Interactions among the atmosphere, hydrosphere, biosphere, and lithosphere control global climate. Global climate represents a balance between the amount of solar radiation received and the amount of this energy retained in a given area. The planet receives about 2.4 times as much heat in the equatorial regions compared to the polar regions. The atmosphere and oceans respond to this unequal heating by setting up currents and circulation systems that redistribute the heat more equally. These circulation patterns are in turn affected by the ever-changing pattern of the distribution of continents, oceans, and mountain ranges.
The amounts and types of gases in the atmosphere can modify the amount of incoming solar radiation, and hence global temperature. For instance, cloud cover can cause much of the incoming solar radiation to be reflected back to space before being trapped by the lower atmosphere. on the other hand, greenhouse gases allow incoming short wavelength solar radiation to enter the atmosphere, but trap this radiation when it tries to escape in its longer wavelength reflected form. This causes a buildup of heat in the atmosphere, and can lead to a global warming known as the greenhouse effect.
The amount of heat trapped in the atmosphere by greenhouse gases has varied greatly over Earth's history. one of the most important greenhouse gases is carbon dioxide (CO2). Plants, which release O2 to the atmosphere, now take up CO2 by photosynthesis. In the early part of Earth's history (in the Precambrian before plants covered the land surface), photosynthesis did not remove Co2 from the atmosphere, with the result that Co2 levels were much higher than at present. Marine organisms remove atmospheric Co2 from 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 CaCO3 (calcite), which is the main component of limestone, a rock composed largely of the dead remains of marine organisms. Approximately 99 percent of the planet's CO2 is presently removed from the atmosphere/ ocean system because it is 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 their greater amount of water, but they redistribute this heat more slowly than the atmosphere. surface currents form in response to wind patterns, but deep ocean currents that move more of the planet's heat follow courses more related to the bathymetry (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 intervals when the Earth had glacial periods, hot dry episodes, hot wet, or cold dry cycles. 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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