.5°F by 2100, depending on the level of future greenhouse gas emissions, and the outcomes from various climate models.
• By 2100, global average temperature is expected to warm at least twice as much as it has during the last 100 years.
• Ground-level air temperatures are expected to continue to warm more rapidly over land than oceans.
• Some parts of the world are projected to see larger temperature increases than the global average.

Future Precipitation and Storm Events

Patterns of precipitation and storm events, including both rain and snowfall are likely to change. However, some of these changes are less certain than the changes associated with temperature. Projections show that future precipitation and storm changes will vary by season and region. Some regions may have less precipitation, some may have more precipitation, and some may have little or no change. The amount of rain falling in heavy precipitation events is likely to increase in most regions, while storm tracks are projected to shift towards the poles. Climate models project the following precipitation and storm changes:

• Global average annual precipitation through the end of the century is expected to increase, although changes in the amount and intensity of precipitation will vary by region.
• The intensity of precipitation events will likely increase on average. This will be particularly pronounced in tropical and high-latitude regions, which are also expected to experience overall increases in precipitation.
• The strength of the winds associated with tropical storms is likely to increase. The amount of precipitation falling in tropical storms is also likely to increase.
• Annual average precipitation is projected to increase in some areas and decrease in others.

Future Ice, Snowpack, and Permafrost

Arctic sea ice is already declining drastically. The area of snow cover in the Northern Hemisphere has decreased since 1970. Permafrost temperature has increased over the last century, making it more susceptible to thawing. Over the next century, it is expected that sea ice will continue to decline, glaciers will continue to shrink, snow cover will continue to decrease, and permafrost will continue to thaw.

For every 2°F of warming, models project about a 15% decrease in the extent of annually averaged sea ice and a 25% decrease in September Arctic sea ice. The coastal sections of the Greenland and Antarctic ice sheets are expected to continue to melt or slide into the ocean. If the rate of this ice melting increases in the 21st century, the ice sheets could add significantly to global sea level rise. Glaciers are expected to continue to decrease in size. The rate of melting is expected to continue to increase, which will contribute to sea level rise.

Future Sea Level Change

Warming temperatures contribute to sea level rise by expanding ocean water, melting mountain glaciers and ice caps, and causing portions of the Greenland and Antarctic ice sheets to melt or flow into the ocean. Since 1870, global sea level has risen by about 8 inches. Estimates of future sea level rise vary for different regions, but global sea level for the next century is expected to rise at a greater rate than during the past 50 years. The contribution of thermal expansion, ice caps, and small glaciers to sea level rise is relatively well-studied, but the impacts of climate change on ice sheets are less understood and represent an active area of research. Thus, it is more difficult to predict how much changes in ice sheets will contribute to sea level rise. Greenland and Antarctic ice sheets could contribute an additional 1 foot of sea level rise, depending on how the ice sheets respond.

Regional and local factors will influence future relative sea level rise for specific coastlines around the world (figure \(\PageIndex{i}\)). For example, relative sea level rise depends on land elevation changes that occur as a result of subsidence (sinking) or uplift (rising), in addition to things such as local currents, winds, salinity, water temperatures, and proximity to thinning ice sheets. Assuming that these historical geological forces continue, a 2-foot rise in global sea level by 2100 would result in the following relative sea level rise:

• 2.3 feet at New York City
• 2.9 feet at Hampton Roads, Virginia
• 3.5 feet at Galveston, Texas
• 1 foot at Neah Bay in Washington state

Future Ocean Acidification

Ocean acidification is the process of ocean waters decreasing in pH. Oceans become more acidic as carbon dioxide (CO 2 ) emissions in the atmosphere dissolve in the ocean. This change is measured on the pH scale, with lower values being more acidic. The pH level of the oceans has decreased by approximately 0.1 pH units since pre-industrial times, which is equivalent to a 25% increase in acidity. The pH level of the oceans is projected to decrease even more by the end of the century as CO 2 concentrations are expected to increase for the foreseeable future. Ocean acidification adversely affects many marine species, including plankton, mollusks, shellfish, and corals. As ocean acidification increases, the availability of calcium carbonate will decline. Calcium carbonate is a key building block for the shells and skeletons of many marine organisms. If atmospheric CO 2 concentrations double, coral calcification rates are projected to decline by more than 30%. If CO 2 concentrations continue to rise at their current rate, corals could become rare on tropical and subtropical reefs by 2050.

Mismatched Interactions

Climate change also affects phenology, the study of the effects of climatic conditions on the timing of periodic lifecycle events, such as flowering in plants or migration in birds. Researchers have shown that 385 plant species in Great Britain are flowering 4.5 days sooner than was recorded earlier during the previous 40 years. In addition, insect-pollinated species were more likely to flower earlier than wind-pollinated species. The impact of changes in flowering date would be mitigated if the insect pollinators emerged earlier. This mismatched timing of plants and pollinators could result in injurious ecosystem effects because, for continued survival, insect-pollinated plants must flower when their pollinators are present.

Likewise, migratory birds rely on daylength cues, which are not influenced by climate change. Their insect food sources, however, emerge earlier in the year in response to warmer temperatures. As a result, climate change decreases food availability for migratory bird species.

Spread of Disease

This rise in global temperatures will increase the range of disease-carrying insects and the viruses and pathogenic parasites they harbor. Thus, diseases will spread to new regions of the globe. This spread has already been documented with dengue fever, a disease the affects hundreds of millions per year, according to the World Health Organization. Colder temperatures typically limit the distribution of certain species, such as the mosquitoes that transmit malaria, because freezing temperatures destroy their eggs.

Not only will the range of some disease-causing insects expand, the increasing temperatures will also accelerate their lifecycles, which allows them to breed and multiply quicker, and perhaps evolve pesticide resistance faster. In addition to dengue fever, other diseases are expected to spread to new portions of the world as the global climate warms. These include malaria, yellow fever, West Nile virus, zika virus, and chikungunya.

Climate change does not only increase the spread of diseases in humans. Rising temperatures are associated with greater amphibian mortality due to chytridiomycosis (see Invasive Species ). Similarly, warmer temperatures have exacerbated bark beetle infestations of coniferous trees, such as pine an spruce.

Climate Change Affects Everyone

Our lives are connected to the climate . Human societies have adapted to the relatively stable climate we have enjoyed since the last ice age which ended several thousand years ago. A warming climate will bring changes that can affect our water supplies, agriculture, power and transportation systems, the natural environment, and even our own health and safety.

Carbon dioxide can stay in the atmosphere for nearly a century, on average, so Earth will continue to warm in the coming decades. The warmer it gets, the greater the risk for more severe changes to the climate and Earth’s system. Although it’s difficult to predict the exact impacts of climate change, what’s clear is that the climate we are accustomed to is no longer a reliable guide for what to expect in the future.

We can reduce the risks we will face from climate change . By making choices that reduce greenhouse gas pollution, and preparing for the changes that are already underway, we can reduce risks from climate change. Our decisions today will shape the world our children and grandchildren will live in.

You can take steps at home, on the road, and in your office to reduce greenhouse gas emissions and the risks associated with climate change. Many of these steps can save you money. Some, such as walking or biking to work, can even improve your health! You can also get involved on a local or state level to support energy efficiency, clean energy programs, or other climate programs.

Suggested Supplementary Reading

Intergovernmental Panel on Climate Change. 2013. 5th Assessment: Summary for Policymakers .

NASA. 2018. Global Climate Change: Vital Signs of the Planet . This website by NASA provides a multi-media smorgasbord of engaging content. Learn about climate change using data collected by NASA satellites and more.

Attributions

Modified by Melissa Ha from the following sources:

• Climate and the Effects of Global Climate Change  from  General Biology  by OpenStax (licensed under  CC-BY )
• Climate Change  from  Environmental Biology  by Matthew R. Fisher (licensed under  CC-BY )
• Carbon Cycle from  Biology  by John W. Kimball (licensed under  CC-BY )

The Greenhouse Effect

Energy from the Sun that makes its way to Earth can have trouble finding its way back out to space. The greenhouse effect causes some of this energy to be waylaid in the atmosphere, absorbed and released by greenhouse gases.

Without the greenhouse effect, Earth’s temperature would be below freezing . It is, in part, a natural process. However, Earth’s greenhouse effect is getting stronger as we add greenhouse gases to the atmosphere. That is warming the climate of our planet.

How Does the Greenhouse Effect Work?

Solar energy absorbed at Earth’s surface is radiated back into the atmosphere as heat. As the heat makes its way through the atmosphere and back out to space, greenhouse gases absorb much of it. Why do greenhouse gases absorb heat? Greenhouse gases are more complex than other gas molecules in the atmosphere, with a structure that can absorb heat. They radiate the heat back to the Earth's surface, to another greenhouse gas molecule, or out to space.

There are several different types of greenhouse gases . The major ones are carbon dioxide, water vapor, methane, and nitrous oxide. These gas molecules all are made of three or more atoms. The atoms are held together loosely enough that they vibrate when they absorb heat . Eventually, the vibrating molecules release the radiation, which will likely be absorbed by another greenhouse gas molecule. This process keeps heat near the Earth’s surface. Most of the gas in the atmosphere is nitrogen and oxygen, which cannot absorb heat and contribute to the greenhouse effect.

A Couple of Common Greenhouse Gases

• Carbon dioxide : Made of one carbon atom and two oxygen atoms, carbon dioxide molecules make up a small fraction of the atmosphere, but have a large effect on climate. There was about 270 parts per million (ppm) of carbon dioxide in the atmosphere in the mid-19th Century at the start of the Industrial Revolution. The amount is growing as burning fossil fuels releases carbon dioxide into the atmosphere. The concentration has been over 400 ppm since 2015. (Check  NOAA Global Monitoring Laboratory  for the latest measurements).
• Methane : A powerful greenhouse gas, able to absorb far more heat than carbon dioxide, methane is made of one carbon and four hydrogen atoms. It is found in very small quantities in the atmosphere but is able to make a big impact on warming. Methane gas is also used as a fuel. When burned, it releases carbon dioxide greenhouse gas into the atmosphere.

Above: (Left) The Earth’s surface, warmed by the Sun, radiates heat into the atmosphere. Some heat is absorbed by greenhouse gases like carbon dioxide and then radiated to space (A). Some heat makes its way to space directly (B). Some heat is absorbed by greenhouse gases and then radiated back towards the Earth’s surface (C). (Right) With more carbon dioxide in the atmosphere later this Century, more heat will be stopped by greenhouse gases, warming the planet. (Image: L.S.Gardiner/UCAR)

More Greenhouse Gases = A Warmer Earth

Even though only a tiny amount of the gases in Earth’s atmosphere are greenhouse gases, they have a huge effect on climate. Sometime during this century, the amount of the greenhouse gas carbon dioxide in the atmosphere is expected to double. Other greenhouse gases like methane and nitrous oxide are increasing as well. The quantity of greenhouse gases is increasing as fossil fuels are burned, releasing the gases and other air pollutants into the atmosphere. Greenhouse gases also make their way to the atmosphere from other sources. Farm animals, for example, release methane gas as they digest food. As cement is made from limestone, it releases carbon dioxide.

With more greenhouse gases in the air, heat passing through on its way out of the atmosphere is more likely to be stopped. The added greenhouse gases absorb the heat. They then radiate this heat. Some of the heat will head away from the Earth, some of it will be absorbed by another greenhouse gas molecule, and some of it will wind up back at the planet’s surface again. With more greenhouse gases, heat will stick around, warming the planet.

• Why Earth Is Warming
• Greenhouse Effect Video
• Why Does Climate Change?
• Carbon dioxide
• Some Greenhouse Gases Are Stronger than Others

Related Links

• UCAR Center for Science Education Climate Activities
• NASA Global Climate Change
• Climate Literacy and Energy Awareness Network
• Climate Sensitivity Calculator

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The Greenhouse Effect

• Greenhouse Gases
• All About Carbon Dioxide

If it were not for greenhouse gases trapping heat in the atmosphere, the Earth would be a very cold place. Greenhouse gases keep the Earth warm through a process called the greenhouse effect. Play the video to learn more » ( Alternative version )

The Earth gets energy from the sun in the form of sunlight. The Earth's surface absorbs some of this energy and heats up. That's why the surface of a road can feel hot even after the sun has gone down—because it has absorbed a lot of energy from the sun. The Earth cools down by giving off a different form of energy, called infrared radiation. But before all this radiation can escape to outer space, greenhouse gases in the atmosphere absorb some of it, which makes the atmosphere warmer. As the atmosphere gets warmer, it makes the Earth's surface warmer, too.

Learn more about radiation . ( Alternative version )

Learn where the term “greenhouse effect” comes from . ( Alternative version )

Greenhouse gases keep the Earth warm through a process called the greenhouse effect.

What Is Radiation?

You might hear the word radiation and think that it's a bad thing. It's true that there are certain types of radiation that are bad for you, but other types of radiation are important parts of your life. When you feel heat from the sun, see all the colors around you, or listen to the radio, you are actually experiencing different types of radiation.

These types of radiation are all part of the electromagnetic spectrum, which means they involve energy traveling in the form of a wave. Different types of radiation have different wavelengths.

What's in a Name? The “Greenhouse Effect”

A greenhouse is a building made of glass that allows sunlight to enter but traps heat inside, so the building stays warm even when it's cold outside. Because gases in the Earth's atmosphere also let in light but trap heat, many people call this phenomenon the “greenhouse effect.” The greenhouse effect works somewhat differently from an actual greenhouse, but the name stuck, so that's how we still refer to it today.

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Understanding Global Change

Discover why the climate and environment changes, your place in the Earth system, and paths to a resilient future.

Greenhouse effect

Life as we know it would be impossible if not for the greenhouse effect, the process through which heat is absorbed and re-radiated in that atmosphere. The intensity of a planet’s greenhouse effect is determined by the relative abundance of greenhouse gases in its atmosphere. Without greenhouse gases, most of Earth’s heat would be lost to outer space, and our planet would quickly turn into a giant ball of ice. Increase the amount of greenhouse gases to the levels found on the planet Venus, and the Earth would be as hot as a pizza oven! Fortunately, the strength of Earth’s greenhouse effect keeps our planet within a temperature range that supports life

On this page

What is the greenhouse effect, earth system models about the greenhouse effect, how human activities influence the greenhouse effect, explore the earth system, investigate, links to learn more.

For the classroom:

• Teaching Resources

Global Change Infographic

The greenhouse effect occurs in the atmosphere, and is an essential part of How the Earth System Works. Click the image on the left to open the Understanding Global Change Infographic . Locate the greenhouse effect icon and identify other topics that cause changes to, or are affected by, the greenhouse effect.

Adapted from the Environmental Protection Agency greenhouse effect file

Greenhouse gases such as methane, carbon dioxide, nitrous oxide, and water vapor  significantly affect the amount of energy in the Earth system, even though they make up a tiny percentage of Earth’s atmosphere.  Solar radiation that passes through the atmosphere and reaches Earth’s surface is either reflected or absorbed . Reflected sunlight doesn’t add any heat to the Earth system because this energy bounces back into space.

However, absorbed sunlight increases the temperature of Earth’s surface, and the warmed surface re-radiates as long-wave radiation (also known as infrared radiation). Infrared radiation is invisible to the eye, but we feel it as heat.

If there were not any greenhouse gases in the atmosphere, all that heat would pass directly back into space. With greenhouse gases present, however, most of the long-wave radiation coming from Earth’s surface is absorbed and then re-radiated in all directions many times before passing back into space. Heat that is re-radiated downward, toward the Earth, is absorbed by the surface and re-radiated again.

Clouds also influence the greenhouse effect. A thick, low cloud cover can enhance the reflectivity of the atmosphere, reducing the amount of solar radiation reaching Earth’s surface, but clouds high in the atmosphere can intensify the greenhouse effect by re-radiating heat from the Earth’s surface.

Altogether, this cycle of absorption and re-radiation by greenhouse gases impedes the loss of heat from our atmosphere to space, creating the greenhouse effect. Increases in the amount of greenhouses gases will mean that more heat is trapped, increasing the amount of energy in the Earth system (Earth’s energy budget), and raising Earth’s temperature. This increase in Earth’s average temperature is also known as global warming.

This Earth system model is one way to represent the essential processes and interactions related to the greenhouse effect. Hover over the icons for brief explanations; click on the icons to learn more about each topic. Download the Earth system models on this page. There are a few ways that the relationships among these topics can be represented and explained using the Understanding Global Change icons ( download examples ).  

The greenhouse effect, which influences Earth’s average temperature, affects many of the processes that shape global climate and ecosystems.  This model shows some of the other parts of the Earth system that the greenhouse effect influences, including the water cycle and water temperature .

Humans directly affect the greenhouse effect through activities that result in greenhouse gas emissions. The Earth system model below includes some of the ways that human activities increase the amount of greenhouse gases in the atmosphere. Releasing greenhouse gases intensifies the greenhouse effect, and increases Earth’s average air temperatures (also known as global warming). Hover over or click on the icons to learn more about these human causes of change and how they influence the greenhouse effect.

Click the scene icons and bolded terms on this page to learn more about these process and phenomena.

Learn more in these real-world examples, and challenge yourself to  construct a model  that explains the Earth system relationships.

• Ancient fossils and modern climate change
• How Global Warming Works
• NASA:  Global Climate Change:  A Blanket Around the Earth
• UCAR Center for Science Education: The Greenhouse Effect
• IPCC:  What is the Greenhouse Effect?
• Indicators of Change (NCA.2014)
• Human influence on the greenhouse effect
• The Carbon Cycle and Earth’s Climate

• Earth Science
• Greenhouse Effect And Global Warming

Greenhouse Effect and Global Warming

The Greenhouse Effect and Global Warming are one of the major problems that the world is facing today. An in-depth explanation of global warming and the greenhouse effect along with its causes and effects is given below.

What is the Greenhouse Effect?

The greenhouse effect is the process thanks to which Earth has a higher temperature than it would have without it. The gases that radiate heat also known as greenhouse gases absorb the energy radiated out by the Earth and reflect a part of it back to Earth. Of all the energy that the Earth receives from the Sun, a part of it around 26% is reflected back to space by the atmosphere and clouds. Some part of it is absorbed by the atmosphere, around 19%.

The rest hits the ground and heats the surface of the Earth. This absorbed energy is radiated out of the earth in the form of Infrared Waves . These IR waves warm the atmosphere above the Earth. The atmosphere again radiates this energy it received from the Earth both upwards and downwards. The energy sent downwards results in a higher equilibrium temperature than if greenhouse gases were absent. This greenhouse effect is essential to supporting life on Earth.

What are Greenhouse Gases?

The greenhouse gases responsible for the greenhouse effect are:

• Water Vapour
• Carbon Dioxide

The excessive burning of fossil fuels such as petrol, coal, etc. have resulted in an increase in the number of greenhouse gases in the atmosphere resulting in a phenomenon known as Global Warming. This is an increase in the ambient temperature of Earth which will negatively affect life on Earth.

How do we know?

If an ideal black body were at the same distance from the Sun as Earth, its temperature would be 5.3 o C. However, Earth reflects 30% of this energy back into space. Including this in the calculation for the temperature of Earth gives us an answer of -18 o C. As we can clearly see, this is far from true. The average temperature is a whopping 33 o C higher at 15 o C. This difference in ambient temperature is caused by greenhouse gases.

Causes of Greenhouse Effect

The following are the factors that are responsible for the cause of greenhouse effect:

• Deforestation: This is considered to be one of the most responsible factors for the cause of the greenhouse effect. This is due to the reduction in the release of oxygen and absorption of carbon dioxide by the plants.
• Fossil fuel burning: Fossil fuels such as coal, oil, and natural gases are used as a means of energy which releases a huge amount of harmful gases into the environment.
• Population: As the population increases, the need for space increases which again results in deforestation.

Prevention of Greenhouse Effect

Now that we have made a list of causes, finding alternatives to these causes becomes by following the below-mentioned preventive measures:

• Afforestation: Afforestation on a large scale area helps in decreasing the release of carbon dioxide in the atmosphere.
• Conservation of energy: Switching to renewable sources of energy such as solar energy, wind energy, etc will reduce the use of fossil fuels. This eventually reduces the release of carbon dioxide into the atmosphere.
• Policy intervention: When the government comes up with strict policies to maintain the overall air quality of the city.

Global warming refers to the increasing temperature of the Earth’s climate system and its related effects. Scientific evidence has conclusively proven that the Earth’s temperature is in fact rising and has risen by 0.85 o C. This has an impact that has affected different regions differently. The effects include rising sea levels, retreating glaciers, loss of sea ice in the poles, warming global temperatures, changing precipitation, expansion of deserts, etc.

This raises significant threats for humans such as food security from decreasing crop yields, and submergence of a low-lying area due to the rising sea. To prevent irreversible damage to the delicate ecosystems on Earth, scientists have decided that global warming should be limited to a maximum of 2.0 o C relative to pre-industrial levels. The greenhouse effect plays an important role in the rising temperature. And hence to restrict global warming we need to limit the greenhouse effect and the gaseous emissions that cause the greenhouse effect.

Fourteen of the fifteen years in the 21 st century have been the hottest years on record with constant occurrences of extreme weather, cyclones, droughts, floods, etc. All these events are some way or the other have an association with the greenhouse effect and global warming.

Frequently Asked Questions – FAQs

What are greenhouse gases, what are the factors that are responsible for the cause of the greenhouse effect.

• Deforestation
• Fossil fuel burning

What are the methods to prevent the greenhouse effect?

• Afforestation: Afforestation on a large scale helps in decreasing the release of carbon dioxide in the atmosphere.

What is global warming?

What are the manmade causes of global warming, watch the video and learn more about the effects of global warming on rainfall rate.

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What causes the greenhouse effect? | 16-18 years

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Reinforce your students’ understanding of the cause of the greenhouse effect using this lesson plan with a demonstration and activities for 16–18 year olds

In this activity, students check and clarify their understanding of the greenhouse effect by drawing and comparing diagrams. After observing a teacher demonstration, students work independently and then in collaboration with a partner to share and evaluate their ideas and agree a joint view. They then compare their view with the ideas of other pairs of students.

This activity is best used to consolidate and to check on learning after students have spent some time on this topic.

Learning objectives

Students will:

• Understand the cause of the greenhouse effect.

Sequence of activities

Demonstration.

• Begin with the greenhouse effect demonstration (from T. Lister’s  Classic chemistry demonstrations , number 68), emphasising the observed temperature changes but without providing an explanation at this stage.
• Invite students to say what phenomenon is illustrated by the demonstration and then share the learning objective with them.

Activity: stage 1

• Hand out a copy of the ’Student sheet’ to each student.
• Ask them to draw, by themselves, an annotated diagram to explain the greenhouse effect.

Activity: stage 2

Divide students into pairs. Ask them to

• Compare their diagrams.
• Justify their ideas, where there are differences.
• Agree a common diagram which they draw on an OHT or produce a PowerPoint slide.
• One group to show and explain their diagram to the class using an OHP or a data projector.
• Other groups to add to it (from their own diagram) until all key points have been described.
• Remind students of the demonstration used to start the session.
• Question them on what the lamp and lead foil represented (Sun and Earth).
• Ensure that students modify their original diagram, where necessary, to take account of what other students have said during the session.

Take in the student diagrams and write comments which reinforce the good features. Identify any points that still need developing and indicate where additional support can be found.

The demonstration is a key part of the session and a key to its objective.

The process of comparing diagrams draws students into assessing themselves and their fellow students. It encourages them to listen to the ideas of others.

In the plenary, the students experience a wider range of ideas. They then reassess the completeness of their original diagram.

Even though this may be a final session on this topic, written feedback remains important. It confirms achievement or guides students on how to clarify their understanding.

Practical notes

Health, safety and technical notes.

• See the  greenhouse effect demonstration  for full kit list, safety instructions and procedure for the teacher demonstration.
• Read our standard health and safety guidance .
• Wear eye protection.
• It is the responsibility of the teacher to carry out an appropriate risk assessment.

Expected explanation of the greenhouse effect

• Visible radiation is emitted from the hot sun.
• Some of this radiation is absorbed by the Earth which is consequently warmed.
• Warm earth emits infrared radiation.
• Some of the radiation is absorbed by greenhouse gases such as carbon dioxide. Carbon-oxygen bonds in carbon dioxide molecules vibrate more vigorously.
• Molecules excited in this way collide with other molecules in the atmosphere and spread absorbed energy around.
• The atmosphere becomes warmer.

What causes the greenhouse effect? student sheet

Additional information.

This lesson plan was originally part of the  Assessment for Learning  website, published in 2008.

Assessment for Learning is an effective way of actively involving students in their learning.  Each session plan comes with suggestions about how to organise activities and worksheets that may be used with students.

Acknowledgement

T. Lister,  Classic chemistry demonstrations . London: Royal Society of Chemistry, 1995.

• 16-18 years
• Demonstrations
• Formative assessment 
• Higher-order thinking and metacognition
• Environmental science
• Society and ethics
• Climate change

Specification

• Greenhouse gases in the atmosphere maintain temperatures on Earth high enough to support life. Water vapour, carbon dioxide and methane are greenhouse gases.
• Students should be able to describe the greenhouse effect in terms of the interaction of short and long wavelength radiation with matter.
• Describe the greenhouse effect in terms of the interaction of radiation with matter.
• 8.24 Describe how various gases in the atmosphere, including carbon dioxide, methane and water vapour, absorb heat radiated from the Earth, subsequently releasing energy which keeps the Earth warm: this is known as the greenhouse effect
• C1.3.1 describe the greenhouse effect in terms of the interaction of radiation with matter
• C6.2c describe the greenhouse effect in terms of the interaction of radiation with matter within the atmosphere
• C6.3c describe the greenhouse effect in terms of the interaction of radiation with matter within the atmosphere
• Hydrocarbons and alcohols burn in a plentiful supply of oxygen to produce carbon dioxide and water.
• (g) the environmental effects and consequences of the emission of carbon dioxide and sulfur dioxide into the atmosphere through the combustion of fossil fuels
• 2.5.28 demonstrate knowledge that the combustion of fuels is a major source of atmospheric pollution due to: combustion of hydrocarbons producing carbon dioxide, which leads to the greenhouse effect causing sea level rises, flooding and climate change;…
• 2.5.26 demonstrate knowledge that the combustion of fuels is a major source of atmospheric pollution due to: combustion of hydrocarbons producing carbon dioxide, which leads to the greenhouse effect causing sea level rises, flooding and climate change…
• Methane as a contributor to the greenhouse effect.
• The greenhouse effect and the influence of human activity on it.
• Greenhouse gases and their relative effects [especially carbon dioxide and water vapour; also methane, chlorofluorocarbons (CFCs)].
• Possible implications of increased greenhouse effect.

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The Greenhouse Effect and our Planet

The greenhouse effect happens when certain gases, which are known as greenhouse gases, accumulate in Earth’s atmosphere. Greenhouse gases include carbon dioxide (CO 2 ), methane (CH 4 ), nitrous oxide (N 2 O), ozone (O 3 ), and fluorinated gases.

Biology, Ecology, Earth Science, Geography, Human Geography

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The greenhouse effect happens when certain gases , which are known as greenhouse gases , accumulate in Earth’s atmosphere . Greenhouse gases include carbon dioxide (CO 2 ), methane (CH 4 ), nitrous oxide (N 2 O), ozone (O 3 ), and fluorinated gases.

Greenhouse gases allow the sun’s light to shine onto Earth’s surface, and then the gases , such as ozone , trap the heat that reflects back from the surface inside Earth’s atmosphere . The gases act like the glass walls of a  greenhouse —thus the name, greenhouse gas .

According to scientists, the average temperature of Earth would drop from 14˚C (57˚F) to as low as –18˚C (–0.4˚F), without the greenhouse effect .

Some greenhouse gases come from natural sources, for example, evaporation  adds water vapor to the atmosphere . Animals and plants release carbon dioxide when they respire, or breathe. Methane is released naturally from decomposition. There is evidence that suggests methane is released in low-oxygen environments , such as  swamps or landfills . Volcanoes —both on land and under the ocean —release greenhouse gases , so periods of high volcanic activity tend to be warmer.

Since the  Industrial Revolution  of the late 1700s and early 1800s, people have been releasing larger quantities of greenhouse gases into the atmosphere. That amount has skyrocketed in the past century. Greenhouse gas emissions increased 70 percent between 1970 and 2004. Emissions of CO 2 , rose by about 80 percent during that time.

The amount of CO 2 in the atmosphere far exceeds the naturally occurring range seen during the last 650,000 years.

Most of the CO 2 that people put into the atmosphere comes from burning  fossil fuels . Cars, trucks, t rains , and planes all burn fossil fuels. Many electric power plants do as well. Another way humans release CO 2 into the atmosphere is by cutting down  forests , because trees contain large amounts of carbon.

People add methane to the atmosphere through  livestock  farming, landfills , and fossil fuel production such as  coal mining  and natural gas processing. Nitrous oxide comes from  agriculture  and fossil fuel burning. Fluorinated gases include chlorofluoro carbons (CFCs),  hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbons (HFCs). They are produced during the manufacturing of refrigeration and cooling products and through aerosols.

All of these human activities add greenhouse gases to the atmosphere . As the level of these gases rises, so does the  temperature  of Earth. The rise in Earth’s average temperature contributed to by human activity is known as  global warming .

The Greenhouse Effect and Climate Change Even slight increases in average global temperatures can have huge effects.

Perhaps the biggest, most obvious effect is that  glaciers and  ice caps melt faster than usual. The  meltwater  d rains into the oceans , causing  sea levels to rise.

Glaciers and ice caps cover about 10 percent of the world’s landmasses. They hold between 70 and 75 percent of the world’s  freshwater . If all of this ice melted, sea levels would rise by about 70 meters (230 feet).

The Intergovernmental Panel on Climate Change states that the global sea level rose about 1.8 millimeters (0.07 inches) per year from 1961 to 1993, and about 3.1 millimeters (0.12 inches) per year since 1993.

Rising sea levels cause  flooding in  coastal cities, which could displace millions of people in low-lying areas such as Bangladesh, the U.S. state of Florida, and the Netherlands.

Millions more people in countries like Bolivia, Peru, and India depend on glacial meltwater for drinking,  irrigation , and  hydroelectric power . Rapid loss of these glaciers would devastate those countries.

Greenhouse gas emissions affect more than just temperature . Another effect involves changes in  precipitation , such as  rain  and  snow .

Over the course of the 20th century, precipitation increased in eastern parts of North and South America, northern Europe, and northern and central Asia. However, it has decreased in parts of Africa, the Mediterranean, and southern Asia.

As climates change, so do the habitats for living things. Animals that are adapted to a certain  climate  may become threatened. Many human societies depend on predictable rain patterns in order to grow specific  crops for food, clothing, and trade. If the climate of an area changes, the people who live there may no longer be able to grow the crops they depend on for survival. Some scientists also worry that tropical diseases will expand their ranges into what are now more temperate regions if the temperatures of those areas increase.

Most climate scientists agree that we must reduce the amount of greenhouse gases released into the atmosphere. Ways to do this, include:

• driving less, using public transportation , carpooling, walking, or riding a bike.
• flying less—airplanes produce huge amounts of greenhouse gas emissions.
• reducing, reusing, and recycling.
• planting a tree—trees absorb carbon dioxide, keeping it out of the atmosphere.
• using less  electricity .
• eating less meat—cows are one of the biggest methane producers.
• supporting alternative energy sources that don’t burn fossil fuels.

Artificial Gas

Chlorofluorocarbons (CFCs) are the only greenhouse gases not created by nature. They are created through refrigeration and aerosol cans.

CFCs, used mostly as refrigerants, are chemicals that were developed in the late 19th century and came into wide use in the mid-20th century.

Other greenhouse gases, such as carbon dioxide, are emitted by human activity, at an unnatural and unsustainable level, but the molecules do occur naturally in Earth's atmosphere.

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Greenhouse Effect

What is Meant by Greenhouse Effect?

Greenhouse Effect is the process of heating of the surface of Earth till the troposphere. It happens because of higher concentration of carbon dioxide, water vapour, methane and other gases.

Sunlight heats up Earth's surface, and subsequently, the energy is reflected back to space in the form of infrared radiation. In the greenhouse effect, the concentrated gases absorb the energy, thereby increasing the global temperature. Hence, greenhouse effect and global warming are correlated.  

[Image: Greenhouse effect ]

Before proceeding with greenhouse effect causes and prevention, let us familiarise with different greenhouse gases. 

Greenhouse Gases 

The concentration of gases that lead to trapping of heat in the atmosphere is known as greenhouse gases. Greenhouse gases include –

Carbon dioxide (CO 2 )

Methane (CH 4 )

Nitrous oxide (N 2 O)

Fluorinated gases like halons, hydrochlorofluorocarbons, chlorofluorocarbons, nitrogen trifluoride, sulphur hexafluoride etc. 

However, the emission scale of these greenhouse gases varies, leading to differences in its concentration in atmosphere. 

[Image: Overview of greenhouse gas emissions in 2018]

Did You Know?

In spite of the damage caused by greenhouse effect and global warming, in its absence, temperature of Earth may slide down to almost -18° Celsius. 

Read on to know more about causes and effects of greenhouse effect. 

What Causes the Greenhouse Effect?

When preparing a short note on greenhouse effect, make sure to have an understanding about factors causing greenhouse effect. 

Fossil fuel burning 

Coal, oil and natural gas are fossil fuel which is utilised for transportation and electricity generation among others. Burning of fossil fuels releases enormous amounts of carbon dioxide into the air. 

Farming 

Fertilisers used during farming releases greenhouse gas nitrous oxide. It is a major cause of global warming. 

Deforestation 

Rampant deforestation is a common cause of greenhouse effect owing to reduction of oxygen release and absorption of carbon dioxide by plants. Moreover, when wood is burnt, the stored carbon is further released into the environment. 

Population increase

Population explosion in different parts of the world has caused enormous pressure on existing resources, which is finite. Higher demand has caused a substantial increase in manufacturing, causing greater emission of harmful gases. 

Landfill and industrial waste 

Landfill of industrial produce and industrial waste emanating from coal mining activities, cement production, and oil extraction among others lead to the generation of harmful greenhouse gases. 

Consequences of Greenhouse Effect

The major consequences of Greenhouse Effect are –

Ozone layer depletion 

Global warming 

Environmental degradation 

Extinction of species 

Prevention of Greenhouse Effect

Conservation of Energy 

Conservation of energy can substantially cut down emission of greenhouse gases. It is due to the fact that maximum industrial process and electricity production is dependent on consumption of fossil fuel. Increased usage of alternative or renewable energy will facilitate energy conservation. 

Afforestation 

Planned afforestation on a large scale will help in higher absorption of carbon dioxide from the atmosphere. Barring at night, green plants absorb carbon dioxide and release oxygen into air.  

Public Transportation 

Close to 30% of greenhouse gases are emitted by various modes of transport. Developed public transportation system helps to reduce number of automobiles that run of regular basis, eventually cutting down harmful gases emission. 

Policy Intervention 

There is a greater need for policy intervention by government both in terms of framing appropriate regulations and enforcement. International cooperation is important to make such policies a success. 

Test Yourself 

i. How much does carbon dioxide contribute to global warming?

(a) 10% to 15%

(b) 20% to 25%

(c) 30% to 35%

(d) 40% to 45%

ii. Ozone layer lies in -

(a) Mesosphere 

(b) Ionosphere 

(c) Stratosphere 

(d) Troposphere

[To check your answer, see the solution mentioned at the end of the article]

Learn more about causes of greenhouse effect and other related topics through our online classes. You can also download free pdf solutions that will definitely enhance your knowledge. All you have to do is install the Vedantu app now!

FAQs on Greenhouse Effect

1. Which Gas is Responsible for the Greenhouse Effect?

Ans. Carbon dioxide is one of the main causes of greenhouse effect. 

2. Write a Short Note on Greenhouse Effect.

Ans. The trapping of higher quantities of gases such as carbon dioxide in the Earth’s atmosphere is a greenhouse effect. These gases act as a screen to trap heat from the sun, causing a substantial rise in temperature.

3. What is Increased by the Greenhouse Effect?

Ans. A higher concentration of greenhouse gases in the atmosphere increases global warming. Research indicates that warming of the atmosphere has increased by 37%. 

[Solutions]

i. (d) 40% to 45%

ii. (c) Stratosphere 

Biology • Class 12

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• Greenhouse Atmosphere: Let's Heat Things Up!

Lesson Greenhouse Atmosphere: Let's Heat Things Up!

Grade Level: 5 (4-6)

Time Required: 45 minutes

Lesson Dependency: None

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• Greenhouse Effect Models: Hot Stuff!
• It's Really Heating Up in Here!
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An excess production of "greenhouse gases" is creating an environment unfit for healthful living. In response to this global warming, engineers of all disciplines are examining how humans activities have caused an increase in greenhouse gases and what they can do to mitigate the effects of greenhouse gases on our environment. Some engineers re-design vehicles and factories to reduce the emissions. Others are working to change manufacturing processes, regulations and practices, in an effort to clean up many chemical sources.

After this lesson, students should be able to:

• Understand that human activities have increased carbon dioxide concentrations (air pollution).
• Explain global warming.
• Understand that carbon dioxide gas is a greenhouse gas, and its increased concentration in the atmosphere is contributing to global warming.
• Describe how global warming may impact an engineer's decisions, their own lives and the Earth.

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(Ask students to do a "splash sheet" about the greenhouse effect and global warming. A "splash sheet" is similar to a brainstorm, but includes pictures, diagrams, or words to help students quickly jot down their ideas. Consider assigning this as homework the night before the lesson.)

(Have students share the ideas from their splash sheets in class.)

The Earth is getting warmer. The annual average global temperature has increased 1.8°F (1°C) from 1901 to 2016. Scientists attribute this observed global warming trend to the increased greenhouse effect. What is the greenhouse effect? Maybe you have already felt a miniature version of the greenhouse effect. Has this happened to you? It's a hot summer day and your parents have parked the car in the sun and no one opened the windows. How does it feel inside the car? It is very hot because the sun's energy is trapped inside the car. When you feel that trapped energy as heat, you have just felt the greenhouse effect. This is why no one should be left in a car on a warm, sunny day (including pets), because the inside temperature of the car can become over 100ºF (37.8°C), even with the windows slightly opened! The sun warms our planet. We feel the sun's energy as heat, but more of this heat is getting trapped near Earth by greenhouse gases like carbon dioxide.

Human activities are changing the Earth's natural greenhouse. Over the last century the burning of fossil fuels like coal and oil has increased the concentration of carbon dioxide in the atmosphere. Scientists predict that the Earth's temperature will continue to rise. According to the Environmental Protection Agency (EPA), electric utility companies, industry, businesses, homes, and transportation cause carbon dioxide levels to build up in our atmosphere.

From where does carbon dioxide come? (As necessary, review the carbon cycle with the students; see The Carbon Cycle Diagram attachment.) Why is it considered such a problem? What does it have to do with the greenhouse effect and global warming?

Lesson Background and Concepts for Teachers

The Earth's climate has changed many times in the past. Subtropical forests have spread from the south to more temperate (milder, cooler climates) areas. Millions of years later, ice sheets spread from the north covering much of the U.S., Europe and Asia with glaciers. Today, our climate appears to be changing again, but this time scientists think that humans activity is the major cause.

The Greenhouse Effect

The greenhouse effect is a naturally occurring phenomenon in which the specific gases in the atmosphere of the Earth trap heat from the sun (see The Greenhouse Effect Diagram attachment). Typically, our atmosphere absorbs just the right amount of heat so that living things can survive. Essentially, the atmosphere acts like the glass in a greenhouse. As a result of this, the Earth's surface is about 58°F (14°C) warmer than it would be without the greenhouse effect. (Refer to the activities Greenhouse Effect Models: Hot Stuff! and It's Really Heating Up in Here! to help students illustrate the development of the greenhouse effect by designing and analyzing simple models.)

Gases that trap heat in the atmosphere are called greenhouse gases, and the primary greenhouse gases on Earth are water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Of these, carbon dioxide, methane, nitrous oxide, and chlorofluorocarbons (CFCs), have an appreciable greenhouse effect, and are being released in large quantities by human activity. These gases, except for CFCs, come from both natural and human-made sources, and the higher their concentration in the atmosphere, the warmer the Earth's temperature becomes.

• Carbon dioxide comes from natural processes (like decaying and living organisms and volcanoes), but it also is released when fossil fuels (like coal and oil) are burned.
• Methane is released from natural decay, wetlands, growing rice, raising cattle, using natural gas, and mining coal.
• Nitrous oxide is not only naturally released by bacteria in the soil and the oceans, but also emitted by certain types of factories, power plants and plant fertilizers. 
• CFCs are a class of human-made chemicals once commonly used in air conditioners, refrigerators and as the pressurizing gas in aerosol spray cans.

Global Warming

Global warming  is the increase in  the Earth's average atmospheric temperature over a long period of time, generally due to increased levels of greenhouse gases caused by human activities. Scientists believe that even a 2-3ºF (0.6-1.1°C) increase in the average temperature of the Earth could trigger disasters.

According to the National Oceanic and Atmospheric Administration (NOAA), between 1880 and 1980, the global annual temperature increased at an average rate of 0.13°F (0.07°C) per decade. Since 1981, the global annual temperature has increased at twice that rate, 0.32°F (0.18°C). This has led to an overall 3.6°F (2°C) increase in global average temperature today compared to the pre-industrial era. In 2019, the average global temperature (over land and ocean) was 1.75°F (0.95°C) above the 20th-century average. This made 2019 the second hottest year on record, behind 2016. 

This rise in average global temperature is caused by human activities, particularly the burning of fossil fuels. The amount of carbon dioxide entering the atmosphere increases when fossil fuels are burned and the excess carbon dioxide cannot be used by plants (especially since we are eliminating them, too). The excess carbon dioxide in the atmosphere absorbs heat from the sun and keeps it near the surface of the Earth, which raises the Earth's temperature. The concentration of carbon dioxide in the atmosphere has doubled in the last 100 years and scientists expect it to double in the next 100 years as well.

Scientists predict that these changes will trigger disaster. For example, a major shift in weather patterns could cause droughts, tropical storms and increase temperatures that would make some currently  habitable  areas of the Earth become uninhabitable. Scientists also predict that melting polar ice caps could cause a rise in sea levels and, in turn, flood low-lying areas, such as coastal cities like New York City and San Francisco. The melting of the icecaps could also dilute marine saline concentrations, threatening marine life.

While most scientists believe that the greenhouse effect will gradually warm the Earth's climate, some scientists predict that as the temperature rises, more water will evaporate from the oceans, resulting in more clouds. This increase in clouds could block out sunlight, causing an overall decrease in the Earth's average temperature. This increased atmospheric reflectivity is called an increase in the Earth's  albedo .

Photosynthesis

Forests have been called the "lungs of the Earth" because animals inhale oxygen and exhale carbon dioxide in the process of breathing, and plants take in carbon dioxide and give off oxygen in the process of photosynthesis . See The Carbon Cycle Diagram attachment.

As of 2019, more than 78 million acres of tropical forest are cut and burned each year to clear land for farming and ranching. According to the World Resources Institute, averaged over 2015 – 2017, global loss of tropical forests contributed about 4.8 billion tonnes of carbon dioxide per year (or about 8-10% of annual human emissions of carbon dioxide).

Engineering

Environmental engineers are concerned about photosynthesis because plants help clean the air of harmful carbon dioxide gas, replacing it with oxygen. With the decreasing numbers of trees in the world, the air is not being cleaned as well. Additionally, the carbon dioxide levels continue to increase due to increasing numbers of automobiles and industrial pollution. Environmental engineers are continually challenged to find methods to reduce carbon dioxide emissions from industry and cars, and find ways to clean our polluted air. Students can explore taking the matter into their own hands with the literacy-based associated activity Pollution Politics .

• Greenhouse Effect Models: Hot Stuff! - Students observe demonstrations, and build and evaluate simple models to understand the greenhouse effect and the role of increased greenhouse gas concentration in global warming.

Watch this activity on YouTube

• Pollution Politics - In this literacy activity, students learn how a bill becomes law in the U.S. Congress, and research legislation related to global warming.

Ask students to create a final, more detailed and thoughtful splash sheet describing their full understanding of the greenhouse effect and global warming. Consider using butcher block paper or large, bulletin board-size paper for the splash sheets. Display these in the classroom or a school common area.

Have the students create a pie chart or bar graph using the data on the Sources of CO 2 Emissions attachment (see the Assessment section for details).

albedo: The reflectivity of a substance, usually a percentage of the amount of incoming radiation that is reflected.

global warming: A gradual increase in the overall temperature of the earth's atmosphere generally attributed to the greenhouse effect caused by increased levels of carbon dioxide, chlorofluorocarbons, and other pollutants.

greenhouse effect: A naturally occurring phenomenon in which the atmosphere of the Earth traps heat from the sun.

habitable: The idea that a place is suitable to live in or on.

photosynthesis: The process by which plants take in carbon dioxide and give off oxygen.

Pre-Lesson Assessment

Splash Sheet : Ask students to create a "splash sheet" about the greenhouse effect and global warming. A splash sheet is similar to a brainstorm, but includes pictures, diagrams, words, etc., to help students quickly jot down their ideas. Consider assigning this as homework the night before the lesson. If done in class, provide students with large sheets of butcher block paper on which to create their splash sheet.

Questions: Have students come up with questions to ask each other about global warming (i.e., what factors have caused the rise in global temperatures over the past century?) After the lesson, have students answer the questions.

Post-Introduction Assessment

Question/Answer : Ask students: From where does carbon dioxide come? (As necessary, review The Carbon Cycle Diagram with the students.) Why is it considered such a problem? What does it have to do with the greenhouse effect and global warming?

Lesson Summary Assessment

Graphing : According to the U.S. Environmental Protection Agency, the sources of carbon dioxide emissions in the U.S. in 2018 were: 34% from transportation, 32% from electricity generation, 15% from industrial, 11% from residential and commercial, and 7% from other non-fossil fuel combustion. Using this data, have students make a pie chart and/or a bar graph. Ask them to include a paragraph explaining what the graph represents and how this information relates to the greenhouse effect and global warming. Refer to the Sources of CO 2 Emissions attachment.

Create a Poem : Have the students write a short poem that expresses what they understand about global warming and how it may affect our environment in the next 50 years.

Lesson Extension Activities

Explore carbon monoxide production in more detail. Will our individual efforts really make a difference or do we need to address carbon dioxide production at another level? Are there other greenhouse gases to which we should be paying more attention?

Have students act out the greenhouse effect. Some students can be a carbon dioxide wall, some can be the Earth, and others can be the trapped energy between the greenhouse gasses and the Earth.

Students observe teacher-led demonstrations, and build and evaluate simple models to understand the greenhouse effect and the role of increased greenhouse gas concentration in global warming.

Students determine their carbon footprints by answering questions about their everyday lifestyle choices. Then they engineer plans to reduce them.

Students are introduced to the concept of energy cycles by learning about the carbon cycle. They learn how carbon atoms travel through the geological (ancient) carbon cycle and the biological/physical carbon cycle.

Students learn how rooftop gardens help the environment and the lives of people, especially in urban areas. They gain an understanding of how plants reduce the urban heat island effect, improve air quality, provide agriculture space, reduce energy consumption and increase the aesthetic quality of ci...

Blashfield, Jean F. and Black, Wallace B. Recycling. Chicago, IL: Children's Press Inc., 1991.

Energy Information Administration. Department of Energy. www.eia.gov. Last accessed August 30, 2020. (For great information and energy statistics)

Environmental Issues . Teacher Created Materials, 1994. Online at Teacher Created Resources. www.teachercreated.com. Last accessed August 30, 2020.

The EPA Global Warming Kids Page. Updated July 12, 2004. U.S. Environmental Protection Agency. www.epa.gov. Last accessed August 30, 2020.

Goodman, Billy. A Kid's Guide to How to Save the Planet . New York, NY: Avon Books, 1990.

Investigations in Science – Ecology . Huntington Beach, CA: Creative Teaching Press, 1995.

Rain Forest – Extended Thematic Unit. Teacher Created Materials, 1995. Online at Teacher Created Resources. http://www.buyteachercreated.com/estore/product/0674. Last accessed on August 30, 2020.

Science Plus – Technology and Society (Level Green) . Holt, Rinehart and Winston Inc., 1997.

Williams, Jack. Understanding Greenhouse Gases. Written November 7, 2000. Updated July 23, 2003. USA Today. Originally found at www.usatoday.com/weather/climate/wco2.htm. Accessed August 17, 2004.

“Climate Change: Vital Signs of the Planet.” NASA, NASA, climate.nasa.gov/. Last accessed September 4, 2020.

"By the Numbers: The Value of Tropical Forests in the Climate Change Equation." World Resources Institute, https://www.wri.org/blog/2018/10/numbers-value-tropical-forests-climate-change-equation. Last accessed September 6, 2020.

Contributors

Supporting program, acknowledgements.

The contents of this digital library curriculum were developed under a grant from the Fund for the Improvement of Postsecondary Education (FIPSE), U.S. Department of Education and National Science Foundation GK-12 grant no. 0338326. However, these contents do not necessarily represent the policies of the Department of Education or National Science Foundation, and you should not assume endorsement by the federal government.

Last modified: September 8, 2020

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Greenhouse Effect and Global Warming

Last updated on March 29, 2024 by ClearIAS Team

Nowadays we are facing many climate-changing issues like the greenhouse effect and Global Warming.

The greenhouse effect is the way in which heat is trapped close to Earth’s surface by “greenhouse gases.” The greenhouse effect leads to global warming.

Table of Contents

What do you mean by the Greenhouse effect?

A greenhouse is a structure where plants that require controlled climate conditions are grown. Its roof and walls are mostly made of transparent material, like glass.

In a greenhouse what is the incident solar radiation?

Visible light and nearby infrared and ultraviolet wavelengths.

Passes through the glass walls and roof and is absorbed by the ground, the floor, and the contents. As the materials warm up, they release the energy as longer-wavelength infrared radiation (heat radiation).

What is the reason infrared radiation cannot escape through radiative transfer?

Because glass and other wall materials used in greenhouses do not transmit infrared energy.

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The building is not exposed to the atmosphere, so heat cannot escape through convection, causing the greenhouse’s interior temperature to rise. This is known as the ‘greenhouse effect ‘.

Natural Greenhouse Effect: Importance

The greenhouse effect is a natural occurrence that has been taking place on earth for millions of years.

How natural greenhouse effect caused?

The natural greenhouse effect caused by the presence of water vapour and small water particles in the atmosphere has made life on earth possible. Together, these produce more than 95 per cent of total greenhouse warming.

• Average global temperatures are maintained at about 15°C due to the natural greenhouse effect.
• Without this phenomenon, the world’s average temperature could have been as low as -17°C, where life would not have been able to develop.

Greenhouse Gases (GHGs)

There are multiple gases responsible for the greenhouse effect. They are listed below.

Which gases are responsible for the greenhouse effect?

It is a result of atmospheric gases like carbon dioxide, methane, nitrous oxide (N2O), water vapour, and chlorofluorocarbons being able to trap the outgoing infrared radiation from the earth’s surface.

Hence these gases are known as greenhouse gases and the heating effect is known as the greenhouse effect.

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Are all oxides of nitrogen greenhouse gas? No

Oxides of Nitrogen with the general formula NOx – NO, NO2 – Nitrogen oxide, Nitrogen dioxide, etc. are global cooling gasses while Nitrous oxide (N2O) is a greenhouse gas.

• If greenhouse gases are not checked, by the turn of the century the temperature may rise by 5°C.
• According to scientists, this temperature increase will harm the environment and cause unusual climatic changes (like an increase in the frequency of El Nino), which will accelerate the melting of the polar ice caps as well as ice caps in other regions, such as the Himalayas.

Cryosphere: The cryosphere is the frozen water part of the Earth’s water system. Polar regions and snow caps of high mountain ranges are all part of the cryosphere.

How does this impact?

This will cause the sea level to rise over a long period, submerging many coastal areas and causing the loss of coastal habitats, including the most crucial ecosystems in terms of ecological services, such as marshes and swamps.

Gas V/S Sources and Causes

Carbon dioxide (CO2) : Burning of fossil fuels, deforestation

Chlorofluorocarbons (CFCs) : Refrigeration, solvents, insulation foams, aero propellants, industrial and commercial uses

Methane (CH4) : Growing paddy, excreta of cattle and other livestock, termites, burning of fossil fuel, wood, landfills, wetlands, and fertilizer factories.

Nitrogen oxides (N2O) : Burning of fossil fuels, and fertilizers; burning of wood and crop residue.

Carbon Monoxide (CO) : Iron ore smelting, burning of fossil fuels, burning e-waste.

Carbon dioxide

• In terms of meteorology, carbon dioxide is a very significant gas because it is transparent to solar radiation coming in but opaque to radiation leaving the earth.
• A portion of the radiation from the earth’s surface is reflected toward the surface after being partially absorbed by it. The greenhouse effect can be largely attributed to it.
• Its concentration is greater close to the earth’s surface as it is denser than air.
• Ozone is another important greenhouse gas. But it is in very small proportions at the surface.
• Most of it is confined to the stratosphere where it absorbs harmful UV radiation.
• Pollutants such as NO2 react with volatile organic compounds at ground level in the presence of sunlight to produce ozone (tropospheric ozone).

Water vapour

Why does water vapour consider a unique greenhouse gas?

Because it absorbs both incoming (part of incoming) and outgoing solar radiation.

It may account for four per cent of the air by volume in the warm and wet tropics, while it may account for less than one per cent of the air in the dry and cold desert and polar regions.

• Water vapour is also a variable gas in the atmosphere, which decreases with altitude.
• Water vapour also decreases from the equator towards the poles.
• Methane is the most important greenhouse gas after carbon dioxide.
• It is produced from the decomposition of animal wastes and biological matter.
• The emission of this gas can be restricted by producing gobar gas from animal waste and biological matter (methane).

Nitrous Oxide (N2O)

• N2O or Nitrous Oxide is a greenhouse gas.
• NO and NO2 (nitric oxide or nitrogen oxide and nitrogen dioxide) emissions cause global cooling by forming (OH) radicals that destroy methane molecules, thereby offsetting the effect of GHGs.

Carbon Monoxide

• Carbon monoxide is a short-lived greenhouse gas (it is less dense than air).
• It has an indirect radiative forcing effect by increasing methane and tropospheric ozone concentrations via chemical reactions with other atmospheric constituents (e.g., the hydroxyl radical, OH.) that would otherwise destroy them.
• Through natural processes in the atmosphere, it is eventually oxidized to carbon dioxide.

Fluorinated gases

Chlorofluorocarbons (cfcs).

• Because of their role in ozone depletion (explained in Geography > Climatology > Polar Vortex), CFCs were phased out through the Montreal Protocol.
• This anthropogenic compound is also a greenhouse gas, with a much greater capacity to amplify the greenhouse effect than CO2.

Know more about Montreal Protocol and the Kigali agreement click here:

Hydrofluorocarbons

• Hydrofluorocarbons are used as refrigerants, aerosol propellants, solvents, and fire retardants.
• These chemicals were developed as a replacement for chlorofluorocarbons (CFCs).
• Unfortunately, HFCs are potent greenhouse gases with long atmospheric lifetimes.

Perfluorocarbons

• Perfluorocarbons are compounds that are produced as a by-product of aluminum production and semiconductor manufacturing.
• Like HFCs, PFCs generally have long atmospheric lifetimes and high global warming potential.

Sulfur hexafluoride

• Sulfur hexafluoride is also a greenhouse gas.
• It is used in the production of magnesium and semiconductors, as well as as a tracer gas for leak detection.
• Sulfur hexafluoride is used in electrical transmission equipment, including circuit breakers.

Black Carbon

• Black carbon (BC) is a solid particle or aerosol (though not a gas) that contributes to global warming.

Is Black carbon and soot are same? Yes

Soot is another name for black carbon. Soot is a form of particulate air pollutant, produced from incomplete combustion.

When deposited on snow and ice, black carbon warms the earth by absorbing heat in the atmosphere and decreasing albedo (the ability to reflect sunlight).

Black carbon is the strongest absorber of sunlight and heats the air directly.

• Furthermore, it darkens snowpacks and glaciers through deposition and causes ice and snow to melt.
• Regionally, Black carbon disrupts cloudiness and monsoon rainfall.
• Black carbon stays in the atmosphere for only several days to weeks.
• As a result, the effects of Black carbon on atmospheric warming and glacier retreat vanish within months of reducing emissions.

Brown Carbon

• As a result, the effects of BC on atmospheric warming and glacier retreat vanish within months of reducing emissions.
• Biomass burning (possibly domestic wood burning) has been identified as a significant source of brown carbon.
• Brown carbon is commonly referred to as a greenhouse gas, while black carbon refers to particles produced by impure combustion, such as soot and dust.

GHG Protocol

• GHG Protocol is creating standards, tools, and online training to assist countries, cities, and businesses in tracking their progress toward their climate goals.
• The Greenhouse Gas Protocol (GHG Protocol) establishes frameworks for measuring and managing greenhouse gas (GHG) emissions from private and public sector operations, value chains, and mitigation actions.
• The GHG Protocol arose in the late 1990s when the World Resources Institute (WRI) and the World Business Council for Sustainable Development (WBCSD) recognized the need for an international standard for corporate GHG accounting and reporting.

 Global Warming – Impacts

Melting of the ice caps refers to?

Melting of the ice caps and glaciers will lead to a rise in sea level.

The thermal expansion also contributes to sea level rise.

Fertile coastal agricultural lands will be submerged, and saline water intrusions will degrade neighbouring land. Groundwater in such areas will be rendered ineffective.

• Populous cities lying on the coasts will be submerged under the sea.
• Flooding in the Himalayas and Ganga plains during the wet season, and drought during the dry season will have a devastating impact on the country.
• The amount of arable land in the high-latitude region is likely to increase as a result of the melting of snow and the reduction of frozen land.
• At the same time, arable land along the coastlines are bound to be reduced as a result of rising sea level and saline water inundations.

Extreme Climatic Events

• The increased likelihood of extreme events such as heat waves, flooding, hurricanes, and so on will cancel out all economic gains.
• Changes in rainfall patterns (E.g. 2015 Chennai floods, and the 2018 Kerala floods) will severely impact agriculture.

Environmental Degradation

• Reduced hydroelectric power generation due to glacier abnormal behaviour will increase reliance on fossil fuels.
• The widespread extinction of animal populations due to habitat loss will add to the list of ‘threatened’ and ‘extinct’ species.

What are the Rising Health-Related Issues?

• The spread of diseases (like malaria, etc.) in the tropics will put more pressure on the healthcare sector.
• The increased frequency and severity of heat waves and other extreme weather events are expected to increase the number of deaths.
• Lack of freshwater during droughts and contamination of freshwater supplies during floods jeopardize hygiene, increasing the prevalence of diseases such as cholera and diarrhea.

How Biodiversity Loss occurred?

• The loss of plankton due to sea-level rise will harm the marine food chain.
• The bleaching of coral reefs (ocean rainforests) will result in a significant loss of marine biodiversity.
• Rising temperatures would necessitate more fertilizer for the same production targets, resulting in higher GHG emissions, ammonia volatilization, and crop production costs.
• Rising temperatures will have an even greater impact on the physical, chemical, and biological properties of freshwater lakes and rivers, threatening many individual freshwater species.

No Food Security

• Climate change affects crops by influencing irrigation, insolation, and pest prevalence.
• Drought, flood, storm, and cyclone frequency are likely to increase agricultural production variability.
• Crop yields in temperate regions are expected to benefit from moderate warming (an increase of 1 to 3°C in mean temperature), while crops in lower latitudes will suffer.
• However, natural disasters caused by global warming may outweigh the benefits in temperature regions.
• Sea level rise will exacerbate water resource constraints in coastal areas due to increased salinization of groundwater supplies.

Deterioration of Carbon sinks

• High-latitude forests store more carbon than tropical rainforests.
• One-third of the world’s soil-bound carbon is in taiga and tundra areas.
• Permafrost melts as a result of global warming, releasing carbon in the form of carbon dioxide and methane.
• The tundra was a carbon sink in the 1970s, but it is now a carbon source due to global warming. (Global warming causes even more global warming.)

Sea Level Change

• Sea level change means the fluctuations in the mean sea level over a considerably long period.

Processes that cause Change in Sea Level

• Eustatic changes happen when the amount of seawater changes as a result of things like
• global warming and melting of ice sheets (rise in sea level) or ice ages (fall in sea level) and
• changes in the volume of mid-oceanic ridges.

2. Tectonic changes occur due to a change in the level of the land.

• The addition or removal of load causes isostatic changes. During the ice ages, the weight of the glacial ice caused the landmass to sag. On the other hand, as the glacial ice is melted, landmasses rise.
• Epeirogenic movement is caused by the large-scale tilting of continents, which may cause one part of the continent to rise while the other part may sink, giving the appearance that the sea level is rising.
• Orogenic movement (mountain building) results in the formation of lofty mountains and an apparent fall in sea level.

Importance of understanding Sea Level Changes

It provides key evidence regarding climate change in the past. It helps in estimating the rates of tectonic upliftment in the past geological periods.

• to determine whether coastal areas are suitable for industrial and agricultural development.
• To protect low-lying countries by building coastal dykes and embankments.
• Only when we are aware of the areas that will likely be affected by future sea level rise can we map the areas that will likely experience storm surges and intermittent flooding.
• It becomes possible to build tidal power generation facilities in suitable locations by identifying the areas that may soon be submerged.

Changes in Global Sea Level

• Short-term changes occur during a year.
• Commonly, seasonal variations of 5-6 cm in sea level are observed in a year.

Short-term sea level change may be due to a complex interaction of the following factors:

• Marine water density:  Seawater density depends on temperature and salinity. Seawater has a high density due to low temperature and high salinity, which results in a falling sea level.
• Atmospheric pressure: Low pressure results in higher local sea levels and vice versa. E.g. Storm surge.
• The velocity of ocean currents:  The edges of fast-moving ocean currents that follow curved paths experience an increase in sea level.
• Generally, a difference of 18 cm in sea level is observed between the two sides of a fast-flowing current.
• Ice formation and fall in sea level:  Sea levels fall during the winter as a result of ocean water being trapped in the icecaps of the northern and southern hemispheres.
• Piling up of water along windward coasts: As a result of an air mass pushing water toward the coast, the sea level rises locally in coastal areas. For instance, during the monsoon season, sea levels rise in south and east Asia as a result of the air mass moving inland.

The twentieth century has observed short-term global sea level rise due to the following factors.

• Ocean water has expanded thermally in the past century as a result of anthropogenic global warming. In other words, in the last 100 years, the sea level has increased by 10 to 15 cm.
• To some extent, the melting of Antarctica’s ice sheets, which account for about 3% of the world’s total ice mass, has contributed to the rise in sea level.
• About 15% of the Greenland ice cap’s total volume melted during the 20th century.
• Other glaciers are estimated to have contributed about 48% of the rise in sea level globally, in addition to these ice-melt regions.
• Only if the major ice sheets melt or the volume of the world’s mid-oceanic ridge significantly changes are global sea level changes of more than 100 m possible.

Impact of Sea Level Fall

• Coral reefs perish when the sea level drops because the continental shelves where they are formed become dry. Therefore, along the edge of the dead corals, new coral reefs appear.
• The decrease in sea level causes more aridity in the continental hinterland where there are shallow continental shelves because there is less surface runoff.
• Ice caps and glacial tongues spread out onto the continental shelves as a result of a decline in sea levels in temperate and high-latitude regions.

Impact of Possible Rise in Sea Level

If the atmosphere’s temperature rises further, Antarctica’s ice melt could become hazardous soon.

Low-lying, densely populated coastal areas, which make up a significant portion of the populated land, will be submerged. Even the tiny islands will perish.

A rise in sea levels will have an impact on the estimated one billion people living on the planet.

• Immense damage may be caused to coastal structures like ports, industrial establishments, etc.
• Nearly 33% of the world’s croplands may be submerged due to the rise in sea level (coastal plains and deltas are made up of very fertile soils).
• Accelerated coastal erosion may cause damage to and destruction of beaches, coastal dunes, and bars.
• As a consequence, a vast section of the coastal land will remain unprotected against the direct attack of sea waves.
• Groundwater resources of the coastal regions will be severely affected by salinization due to marine water intrusion.

The destruction of the reefs, coral atolls, and deltas will cause significant harm to the ecosystem. On the periphery of the dead corals, new coral reefs will form.

The mouths of drainage basins will be submerged due to the rise in sea level. The long profiles of the rivers will need to be readjusted as a result, and they probably show a rise.

• The recent rise in sea level has had the greatest impact on islands. The Carteret Islands, which are in the Pacific Ocean northeast of Papua New Guinea, and the Tuvalu Islands, which are in the South Pacific about 1000 km north of Fiji, are two of the islands that are impacted.

The United Nations Environment Programme ( UNEP ) established the “Oceans and Coastal Areas Programme Activity Centre” in 1987 to investigate the phenomenon of sea level rise and to determine which nations are most at risk of submersion.

Reducing Carbon in the Atmosphere to Fight Climate Change.

Half of the world’s electricity is generated by burning coal. Coal will remain a dominant energy source for years to come.

• CO2 and CO (carbon monoxide) are the major greenhouse gas that is released during the burning of coal.
• Along with the above gases, nitrogen oxides (destroys ozone) and sulphur oxides (acid rains) are also released.
• Clean coal technology seeks to reduce harsh environmental effects by using multiple technologies to clean coal and contain its emissions.
• Some clean coal technologies purify the coal before it burns.

One type of coal preparation, coal washing, removes unwanted minerals by mixing crushed coal with a liquid and allowing the impurities to separate and settle.

Other systems control the coal burn to minimize emissions of sulphur dioxide, nitrogen oxides, and particulates.

• Electrostatic precipitators remove particulates by charging particles with an electrical field and then capturing them on collection plates.
• Gasification avoids burning coal altogether. With gasification, steam and hot pressurized air or oxygen combine with coal in a reaction that forces carbon molecules apart.
• The resulting syngas, a mixture of carbon monoxide and hydrogen, is then cleaned and burned in a gas turbine to make electricity.

Wet scrubbers, or flue gas desulfurization systems, remove sulphur dioxide, a major cause of acid rain, by spraying flue gas with limestone and water.

Low-NOx (nitrogen oxides) burners reduce the creation of nitrogen oxides, a cause of ground-level ozone, by restricting oxygen and manipulating the combustion process.

Carbon capture and storage

‘ Carbon capture and storage catches and sequesters (hides) carbon dioxide (CO2) from stationary sources like power plants.

• Capture: Flue-gas separation removes CO2 and condenses it into a concentrated CO2 stream.
• After capture, secure containers sequester the collected CO2 to prevent or stall its re-entry into the atmosphere.
• The two storage options are geologic and oceanic (must hide the CO2 until peak emissions subside hundreds of years from now).
• Due to this rise in atmospheric carbon, much emphasis has been placed on and hope placed on soil, plants, and trees’ capacity to temporarily store the carbon that burning fossil fuels releases into the atmosphere.
• The Kyoto Protocol, the primary tool used by the international community to stop global warming, suggests that reducing carbon dioxide emissions from fossil fuels while also allowing trees and soil to absorb carbon dioxide is a valid strategy.

Read:  Black carbon emissions

Article Written by: Remya

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Policy Implications of Greenhouse Warming: Mitigation, Adaptation, and the Science Base (1992)

Chapter: a questions and answers about greenhouse warming, appendix a questions and answers about greenhouse warming, the greenhouse effect: what is known, what can be predicted.

1. What is the "greenhouse effect"?

In simplest terms, "greenhouse gases" let sunlight through to the earth's surface while trapping "outbound" radiation. This alters the radiative balance of the earth (see Figure A.1) and results in a warming of the earth's surface. The major greenhouse gases are water vapor, carbon dioxide (CO 2 ), methane (CH 4 ), chlorofluorocarbons (CFCs) and hydrogenated chlorofluorocarbons (HCFCs), tropospheric ozone (O 3 ), and nitrous oxide (N 2 O). Without the naturally occurring greenhouse gases (principally water vapor and CO 2 ), the earth's average temperature would be nearly 35°C (63°F) colder, and the planet would be much less suitable for human life.

2. Why is it called the "greenhouse" effect?

The greenhouse gases in the atmosphere act in much the same way as the glass panels of a greenhouse, which allow sunlight through and trap heat inside.

3. Why have experts become worried about the greenhouse effect now?

Rising atmospheric concentrations of CO 2 , CH 4 , and CFCs suggest the possibility of additional warming of the global climate. The panel refers to warming due to increased atmospheric concentrations of greenhouse gases as "greenhouse warming." Measurements of atmospheric CO 2 show that the 1990 concentration of 353 parts per million by volume (ppmv) is about one-quarter larger than the concentration before the Industrial Revolution (prior

FIGURE A.1 Earth's radiation balance. The solar radiation is set at 100 percent; all other values are in relation to it. About 25 percent of incident solar radiation is reflected back into space by the atmosphere, about 25 percent is absorbed by gases in the atmosphere, and about 5 percent is reflected into space from the earth's surface, leaving 45 percent to be absorbed by the oceans, land, and biotic material (white arrows).

Evaporation and mechanical heat transfer inject energy into the atmosphere equal to about 29 percent of incident radiation (grey arrow). Radiative energy emissions from the earth's surface and from the atmosphere (straight black arrows) are determined by the temperatures of the earth's surface and the atmosphere, respectively. Upward energy radiation from the earth's surface is about 104 percent of incident solar radiation. Atmospheric gases absorb part (25 percent) of the solar radiation penetrating the top of the atmosphere and all of the mechanical heat transferred from the earth's surface and the outbound radiation from the earth's surface. The downward radiation from the atmosphere is about 88 percent and outgoing radiation about 70 percent of incident solar radiation.

Note that the amounts of outgoing and incoming radiation balance at the top of the atmosphere, at 100 percent of incoming solar radiation (which is balanced by 5 percent reflected from the surface, 25 percent reflected from the top of the atmosphere, and 70 percent outgoing radiation), and at the earth's surface, at 133 percent (45 percent absorbed solar radiation plus 88 percent downward radiation from the atmosphere balanced by 29 percent evaporation and mechanical heat transfer and 104 percent upward radiation). Energy transfers into and away from the atmosphere also balance, at the atmosphere line, at 208 percent of incident solar radiation (75 percent transmitted solar radiation plus 29 percent mechanical transfer from the surface plus 104 percent upward radiation balanced by 50 percent of incoming solar continuing to the earth's surface, 70 percent outgoing radiation, and 88 percent downward radiation). These different energy transfers are due to the heat-trapping effects of the greenhouse gases in the atmosphere, the reemission of energy absorbed by these gases, and the cycling of energy through the various components in the diagram. The accuracy of the numbers in the diagram is typically ±5.

This diagram pertains to a period during which the climate is steady (or unchanging); that is, there is no net change in heat transfers into earth's surface, no net change in heat transfers into the atmosphere, and no net radiation change into the atmosphere-earth system from beyond the atmosphere.

to 1750). Atmospheric CO 2 is increasing at about 0.5 percent per year. The concentration of CH 4 is about 1.72 ppmv, or slightly more than twice that before 1750. It is rising at a rate of 0.9 percent per year. CFCs do not occur naturally, and so they were not found in the atmosphere until production began a few decades ago. Continued increases in atmospheric concentrations of greenhouse gases would affect the earth's radiative balance and could cause a large amount of additional greenhouse warming. Increasing the capture of energy in this fashion is also called "radiative forcing." Other factors, such as variation in incoming solar radiation, could be involved.

4. Has there been greenhouse warming in the recent past?

Best estimates are that the average global temperature rose between 0.3° and 0.6°C over about the last 100 years. However, it is not possible to say with a high degree of confidence whether this is due to increased atmospheric concentrations of greenhouse gases or to other natural or human causes. The temperature record much before 1900 is not reliable for estimates of changes smaller than 1°C–1.8°F).

5. What about CO 2 and temperature in the prehistoric past?

According to best estimates based on analysis of air bubbles trapped in ice sheets, ocean and lake sediments, and other records from the geologic past, there have been three especially "warm" periods in the last 4 million years. The Holocene optimum occurred from 6,000 to 5,000 years ago. During that period, atmospheric concentrations of CO 2 were about 270 to 280 ppmv, and average air temperatures about 1°C (1.8°F) warmer than modern times. The Eemian interglacial period happened with its midpoint about 125,000 years ago. Atmospheric concentrations of CO 2 were 280 to 300 ppmv, and temperatures up to 2°C (3.6°F) warmer than now. The Pliocene climate optimum occurred between 4.3 and 3.3 million years ago. Atmospheric concentrations of CO 2 have been estimated for that period to be about 450 ppmv, with temperatures 3° to 4°C (5.4° to 7.2°F) warmer than modern times. The prehistoric temperature estimates are from evidence dependent

on conditions during growing seasons and probably are better proxies for summer than winter temperatures. The estimate for the Pliocene period is especially controversial.

6. What natural things affect climate in the long run?

On the geologic time scale, many things affect climate:

• Changes in solar output

• Changes in the earth's orbital path

• Changes in land and ocean distribution (tectonic plate movements and the associated changes in mountain geography, ocean circulation, and sea level)

• Changes in the reflectivity of the earth's surface

• Changes in atmospheric concentrations of trace gases (especially CO 2 and CH 4 )

• Changes of a catastrophic nature (such as meteor impacts or extended volcanic eruptions)

7. What is meant by ''atmospheric lifetime" and "sinks"?

These concepts can be illustrated by referring to what is called the "carbon cycle." When CO 2 is emitted into the atmosphere, it moves among four main sinks, or pools, of stored carbon: the atmosphere, the oceans, the soil, and the earth's biomass (plants and animals). The movement of CO 2 among these sinks is not well understood. About 45 percent of the total emissions of CO 2 from human activity since preindustrial times is missing in the current accounting of CO 2 in the atmosphere, oceans, soil, and biomass. Three possible sinks for this missing CO 2 have been suggested. First, more CO 2 may have been absorbed into the oceans than was thought. Second, the storage of CO 2 in terrestrial plant life may be greater than estimated. Third, more CO 2 may have been absorbed directly into soil than is thought. However, there is no direct evidence for any of these explanations accounting for all the missing CO 2 . CO 2 in the atmosphere is relatively "long-lived" in that it does not easily break down into its constituent parts. CH 4 , by contrast, decomposes in the atmosphere in about 10 years. The greenhouse gas with the longest atmospheric lifetime (except for CO 2 ), CFC-115, has an average atmospheric lifetime of about 400 years. The overall contribution of greenhouse gases to global warming depends on their atmospheric lifetime as well as their ability to trap radiation. Table A.1 shows the relevant characteristics of the principal greenhouse gases.

8. Do all greenhouse gases have the same effect?

Each gas has different radiative properties, atmospheric chemistry, typical atmospheric lifetime, and atmospheric concentration. For example, CFC-12 is roughly 15,800 times more efficient molecule for molecule at trapping heat than CO 2 . Because CFC-12 is a large, heavy molecule with many atoms and a

TABLE A.1 Key Greenhouse Gases Influenced by Human Activity

CO 2 molecule is small and light in comparison, there are fewer molecules of CFC-12 in each ton of CFC-12 emissions than CO 2 molecules in each ton of CO 2 emissions. Each ton of CFC-12 emissions is about 5,750 times more efficient at trapping heat than each ton of CO 2 . The comparatively greater amount of CO 2 in the atmosphere, however, means that it accounts for roughly half of the radiative forcing associated with the greenhouse effect.

9. Do greenhouse gases have different effects over time?

Yes. Figure A.2 shows projected changes in radiative forcing for different greenhouse gases between now and 2030. The potential increase for each

FIGURE A.2 Additional radiative forcing of principal greenhouse gases from 1990 to 2030 for different emission rates. The horizontal axis shows changes in greenhouse gas emissions ranging from completely eliminating emissions (-100 percent) to doubling current emissions (+100 percent). Emission changes are assumed to be linear from 1990 levels to the 2030 level selected. The vertical axis shows the change in radiative forcing in watts per square meter at the earth's surface in 2030. Each asterisk indicates the projected emissions of that gas assuming no additional regulatory policies, based on the Intergovernmental Panel on Climate Change estimates and the original restrictions agreed to under the Montreal Protocol, which limits emissions of CFCs. Chemical interactions among greenhouse gas species are not included.

For CO 2 emissions remaining at 1990 levels through 2030, the resulting change in radiative forcing can be determined in two steps: (1) Find the point on thecurvelabeled "CO 2 " that is vertically above 0 percent change on the bottom scale. (2) The radiative forcing on the surface-troposphere system can be read in watts per square meter by moving horizontally to the left-hand scale, or about 1 W/m 2 . These steps must be repeated for each gas. For example, the radiative forcing for continued 1990-level emissions of CH 4 through 2030 would be about 0.2W/m 2 .

SOURCE : Courtesy of Michael C. MacCracken.

gas is plotted for different emissions of each gas compared to 1990 emission levels. The figure shows the impact of different percentage changes in emissions (compared to 1990 emission rates) on the radiative forcing. Figure A.3 extends this to show the impact on equilibrium temperature for different sensitivities of the climatic system (in degrees Celsius).

10. What is meant by a "feedback" mechanism?

One example of a greenhouse warming feedback mechanism involves water vapor. As air warms, each cubic meter of air can hold more water vapor. Since water vapor is a greenhouse gas, this increased concentration of water vapor further enhances greenhouse warming. In turn, the warmer air can hold more water, and so on. This is an example of a positive feedback, providing a physical mechanism for "multiplying" the original impetus for change beyond its initial force.

Some mechanisms provide a negative feedback, which decreases the initial impetus. For example, increasing the amount of water vapor in the air may lead to forming more clouds. Low-level, white clouds reflect sunlight, thereby preventing sunlight from reaching the earth and warming the surface. Increasing the geographical coverage of low-level clouds would reduce greenhouse warming, whereas increasing the amount of high, convective clouds could enhance greenhouse warming. This is because high, convective clouds absorb energy from below at higher temperatures than they radiate energy into space from their tops, thereby effectively trapping energy. Satellite measurements indicate that clouds currently have a slightly negative effect on current planetary temperature. It is not known whether increased temperatures would lead to more low-level clouds or more high, convective clouds.

11. Can the temperature record be used to show whether or not greenhouse warming is occurring?

The estimated warming of between 0.3° and 0.6°C (0.5° and 1.1°F) over the last 100 years is roughly consistent with increased concentrations of greenhouse gases, but it is also within the bounds of "natural" variability for weather and climate. It cannot be proven to a high degree of confidence that this warming is the result of the increased atmospheric concentrations of greenhouse gases. There may be an underlying increase or decrease in average temperature from other, as yet undetected, causes.

12. What is the basis for predictions of global warming?

General circulation models (GCMs) are the principal tools for projecting climatic changes. GCMs project equilibrium temperature increases between 1.9° and 5.2°C (3.4° and 9.4°F) for greenhouse gas concentrations equivalent to a doubling of the preindustrial level of atmospheric CO 2 . The midpoint of this range corresponds to an average global climate warmer than

FIGURE A.3 Commitment to future warming. An incremental change in radiative forcing between 1990 and 2030 due to emissions of greenhouse gases implies a change in global average equilibrium temperature (see text). The scales on the right-hand side show two ranges of global average temperature responses. The first corresponds to a climate whose temperature response to an equivalent of doubling of the preindustrial level of CO 2 is 1°C; the second corresponds to a rise of 5°C for an equivalent doubling of CO 2 . These scales indicate the equilibrium commitment to future warming caused by emissions from 1990 through 2030. Assumptions are as in Figure A.2.

To determine equilibrium warming in 2030 due to continued emissions of CO 2 at the 1990 level, find the point on the curve labeled "CO 2 " that is vertically above 0 percent change on the bottom scale. The equilibrium warming on the right-hand scales is about 0.23°C (0.4°F) for a climate system with 1° sensitivity and about 1.2°C (2.2°F) for a system with 5° sensitivity. For CH 4 emissions continuing at 1990 levels through 2030, the equilibrium warming would be about 0.04°C (0.07°F) at 1°sensitivity and about 0.25°C (0.5°F) at 5° sensitivity. These steps must be repeated for each gas. Total warming associated with 1990-level emissions of the gases shown until 2030 would be about 0.41°C (0.7°F) at 1°sensitivity and about 2.2°C (4°F) at 5° sensitivity.

Scenarios of changes in committed future warming accompanying different greenhouse gas emission rates can be constructed by repeating this process for given emission rates and adding up the results.

any in the last 1 million years. The consequences of this amount of warming are unknown and may include extremely unpleasant surprises.

13. What is "equilibrium temperature"?

The oceans, covering roughly 70 percent of the earth's surface, absorb heat from the sun and redistribute it to the deep oceans slowly. It will be decades, perhaps centuries, before the oceans and the atmosphere fully redistribute the absorbed energy and the currently "committed" temperature rise is actually "realized." The temperature at which the system would ultimately come to rest given a particular level of greenhouse gas concentrations is called the "equilibrium temperature.'' Since atmospheric concentrations of greenhouse gases are constantly changing, the temperature measured at any time is the "transient" temperature, which lags behind the committed equilibrium warming. The lag depends in part on the sensitivity of the climate system and is believed to be between 10 and 100 years. This phenomenon makes it difficult to use temperature alone to "prove" that greenhouse warming is occurring.

14. How can we know when greenhouse warming is occurring?

The only tools we have for trying to produce credible scientific results are observations combined with theoretical calculation. Detecting additional greenhouse warming will require careful monitoring of temperature and other variables over years or even decades. Further development of numerical models will help characterize the climatic system, including the atmosphere, oceans, and land-based elements like forests and ice fields. However, only careful interpretation of actual measurements can reveal what has occurred and when.

15. How can credible estimates of future global warming be made?

Several approaches can be used. Scientific "first principles" can be used to estimate physical bounds on future trends. GCMs can be used to conduct "what if" experiments under differing conditions. Comparisons can be made with paleoclimatic data of previous interglacial periods. None of these methods is absolutely conclusive, but it is generally agreed that GCMs are the best available tools for predicting climatic changes. Substantial improvements in GCM capabilities are needed, however, for GCM forecasts to increase their credibility.

16. What influences future warming?

The amount of climatic warming depends on several things:

• The amount of sunlight reaching the earth

• Emission rates of greenhouse gases

• Chemical interactions of greenhouse gases in the atmosphere

• Atmospheric lifetimes of greenhouse gases until they decompose or transfer into sinks

• Effectiveness of positive or negative feedback mechanisms that enhance or reduce warming

• Human actions, which effect radiative forcing in both positive and negative directions

17. What are the major "unknowns" in predictions?

Major uncertainties include:

• Future emissions of greenhouse gases

• Role of the oceans and biosphere in uptake of heat and CO 2

• Amount of CO 2 and carbon in the atmosphere, oceans, biota, and soils

• Effectiveness of sinks for CO 2 and other greenhouse gases, especially CH 4

• Interactions between temperature change and cloud formation and the resulting feedbacks

• Effects of global warming on biological sources of greenhouse gases

• Interactions between changing climate and ice cover and the resulting feedbacks

• Amount and regional distribution of precipitation

• Other factors, like variation in solar radiation

18. How can the uncertainties best be handled?

Data can be arrayed to validate components of the models. Increasing the number of data sets can also help. In addition, the variation in GCM results can be compared to provide a sense of their "robustness." A major "intercomparison" of GCMs is being conducted, and has shown large differences in regional precipitation and reduction of snow and ice fields at high latitudes.

19. Are these changes associated with an equivalent doubling of the preindustrial level of atmospheric CO 2 that can be stated with confidence?

Because of the uncertainty in our understanding of various factors, projections reflect different levels of confidence.

20. What about storms and other extreme weather events?

The factors governing tropical storms are different from those governing mid-latitude storms and need to be considered separately.

One of the conditions for formation of typhoons or hurricanes today is a sea surface temperature of 26°C (79°F) or greater. With higher global average surface temperature, the area of sea with this temperature should be larger. Thus the number of hurricanes could increase. However, air pressure, humidity, and a number of other conditions also govern the creation and propagation of tropical cyclones. The critical temperature for their creation may increase as climate changes these other factors. There is no consistent indication whether tropical storms will increase in number or intensity as climate changes. Nor is there any evidence of change over the past several decades.

Mid-latitude storms are driven by equator-to-pole temperature contrast. In a warmer world, this contrast will probably weaken since surface temperatures in high latitudes are projected to increase more than at the equator (at least in the northern hemisphere). Higher in the atmosphere, however, the temperature contrast strengthens. Increased atmospheric water vapor could also supply extra energy to storm development. We do not currently know which of these factors would be more important and how mid-latitude storms would change in frequency, intensity, or location.

21. Can projections be improved?

Better computers alone will not solve the problems associated with positive and negative feedbacks. Better understanding of atmospheric physics and chemistry and better mathematical descriptions of relevant mechanisms in the models are also needed, as are data to validate models and their subcomponents. Significant improvements may require decades.

22. Is it possible to avoid the projected warming?

It is possible only at great expense or by incurring risks not now understood, unless the earth is itself self-correcting. Continued increases in atmospheric concentrations of greenhouse gases would probably result in additional global warming. Avoiding all future warming either would be very costly (if we significantly reduce atmospheric concentrations of greenhouse gases) or potentially very risk (if we use climate engineering). However, a comprehensive action program could slow or reduce the onset of greenhouse warming.

A Framework for Responding to Additional Greenhouse Warming

23. What kinds of responses to potential greenhouse warming are possible?

Human interventions in natural and economic activities can affect the net rate of change in the radiative forcing of the earth. It is useful to categorize the possible types of intervention into three types:

• Actions to eliminate or reduce emissions of greenhouse gases

• Actions to "offset" such emissions by removing such gases from the atmosphere, blocking solar radiation, or altering the earth's reflectivity or absorption of energy

• Actions to help human and ecologic systems adjust to new climatic conditions and events

In this study the panel analyzes the first two types of action together under the label of "mitigation," since they are aimed at avoiding or reducing greenhouse warming. The third type of action is here called "adaptation."

24. How can response options be evaluated?

The choice of response options to potential greenhouse warming can be guided by a standard cost-benefit approach, augmented to handle some important aspects of the issues involved. The anticipated impacts (both adverse and beneficial) can be arrayed to produce a "damage function" showing the anticipated costs (or benefits) associated with projected climatic changes. The mitigation and adaptation options can be arrayed similarly according to their respective costs and effectiveness to produce an "abatement cost function." Optimal policies involve balancing incremental costs and benefits, which is called cost-benefit balancing. A necessary condition for an optimal policy is that the level of policy chosen should be cost-effective (any step undertaken minimizes costs). Employing such guidelines requires estimating both the anticipated damages and the cost-effectiveness of alternative response options, and choosing a discount rate to use for assessing the current value of future expenditures or returns.

In practice, a full cost-benefit approach can only be approximated. It is impossible to determine in detail the impacts of climatic changes that will not occur for 40 or 100 years. Thus the damage function can be only roughly approximated. Estimation of the abatement cost function is considerably easier.

Responses to greenhouse warming should be regarded as investments in the future. Cost-effectiveness and cost-benefit balancing should guide the selection of options. In general, a mixed strategy employing some investment in many different alternatives will be most effective.

Impacts of Additional Greenhouse Warming

25. Can impacts of expected climatic changes be projected?

It currently is not possible to predict regional temperature, precipitation, and other effects of climate change with much confidence. And without quantitative projections of regional and local climatic changes, it is not possible to produce quantitative projections of the consequences of greenhouse warming.

Instead, the degree of "sensitivity" of affected human and natural systems to the projected changes can be estimated. The sensitivity of a particular system to the climate changes expected to accompany different amounts of additional greenhouse warming can be used to estimate the impacts of those changes.

A crucial aspect of the sensitivity of a system is the speed at which it can react. For example, investment decisions in many industries typically have a "life-cycle" of 10 years or less. Climatic changes associated with additional greenhouse warming are expected to emerge slowly enough that these industries may be expected to adjust as climate changes. Some industries, such as electric power production, have longer investment cycles, and might have more difficulty responding as quickly. Natural ecological systems would not be expected to anticipate climate change and probably would not be able to adapt as quickly as climatic conditions change.

The impacts of climate change are thus hard to assess because the response of human and natural systems to climate change must be included.

26. How can the impacts on affected systems be classified?

Likely impacts of climate change can be divided into four categories:

• Low sensitivity. The projected changes would likely have little effect on the system. An example is most industrial production not requiring large quantities of water. Temperature changes of the magnitude projected would not matter much for most industrial processes. These impacts do not give rise to much concern.

• High sensitivity, but adaptation possible at some cost. The system would likely adapt or otherwise cope with the projected changes without completely restructuring the system. An example is American agriculture. Although some crops would likely move into new locations, agricultural scientists and plant breeders would almost certainly develop new crops suitable for changed growing conditions. There would be costs, but food supply would not be interrupted. As a class, these impacts give rise to concern because the affected systems may have difficulty adapting.

• High sensitivity, and adaptation problematic. The system would be seriously affected, and adaptation would probably not be easy or effective. Natural communities of plants and animals would probably lose their current structure, and reformulate with different mixes of species. Some individual species, especially animals, would move to new locations. The natural landscape as we know it today would almost certainly be altered by a climate change at or above the midpoint of the range used in this study. These impacts are of considerable concern because the affected systems may not be able to adapt without assistance.

• Uncertain sensitivity, but cataclysmic consequences. The sensitivity of the system cannot be assessed with certainty, but the consequences would be extremely severe. An example is the possible shifting, slowing, or even stopping of major ocean currents like the Gulf Stream or the Japanese Current. These ocean currents strongly affect weather patterns, and changes in them could drastically alter weather in Europe or the West Coast of the United States. We have no credible way, however, of assessing the conditions that could lead to such shifts.

27. What are the likely impacts of climate change?

Human societies exhibit a wide range of adaptive mechanisms in the face of changing climatic events and conditions. Projected climatic changes, especially at the upper end of the range, may overwhelm human adaptive mechanisms in areas of marginal productivity and in countries where traditional coping mechanisms have been disrupted. In general, natural ecosystems would be much more sorely stressed, probably beyond their capacities for adjustment. For example, even temperature changes at the lower end of the range would result in shifts of local climates at rates faster than the movement of long-lived trees with large seeds.

A comprehensive catalog of beneficial and harmful impacts is not available. Nor is an estimation of the magnitude of the likely impacts of projected climatic changes. Table A.2 summarizes impacts to human and natural systems in the United States according to the sensitivity categories.

28. Can costs be calculated for the various impacts of projected climate changes?

Not directly. The climatic changes likely to occur in the future cannot be directly measured. The costs and benefits associated with some aspects of certain changes can be estimated, however. These can be used to produce very rough estimates of the costs of climatic impacts. However, these must be recognized as very imprecise indicators.

In general, the costs in the United States associated with the first category of sensitivity are low in relation to overall economic activity. The

TABLE A.2 The Sensitivity and Adaptability of Human Activities and Nature

costs associated with the second category are higher but still should not result in major disruption of the economy. Appropriate adjustments could probably be accomplished without replacing current systems. Costs associated with the third category are much larger, and the adjustments could involve disruption. Some type of anticipation for meeting them may be justified. The category of extremely adverse impacts would be associated with high potential costs and would disrupt most aspects of the system in question. These outcomes, however, are extremely difficult to assess. Table A.3 summarizes some "benchmark" costs illustrative of impacts similar to those that might be associated with climate change.

TABLE A.3 Illustrative Costs of Impacts and Adaptations

29. Are there possible consequences of greenhouse warming with highly adverse impacts?

Two have been identified.

• Deep ocean currents could be interrupted. Increased freshwater runoff in the Arctic might alter the salinity of northern oceans, thereby reducing or stopping the vertical flow of water into the deep ocean along Greenland and Iceland. This might interrupt a major deep ocean current running from the North Atlantic around the Cape of Good Hope and through the Indian Ocean to the Pacific. This could affect temperature and precipitation, with repercussions that might be catastrophic. Very little is currently known about the potential of this phenomenon.

• The West Antarctic Ice Sheet could surge. The Antarctic and Greenland ice sheets combined make up the world's largest reservoir of fresh water. The West Antarctic Ice Sheet alone contains enough water to raise global average sea level about 7 meters (23 feet). Warming could affect the speed at which the ice sheet flows to the sea and breaks off into icebergs. A large subsequent influx of fresh water could alter the salinity of the world's oceans, affecting currents and plant and animal populations alike. The ramifications are extreme, and it might lead to disruption of deep ocean currents and all that that entails. The timing of such a possibility is controversial. Current thinking is that it would take centuries, but there is little empirical evidence on which to base estimates.

30. What are appropriate responses to very uncertain, but highly adverse impacts?

Both individuals and societies must decide how to handle events that are very unlikely but which have severe consequences. Homeowners purchase insurance against the very unlikely event of fire. In essence, insurance is a cost today (the insurance premium) to avoid undesirable consequences later (losing one's possessions to fire). If we want to avoid unsure adverse impacts of possible climate change, we might want to spend money now that would reduce the likelihood that those things can happen. In principle, there are two different kinds of "climate insurance." We could do things that reduce the likelihood that the climate will change (mitigation options), or we could do things that reduce the sensitivity of affected human and natural systems to future climate change (adaptation options).

31. Does looking at potential impacts tell us where to set priorities for responding to greenhouse warming?

Partly. The examination of potential impacts can help provide rough estimates of the cost at which adaptation could be accomplished should climate change. This is an approximation of the "damage function" and can be used

to assess how much to spend on emission reductions or offsets. However, all estimates are approximations with very little precision. The amount to allocate to prevent additional greenhouse warming depends significantly on the preferred degree of risk aversion.

Preventing or Reducing Additional Greenhouse Warming

32. What are the sources of greenhouse gas emissions?

All of the major greenhouse gases except CFCs are produced by both natural processes and human activity. Table A.4 summarizes the principal sources of greenhouse gases associated with human activity.

33. What interventions could reduce greenhouse warming?

It is useful to examine two different aspects of reducing emissions or offsetting emissions:

• ''Direct" reduction or offsetting of emissions through altering equipment, products, physical processes, or behaviors

• "Indirect" reduction or offsetting of emissions through altering the behavior of people in their economic or private lives and thus affecting the overall level of activity leading to emissions

It is much easier to estimate potential effectiveness and costs of direct reductions than of indirect incentives on human behavior. This is mostly because of the many factors that affect behavior in addition to the incentives in any particular program.

34. How can specific mitigation options be compared?

Mitigation options can be compared quantitatively in terms of their cost-effectiveness and qualitatively in terms of the obstacles to their implementation and in terms of other benefits and costs.

The standard quantitative unit used to compare mitigation options is the cost per metric ton of carbon emissions reduced or per metric ton of carbon removed from the atmosphere. The amount of carbon can be converted to the amount of CO 2 in the atmosphere by multiplying by 3.67, which is the ratio of the molecular weights of carbon and CO 2 . Other greenhouse gases can be "translated" to CO 2 equivalency by using two calculations. First, the amount of radiative forcing caused by a specific concentration of the gas is estimated in terms of the change in energy reaching the surface (in watts per square meter). This estimate accounts for atmospheric chemistry, atmospheric lifetime of the gas, and other relevant factors affecting the total contribution of that gas to greenhouse warming. Second, the amount of

TABLE A.4 Estimated 1985 Global Greenhouse Gas Emissions from Human Activities

CO 2 that would produce the same amount of forcing at the surface is calculated. This is the CO 2 equivalent for that specific concentration of the other greenhouse gas. The respective costs per ton for different options can then be compared directly. It is important to recognize, however, that these calculations allow comparison only of initial contributions. They do not account for changes in energy-trapping effectiveness over the various lifetimes of these gases in the atmosphere.

35. What mitigation options are most cost-effective?

The panel ranks options for reducing greenhouse gas emissions or removing greenhouse gases from the atmosphere according to their cost-effectiveness. Some of these options have net savings or very low net implementation costs compared to other investments. The options range from net savings to more than $100 per metric ton of CO 2 -equivalent emissions avoided or removed from the atmosphere. The most cost-effective mitigation options are presented in Table A.5.

36. What are examples of options with large potential to reduce or offset emissions?

The so-called geoengineering options have the potential of substantially affecting atmospheric concentrations of greenhouse gases. They have the ability to screen incoming sunlight, stimulate uptake of CO 2 by plants and animals in the oceans, or remove CO 2 from the atmosphere. Although they appear feasible, they require additional investigation because of their potential environmental impacts.

37. How much would it cost to significantly reduce current U.S. greenhouse gas emissions?

It depends on the level of emission reduction desired and how it is done. The most cost-effective options are those that enhance efficient use of energy: efficiency improvements in lighting and appliances, white roofs and paving to enhance reflectivity, and improvement in building and construction practices.

Figure A.4 compares mitigation options, and Table A.5 gives the panel's estimates of net cost and emission reductions for several options. It must be emphasized that the table presents the panel's estimates of the maximum technical potential for each option. The calculation of cost-effectiveness of lighting efficiency, for example, does not consider whether the supply of light bulbs could meet the demand with current production capacities. Nor does it consider the trade-off between expenditures on light bulbs and on health care, education, or basic shelter for low-income families. In addition, there is a danger of some "double counting." For example, in the area of energy supply both nuclear and natural gas energy options assume replacement

TABLE A.5 Comparison of Selected Mitigation Options in the United States

of the same coal-fired power plants. Table A.5, however, presents only options that avoid double counting. Finally, although there is evidence that efficiency programs can pay, there is no field evidence showing success with programs on the massive scale suggested here. Thus there may be very good reasons why "negative cost options" on the figure are not implemented today.

The United States could reduce its greenhouse gas emissions by between 10 and 40 percent of the 1990 levels at low cost, or perhaps some net savings, if proper policies are implemented.

FIGURE A.4 Comparison of mitigation options. Total potential reduction of CO 2 equivalent emissions is compared to the cost in dollars per ton of CO 2 reduction. Options are ranked from left to right in CO 2 emissions according to cost. Some options show the possibility of reductions of CO 2 emissions at a net savings.

Adapting to Additional Greenhouse Warming

38. Will human and natural systems adapt without assistance?

Farmers adjust their crops and cultivation practices in response to weather patterns over time. Natural ecosystems also adapt to changing conditions. The real issue is the rate at which human and natural systems will be able to adjust.

39. At what rates can human and natural systems adapt?

Many human systems have decision and investment cycles that are shorter than the time in which impacts of climate change would become manifest. These systems in the United States should be able to adjust to climate change without governmental intervention, as long as it is gradual and information about the rates of change is widely available. This applies to agriculture, commercial forestry, and most of industry. Industrial sectors with extremely long investment cycles (e.g., transport systems, urban infrastructure, and major structures and facilities) or requiring high volumes of water may require special attention. Coastal urban settlements would be

able to react quickly (within 3 to 5 years) if sea level rises. Response would be much more difficult, however, where financial and other resources are limited, such as in many developing countries.

Some natural systems adjust at rates an order of magnitude or more slower than those anticipated for global-scale temperature changes. For example, the observed and theoretical migration of large trees with heavy seeds is an order of magnitude slower than the anticipated change in climate zones. Furthermore, natural ecosystems cannot anticipate climate change but must wait until after conditions have changed to respond.

40. What is the value of the vulnerable natural ecosystems?

Natural ecosystems contribute commercial products, but their value is generally considered to exceed this contribution to the economy. For example, genetic resources are generally undervalued because people cannot capture the benefits of investments they might make in preserving biodiversity. Many species are unlikely to ever have commercial value, and it is virtually impossible to predict which ones will become marketable.

In addition, some people value natural systems regardless of their economic value. Loss of species, in their view, is undesirable whether or not those species have any commercial value. They generally hold that preservation of the potential for evolutionary change is a desirable goal in and of itself. Humanity, they claim, should not do things that alter the course of natural evolution. This view is sometimes also applied to humanity's cultural heritage—to buildings, music, art, and other cultural artifacts.

41. How much would it cost to adapt to the anticipated climatic changes?

The panel's analysis suggests that some human and natural systems are not very sensitive to the anticipated climatic changes. These include most sectors of industry. Other systems are sensitive to climatic changes but can be adapted at a cost whose present value is small in comparison to the overall level of economic activity. These include agriculture, commercial forestry, urban coastal infrastructure, and tourism. Some systems are sensitive, and their adaptation is questionable. The unmanaged systems of plants and animals that occupy much of our lands and oceans adapt at a pace slower than the anticipated rate of climatic change. Their future under climate change would be problematic. Poor nations may also adapt painfully. Finally, some possible climatic changes like shifts in ocean currents have consequences that could be extremely severe, and thus the costs of adaptation might be very large. However, it is not currently possible to assess the likelihood of such cataclysmic changes.

No attempt has been made to comprehensively assess the costs of anticipated climatic changes on a global basis.

42. How much should be spent in response to greenhouse warming?

The answer depends on the estimated costs of prevention and the estimated damages from greenhouse warming. In addition, the likelihood and severity of extreme events, the discount rate, and the degree of risk aversion will modify this first-order approximation.

The appropriate level of expenditure depends on the value attached to the adverse outcomes compared to other allocations of available funds, human resources, and so on. In essence, the answer depends on the degree of risk aversion attached to adverse outcomes of climate change. The fact that less is known about the more adverse outcomes makes this a classic example of dealing with high-consequence, low-probability events. Programs that truly increase our knowledge and monitor relevant changes are especially needed.

Implementing Response Programs

43. What policy instruments could be used to implement response options?

A wide array of policy instruments of two different types are available: regulation and incentives. Regulatory instruments mandate action, and include controls on consumption (bans, quotas, required product attributes), production (quotas on products or substances), factors in design or production (efficiency, durability, processes), and provision of services (mass transit, land use). Incentive instruments are designed to influence decisions by individuals and organizations and include taxes and subsidies on production factors (carbon tax, fuel tax), on products and other outputs (emission taxes, product taxes), financial inducements (tax credits, subsidies), and transferable emission rights (tradable emission reductions, tradable credits). The choice of policy instrument depends on the objective to be served.

44. At what level of society should actions be taken?

Interventions at all levels of human aggregation could effectively reduce greenhouse warming. For example, individuals could reduce energy consumption, recycle goods, and reduce consumption of deleterious materials. Local governments could control emissions from buildings, transport fleets, waste processing plants, and landfill dumps. State governments could restructure electric utility pricing structures and stimulate a variety of efficiency incentives. National governments could pursue action in most of the policy areas of relevance. International organizations could coordinate programs in various parts of the world, manage transfers of resources and technologies, and facilitate exchange of monitoring and other relevant data.

45. Is international action necessary?

The greenhouse phenomenon is global. Unilateral actions can contribute significantly, but national efforts alone would not be sufficient to eliminate the problem. The United States is the largest contributor of CO 2 emissions (with estimates ranging from 17 to 21 percent of the global total). But even if this country were to totally eliminate or offset its emissions, the effect on overall greenhouse warming might be lost if no other countries acted in concert with that aim.

46. What about differences between rich and poor countries?

Poor and developing countries are likely to be the most vulnerable to climate change. In addition, many developing countries today are sorely pressed in a variety of other ways. They may conclude that other issues have more immediate consequences for their citizens. Incentives in all parts of the world for intervention in the area of greenhouse warming may thus draw heavily on the industrialized nations. They may be called upon to help poor countries stimulate economic development and thus become better able to cope with climate change. They may also be asked to provide expertise and technologies to help poor countries adapt to the conditions they face.

Actions to be Taken

47. Do scientific assessments of greenhouse warming tell us what to do?

Current scientific understanding of greenhouse warming is both incomplete and uncertain. Response depends in part on the degree of risk aversion attached to poorly understood, low-probability events with extremely adverse outcomes. Lack of scientific understanding should not be used as a justification for avoiding reasoned decisions about responses to possible additional greenhouse warming.

48. Is it better to prevent greenhouse warming now or wait and adapt to the consequences ?

This complicated question has several parts.

• First, will it be possible to live with the consequences if nothing is done now? The panel's analysis suggests that advanced, industrialized countries will be able to adapt to most of the anticipated consequences of additional greenhouse warming without great economic hardship. In some regions, climate and related conditions may be noticeably worse, but in other regions better. Countries that currently face difficulty coping with extreme climatic events, or whose traditional coping mechanisms are breaking down, may be sorely pressed by the climatic changes accompanying an equivalent doubling of atmospheric CO 2 concentrations. It is important to recognize that there may be dramatic improvement or disastrous deterioration in specific locales. In addition, this analysis applies to the next 30 to 50 years. The situation may be different beyond that time horizon.

Natural communities of plants and animals, however, face much greater difficulties. Greenhouse warming would likely stress such ecosystems sufficiently to break them apart, resulting in a restructuring of the community in any given locale. New species would be likely to gain dominance, with a different overall mix of species. Some individual species would migrate to new, more livable locations. Greenhouse warming would most likely change the face of the natural landscape. Similar changes would occur in lakes and oceans.

In addition, there are possible extremely adverse consequences, such as changing ocean currents, that are poorly understood today. The response to such possibilities depends on the degree of risk aversion concerning those outcomes. The greater the degree of risk aversion, the greater the impetus for preventive action.

• Second, does it matter when interventions are made? Yes, for three different kinds of reasons. Because greenhouse gases have relatively long lifetimes in the atmosphere, and because of lags in the response of the system, their effect builds up over time. These time-dependent phenomena lead to the long-term "equilibrium" warming being greater than the "realized" warming at any given point in time. These dynamic aspects of the climate system show the importance of acting now to change traditional patterns of behavior that we have recently recognized to be detrimental, such as heavy reliance on fossil fuels. In addition, the implications of intervention programs for the overall economy vary with time. Gradual imposition of restraints is much less disruptive to the overall economy than their sudden application. Finally, the length of investment cycles can be crucial in determining the costs of intervention. In addition, some investments can be thought of as insurance, or payments now to avoid undesirable outcomes in the future. The choice is made more complicated by the fact that the outcomes are highly uncertain.

• Third, what discount rate should be used? The selection of a discount rate is very controversial. Macroeconomic calculations for the United States show a return on capital investment of 12 percent. The choice of discount rate reflects time preference. The panel has used discount rates of 3, 6, and 10 percent in its analysis. Finally, consumers often behave as if they have used a discount rate closer to 30 percent. The panel has also included this rate for comparison when options involve individual action.

49. Are there special attributes of programs appropriate for response to greenhouse warming?

Yes. The uncertainties present in all aspects of climate change and our understanding of response to potential greenhouse warming place a high premium on information. Small-scale interventions that are both reversible and yield information about key aspects of the relevant phenomena are especially attractive for both mitigation and adaptation options. Monitoring of emission rates, climatic changes, and human and ecologic responses should yield considerable payoffs.

Perhaps the most important attribute of preferred policies is that they be able to accommodate surprises. They should be constructed so that they are flexible and can change if the nature or speed of stress is different than anticipated.

50. What should be done now?

The panel developed a set of recommended options in five areas: reducing or offsetting emissions, enhancing adaptation to greenhouse warming, improving knowledge for future decisions, evaluating geoengineering options, and exercising international leadership. The panel recommends moving decisively to undertake all of the actions described under questions 51 through 55 below.

51. What can be done to reduce or offset emissions of greenhouse gases?

Three areas dominate the panel's analysis of reducing or offsetting current emissions: eliminating CFC emissions and developing substitutes that minimize or eliminate greenhouse gas emissions, changing energy policy, and utilizing forest offsets. Eliminating CFC emissions has the biggest single contribution. Recommendations concerning energy policy are to examine how to make the price of energy reflect all health, environmental, and other social costs with a goal of gradual introduction of such a system; to make conservation and efficiency the chief element in energy policy; and to consider the full range of supply, conversion, end use, and external effects in planning future energy supply. Global deforestation should be reduced, and a moderate domestic reforestation program should be explored.

52. What can be done now to help people and natural systems of plants and animals adapt to future greenhouse warming?

Most of the actions that can be taken today improve the capability of the affected systems to deal with current climatic variability. Options include maintaining agricultural basic, applied, and experimental research; making water supplies more robust by coping with present variability; taking into consideration possible climate change in the margins of safety for long-lived structures; and reducing present rates of loss in biodiversity.

53. What can be done to improve knowledge for future decisions?

Action is needed in several areas. Collection and dissemination of data that provide an uninterrupted record of the evolving climate and of data that are needed for the improvement and testing of climate models should be expanded. Weather forecasts should be improved, especially of extremes, for weeks and seasons to ease adaptation to climate change. The mechanisms that play a significant role in the responses of the climate to changing concentrations of greenhouse gases need further identification, and quantification at scales appropriate for climate models. Field research should be conducted on entire systems of species over many years to learn how CO 2 enrichment and other facets of greenhouse warming alter the mix of species and changes in total production or quality of biomass. Research on social and economic aspects of global change and greenhouse warming should be strengthened.

54. Do geoengineering options really have potential?

Preliminary assessments of these options suggest that they have large potential to mitigate greenhouse warming and are relatively cost-effective in comparison to other mitigation options. However, their feasibility and especially the side-effects associated with them need to be carefully examined. Because the geoengineering options have the potential to affect greenhouse warming on a substantial scale, because there is convincing evidence that some of these cause or alter a variety of chemical reactions in the atmosphere, and because the climate system is poorly understood, such options must be considered extremely carefully. If greenhouse warming occurs, and the climate system turns out to be highly sensitive to radiative forcing, they may be needed.

55. What should the United States do at the international level?

The United States should resume full participation in international programs to slow population growth and contribute its share to their financial and other support. In addition, the United States should participate fully in international agreements and programs to address greenhouse warming, including representation by officials at an appropriate level.

Global warming continues to gain importance on the international agenda and calls for action are heightening. Yet, there is still controversy over what must be done and what is needed to proceed.

Policy Implications of Greenhouse Warming describes the information necessary to make decisions about global warming resulting from atmospheric releases of radiatively active trace gases. The conclusions and recommendations include some unexpected results. The distinguished authoring committee provides specific advice for U.S. policy and addresses the need for an international response to potential greenhouse warming.

It offers a realistic view of gaps in the scientific understanding of greenhouse warming and how much effort and expense might be required to produce definitive answers.

The book presents methods for assessing options to reduce emissions of greenhouse gases into the atmosphere, offset emissions, and assist humans and unmanaged systems of plants and animals to adjust to the consequences of global warming.

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