Greenhouse Effect

Sunlight passes through the atmosphere and warms the Earth's surface. "Greenhouse" gases (water vapor, carbon dioxide, and other gases) trap some of the Sun's energy as it is being radiated back into space, causing the atmosphere to warm (see diagram).¹ This action is similar to the way the glass panels in a greenhouse let sunlight through but then trap a portion of the Sun's energy inside the building as heat.

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As long as the mixture of greenhouse gases in the atmosphere remains relatively stable, there is no change in the conditions at the Earth's surface or in the atmosphere, and the amount of energy coming to the Earth from the Sun remains stable, the amount of heat trapped in the atmosphere remains relatively constant from year to year.

This natural "greenhouse effect" maintains an average annual temperature of about 59 degrees Fahrenheit at the surface of the Earth. Without the greenhouse effect, the average annual temperature at the Earth's surface would be around zero degrees Fahrenheit! ²

References:

1. Diagram of Greenhouse Effect, Wes Peck, EERC
   
2. www.physicalgeography.net/fundamentals/7h.html (accessed August 2006).