Originally Posted by BroncoBeavis
The stability of the vapor makes zero difference. Only it's average contribution to the makeup of the atmosphere. If I could invent a machine that sequestered atmospheric CO2 but then spit an equal amount of CO2 out the other side, you'd say this did nothing to prevent greenhouse warming. Water vapor becoming water and then vapor again is essentially the same thing.
Remove all the H2O from the atmosphere, and the CO2 is still there and keeping temperatures above the freezing point of H2O. Well above.
Remove all the CO2 from the atmosphere, and the temperature will drop until the H2O condenses out and eventually freezes, becoming completely unimportant to the radiative balance of the climate system. Eventually every lake, sea, and ocean is frozen all the way to the bottom.
CO2 is by far the more important forcing in the climate system. H2O is just along for the ride.