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6 Infobase Publishing is carbon dioxide (C02). Plants, which release oxygen gas (02) to the atmosphere, take up C02 during photosynthesis. In the early part of Earth's history (in the Precambrian, before plants covered the land surface), photosynthesis did not remove C02 from the atmosphere, with the result that Co2 levels were much higher than at present. Marine organisms also take up atmospheric Co2 by removing it from the ocean surface water (which is in equilibrium with the atmosphere) and use the Co2 along with calcium to form their shells and mineralized tissue. These organisms make CaC03 (calcite is the most common mineral form of calcium carbonate), which is the main component of limestone, a rock composed largely of the dead remains of marine organisms. The atmosphere-ocean system presently has approximately 99 percent of the planet's Co2 locked up in rock deposits of limestone on the continents and on the seafloor. If this amount of Co2 were released back into the atmosphere, the global temperature would increase dramatically. In the early Precambrian, when this Co2 was free in the atmosphere global temperatures averaged about 550°F (290°C).

The atmosphere redistributes heat quickly by forming and redistributing clouds and uncondensed water vapor around the planet along atmospheric circulation cells. oceans are able to hold and redistribute more heat because of the greater amount of water in the oceans, but they redistribute this heat more slowly than the atmosphere. Surface currents are formed in response to wind patterns, but deep ocean currents that move more of the planet's heat follow courses that are more related to the bathym etry (topography of the seafloor) and the spinning of the Earth than they are related to surface winds.

The balance of incoming and outgoing heat from the Earth has determined the overall temperature of the planet through time. Examination of the geological record has enabled paleoclimatologists to reconstruct periods when the Earth had glacial periods, hot dry periods, hot wet periods, or cold dry periods. In most cases the Earth has responded to these changes by expanding and contracting its climate belts. Warm periods see an expansion of the warm subtropical belts to high latitudes, and cold periods see an expansion of the cold climates of the poles to low latitudes.

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