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The release of greenhouse gases and aerosols resulting from human activities are changing the
amount of radiation coming into and leaving the atmosphere, likely contributing to changes in climate.

Greenhouse Gases :

Greenhouse gas concentrations in the atmosphere have historically varied 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 gases 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 increase is not
known with absolute certainty. Changes in the atmospheric concentration of the major
greenhouse gases are described below:

Carbon dioxide (CO2) : concentrations in the atmosphere
increased from approximately 280 parts per million (ppm)
in pre-industrial times to 382 ppm in 2006 according to the
National Oceanic and Atmospheric Administration’s (NOAA)
Earth Systems Research Laboratory, a 36 percent
increase. Almost all of the increase is due to human
activities (IPCC, 2007). The current rate of increase in CO2
concentrations is about 1.9 ppmv/year. Present CO2
concentrations are higher than any time in at least the last
650,000 years (IPCC, 2007). 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

Methane (CH4) 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 during most of the 20th
century and are now 148% 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 Site.

Nitrous oxide (N2O): 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 concentration of N2O
varied only slightly. It increased relatively rapidly toward the end of
the 20th century (IPCC, 2007).

* 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, 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 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 (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 gases” because, molecule for molecule, they trap more heat than CO2. For more information,
visit EPA’s High Global Warming Potential Gases Site.

Aerosols :

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 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.

Green earth which usually utters greenhouse. 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.