On Earth, atmospheric CO2 is absorbed by precipitation- --by rain---and forms a very weak solution of carbonic acid, a very mild form of acid rain. This acid rain falls on surface rocks, many of which contain calcium, and the carbonic acid dissolves a tiny bit of the calcium. Eventually the water, containing both carbonic acid and calcium ions, washes down to the ocean. In our oceans tiny plants and animals, plankton, incorporate the calcium and carbonic acid into shells of calcium carbonate. When the animals die, their calcium carbonate exoskeletons drift to the ocean floor. When enough of these carbonate deposits build up, they form carbonate rocks, such as limestone, which are composed of the skeletons of trillions of dead plankton. In short, the action of water removes CO2 from the atmosphere and puts it into the crust of the Earth. The Earth has roughly the same amount of CO2 as does Venus, but it is nearly all locked up in the crust as carbonate sediments.
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This is the complete carbon cycle: rainwater removes CO2 from the atmosphere and puts it in the crust, and volcanic action releases CO2 from the crust and puts it back in the atmosphere.
What happens on Venus? Venus has no water! Early in its history Venus may have had water, but it is too close to the Sun to retain it. When water molecules rise high in an atmosphere, ultraviolet radiation split the water molecules into its component gases, oxygen and hydrogen, and the lighter hydrogen molecules escape into space. While Earth's lower atmosphere is about one percent water vapor (although it seems much higher in the humid Louisiana summers), the upper atmosphere, where ultraviolet radiation can penetrate, is very dry: a cold trap, a combination of pressure and temperature, prevents water vapor from rising high in the earth's atmosphere. Venus has a cold trap, too, but because Venus is closer to the Sun its cold trap is much higher in the atmosphere and any Venusian water molecules rise high enough to be broken apart by ultraviolet radiation.
Therefore the carbon cycle is incomplete on Venus: without water, CO2 cannot be removed from the atmosphere. Venus does have volcanoes, however. Radar mappings of Venus by interplanetary probes indicate volcano-like mountains, and there is other evidence for volcanoes as well. The atmosphere of Venus is full of sulfur dioxide and sulfur particulates. Sulfur and sulfur dioxide is highly reactive and cannot remain long in an atmosphere; therefore something (volcanoes) must be regularly replenishing the sulfur. This theory is bolstered by data from interplanetary probes, which have detected large fluctuations in the sulfur content of the Venusian atmosphere, as well as radio signals reminiscent of lightning--and lightning is often found in volcanic plumes.
http://www.phys.lsu.edu/faculty/cjohnson/climate.htmlSo 'Venusian' probably just can't happen here, whatever we do.
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