Is the atmosphere drying up?

A recent set of posts at Anthony Watt’s blog, particularly this one has sparked some interest over the internet as of late. From a quick glance, it looks like negative trends in specific humidity over the last half a century. Readers were quick to pick up on the connection to water vapor feedback, which is expected to at least double the sensitivity of climate to external perturbations (e.g., by human released CO2).

The accepted theory of water vapor feedback in climate change is that in a warming world the global relative humidity will not change much, and an increase in temperature with little change in relative humidity means an increase in specific humidity. So any increase in temperature caused by something (maybe CO2, increased solar irradiance, etc) increases the saturation vapor pressure which allows water vapor concentration to go up, which further amplifies the greenhouse effect since H2O is also a strong greenhouse gas being the most important gaseous source of infrared opacity in the atmosphere. Most of the water vapor feedback relevant for global warming occurs in the upper layers of the atmosphere, which are considerably drier than the boundary layer. Only about 10% of the water vapor feedback is from below 800 mb.

So what do the plots in Anthony Watts blog show? Well, they show specific humidity as a function of time, with multiple graphs corresponding to different altitudes in the atmosphere. For example, at 700 mb

600 mb

and so on…

The figures come from plotting annual specific humidity at global coverage from this site. Now where is this data coming from and how exactly does it relate to reality? The constructed timeseries is from the NCEP Reanalysis Dataset. The goal of the NCEP/NCAR reanalysis project is to “produce new atmospheric analyses using historical data (1948 onwards) and as well to produce analyses of the current atmospheric state.” Application has been used to process multi-decadal sequences of past observations using modern data assimilation techniques, which brings its share of observational and model problems (coverages and bias varying over time, biases in background forecasts which are combined with observations over a short time period to get an analysis of the state of the atmosphere, etc). Radiosondes provide water vapor information in the atmosphere since the 1940’s, but earlier products had a lot of biases, and since then changes in instrumentation have taken place which may lead to discontinuities in the data, and problems arose especially for upper atmospheric data. Reanalysis products contain discontinuities from changes in observational data available, changes in error characteristics of data, changes in sampling, better assimilation techniques, and other things. These make them virtually unusable for examining trends and low-frequency variability. Reanalysis products are not reliable sources for trends on water vapor, precipitation, clouds, etc though estimates other climatic variables like temperature do well after the satellite era. Here is a quote from Soden et al (2005),

“Although an international network of weather
balloons has carried water vapor sensors for
more than half a century, changes in instrumentation
and poor calibration make such
sensors unsuitable for detecting trends in
upper tropospheric water vapor (27). Similarly,
global reanalysis products also suffer
from spurious variability and trends related to
changes in data quality and data coverage (24).”

Satellites are a nice tool though. By measuring the upwelling radiance in different spectral bands which water vapor absorbs, you can obtain measurements of water vapor concentrations in various parts of the atmosphere. The Soden paper, for example, uses satellites to detect upper tropospheric moistening from 1982 to 2004 as evidenced by changing emission levels in T12 (the High Resolution Infrared radiation sounder channel 12) because of increased opacity to water vapor. As the IPCC AR4 report discusses in Chapter 3, interannual variability is not often captured well by reanalysis techniques, even after the satellite era. Probably the best paper discussing upper atmospheric trends is the above paper, and IPCC AR4 (Chapter 3) also summarizes the literature and supports the conclusion of no detectable changes in relative humidity, but trends in increased specific humidity.

Putting aside the data and model conclusions, from a purely theoretical framework, a lack of water vapor response to global warming (especially in the upper troposphere) would mean a very insensitive climate system, as the discussion here (see final figure as well) goes over. The water vapor effect also works in sync with other feedback effects. For example, more water vapor means more warming which means more of an ice-albedo effect, and that in turn means more warming. If climate sensitivity were very low (they would be on the low end of the IPCC scale, or lower if the WV feedback effect was not substantial) then we would not be able to explain the variability over the paleoclimatic record, or the 20th century rise. But as a lot of research has shown (see empirical evidence by James Annan for instance) there is unlikely to be anything wrong with the estimates of 2 to 4.5 C per doubling of CO2. Overall, I’ve not been convinced that there is anything significantly wrong with our understanding of water vapor feedback, or that (as some of Watts’ readers proclaim) that this is some nail in the coffin for AGW.


8 responses to “Is the atmosphere drying up?

  1. When I looked at the NCEP/NCAR reanalysis data, I reached the opposite conclusion.

    Response- I didn’t do any original analysis, and this is not different from IPCC conclusions; I also contacted Brian Soden who seemed convinced that (at least for upper level water vapor, maybe not temperature), the reanalysis data was not reliable. Having him say that in Science, and in informal exchange, suggests to me there may be some issues. The trends in Watts’ plots make no sense.– chris

  2. Hi Chris, (cross post) Thanks for your response. As you seem to have some familiarity with the issue I welcome your contribution and potential resolution of this issue.

    I read your post, but it seems to just restate WG1 Sect. 3.4. Specifically I would welcome clarification of a number of apparent contradictions in the arguments:

    1. “there is very strong reason to suspect that the data is simply not reliable,”
    This doesn’t cut it. What is the confidence interval? Are they any worse than global average surface temperatures that also have changes in instrumentation? These data are averaged over many problematic measurements and still broadly useful.

    2. Sect 3.4 states: “that the available data do not indicate a detectable trend in upper-tropospheric relative humidity”. “available data” also includes radiosonde data. How do you account for what appears to be denying the existence of “inconvenient” data.

    3. Related to the above, the relative humidity trends look much more reliable at all altitudes than specific humidity.

    4. Though I haven’t really nailed down the levels of the atmosphere, without wanting to get into ambiguity over that, the levels they model at 200-300hPa are the same levels that models are supposed to exhibit maximum EGE warming. See the figure from Douglass above. Therefore this is exactly the region that should be identified for evaluating the accuracy of GCM modeling of enhanced greenhouse due to feedbacks.

    5. The spectroscopic effects are very small, and the stats suspect. See my post on the Harries paper here and John Daly extensive review I haven’t read Sodon’s paper yet.

    Sorry to seem like I am hammering on this. I really would appreciate clarification, and we try to be civil around here. Regards.

  3. For any of my readers who may be curious by the above, we were discussing the Minschwaner and Dessler paper on upper atmospheric moistening in the tropics at his site. The paper only treats a very small portion of the overall feedback, and Soden et al. provides a more extensive treatment. Concerning further discussion on data quality, these issues are discussed much in AR4 Chapter 3 and in the supplementary info.

  4. Hi Chris,

    This is a late post, hope you see it. You’ve answered my questions on some other blogs and I have a conceptual question for you.

    I’ve recently read that despite an indicated cooling of the mean ocean temperatures since ~2003, because sea levels have actually been rising it has been hypothesized that the measurements are inaccurate and the water is warming and therefore expanding. That makes sense.

    But I see now that according to sea levels have been falling for approx. 2 years. [Way to short for a trend and I won’t put any deeper thought in to it, but… ] It’s always said that there is “warming in the pipeline”.

    Well surface temps have been flat or slightly cooling for some period of time, say 8 years. Ocean’s have been reported to be cooling for say 5 years. Now sea level has been reported as dropping for 2 years. Logically doesnt this all make sense and could it not paint the picture that the climate is cooling and there isn’t more heat in the “pipeline”?

    If the energy imbalance peaked in the 1990’s and the surface temperatures started to drop, ocean temps lagged and peaked a few years later, then the ice melt input fell behind the volume from thermal contraction so the sea level started to drop. It makes a logical picture no?

    Response- Check this out. – C

  5. I remember reading in Chirs Mooney’s book “Storm World” that ocean temperatures can be altered by hurricanes which we all know, and it alluded to the fact that 2005 had so many strong storms churnning the ocean that it literally “cooled off the ocean” by mixing the upper layers of the warm ocean water with the cooler water before it. We’ve seen before how one hurricane or strong TS can inhibit the development of another trailing behind it even if atmospheric conditions are favorable. I forget his name, but there’s a scientist (its on the tip of my tonque) who believes hurricanes play a bigger role in ocean heat transfer then we may realize.

    Hey Chris! How’s it going! 🙂 Phys 2 is annoying. Good luck! 🙂

    Response- Hi Chris. You might be referring to this piece by Robert Korty, Kerry Emanuel, and Jeffrey R. Scott

    Clearly hurricanes play a role in heat transport from the equator to the poles, so changing hurricane intensity/frequency may play a role in the temperature contrast with latitude. Interesting hypothesis, and I’m glad to see the paper touched upon the implications to paleoclimate (are you familiar with the pole-equator temperature gradient issue during the Cretaceous and other hothouse climates?) — chris

  6. “”(are you familiar with the pole-equator temperature gradient issue during the Cretaceous and other hothouse climates?) — chris””


    Kerry Emanuel. It was on the tip of my damn tongue.

  7. You say: “For example, more water vapor means more warming which means more of an ice-albedo effect, and that in turn means more warming. If climate sensitivity were very low (they would be on the low end of the IPCC scale, or lower if the WV feedback effect was not substantial) then we would not be able to explain the variability over the paleoclimatic record, or the 20th century rise.”

    So in addition to your very odd assertion that we can safely ignore the observational data in favour of theory – you made one mistake and 2 leaps of faith:
    1. It’s well known that there is not much actual water vapour over the Arctic. Most water vapour feedback is expected over the tropics. Any water vapour in the arctic turns to snow, giving negative ice-albedo feedback – the opposite of what you say. Ice albedo would give positive feedback via warmer seas but that is a different argument.
    2. The idea that the paleo record cannot be explained assumes the very simplistic theory that climate operates like an electrical circuit is correct. A simple correction to the theory can easily be made to explain whatever you fancy.
    3. CO2 plus water vapour feedback cannot explain the 20th century warming without adding aerosols and natural variations. And even then it isn’t a great fit. But I think you’ll find that Roy Spencer has a new theory that fits 20th century warming quite well.

    Response– James, James…would you believe anything you hear?- c

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