Contrary to the popular talking point, climate models do take into account H2O as a greenhouse gas. In fact, it is the largest single feedback factor in the climate system. And also contrary to another popular talking point, models are being validated in many ways.
Go have a read at Gristmill for a post by Andrew Dessler on a recent paper he co-authored[PDF] that seeks to assess the state of current climate science literature on the topic of water vapor feedback in models and the climate system. He describes papers raw material as a “mountain of evidence” supporting a strongly positive effect. He also has this observation:
Interestingly, it seems that just about everybody now agrees water vapor provides a robustly strong and positive feedback. Roy Spencer even sent me email saying that he agrees.
Note that Spencer is one of the climate septic poster boys who, like most of the actual scientists on the Heartland Institutes call list don’t actually disagree that global warming is real and anthropogenic. They just tend to hand wave away the apparently dire consequences that more sober scientific assessments indicate are very likely.