the temp decline translated into a decline in relative humidity should indicate the water vapour feedback is, in fact, higher than projected.

Not really. The effect of additional cloud formation due to higher RH is generally believed to provide a negative feedback, and must be factored in accurately before a statement like this can be made.

Global warming theory says that relative humidity should remain broadly constant. (The models do show some changes in the various levels of the atmosphere but overall it is supposed to be very close to constant.)

And yes, the fact that the temp decline translated into a decline in relative humidity should indicate the water vapour feedback is, in fact, higher than projected. None of the models would work if there was a positive water vapour feedback. The Earth’s climate would just be runaway greenhouses and runaway ice planets if that were the case.

Other studies have also shown wide-ranging results so I think this one just fits into the mode of relative humidity varies somewhat and we still don’t know what causes that variation. It might be constant or it might vary slightly with temperature as a negative feedback.

I guess Antarctica would be a good place to test the theory. There should be no water vapour there at all given the average temperature is below the -18C that Earth would be without a greenhouse effect.

This subject brings a feature of blogs that I find frustrating and that is to properly learn I need someone to summarize what has been posted in many dispersed locations on the blog. When someone does it (rarely), I find it really helps and particularly when it brings some subtle part of the topic to the fore that can get lost in individual posts.

I appreciate the compliment, but I’m not sure that I can live up to the billing. Off the top of my head, band models like MODTRAN do well in the troposphere and less well at higher altitudes where the lines narrow substantially. OTOH, the troposphere is probably more important to the greenhouse effect. AFAIK, the GCM’s don’t even use band models because they still have too much computational overhead. There was some study a while back that found that the radiation parameterizations in some of the models were way off, yet they still seemed to be able to tune them to hindcast the GMST.

Re: John Lang (#200),

I think the argument that Dessler would make is that if the RH declines with a temperature drop, then it would go up with a temperature increase so rather than constant relative humidity, which would be bad enough, you would get increasing RH with temperature, which would be much worse. There would be additional feedback if the altitude of the tropopause increased with increasing temperature. But it’s still just one year’s data and the change in RH may have been the result of some other factor than temperature.

The results are not quite what is being portrayed.

The main result was measuring the change in water vapour across all levels of the troposphere as temperatures changed in the winter (DJF) of 2007 to the winter of 2008. Temperatures declined over this period by about 0.4C.

The results showed there was a decline in relative humidity of 1.5% (percentage points that is) in the very lower troposphere and an increase of 1.5% in the very upper troposphere. The middle was constant.

Now there is much more water vapour in the lower troposphere than the upper so the study really found a decline in the overall relative humidity when there was a (historically) large decline in temperature.

In my mind this does not at all prove that relative humidity stays broadly constant with changes in temperature. If anything the result shows that relative humidity has lots of variation.

One should also note that thanks are provided at the end of the paper to gavin and the usual realclimate suspects and to NASA for funding the study.

Dessler previously undertook a longer study (covering less of the troposphere) which showed that relative humidity was not keeping up with the temperature increases (so I suppose he has now been bought off.)

DeWitt, I judge that you are the most qualified poster I have read at CA (basic knowledge plus the ability to articulate it) to give an engineering exposition on the effects of 2XCO2 on temperature. If you are not up to an exposition, I would be interested in general comments on items such as how well can a band treatment approximate a line by line calculation and other limitations in data availablity and computational complexities.

Climate models have estimated the strength of water vapor feedback, but until now the record of water vapor data was not sophisticated enough to provide a comprehensive view of at how water vapor responds to changes in Earth’s surface temperature.

Hmm, then why isn’t it getting hotter? And why don’t the tropics ever get over 33 C?

Yes, I note there is mention of ‘until now the record of water vapor data was not sophisticated enough’. Oh yes, that is your comment.

Two questions: Isn’t Roy Spencer also looking at this water vapor data and finding a different result? Doesn’t a ‘spiraling cycle’ imply the sort of feedback that leads to dangerous instability of climate, something not noted in the past from water vapor?
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“The answer can be found by estimating”. Hmmmm. How about measuring instead of estimating?
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Important new release from NASA at http://earthobservatory.nasa.gov/Newsroom/view.php?id=35952 :

Andrew Dessler and colleagues from Texas A&M University in College Station confirmed that the heat-amplifying effect of water vapor is potent enough to double the climate warming caused by increased levels of carbon dioxide in the atmosphere…..

The answer can be found by estimating the magnitude of water vapor feedback. Increasing water vapor leads to warmer temperatures, which causes more water vapor to be absorbed into the air. Warming and water absorption increase in a spiraling cycle.

Water vapor feedback can also amplify the warming effect of other greenhouse gases, such that the warming brought about by increased carbon dioxide allows more water vapor to enter the atmosphere.

“The difference in an atmosphere with a strong water vapor feedback and one with a weak feedback is enormous,” Dessler said.

Climate models have estimated the strength of water vapor feedback, but until now the record of water vapor data was not sophisticated enough to provide a comprehensive view of at how water vapor responds to changes in Earth’s surface temperature.