Like the glass in a greenhouse, certain gases that occur naturally in the atmosphere tend to trap the sun's heat. This natural "greenhouse effect" helps keep the Earth's average temperature at a comfortable 59° Fahrenheit. Without these greenhouse gases, the Earth would be about 0° F, like a deep- frozen snowball. The earth's climate is predicted to change because human's are changing the chemical composition of the atmosphere by the buildup of greenhouse gases—which mostly is carbon dioxide, methane, and nitrous oxide. These gases trap heat which is well known. But it is uncertain about how the earth's clim