[anthonyb]

Water vapour has a very short life cycle in the atmosphere, so while it's theoretically the worst, it's not actually particularly bad in the long term, and it's not likely to change by much. There's also some question as to the cooling effect of clouds.

CO2 on the other hand, stays in the atmosphere for quite a few years (30-95 according to wikipedia), so it's far worse in the long term.