Greenhouse Effect

A process through which solar energy received by the earth is trapped as heat in the atmosphere near the earth's surface.

Some of the heat flowing back toward space from the earth's surface is absorbed by certain compounds in the atmosphere, referred to as greenhouse gases, and then radiated back toward the earth's surface. This is a natural process that keeps the earth's atmosphere about 59°F warmer than it would be otherwise. If atmospheric concentrations of greenhouse gases rise, the greenhouse effect suggests that the average temperature of the lower atmosphere will gradually increase.

Source: http://www.mondaq.com/unitedstates/article.asp?articleid=87596