Comparing the nightly 10F temperature drop in near 100% humidity San Diego, CA to the near 100F nightly temperature drop in 10% humidity sea level Death Valley, CA will quickly dispel the myth that CO2 is the greatest green house gas in our environment.
CO2 is not the “greatest greenhouse gas”, it is the “greatest greenhouse gas that behaves as a forcing”. Water vapor behaves as a feedback, not a forcing.
When the planet warms up, ocean and lake water evaporates at a higher rate, increasing the amount of water vapor in the atmosphere. This increase of water vapor quickly doubles the effect of any warming that occurs (regardless of the cause of the original warming — whether the original warming was caused by CO2, by the sun, or by slight changes in Earth’s orbit).
(And of course CO2 is only the greatest GHG forcing in terms of absolute effect; marginal increases of other GHGs such as methane can have a much greater effect than CO2)