Water vapor constitutes Earth's most significant greenhouse gas, accounting for about 95% of Earth's greenhouse effect (4). Interestingly, many "facts and figures' regarding global warming completely ignore the powerful effects of water vapor in the greenhouse system, carelessly (perhaps, deliberately) overstating human impacts as much as 20-fold.
I've seen that analysis and it's a valid description of the current static situation. But man's CO2 emissions are still increasing (although 1st world countries are steady). So it's the change in concentration and subsequent rise in temperature (1 degree C) that is generally agreed upon. From there however, the alarmists say that increased water vapor will increase temperature much more in a positive feedback. The problem with their theory is that it can't be analyzed statically because water vapor is unevenly distributed by the weather (winds, evaporation, rain). The climate models do not adequately model weather (among other inadequacies) and therefore don't have much if any useful predictive value.