The greenhouse effect is a natural process by which the Earth's lower atmosphere is warmed. The term "greenhouse effect" has a negative connotation because of its association with air pollution and global warming. However, it is only because of the greenhouse effect that the Earth's surface is warm enough to sustain life. In the absence of the greenhouse effect, Earth's average surface temperature would be the same as its outer atmosphere, about 0° Fahrenheit (-18° Celsius)—a far cry from our actual average surface temperature of 59° Fahrenheit (33° Celsius)!
"Greenhouse gases" in the atmosphere—primarily water vapor and carbon dioxide and secondarily methane, nitrous oxide, and chlorofluorocarbons (CFCs)—trap heat in the same way as the glass of a greenhouse. Specifically, solar radiation enters the atmosphere in the form of visible light, is absorbed by the Earth's surface, and is converted to heat. Some of that heat radiates upward and is absorbed by greenhouse gases in the atmosphere. The gases reradiate heat outward in all directions, including back toward the surface. In this way the Earth's lower atmosphere is heated continuously, even at night when there is no incoming sunlight.
Since the Industrial Revolution (in the mid-1700s), levels of carbon dioxide and other greenhouse gases in the atmosphere have been increasing. This is primarily due to the burning of coal, oil, gas, and wood. The increase of greenhouse gases means that more heat is being trapped and returned to Earth, creating a condition known as "global warming."
Sources: Engelbert, Phillis. The Complete Weather Resource, vol. 3, pp. 500-4; Gay, Kathlyn. The Greenhouse Effect, pp. 2, 122-25; McGraw-Hill Encyclopedia of Science and Technology, 7th ed., vol. 8, pp. 225-26.
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