The increase (decrease) in specific humidity under a global warming (cooling) situation represents the most powerful climate feedback as a response to radiative perturbations, effectively doubling the sensitivity of Earth’s climate.
The absorption of longwave radiation goes up as the logarithm of water vapor concentrations, so like CO2, the fractional change in water vapor is more important than the absolute changes. Most of the water vapor feedback occurs in the upper tropical troposphere, and most of the water vapor feedback occurs in the longwave component of the spectrum (at the poles, there is a significant shortwave component as well).
The water vapor increases in the atmosphere come from the fact that the partial pressure of atmospheric water vapor can increase as a function of temperature (following the Clausius-Clapeyron equation). C-C provides an upper limit to the saturation pressure, which as a result of circulation patterns, is not what is actually observed (i.e., global relative humidity is well under 100%) so understanding how water vapor increases also requires knowing how incremental changes in water vapor actually follow with the C-C equation. Here we are in 2009 where observations and models produce a roughly constant relative humidity (globally averaged) in climate change situations, suggesting increases of saturation pressure about 7%/degree K of warming.
In a recent issue of Science, Andrew Dessler and Steve Sherwood provide a perspectives article (subscription required for full text) briefly summarizing some history and current state of understanding behind the water vapor feedback. The authors seem confident that this feedback is well understood, and observational, theoretical, and model-based understanding has improved remarkably over the last few decades. As it stands, there is no reason to doubt the general description of a powerful positive feedback due to humidity increases as the globe warms, which now has strong support from a large and growing body of literature.
Humidity is largely controlled by the large-scale wind and temperature fields, which can be calculated explicitly in climate models; the dependance of the overall water vapor feedback on microphysical interactions (which must be paramaterized) is comparatively small, so the strength of the water vapor feedback associated with cloud microphysical processes is unlikely to exceed a few percent.
Climate Change 
Chris: As your post infers and references point out, could water vapour cause a doubling of warming by the end of the century, caused by Global Warming. Conversely on Watts Up they say that water vapour will intercept the warming, and therefore cause cooling. Since water vapour is an absorber of heat, would you agree that it would cause a doubling of warming from GHG’s by the end of this century. Great post, Kipp
Response– I’m not aware of what WUWT specifically said on that matter, but coming from that source, it’s very likely to be wrong or very misleading. WV does provide a strong positive feedback to global warming, and if anyone says otherwise they probably don’t know what they are talking about– chris
A reprint of the Science paper is available on Dessler’s Texas A&M website:
http://atmo.tamu.edu/profile/sub/239
Nice post. Thanks. And thanks Miguelito for the link to the Science paper.
jg
Chris, I actually followed up on this and found that the state of the science is nowhere near as settled as Dessler and Sherwood claimed.
One place to start is this book chapter by Pierrehumbert et al. They try to give support for alarm by modeling how much the Earth would heat up with a fully saturated atmosphere, which is a completely unrealistic scenario. The rest of the chapter describes very interesting stochastic modeling attempts that serve mostly reinforce how little understanding we really have of the humidity question. [Off topic, they present some very wordy explanations of why OLR can increase even when the mean humidity does not change. Apparently they’ve not heard of Jensen’s Inequality which is instantly recognizable to any serious theoretical physicist or mathematician. My advice to Dessler or Sherwood if you’re reading: just refer to Jensen’s inequality next time. You can thank me for the pointer].
Over and over and over again I read these summary articles by climate scientists claiming that the science is settled and the consensus is clear only to find on delving deeper that the opposite is true. And they wonder why we’re skeptical.
Response– Saying the “science is settled” is usually just a distraction and strawman that is put up by wingnut sources, rather than a positive statement made by anyone within the physical sciences. I’m not even sure what that is supposed to mean. There are always interesting questions as to ecosystem responses to climate change, the magnitude and rate of Greenland ice loss, changing low vs. high cloud cover in a warmer climate, how much sea level will rise, possible thresholds of abrupt climate change, how to better improve observational data and models, what to do about climate change, etc, etc. No one would be doing any research if all of these questions were “settled” so please don’t make things up.
However, for some odd reason most of the blogosphere’s main focus is on the questions which are in fact settled (as much as science can “settle” anything) like “does CO2 cause warming” or “is the water vapor feedback positive” or “are humans causing CO2 to rise.” The questions are just not interesting anymore and are usually accompanied with a string of talking points that are very easy to refute. Still, that has no effect in how the public believes them, or how often they are repeated in secondary sources.– chris
The leading source of “debate” in AGW is on blogs, usually run by non-climate specialsits. Some are half way competent like McIntyre, and some are just fradulent like WUWT, Lubos, Inhofe, Morohasy. Likewise some skeptics like Lindzen are worht paying attention to, and some are just fraudulent like Tim Ball. It amazes me how many readers cannot see this, and to distinguish between the quality of arguments usually does take some background in climate, but should be evident to those with at least undergraduate-level scientific training. Actually, most of the arguments involves thinking it through logically even before scientific evaluation.
The problem with so-called “skeptics” is that they have no faith in physics and they don’t want to read and digest peer-reviewed articles. They want quick bits of information from someones website. Admittedly, climatologists haven’t done the best job of communicating outside the technical documents (IMHO).
There are hundreds of documents in the literature supporting AGW (and at the same time showing the lack of disagreement amongst scientists), with only a few exceptions (usually authored by familiar names like Roy Spencer or Richard Lindzen). The arguments deserve attention, but those scientists should have an obligation not to tell half-truths in the lay public.
” No one would be doing any research if all of these questions were “settled” so please don’t make things up.”
You’re living on a different planet. “Consensus view”, “settled science”, etc are popular alarmist catchphrases, from Al Gore to James Hansen to Steven Schneider. I am not making anything up.
“However, for some odd reason most of the blogosphere’s main focus is on the questions which are in fact settled (as much as science can “settle” anything) like “does CO2 cause warming” or “is the water vapor feedback positive” or “are humans causing CO2 to rise.”
Now you are making stuff up. Wingnut skeptics are focused on those questions. Serious skeptics are not. (Except possibly for the water vapor question since clouds are water too). The main problem is there is very little evidence for the water vapor multiplier being 3 or more, which is what the alarmists need.
“The problem with so-called “skeptics” is that they have no faith in physics and they don’t want to read and digest peer-reviewed articles.”
Baloney John Grey. That’s a problem with some skeptics but in fact, a far greater problem with alarmists most of whom do not read peer-reviewed papers at all (you think Al Gore has the foggiest idea how to model humidity using bounded Brownian motion?). I read the papers. They do not support the alarmist view.
Perhaps you can tell me who these “serious skeptics” are and where these “papers” are that don’t support the “alarmist” view (and a defintion of whatever that may be). These kind of words are intentionally vague enough to allow for any interpretation, which I guess is the point.
“Saying the “science is settled” is usually just a distraction and strawman that is put up by wingnut sources…”
OK, fine.
“…the blogosphere’s main focus is on the questions which are in fact settled (as much as science can “settle” anything) like “does CO2 cause warming” or “is the water vapor feedback positive” ….
I worried that the lack of introspective curiosity in the climate research community has led to a culture of subtle, but persuasive self-deception, which is dampening our ability to produce first rate empirically-based discourse.
As a curious member of the lay audience I am unqualified to evaluate the science on water vapour feedback, however, I am qualified to assess the self-awareness and logic employed by the debaters pro and con.
“As it stands, there is no reason to doubt the general description of a powerful positive feedback due to humidity increases as the globe warms, which now has strong support from a large and growing body of literature.”
I worry when the reasons to doubt are forgotten. I’d feel better if you were vigilant and curious.
You know, I know this post is months old, but while intemperately replying to Thomas Fuller’s Examiner column, I discovered that the Dessler and Sherwood paper is online: A matter of humidity (in PDF Form)
I was replying to Fuller in this article:
Evidence of a desperate push to pump global warming up… and up