Greenhouse gas - (n.)

These atmospheric gases tend to absorb the infrared radiation from the Sun as it is reflected back towards space, thus trapping the heat in the atmosphere. The major greenhouse gases include both naturally occurring species, like water vapor (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and several anthropogenically designed gases, like chlorofluorocarbons (CFCs), perfluorocarbons (PFCs) and sulfur hexafluoride (SF6). Their relative ability to trap heat can be measured in terms of their Global Warming Potential (GWP), the ratio of global warming both direct and indirect from one unit mass of a greenhouse gas to that of one unit mass of carbon dioxide over a period of time. Table 1 summarizes the GWP of some of the major greenhouse gases. Table 2 shows the latest changes in US emissions, in terms of the equivalent amounts of carbon.

Table 1: Global Warming Potentials on a 100 Year Time Horizon (Source:IPCC 1996)
Table 2: U.S. Anthropogenic Greenhouse Gas Emissions, 1990-1999 (Source: US Department of Energy; http://www.eia.doe.gov/oiaf/1605/ggrpt/ )