How is it that all the denialist's liturgy is either based on misinformation or hypotheses without either a theoretical explanation or empirical evidence. CO2 as a cause has the support of both. And, surprise surprise, zzedzz water vapour comment is one of the often repeated bits of misinformation. I only wish that the denialists would apply the same standards of truth to their own work that they demand from climatologists.
Climate Scientists Hide Water Vapor (Part of the How to Talk to a Climate Sceptic guide)
Objection: Climate scientists never talk about water vapor, which is the strongest Greenhouse Gas, because it undermines their CO2 theory.
This is fairly common and fairly straightforward to answer, it just isn't true. This is an effective attack though because it is true that the IPCC report does not spend much time discussing water vapor's greenhouse effect because it is considered as a feedback effect, rather than a forcing of the climate system.
Answer: There is no climate model or climate textbook that does not discuss the role water vapor plays in the Greenhouse Effect. It is the strongest Greenhouse gas, contributing 66% to 85% to the overall effect when you include clouds, 36% - 66% for vapor alone. It is however, not considered as a climate "forcing" because the amount of H2O in the air varies basically 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), similarily, due to the abundance of sea surface, if you somehow removed water from the air it would quickly be replaced through evaporation. This has the interesting consequence that if one could somehow instantly remove all CO2 from the atmosphere, the temperature would begin to drop, causing percipitation to remove H2O from the air causing even further drops, in a feedback effect that would not end until no water was left unfrozen on the ground.
CO2 put into the air by burning fossil fuels, on the other hand, has an atmospheric lifetime of centuries before natural sinks will significantly absorb any excess from the air. This is plenty of time to have substantial and even longer lasting effects of the climate system.
This article from Real Climate has an good discussion of this "Water vapor, forcing or feedback?"
