Forces of Change
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Blanket against the Cold

Without its atmosphere to retain the sun’s heat, Earth would be cold and nearly lifeless. The average temperature would be about -18° C (0° F) instead of the 15° C (60° F) it is now. Carbon dioxide, water vapor, and methane are greenhouse gases that make Earth habitable.

Greenhouse gases are made up of at least two atoms. Bonds between the atoms absorb heat that radiates from Earth’s surface and would otherwise be lost in space. Greenhouse gases keep Earth warm much as a greenhouse keeps plants warm in winter.

How Greenhouse Gases Work

Some heating of the atmosphere occurs when molecules, aerosols, and particles in the air absorb radiation from the sun. Some heating occurs from contact with Earth’s surface, which absorbs most of the sun’s energy and emits long-wave radiation back into the atmosphere. Greenhouse gases reflect that long-wave radiation back to Earth’s surface and make it hotter still.