Busy Week for Water Vapor

The accuracy of the media coverage of Phillipona et al. is decidedly mixed. The BBC got the scientific story straight (warming due to water vapor amplifying anthropogenic effects, everything working as it should, no worries about the physics, mate.), but their otherwise sound article was published with the unfortunate sub-header "Water vapour rather than carbon dioxide in the atmosphere is the main reason why Europe’s climate is warming, according to a new study." This gives the casual reader the erroneous impression that the study concludes CO2 is unimportant. It feeds the old, discredited skeptics’ notion that the water vapor greenhouse effect is so dominant that there’s no need to be concerned about CO2. National Geographic is a little breathless: " The latest villain on global warming’s most-wanted list is all wet—and a little surprising. Water vapor, experts say, is the culprit behind Europe’s rapidly rising temperatures." However, they get the basic scientific story straight, quoting Philipona as saying "It is an experiment that clearly shows which factors are driving the higher temperatures. It is not the clouds, not the sun, not the aerosols. It is the increased greenhouse gases and the strong water vapor impact." UPI is probably the worst of the bunch. They state "Swiss scientists say Europe’s recent rapid temperature increase is likely due to an unexpected greenhouse gas: water vapor." Unexpected? If they were readers of RealClimate, they’d know better.

All of this was relatively harmless, but all the coverage missed the boat in the same way. Press reports failed to note that the water vapor feedback discussed in Philipona et al. is not the same water vapor feedback usually discussed in connection with global warming. It is instead a surface water vapor feedback which adds additional surface warming on top of the usual things we talk about. The effect is already incorporated in the climate models used in IPCC forecasts, but the new observational study will be useful as a reality-check.

Phillipona et al. analyzed trends in the energy budget of the Earth’s surface. While this is definitely an aspect of climate change, it comes as a surprise to many that the surface energy budget plays a decidedly secondary role in climate change compared to the top-of-atmosphere energy budget. The fact is, that even if the diligent Swiss authors of this paper had found that increasing CO2 contributed nothing to the changes in the surface budget, this would have in no way contradicted our understanding of the way anthropogenic greenhouse gases influence climate. For the most part, surface temperature changes are determined by perturbations to the top-of-atmosphere budget, and the surface budget is just dragged along, accomodating itself to whatever changes in surface temperature are demanded in order to be able to satisfy the top of atmosphere budget. It is impossible to understand the greenhouse effect without thoroughly understanding this point. Even the authors of Phillipona et al. seem to be a little fuzzy on this matter. They seem to think they are looking at the same water vapor feedback discussed in various review articles on the subject (e.g. Held and Soden (Annu. Rev. Energy Environ., 25, 441– 475. (2000)), Pierrehumbert et al. ("On the Relative Humidity of the Earth’s Atmosphere" in The General Circulation, T. Schneider and A. Sobel, eds. Princeton U. Press 2005,) Pierrehumbert (Subtropical water vapor as a mediator of rapid global climate change. . in Clark PU, Webb RS and Keigwin LD eds. Mechanisms of global change at millennial time scales. American Geophysical Union:Washington, D.C. Geophysical Monograph Series 112, 394 pp1999), and the RealClimate article on the subject). but they are not. I shall try to explain.

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