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					Recent Climate Change - Atmosphere Changes | Science | Climate Change | U.S. EPA                                                    3/20/09 5:45 PM

                                                                                         Last updated on Wednesday, December 17th, 2008.
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               Recent Climate Change
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    Atmosphere Changes
                                                                                                                    Related Links
    Greenhouse Gases | Aerosols | Radiative Forcing
                                                                                                               Future Atmosphere
    The release of greenhouse gases and aerosols resulting from human                                          Changes
    activities are changing the amount of radiation coming into and leaving                                    Greenhouse Gas Emissions
    the atmosphere, likely contributing to changes in climate.                                                 Methane Site
                                                                                                               High Global Warming
                                                                                                               Potential Gases Site
    Greenhouse Gases                                                                                           Nitrous Oxide Site
    Greenhouse gas concentrations in the atmosphere have historically
                                                                                                               Methane as a Greenhouse
    varied as a result of many natural processes (e.g. volcanic activity,                                      Gas
    changes in temperature, etc). However, since the Industrial Revolution                                     Product 2.3 - Aerosol
    humans have added a significant amount of greenhouse gases in the                                          properties and their
                                                                                                               impacts on climate
    atmosphere by burning fossil fuels, cutting down forests and other
    activities. Because greenhouse gases absorb and emit heat, increasing                                   CDIAC: Information on
                                                                                                            primary global-change data
    their concentrations in the atmosphere will tend to have a warming
                                                                                                            and information analysis
    effect. But the rate and amount of temperature increase is not known
    with absolute certainty. Changes in the atmospheric concentration of the                                NASA
    major greenhouse gases are described below:                                                                Aerosols and Climate
                                                                                                               Atmospheric Methane
                                                                                                               Methane's Impacts on
                                                                                                               Climate Change May Be
                                                     Carbon dioxide (CO 2) concentrations                      Twice Previous Estimates
                                                     in the atmosphere increased from                          Soot has Impact on Global
                                                     approximately 280 parts per million                       Climate

      Figure 1 - Carbon Dioxide:                     (ppm) in pre-industrial times to 382                   NOAA
      Click on Thumbnail for full                    ppm in 2006 according to the National                     Earth System Regional
                                                     Oceanic and Atmospheric                                   Laboratory
      size image                                                                                               Interactive Atmospheric
                                                     Administration's (NOAA) Earth Systems
                                                                                                               Visualization Tool
                                                     Research Laboratory, a 36 percent
                                                                                                            U.S. Carbon Cycle Research
                                                     increase. Almost all of the increase is
                                                     due to human activities (IPCC, 2007).
                                                     The current rate of increase in CO 2
                                                     concentrations is about 1.9 ppmv/year.
                                                     Present CO 2 concentrations are higher
                                                     than any time in at least the last
                                                     650,000 years (IPCC, 2007). See
                                                     Figure 1 for a record of CO 2
                                                     concentrations from about 420,000
                                                     years ago to present. For more
                                                     information on the human and natural                                                                                   Page 1 of 4
Recent Climate Change - Atmosphere Changes | Science | Climate Change | U.S. EPA                         3/20/09 5:45 PM

                                                     information on the human and natural
                                                     sources of CO 2 emissions, see the
                                                     Emissions section and for actions that
                                                     can reduce these emissions, see the
                                                     What You Can Do Section.

                                                     Methane (CH 4) is more abundant in
                                                     the Earth’s atmosphere now than at
                                                     any time in at least the past 650,000
                                                     years (IPCC, 2007). Methane
                                                     concentrations increased sharply
      Figure 2 - Methane: Click on                   during most of the 20th century and
                                                     are now 148% above pre-industrial
      Thumbnail for full size image                  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 Site.
                                                     Nitrous oxide (N 2O) has increased
                                                     approximately 18 percent in the past
                                                     200 years and continues to increase
                                                     (see Figure 3). For about 11,500 years
                                                     before the industrial period, the
      Figure 3 - Nitrous Oxide:                      concentration of N2O varied only
                                                     slightly. It increased relatively rapidly
      Click on Thumbnail for full
                                                     toward the end of the 20th century
      size image                                     (IPCC, 2007). For more information on
                                                     N2O emissions and sources, see EPA’s
                                                     Nitrous Oxide Site .

              How are Greenhouse Gas Concentrations from Thousands of Years Ago

              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 (CO 2, 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 Method"
              for more information.

               Tropospheric ozone (O 3) 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 O 3 has increased by about 36% since the pre-industrial era, although substantial
               variations exist for regions and overall trends (IPCC, 2007). Besides being a greenhouse gas,
               ozone can also be a harmful air pollutant at ground level, especially for people with respiratory
               diseases and children and adults who are active outdoors. Measures are being taken to reduce                                                        Page 2 of 4
Recent Climate Change - Atmosphere Changes | Science | Climate Change | U.S. EPA                       3/20/09 5:45 PM

               diseases and children and adults who are active outdoors. Measures are being taken to reduce
               ozone emissions 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 Layer.
               Fluorinated gases such as hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur
               hexafluoride (SF 6) 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 gases" because, molecule for molecule, they trap more heat than CO 2. For
               more information, visit EPA’s High Global Warming Potential Gases Site.


    The burning of fossil fuels and biomass (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 describes how aerosols
    can also affect how clouds form.

               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 dioxide in the United States and other countries.
               Black carbon (or soot) results from the incomplete combustion of fossil fuels and biomass
               burning (forest fires and land clearing) and is believed to contribute to global warming (IPCC,
               2007). Though global concentrations are likely increasing, there are significant regional
               differences. In the United States and many other countries, efforts to reduce particulate matter
               (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

    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 forcing in the year 2005 relative to 1750 for
    different components of the climate (IPCC, 2007):

               The radiative forcing contribution (since 1750) from increasing concentrations of well-mixed
               greenhouse gases (including CO 2, CH4, N2O, CFCs, HCFCs, and fluorinated gases) is
               estimated to be +2.64 Watts per square meter - over half due to increases in CO 2 (+1.66
               Watts per square meter), strongly contributing to warming relative to other climate components
               described below.
               The radiative forcing contribution from increasing tropospheric 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 stratospheric ozone is                                                      Page 3 of 4
Recent Climate Change - Atmosphere Changes | Science | Climate Change | U.S. EPA                         3/20/09 5:45 PM

               The radiative forcing contribution from the observed depletion of stratospheric ozone is
               estimated to be -0.05 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. The best estimate of
               aerosols’ direct cooling effect is -0.5 Watts per square meter; the best estimate for their
               indirect cooling effect (by increasing the reflectivity of clouds) is -0.7 Watts per square meter,
               with an uncertainty range of -1.8 to -0.3 Watts per square meter. Therefore, the net effect of
               changes in aerosol radiative forcing has likely resulted in a small to relatively large cooling
               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 albedo). 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 -0.2
               Watts per square meter (IPCC, 2007), 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 +0.12 Watts per square
               meter, resulting in a relatively small warming effect.

    NOAA’s Annual Greenhouse Gas Index (AGGI), 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 CO 2. 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 Working Group I’s contribution to the Intergovernmental Panel
              on Climate Change’s Fourth Assessment Report (2007), Chapter 2, “Changes in
              Atmospheric Constituents and Radiative Forcing,” pp. 133-134 (PDF, 8.6 MB, 106 pp.).

               IPCC, 2007: Climate Change 2007: The Physical Science Basis.              Contribution of
               Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate
               Change [Solomon, S., D. Qin, M. Manning (eds.)].                                                        Page 4 of 4

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