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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 CO2 emissions, see the Emissions section and for actions that can reduce these emissions, see the What You Can Do Section.

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). For more information on N2O emissions and sources, see EPA’s Nitrous Oxide Site .