Originally Posted by BroncoBeavis
I understand the theory. Increased heat from CO2 will lead to more H2O in the atmosphere as well.
CO2 doesn't create heat - its radiative properties lessen the amount of outgoing IR from the surface that can be emitted to space. Surface temperatures then increase until the energy balance is attained. Since warmer air can hold more water vapor, as the atmosphere warms, more H2O is evaporated and thus increases.
These two facts were known long before climate models existed.