Schmidt’s study found that water vapor is indeed the largest single contributor to natural global warming, responsible for about 50 percent of the greenhouse effect. Following water vapor are clouds, contributing about 25 percent, and next comes carbon dioxide with 20 percent.
This is a very simple experiment in RRTM. I simply zeroed out the CO2 concentration in each layer of the atmosphere. It turns out that in the tropical atmosphere, CO2 only accounts for 1.3% of the greenhouse effect. Downwelling LW radiation dropped from 397 watts/m^2 to 392 watts/m^2.
In other words, the loss of all CO2 from the atmosphere would have the same effect as a small change in the humidity.
Of course life would also cease to exist, as plant growth would stop.