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Thursday, March 12, 2009


Global warming: is relative humidity increasing or decreasing?

posted by Aziz P. at Thursday, March 12, 2009 permalink View blog reactions
I am not a global warming expert but I do follow the debates with reasonable attention to detail. I think my scientific training provides me the tools to assess the scientific claims to a reasonable degree even though I lack the training to go much deeper than abstracts, figures, and conclusions of papers on the topic.

Steve McIntyre at Climate Audit (the global warming skeptic's counter-site to the Real Climate project) points to a new paper that purports to show that the relative humidity of the upper atmosphere is declining rather than increasing as one might expect based on climate models. The implication pertains to how much feedback there is with increase in atmospheric CO2 - a positive feedback (increasing relative humidity) would imply greater temperature change per doubling of CO2 concentration increase, ie making global warming worse, whereas a negative feedback would imply the opposite, that global warming is less of a threat. Note that the debate is only about the magnitude of the increase in temperature change with increased CO2, not a repudiation of global warming outright. In fact the whole issue of relative humidity essentially validates the main assumption of global warming as caused by CO2 in the first place.

Still, the paper is getting attention from committed GW skeptics like Dave Price at Dean's World, who interprets the results as a "collapse" of a "central pillar" of global warming theory. Anthony Watts, another GW skeptic, also suggests that the paper's findings "could be significant". However, a reading of the paper's abstract and conclusions show a far milder assertion, related to data collection protocols using balloons in the atmosphere, rather than any such monumental toppling of the central GW dogmas.

The paper makes the following claims (see the abstract reprinted at Climate Audit):

- relative humidity has been thought to be increasing with global temperature (ie, positive feedback)
- in the upper atmosphere, according to balloon data, the paper reports that relative humidity has actually been decreasing (ie, negative feedback, with the warming trend that the authors explicitly acknowledge exists)

the authors themselves do acknowledge in the abstract itself that balloon data must be "treated with great caution, particularly at altitudes above the 500 hPa pressure level", though Anthony Watts does not mention this admission in his summary. The abstract only reports a negative feedback trend above 850 hPa for the tropics and southern midlatitudes, and above 600 hPa for the northern midlatitudes. The abstract goes on to explicitly state that the humidity trend is "significantly positive below 850 hPa in all three zones" - which is where the vast bulk of the water vapor in the atmosphere resides.

the main conclusion of the paper is not as bold as Dave implies - they merely state that the data suggests that there might need to be a review of data collection protocols using balloons, since the data is so iffy indeed and at odds with satellite measurements (which contrary to Watts' assertion, are probably more accurate for annual average measurements than spot measurements by balloon. see notes below for an analogy as to why). Here's the first paragraph of the conclusion proper from the article full text (courtesy of Watts):

It is of course possible that the observed humidity trends from the NCEP data are simply the result of problems with the instrumentation and operation of the global radiosonde network from which the data are derived. The potential for such problems needs to be examined in detail in an effort rather similar to the effort now devoted to abstracting real surface temperature trends from the face-value data from individual stations of the international meteorological networks. As recommended by Elliot and Gaffen (1991) in their original study of the US radiosonde network, there needs to be a detailed examination of how radiosonde instrumentation, operating procedures, and recording practices of all nations have changed over the years and of how these changes may have impacted on the humidity data.

emphasis mine. The conclusion goes on to state that "trends of water vapor shown by the NCEP data for the middle and upper troposphere should not be “written off” simply on the basis that they are not supported by climate models". I agree, but they should heed their own advice when they state a few sentences later, "it is important that as much information as possible be retrieved from within the “noise” of the potential errors." If the balloon data is inherently noisy (partly due to collection protocols that the authors themselves acknowledge must be refined and reviewed) then making claims about the trends of relative humidity from that data alone is essentially treating noise as data. In light of the problems with the balloon data - again, explicitly acknowledged by the authors in their own abstract and conclusion - it's perfectly reasonable that review papers continue to assert that there is a scientific consensus that water vapor provides a positive feedback.

The authors have made a good case for a review of balloon data collection protocols, but have made no claims on whether the data we have from balloons so far is of any value. The "central pillar" (not really) of GW theory has certainly not been "collapsed" by this paper in any sense.

Incidentally there is a lot more information about humidity and water vapor effects available from other science bloggers. See:

As far as the meta arguments about science go, note that the paper in question was accepted but according to Watts is being "ignored". As far as i am aware, publication is the exact opposite of being "ignored". One of the authors of the paper in question points to a single vitriolic and political comment by a reviewer as evidence that the paper's refusal at a more prestigious journal is evidence of a deeper agenda at work; he may not be aware that reviewers are also human, that sometimes prestigious journals refuse perfectly valid papers for trivial reasons. Since they haven't released the full text of the reviewers' comments, we are not able to discern whether there were perhaps more substantive reasons for the refusal of the paper at the big journal.

At any rate the simple fact of the paper being accepted (granted, by a "lesser" journal than the author's ego would prefer) puts the truth to the lie that anti-GW skeptics are being shut out by the all-powerful, money-grubbing GW consensus conspiracy.

finally, one note regarding satellite vs balloon measurements for long term averaged regional data. Balloons provide a single spot measure whereas satellites can integrate over a far larger area. The former are very vulnerable to local variation and noise whereas the latter can average these out by simple virtue of broader field. It's like trying to assess whether a patient has fatty infiltrating liver disease by taking a liver biopsy: you take a tissue sample over here, but what if the diseased tissue is over there? if you image the whole liver at once (analogous to satellites), then you get the broad picture and can make a global (liver) assessment more accurately and safe from measurement variation.



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Nation-Building was founded by Aziz Poonawalla in August 2002 under the name Dean Nation. Dean Nation was the very first weblog devoted to a presidential candidate, Howard Dean, and became the vanguard of the Dean netroot phenomenon, raising over $40,000 for the Dean campaign, pioneering the use of Meetup, and enjoying the attention of the campaign itself, with Joe Trippi a regular reader (and sometime commentor). Howard Dean himself even left a comment once. Dean Nation was a group weblog effort and counts among its alumni many of the progressive blogsphere's leading talent including Jerome Armstrong, Matthew Yglesias, and Ezra Klein. After the election in 2004, the blog refocused onto the theme of "purple politics", formally changing its name to Nation-Building in June 2006. The primary focus of the blog is on articulating purple-state policy at home and pragmatic liberal interventionism abroad.