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There is a natural greenhouse effect that contributes to warming. Greenhouse gases trap heat and thus warm the Earth because they prevent a significant portion of infrared radiation from escaping into space.

"President Bush Announces Global Warming Initiatives". CNN Live Event/Special, www.cnn.com. June 11, 2001.
There is a natural greenhouse effect that contributes to warming. Greenhouse gases trap heat and thus warm the Earth because they prevent a significant portion of infrared radiation from escaping into space.