The Sceptics Say: ”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. 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), similarly, due to the abundance of sea surface, if you somehow removed water from the air it would quickly be replaced through evaporation.

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.

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