(Part of the How to Talk to a Global Warming Skeptic guide)
Objection: Climate scientists never talk about water vapor — the strongest greenhouse gas — because it undermines their CO2 theory.
Answer: Not a single climate model or climate textbook fails to discuss the role water vapor plays in the greenhouse effect. It is the strongest greenhouse gas, contributing 36% to 66% to the overall effect for vapor alone, 66% to 85% when you include clouds. It is however, not considered a climate “forcing,” because the amount of H2O in the air basically varies as a function of temperature.
If you artificially increase the level of H2O in the air, it rains out immediately (in terms of climate response times). Similarly, due to the abundance of ocean on the earth’s surface, if you somehow removed all the water from the air, it would quickly be replaced through evaporation.
This has the interesting consequence that if you could somehow instantly remove all CO2 from the atmosphere, the temperature would begin to drop, causing precipitation to remove H2O from the air, causing even further drops, in a feedback effect that would not end until no liquid water was left, only ice sheets and frozen oceans.
CO2 put into the air by burning fossil fuels, on the other hand, stays in the atmosphere for centuries before natural sinks finish absorbing the excess. This is plenty of time to have substantial and long-lasting effects on the climate system. As the climate warms in response to CO2, humidity rises and increased H2O concentration acts as a significant amplifier of CO2-driven warming, basically doubling or tripling its effect.
An article from RealClimate — “Water vapor: feedback or forcing?” — has a good discussion of this subject.