It should also be noted that the total atmosphere (dry + water) is about 3% water vapor by weight and H2O makes up the majority of the greenhouse effect.
CO2 isn't a problem because it's the biggest greenhouse gas, it's a problem because it's the one (the biggest anthroprogenic one) pushing us over the equilibrium where net warming is zero.
This is actually an incredibly important point, because higher temperatures mean more water vapor in the atmosphere. However, that also means more clouds, which counteracts warming to some degree.
The impact of clouds on climate change is one of the biggest difficulties in modeling climate change. The IPCC report goes into it a bit. My understanding is that computationally it's a bit like predicting the weather into the future - a lot of uncertainty due to the chaotic nature of the system.
For a long-term climate model, it might work to average out (over space and time) some critical phenomema and assume a steady function from e.g. average air temperature to ice cap size and cloud cover.
(source: wrote some homework-grade numerical simulations, physics major.)