They're in the news almost as much as Brad and Angelina, and you know they have something to do with global warming. But what exactly is a greenhouse gas? In a nutshell, they're gases in the atmosphere—both naturally occurring and caused by humans—that absorb and emit radiation.

Ultraviolet radiation and sunlight hit the Earth and its atmosphere. Most of this is reflected back into outer space, thanks to the ozone layer, clouds, and ice on the Earth's surface. Approximately 25 percent i...


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