One commonly heard argument in the Global Warming Debate is that since water vapor makes up the largest greenhouse gas (GHG), CO2 emissions can't be having that much effect on global temperature. Water vapor is the most important GHG, but focusing on this fact ignores the difference between forcings and feedback.
Fossil fuel emissions, a forcing, kicks off a positive feedback loop involving water vapor. Atmospheric CO2 increases temperature (since CO2 is a GHG) which increases the saturation capacity of the atmosphere which increases water vapor which in turn increases temperature. What is less clear is the effect of increases in water vapor on cloud formation. Clouds are water droplets and the formation of those droplets and the level they form in the atmosphere determine whether clouds have a positive or negative effect on temperature. The effect is called the Iris hypothesis and it is currently being tested by climate scientists, with mixed results. The formation and effect of clouds is poorly understood and presents a challenge for current Global Circulation Models (GCMs) used to make climate change predictions. This doesn't mean the GCMs are wrong (a subject for a later post) just incomplete.
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