INDICATORS IN THIS CHAPTERGreenhouse U.S. Greenhouse Gas EmissionsGlobal Greenhouse Gas EmissionsAtmospheric Concentrations of Greenhouse GasesMajor Greenhouse Gases Associated with Human ActivitiesGreenhouse gasHow it’s producedAverage lifetime in the atmosphere100-year global warming potentialCarbon dioxideEmitted primarily through the burning of fossil fuels (oil, natural gas, and coal), solid waste, and trees and wood products. Changes in land use also play a role. Deforestation and soil degradation add carbon dioxide to the atmo-sphere, while forest regrowth takes it out of the atmosphere. see below*1MethaneEmitted during the production and transport of coal, natural gas, and oil. Methane emissions also result from livestock and agricultural prac-tices and from the anaerobic decay of organic waste in municipal solid waste landfills.12 years21Nitrous oxideEmitted during agricultural and industrial activi-ties, as well as during combustion of fossil fuels and solid waste.114 years310Fluorinated gasesA group of gases that includes hydrofluorocar-bons, perfluorocarbons, and sulfur hexafluoride, among other chemicals. These gases are emitted from a variety of industrial processes and com-mercial and household uses, and do not occur naturally. Sometimes used as substitutes for ozone-depleting substances such as chlorofluoro-carbons (CFCs).A few weeks to thousands of yearsVaries (the highest is sulfur hexafluo-ride at 23,900)This table shows 100-year global warming potentials, which describe the effects that occur over a period of 100 years after a particular mass of a gas is emitted. EPA uses global warming potentials from the Intergovernmental Panel on Climate Change’s (IPCC’s) Second Assessment Report,1as countries have agreed to do under current international guidelines within the United Nations Framework Convention on Climate Change (UNFCCC). Lifetimes come from the IPCC’s Fourth Assessment Report.2* Carbon dioxide’s lifetime is poorly defined because the gas is not destroyed over time, but instead moves among different parts of the ocean–atmosphere–land system. Some of the excess carbon dioxide will be absorbed quickly (for example, by the ocean surface), but some will remain in the atmosphere for thousands of years, due in part to the very slow process by which carbon is transferred to ocean sediments.10
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, mak-ing the Earth’s surface warmer than it otherwise would be.