> Water Vapor Feedback and Global Warming
>
http://www.dgf.uchile.cl/~ronda/GF3004/helandsod00.pdf
> Abstract: Water vapor is the dominant greenhouse gas, the most important gaseous
> source of infrared opacity in the atmosphere. As the concentrations of other greenhouse
> gases, particularly carbon dioxide, increase because of human activity, it is centrally
> important to predict how the water vapor distribution will be affected. To the extent that
> water vapor concentrations increase in a warmer world, the climatic effects of the other
> greenhouse gases will be amplified. Models of the Earth’s climate indicate that this
> is an important positive feedback that increases the sensitivity of surface temperatures
> to carbon dioxide by nearly a factor of two when considered in isolation from other
> feedbacks, and possibly by as much as a factor of three or more when interactions with
> other feedbacks are considered. Critics of this consensus have attempted to provide
> reasons why modeling results are overestimating the strength of this feedback.
> Our uncertainty concerning climate sensitivity is disturbing. The range most often
> quoted for the equilibrium global mean surface temperature response to a doubling
> of CO2 concentrations in the atmosphere is 1.5° C to 4.5° C. If the Earth lies near
> the upper bound of this sensitivity range, climate changes in the twenty-first century
> will be profound. The range in sensitivity is primarily due to differing assumptions
> about how the Earth’s cloud distribution is maintained; all the models on which these
> estimates are based possess strong water vapor feedback. If this feedback is, in fact,
> substantially weaker than predicted in current models, sensitivities in the upper half of
> this range would be much less likely, a conclusion that would clearly have important
> policy implications. In this review, we describe the background behind the prevailing
> view on water vapor feedback and some of the arguments raised by its critics, and
> attempt to explain why these arguments have not modified the consensus within the
> climate research community.
See:
http://www.dgf.uchile.cl/~ronda/GF3004/helandsod00.pdf