The water vapor feedback is one of the most important processes in our climate. In fact, the feedback is responsible for a significant part of the warming predicted to occur over the next century.
A reader asked an interesting question in response to a recent post on the water vapor feedback: What peer-reviewed evidence exists for a positive water vapor feedback? What about a negative water vapor feedback?
Let's take the negative feedback first. I did a quick search on the Web of Science and found out these statistics:
70 papers contained the phrase “water vapor feedback”
18 of them contained “negative” and “water vapor feedback”
If you go through the abstracts, you find that only four articles talk about a negative water vapor feedback (in other abstracts, the word “negative” was modifying another phrase). I’m adding a fifth paper that was not flagged in my search because it was published in 1990, before the WOS included abstracts. Also, I’m dropping one paper for reasons I won’t go into here.
Here is the resulting list of peer-reviewed literature on the negative water vapor feedback:
1. Lindzen, R. S. (1990), Some coolness concerning global warming, Bull. Amer. Meteor. Soc., 71, 288-299.
2. Sun, D.-Z., and R. S. Lindzen (1993), Distribution of tropical tropospheric water vapor, Journal of Atmospheric Sciences, 50, 1643-1660.
3. Sun, D. Z., and R. S. Lindzen (1993), Water-vapor feedback and the ice-age snowline record, Annales Geophysicae, 11, 204-215.
4. Lindzen, R. S., M.-D. Chou, and A. Y. Hou (2001), Does the Earth have an adaptive iris?, Bull. Am. Met. Soc., 82, 417-432.
Hmmm. There’s a pattern here that I just can’t quite figure out. Just joking. The pattern, of course, is that only Dick Lindzen has been able to publish a paper arguing for a negative water vapor feedback. But if you look carefully at the papers, they make a much weaker argument than you might expect.
Paper 1 is considered wrong by everyone, including Lindzen. In fact, paper 2 was written to correct the defect in paper 1. Paper 2 mainly discusses how tropospheric water vapor is regulated. Its discussion of the water vapor feedback is limited to a short discussion of how a negative water vapor feedback might plausibly arise, but no evidence of such a feedback is given. I cannot remember what’s in paper 3, and I can’t find a copy. However, based on the date and author list, it’s likely that it contains much of the same as in the other 1993 Sun and Lindzen paper.
In paper 4, Lindzen resurrects his idea from paper 2, and provides some data to argue that indeed a negative water vapor feedback does exist. Unfortunately (for him), subsequent tests of other scientists failed to verify this idea. At the present time, there is virtually no support in the community for it. Lindzen still gives talks on this and claims that the iris hypothesis is still viable. However, as far as I can tell, no one, including Lindzen, is working on it, so that indicates exactly how vibrant the idea is.
So let’s summarize. There have been a very small number of articles written that argue for a negative water vapor feedback. Virtually all have been written by Dick Lindzen. None have stood the test of time.
Now let’s turn to the other side. What evidence is there for a positive water vapor feedback. First, there are 25 papers that contain “positive” and “water vapor feedback”. Many of these papers are recent (written in the last 2-3 years). Going through the abstracts reveals that most of these papers argue in favor of a water vapor feedback. Many conclude this directly from data, not from any type of GCM analysis. In addition, the papers were written by a large number of different scientists.
Here are a few positive-feedback papers that you might want to take a look at (just a small subset of the literature):
1. Minschwaner, K., and A. E. Dessler (2004), Water vapor feedback in the tropical upper troposphere: Model results and observations, J. Climate, 17, 1272-1282.
2. Minschwaner, K., A. E. Dessler, and P. Sawaengphokhai (2006), Multi-model analysis of the water vapor feedback in the tropical upper troposphere, J. Climate, accepted.
3. Soden, B. J., et al. (2005), The radiative signature of upper tropospheric moistening, Science, 310, 841-844.
4. Sherwood, S. C., and C. L. Meyer (2006), The general circulation and robust relative humidity, J. Climate, in press.
5. Dessler, A.E., and K. Minschwaner, An analysis of the regulation of tropical tropospheric water vapor, J. Geophys. Res., submitted.
[pre-prints of the Sherwood paper can be found on his web page; if you want a pre-print of paper 2 or 5, let me know].
These are just a few of the more recent papers. A bunch more exist, written by different scientists using different data.
Thus, the evidence in favor of a positive water vapor feedback is strong, with multiple peer-reviewed analyses reaching this conclusion. The evidence that the feedback is negative is weak: only Dick Lindzen argues it, and his arguments have been roundly rejected by the scientific community.
Also note that Bill Gray hasn’t published anything on this topic. That’s because he does not have a testable hypothesis nor any data, both of which would be required. Nor can he criticize the published literature on this subject because he has not read these papers.
The upshot: we can conclude that the scientific community agrees that the water vapor feedback is positive. Arguments to the contrary are distortions of the science.
[Note: I recognize that these WOS-type analyses have their pitfalls. But as a scientist who publishes in this area and who has read (I think) all of the relevant literature, I can attest to the fact that this WOS analysis has got it right: there is just about zero evidence to support a negative water vapor feedback.]