Water Vapor as a Greenhouse Gas
Those opposed to the concept of anthropogenic climate warming often point out that water vapor is the most abundant greenhouse gas. Some claim (incorrectly) that it accounts for 98 percent of the greenhouse effect. With the exception of small amounts in the stratosphere, though, water vapor (considered separately from clouds - see main page, section 21) is strictly a feedback, responding quickly to temperature change. So it can amplify a trend, but not force one. And it's included as such in climate models, contrary to claims that it's ignored. Evaporation and precipitation cycle water vapor through the atmosphere about every ten days, maintaining an overall balance. Since it's concentrations are temperature-regulated on short timescales, if humans directly pump extra water vapor into the air, most of it soon precipitates out. There must be warming to increase atmospheric concentrations. Carbon dioxide, on the other hand, can build up and persist for decades to centuries (determined by various uptake processes), even without a preceding change in temperature. Large accumulations and the resulting feedbacks can extend the persistence of a CO2 anomaly to millennia. For more, see Water vapour: feedback or forcing?
Update: New research affirms water vapor is a strong amplifier.
Update2: Lawrence National Laboratory study shows humans are already influencing average water vapor concentrations, via other greenhouse gases.
Update 3: A discussion of a new study, still under peer review, finding a drop in stratospheric water vapor levels in recent years. Contrary to some media interpretations, the study does not conclude that this phenomenon is a climate forcing, or a "negative feedback" that will offset global warming in the longer-term averages. It's more likely to be a temporary response to decadal variability in the climate system (like ocean cycles and/or the 11 year solar cycle). Even if that weren't the case, the question would remain of how much long-term temperature offset an already dry stratosphere could provide.