More evidence of a positive WV feedback?

Question

I know I've already asked several questions on the water vapor feedback before, and I've personally found the evidence of a positive WV feedback quite compelling for some time now, but a paper was recently accepted to JGR atmos which effectively rebuts the Paltridge 2009 paper on humidity trends from the NCEP reanalysis:http://www.gerkynet.com/meteo/paltrigde08.pdf

P09 purported to find negative trends in specific humidity above 850 hPa, meaning a negative water vapor feedback. However, in their new paper "Trends in tropospheric humidity from reanalysis systems", Dessler et al. study other, more modern reanalyses, and discuss the many issues with the NCEP reanalysis data. They conclude:

"P09 calculated trends of specific humidity with time over the past few decades in the NCEP/NCAR reanalysis and found that the specific humidity, in particular in the tropical mid and upper troposphere, was decreasing.  They concluded that this potentially cast doubt on the general consensus that the global water vapor feedback was strongly positive [e.g., Dessler and Sherwood, 2009]. We have extended the analysis of P09 by addressing two crucial issues, namely whether other reanalyses reproduce this and what time scale of climate fluctuation is associated with the negative water vapor trends. We have analyzed five different reanalyses, including the NCEP/NCAR 
reanalysis.  Rather than calculate trends with time, as P09 did, we instead regressed atmospheric humidity against surface temperatures for the tropics and mid-latitudes.  In response to short time-scale climate variations (e.g., ENSO cycles), there is good agreement among the reanalyses on the connection between atmospheric water vapor and surface temperature: specific humidity increases with increasing surface temperature in the tropical mid and upper troposphere, as well as almost everywhere else. This is in good agreement with both theory and observation.   
The picture is different when long-term climate variations are considered.  The NCEP/NCAR reanalysis shows decreases in specific humidity in the tropical mid and upper troposphere with increasing tropical surface temperature.  Such behavior implies that the water vapor feedback in response to long-term climate fluctuations would be negative, and would therefore have a different sign depending on the time-scale of the climate variation.  No theory or model supports this, nor do analyses of long-term water vapor measurements or paleoclimate data.   
In addition, the other reanayses, including the newest reanalyses (ERA-interim and MERRA), which were specifically designed to better reproduce long-term trends, do not manifest this behavior.  Finally, we pointed out that the NCEP/NCAR reanalysis contains large biases in the tropical mid and upper tropospheric specific humidity, and does not reproduce the moistening of the tropical upper troposphere during the strong 1998 El Nino, further casting doubt on that reanalysis’ water vapor fields in that region. 
Based on the available evidence, it is our judgment that negative trends in the tropical mid and upper troposphere in response to long-term climate change are spurious.  This is clearly the most parsimonious explanation, and it is in accord with virtually all of the independent lines of evidence (models, observations, theory, newer reanalyses)."

Add this to the studies using satellite data; e.g.
Soden 2005: http://www.gfy.ku.dk/~kaas/forc&feedb2008/Articles/Soden.pdf
Dessler and Zhang 2008: http://www.agu.org/pubs/crossref/2008/2008GL035333.shtml
And it looks like the case for a strong and positive water vapor feedback is becoming harder and harder to deny.

Thoughts? Any other useful references on the topic?

Dana1981 · Accepted Answer

I have the same reaction as Paul.  A negative water vapor feedback seems completely implausible, and would make past climate changes very hard to explain.  Dessler is a pretty prominent atmospheric scientist who has done some good work in the past.  I'd like to read the paper if you can send me a copy - my username @ yahoo.com.

In response to David's answer, I also find any study which puts climate sensitivity at under 1.5°C for a doubling of CO2 completely implausible for the same reasons, let alone 0.6°C, which frankly is a patently absurd result.  0.6°C means a strongly net negative feedback, which again runs contrary to past climate changes.
http://www.greenoptions.com/wiki/climate-sensitivity

*edit* thanks for the paper.  It looks like a very thorough study, examining 5 different reanalyses of the data.  I thought the final point in their abstract was an important one:

"And finally, we point out that there exists no theoretical support for having a positive short-term water vapor feedback and a negative long-term one."

In other words, Paltridge 2009 is not only the outlier, but its results have no physical explanation.  I think the obvious conclusion for a truly skeptical person to arrive at is that the Paltridge result was indeed spurious, as Dessler concludes.

David · Answer

Water vapor in the form of increased specific humidity should be a positive feedback.

Water vapor in the form of increased cloud cover will more likely be a negative feedback. Although some types of high clouds and nighttime cloud cover would be a positive feedback.

Paltridge 2009 found that the radiosonde data from 1948-2008 showed a steady decline in relative humidity from 300-700 mb. If the Earth was warming over that period and water vapor was a net positive feedback, relative humidity should not have been declining. At the very least, it should have remained flat while specific humidity was rising.

Spencer & Braswell. 2010 find that there is no way to attribute specific feedback mechanisms; but that the only clear feedback signal is strongly negative...

<QUOTE>
As we show in the new paper, the only clear signal of feedback we ever find in the global average satellite data is strongly negative, around 6 Watts per sq. meter per degree C. If this was the feedback operating on the long-term warming from increasing CO2, it would result in only 0.6 deg. C of warming from 2XCO2. (Since we have already experienced this level of warming, it raises the issue of whether some portion — maybe even a majority — of past warming is from natural, rather than anthropogenic, causes.)

--Roy Spencer
<END QUOTE>

pegminer · Answer

That result would seem to agree with Reitan's 1963 paper "Surface Dew Point and Water Vapor Aloft". 

http://journals.ametsoc.org/doi/abs/10.1175/1520-0450%281963%29002%3C0776%3ASDPAWV%3E2.0.CO%3B2

Hopefully his paper is in their references.

EDIT: I just wanted to correct a confusion of David's, while clouds have water vapor in them, the defining quality of clouds is liquid or frozen water (not water vapor).

Anonymous · Answer

James Hansen was unsure of the feedback of water vapor. Depending on the cloud types, it can be positive or negative.

flossie · Answer

A NEW report suggests that it is water vapour, not CO2, that is driving global warming. Since the late 1940s people here have had access to domestic washing machines. Imagine how much laundry these machines do, and compare it to the amount that was done by laundering using traditional methods. Nobody changed their clothes every day as we do now. Wet washing is dried, either by hanging out or in the tumble drier. Trillion of tons of fresh water is released into the atmosphere every week; it must fall as rain somewhere. Excess moisture also changes the way winds blow and air currents circulate around the globe. The more the wind blows, the warmer it becomes.
That's the enemy, there's your problem: mum doing the weekly wash!

More evidence of a positive WV feedback?

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