Greenhouse gases are trace gases in the atmosphere that absorb outgoing infrared radiation from Earth and thereby, like a greenhouse, warm the planet. Naturally occurring greenhouse gases (primarily water vapor and carbon dioxide) make the planet habitable for life as we know it. Anthropogenic greenhouse gases contribute to further warming, referred to as global warming.

Carbon dioxide (CO₂) is both a natural and anthropogenic greenhouse gas. Anthropogenic inputs of CO₂ mainly from the burning of fossil fuels and deforestation continue to rise, making it the number-one contributor to global warming. Other anthropogenic greenhouse gases include methane (CH₄), nitrous oxide (N₂O), sulfur hexafluoride (SF₆), chlorofluorocarbons (CFCs), and hydrofluorocarbons (HFCs). The last three compounds are synthetic greenhouse gases, which did not exist in the atmosphere before the twentieth century. Molecule for molecule, these gases trap more energy than CO₂, but are less abundant in the atmosphere. One molecule of CH₄, for example, traps as much heat as twenty-three molecules of CO₂. SF₆ traps as much heat as 22,200 molecules of CO₂.

In 1997 the Kyoto Protocol proposed legally binding restrictions on greenhouse gas emissions, targeting a 5-percent reduction over 1990 levels by 2012. As of December 2001, 186 countries had ratified the protocol. The United States, however, is not one of them.

Bibliography

Turco, Richard P. (1997). Earth under Siege: From Air Pollution to Global Change. New York: Oxford University Press.

Internet Resources

U.S. Environmental Protection Agency. "Global Warming." Available from http://www.epa.gov/globalwarming.

United Nations Framework Convention on Climate Change. "Greenhouse Gas Emissions." Available from http://unfccc.int/resource.