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Originally Posted by Tim
If everyone knows that water vapor is the dominant greenhouse gas, why do Al Gore and so many others focus on CO2? Call it the politics of the possible. Water vapor is almost entirely natural. It’s beyond the reach of man’s screwdriver. But when the delegates of 189 countries met at Kyoto in December 1997 to discuss global climate change, they could hardly vote to do nothing. So instead, they agreed that the developed countries of the world would reduce emissions of six man-made greenhouse gases. At the top of the list is CO2, a trivial influence on global warming compared with water vapor, but unquestionably man’s larges contribution.
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Emitting water vapor from a car does not cause an increase in atmospheric water vapor concentrations. Here on earth, the water is in constant (and relatively fast) equilibrium between solid, liquid, and gas phases. If you inject some water vapor into the atmosphere, it will just work it's way back to solid or liquid form. In general, the only way to increase the amount of water vapor in the atmosphere is to increase the temperature.
On the other hand, CO2 is not in a natural equilibrium with various phases. It mostly stays in the gas phase. I believe I have read that it is in a very slow equilibrium with dissolved CO2 in bodies of water (mainly ocean). However, CO2 is being produced at a much faster rate than the ocean can take up.