Also, absorption of radiation follows a logarithmic curve. It is generally accepted that the concentration of carbon dioxide in the atmosphere is already high enough to absorb most of the infrared radiation in the main carbon dioxide absorption bands over a distance of only a few km. Thus, even if the atmosphere were heavily laden with carbon dioxide, it would still only cause a relatively small increase in the amount of infrared absorption over current levels. The situation with Venus could not happen here. The atmosphere of Venus is 90 times thicker than Earth's and is 96% carbon dioxide, making the atmospheric carbon dioxide concentration on Venus 300,000 times higher than on Earth. Even so, the high temperatures on Venus are only partially caused by carbon dioxide. A major contributor is the thick bank of clouds containing sulfuric acid. Although these clouds give Venus a high reflectivity in the visible region, the Galileo probe showed that the clouds appear black at infrared wavelengths of 2.3 microns due to strong infrared absorption. Thus, Venus's high temperature might be entirely explainable by direct absorption of incident light, rather than by any greenhouse effect. The infrared absorption lines by carbon dioxide are also broadened by the high pressure on Venus, making any comparison with Earth invalid. I think some poster here tried to use Venus as an example why CO2 is the main cause of global warming. But I did say comparing the atmospheres of two planets of such widely differences in climates is pointless. Just goes to show the climate is far more complex than us mere mortals can understand at this time. Talking of planets, it has been noticed that various planets (and moons) have increased and decreased in surface temperature in sympathy with the earth over the recent past (eg, http://www.agu.org/pubs/crossref/2007/2006GL028764.shtml for the case of Neptune). However, I don't think one can make a definitive statement about why as yet given the observations have been over too short a period.
