I agree that water vapour may play an important role. I've read speculation that the increased evaporation caused by higher ocean temperatures could have a positive feedback effect on temperatures (since water vapour is also a greenhouse gas) - i.e. more water vapour cause higher temps causes more water vapour etc.
On the flipside, more water vapour may cause more clouds hence reflecting more sunlight and decreasing temps...
I have no idea how they take these effects into account in their models, but scientists are a pretty clever bunch.
