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Our atmosphere is 76% nitrogen, 21% oxygen, and 3% trace gases, which include the greenhouse gases carbon dioxide (CO2) and water. Carbon dioxide has a large infrared absorption at 16 microns, the maximum of the blackbody radiation spectrum of our atmosphere. Carbon dioxide thus absorbs heat radiation and re-emits it back to the surface of our earth to keep it warm. Water vapor increases with temperature amplifying this greenhouse effect.
Non-condensing CO2 provides the stable temperature structure that sustains atmospheric water vapor and clouds (1). When the earth’s orbit and tilt triggered a cooling trend 110,000 years ago, CO2 decreased and water condensed and froze, plunging the earth into an ice age. About 18,000 years ago, orbital changes warmed the oceans which emitted dissolved CO2 gas. This greenhouse forcing warmed the earth and melted the ice.
(1) Andrew A. Lacis, et. al. “Atmospheric CO2: Principal Control Knob Governing Earth’s Temperature.” Science, 330, 356-359, 15 October 2010