Greenhouse gases absorb infrared radiation--the heat produced when the sunlight that strikes the earth's surface is radiated back into the atmosphere. Among these gases are carbon dioxide, produced by decaying vegetable and animal matter, animal respiration, and the burning of fossil fuels; methane, known as "swamp gas," produced by bacteria in the guts of cattle and other animals, and in landfills, rice paddies, and wetlands; nitrous oxide, created by modern agricultural methods; and CFCs, among the most efficient greenhouse gases.

While the absorption process--the famous "greenhouse effect"--is natural and even essential for life on earth, human activity since the Industrial Revolution has put more and more of these gases into the atmosphere, leading to the present concern.