Water vapor feedback that affects the top of atmosphere radiation budget

( B. J. Soden, D. L. Jackson, V. Ramaswamy, M. D. Schwarzkopf, X. Huang, Science 310, 841 (2005); October 2005 (10.1126/science.1115602)., subscription required for full text) has been more or less ignored by the media. Soden et al. deal with the aspect of water vapor feedback that affects the top of atmosphere radiation budget. The analysis consists in using various satellite observations to compare the behavior of mid to upper tropospheric water vapor between a general circulation model and reality. The analysis is carried out for the period 1982-2004, corresponding to the period of satellite data availability. The basic technique is the "model to satellite" method, in which the model temperature and humidity are used to directly simulate the brightness of radiation that would be observed by satellites looking at the atmosphere in various wavelength bands. By choosing satellite observations that are sensitive to the higher-altitude water vapor distribution, one can zero in on how well the model is doing in these all-important regions. Because of the relatively short period of the comparison, this exercise should not be regarded as an attempt to detect a trend in atmospheric water vapor and compare it with models. Rather, it is a check on whether the model does the same thing to upper layer water vapor as the real world, under varying year-to-year conditions (which do contain a trend over this period, as well as other things, e.g. El Nino).

By examining infrared satellite data, Soden et al. find that upper-level moisture increases in warmer conditions, in much the same way as predicted by the model. Further, by artificially suppressing moisture changes in the computation of the synthetic satellite data, they decisively reject the hypothesis that the atmospheric upper layer water content stays fixed as temperature changes. Synthetic satellite data computed on the basis of this hypothesis look nothing at all like the real thing. The authors take their analysis even further. Because the radiation measured by the satellites depends both on moisture and temperature, there is the possibility that faults in the climate model's upper level temperature predictions might be leading to spurious agreement with the infrared satellite data. To rule this out, they make use of microwave satellite data that is sensitive to the mid to upper tropospheric temperature, in order to formulate a diagnostic that is primarily sensitive to upper level moisture changes rather than temperature changes. Again, they find that the data demand that the upper troposphere get moister in warmer conditions. They conclude: "Reproduction of the observed radiance record requires a global moistening of the upper troposphere in response to atmospheric warming that is roughly equivalent in magnitude to that predicted under the assumption of constant relative humidity." This is probably the most direct evidence to date that there is nothing terribly wrong about the way general circulation models handle water vapor feedback. This is quite remarkable, given the potential role of small scale cloud processes in moistening the atmosphere. To be sure, the analysis only deals with clear sky regions, but the moisture in these regions originates in the cloudy convective regions, and so it provides a fair test. In any event, within the cloudy regions themselves, the clouds rather than water vapor have the dominant effect on the radiation budget.

There would appear to be less and less room for skeptics to dismiss climate model predictions on the grounds that we aren't sure they do water vapor feedback right. The picture is about to become even clearer, as researchers begin analyzing microwave upper level water vapor data, which will allow the analysis to be taken deeper into the convective, cloudy regions. To be sure, there is still a gap in understanding what the models are actually doing, in that it is far from clear why such complex processes boil down to a simple behavior: that the water vapor over a deep region of the troposphere changes in such a way as to keep relative humidity approximately constant. I have some ideas on this myself, but the general picture is still very much a work in progress. Meanwhile, it becomes increasingly clear that whyever the models do what they do to upper level water vapor, there can't be anything too terribly wrong with what they are doing.