Climate Change







Greenhouse Gases

Greenhouse gas concentrations in the atmosphere have historically varied20 as a result of many natural processes (e.g. volcanic activity, changes in temperature, etc). However, since the Industrial Revolution humans have added a significant amount of greenhouse gases6 in the atmosphere by burning fossil fuels, cutting down forests and other activities. Because greenhouse gases absorb and emit heat, increasing their concentrations in the atmosphere will tend to have a warming effect. But the rate and amount of temperature increase21 is not known with absolute certainty. Changes in the atmospheric concentration of the major greenhouse gases are described below:

Carbon Dioxide22

Figure 1 - Carbon Dioxide: Click on Thumbnail for full size image

Carbon dioxide (CO2)23 concentrations in the atmosphere increased from approximately 280 parts per million (ppm) in pre-industrial times to 379 ppm in 2005 according to the National Oceanic and Atmospheric Administration's (NOAA) 2005 State of the Climate Report23, a 35 percent increase. Almost all of the increase is due to human activities (IPCC, 2001). The current rate of increase in CO2 concentrations is about 1.8ppmv/year. Present CO2 concentrations are higher than any time in at least the last 420,000 years (IPCC 2001). See Figure 1 for a record of CO2 concentrations from about 420,000 years ago to present. For more information on the human and natural sources of CO2 emissions, see the Emissions section6 and for actions that can reduce these emissions, see the What You Can Do Section24.

Methane Molecule25

Figure 2 - Methane: Click on Thumbnail for full size image

Methane (CH4)26 is more abundant in the Earth’s atmosphere now than at any time in at least the past 420,000 years (IPCC, 2001). Methane concentrations increased sharply during most of the 20th century and are now 151% above pre-industrial levels. In recent decades, the rate of increase has slowed considerably (see Figure 2). For more information on CH4 emissions and sources, and actions that can reduce emissions, see EPA’s Methane Site7.
Nitrous Oxide27

Figure 3 - Nitrous Oxide: Click on Thumbnail for full size image

Nitrous oxide (N2O)28 has increased approximately 18 percent in the past 200 years and continues to increase (see Figure 3). The present concentration of N2O has not been exceeded during at least the last 1,000 years. For more information on N2O emissions and sources, see EPA’s Nitrous Oxide Site 9.


How are Greenhouse Gas Concentrations from Thousands of Years Ago Determined?

Portions of the Antarctic ice sheet are several miles deep, consisting of ice that has accumulated over hundreds of thousands of years or longer. Paleoclimatologists (scientists who study the history of the Earth's climate) drill holes in this ice to extract what are called "cylindrical cores," or "ice cores."

Ice cores can provide valuable information about the Earth’s past. For example, the cores contain trapped air bubbles that can be analyzed to obtain snapshots of the composition of the atmosphere at the time the ice accumulated. Through this analysis, concentrations of greenhouse gases (CO2, CH4, N2O) dating back thousands of years or longer can be obtained with a high level of confidence. See the National Aeronautics and Space Administration’s (NASA) Earth Observatory feature "Paleoclimatogy: The Ice Core Method29" for more information.


  • Tropospheric ozone (O3) is created by chemical reactions from automobile, power plant and other industrial and commercial source emissions in the presence of sunlight. It is estimated that O3 has increased by about 36% since the pre-industrial era, although substantial variations exist for regions and overall trends (IPCC, 2001). Besides being a greenhouse gas, ozone can also be a harmful air pollutant30 at ground level, especially for people with respiratory diseases and children and adults who are active outdoors. Measures are being taken to reduce ozone emissions31 in the U.S. (through the Clean Air Act) and also in other countries.
  • Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are used in coolants, foaming agents, fire extinguishers, solvents, pesticides and aerosol propellants. These compounds have steadily increased in the atmosphere since their introduction in 1928. Concentrations are slowly declining as a result of their phaseout via the Montreal Protocol on Substances that Deplete the Ozone Layer32.
  • Fluorinated gases33 such as hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6) are frequently used as substitutes for CFCs and HCFCs and are increasing in the atmosphere. These various fluorinated gases are sometimes called "high global warming potential greenhouse gases33" because, molecule for molecule, they trap more heat than CO2. For more information, visit EPA’s High Global Warming Potential Gases Site8.



The burning of fossil fuels and biomass34 (living matter such as vegetation) has resulted in aerosol emissions into the atmosphere. Aerosols absorb and emit heat, reflect light and, depending on their properties, can either cool or warm the atmosphere. NASA’s Earth Observatory describes35 how aerosols can also affect how clouds form35.

  • Sulfate aerosols are emitted when fuel containing sulfur, such as coal and oil, is burned. Sulfate aerosols reflect solar radiation back to space and have a cooling effect. These aerosols have decreased in concentration in the past two decades resulting from efforts to reduce the coal-fired power plant emissions of sulfur dioxide36 in the United States and other countries.
  • Black carbon (or soot) results from the incomplete combustion of fossil fuels and biomass burning37 (forest fires and land clearing) and is believed to contribute to global warming (NRC, 2001). Though global concentrations are likely increasing, there are significant regional differences. In the United States and many other countries, efforts to reduce particulate matter38 (of which black carbon is a part) are lowering black carbon concentrations.
  • Other aerosols emitted in small quantities from human activities include organic carbon and associated aerosols from biomass burning. Mineral dust aerosols (e.g., from deserts and lake beds) largely originate from natural sources, but their distribution can be affected by human activities.

Radiative Forcing

Radiative forcing is the change in the balance between solar radiation entering the atmosphere and the Earth's radiation going out. On average, a positive radiative forcing tends to warm the surface of the Earth while negative forcing tends to cool the surface. Radiative forcing is measured in Watts per square meter, which is a measure of energy. For example, an increase in radiative forcing of +1 Watt per square meter is like shining one small holiday tree light bulb over every square meter of the Earth.

Greenhouse gases have a positive radiative forcing because they absorb and emit heat. Aerosols can have a positive or negative radiative forcing, depending on how they absorb and emit heat and/or reflect light. For example, black carbon aerosols - which have a positive forcing - more effectively absorb and emit heat than sulfates, which have a negative forcing and more effectively reflect light. The following are estimates of the change in radiative forcing39 Exit EPA Disclaimer40 in the year 2000 relative to 1750 for different components of the climate (IPCC, 2001):

  • The radiative forcing contribution (since 1750) from increasing concentrations of well-mixed greenhouse gases (including CO2, CH4, N2O, CFCs, HCFCs, and fluorinated gases) is estimated to be +2.43 Watts per square meter - over half due to increases in CO2 (+1.46 Watts per square meter), strongly contributing to warming relative to other climate components described below.
  • The radiative forcing contribution from increasing tropospheric41 ozone, an unevenly distributed greenhouse gas, is estimated to be +0.35 Watts per square meter (on average), resulting in a relatively small warming effect. This forcing varies from region to region depending on the amount of ozone in the troposphere at a particular location.
  • The radiative forcing contribution from the observed depletion of stratospheric42 ozone is estimated to be -0.15 Watts per square meter, resulting in a relatively small cooling effect.
  • While aerosols can have either positive or negative contributions to radiative forcing, the net effect of all aerosols added to the atmosphere has likely been negative, with estimates ranging from -0.2 to -2.0 Watts per square meter. Therefore, the net effect of changes in aerosol radiative forcing has likely resulted in a small to relatively large cooling effect.
  • Land use change (including urbanization, deforestation, reforestation, desertification, etc) can have significant effects on radiative forcing (and the climate) at the local level by changing the reflectivity of the land surface (or albedo43). For example, because farmland is more reflective than forests (which are strong absorbers of heat), replacing forests with farmland would negatively contribute to radiative forcing or have a cooling effect. Averaged over the Earth, the net radiative forcing contribution of land use changes, while uncertain, is estimated to be -0.25 Watts per square meter (IPCC, 2001), resulting in a relatively small cooling effect.
  • Based on a limited, 25-year record, the effect of changes in the sun's intensity on radiative forcing is estimated to be relatively small, or a contribution of about +0.2 Watts per square meter, resulting in a relatively small warming effect.

NOAA’s Annual Greenhouse Gas Index (AGGI)44, which tracks changes in radiative forcing from greenhouse gases over time, shows that radiative forcing from greenhouse gases has increased 21.5% since 1990 as of 2006. Much of the increase (63%) has resulted from the contribution of CO2. The contribution to radiative forcing by CH4 and CFCs has been nearly constant or declining, respectively, in recent years.

How Is Radiative Forcing Determined?

For well-mixed greenhouse gases, mathematical equations are used to compute radiative forcing based on changes in their concentration relative to 1750 (or 1990 for NOAA's AGGI) and the known radiative properties of the gases. Confidence in these calculations is high due to reliable current and historic concentration data and well-established physics.

Due to limited measurements and regional variation, changes in tropospheric ozone, aerosols, land use and the sun’s intensity are much more uncertain. In the case of aerosols, uncertainty is increased due to an incomplete understanding of how aerosols interact with clouds and the effects the interactions have on aerosol radiative forcing.

For more information, see the National Research Council report “Radiative Forcing of Climate Change  (NRC, 2005)".



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