Climate Change
Climate Change Indicators in the United States
Energy from the sun drives the Earth's weather and climate. The Earth absorbs some of the energy it receives from the sun and radiates the rest back toward space. However, certain gases in the atmosphere, called greenhouse gases, absorb some of the energy radiated from the Earth and trap it in the atmosphere. These gases essentially act as a blanket, making the Earth’s surface warmer than it would be otherwise.
The “greenhouse effect” occurs naturally, making life as we know it possible. During the past century, however, human activities have substantially increased the amount of greenhouse gases in the atmosphere, changing the composition of the atmosphere and influencing climate. Some greenhouse gases are almost entirely man-made. Other greenhouse gases come from a combination of natural sources and human activities. For example, carbon dioxide is a greenhouse gas that occurs naturally because of volcanoes, forest fires, and biological processes (such as breathing), but is also produced by burning fossil fuels in power plants and automobiles. Other major sources of greenhouse gases include industrial and agricultural processes, waste management, and land use changes.
The major greenhouse gases emitted into the atmosphere through human activities are carbon dioxide, methane, nitrous oxide, and fluorinated gases (see box below). Many of these gases can remain in the atmosphere for tens to hundreds of years after being released. Thus, to get a more complete picture of the amount of greenhouse gases in the atmosphere, both emissions (how much of a given greenhouse gas is produced and emitted into the air) and concentrations (the amount of a greenhouse gas present in a certain volume of air) are measured. Long-lived greenhouse gases become globally mixed in the atmosphere, reflecting both past and recent contributions from emission sources worldwide.
Greenhouse Gases Associated With Human Activities
The principal greenhouse gases that enter the atmosphere because of human activities are:
- Carbon dioxide is emitted primarily through the burning of fossil fuels (oil, natural gas, and coal), solid waste, and trees and wood products. Changes in land use, such as growing new forests or disturbing soils, can lead to the addition or removal of carbon dioxide to/from the atmosphere.
- Methane is emitted during the production and transport of coal, natural gas, and oil. Methane emissions also result from livestock and agricultural practices and from the decay of organic waste in municipal solid waste landfills.
- Nitrous oxide is emitted during agricultural and industrial activities, as well as during combustion of fossil fuels and solid waste.
- Fluorinated gases, such as hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride, are emitted from a variety of industrial processes and commercial and household uses. Fluorinated gases are sometimes used as substitutes for ozone-depleting substances such as chlorofluorocarbons (CFCs).
Some short-lived greenhouse gases, such as tropospheric ozone, and aerosols (or particles in the atmosphere), such as black carbon and sulfates, are relevant to climate change. While this report focuses only on major, long-lived greenhouse gases, these shorter-lived substances might be included in future editions of this report. For the latest trends and information on these gases, visit EPA's Air Trends Report at: www.epa.gov/airtrends/index.html.