To follow-up on the excellent points made by CraigD, water vapour is very important, but the reason it is the cause for less concern (besides the fact that it is a "feedback" and not a "forcing") is the relatively short time that it stays in the atmosphere (roughly 10 days), whereas CO
2 stays in the atmosphere for decades, and even over 100 years.
So, inputting water vapour into the atmosphere has a relatively short lived effect, whereas CO
2 concentrations just keeps building upon themselves without going away so quickly.
Freeztar gave a link in post #81 which explains this issue very clearly.
RealClimate - Water Vapor in comparison
Quote:
While water vapour is indeed the most important greenhouse gas, the issue that makes it a feedback (rather than a forcing) is the relatively short residence time for water in the atmosphere (around 10 days).
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At Day 0 there is zero water, but after only 14 days, the water is back to 90% of its normal value, and after 50 days it's back to within 1%. That's less than 3 months. Compared to the residence time for perturbations to CO2 (decades to centuries) or CH4 (a decade), this is a really short time.
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