In Which The RealClimate Scientists Give William Gray A Right Smarte Flogging:
The heart of the problem with Gray's new version of the THC story is that he labors under the misconception that the THC primarily upwells in the tropics, so that any reduction in the THC cools the North Atlantic but warms the tropics. This conception is at least 50 years out of date. The tropical upwelling is a shallow wind-driven cell that does not connect to the THC. It is almost impossible for cold deep water to upwell in the tropics, because it takes too much energy to bring it up; the main THC connection is with the Southern Ocean, as described by Marotzke and references therein (for more general background, see also en.wikipedia.org/wiki/Thermohaline_circulation). There are only a few very limited regions where moderately deep water can upwell in the Tropics. Simulations by Vellinga and Wood (Climatic Change, 2002) in fact show that a THC shutdown causes a cooling right into the Northern Subtropical Atlantic (the birthplace of hurricanes), and in fact only very weak warming in a few spots elsewhere in the tropics. On a longer time scale, the classic study of Manabe and Stouffer (Paleoceanography 1997) shows virtually no impact of THC shutdown in the tropics, but a considerable remote impact in the Southern Ocean.
No doubt, Gray would object that these are only models, but why should we believe that Gray's drawing of circles and arrows on a map yields a better prediction than a simulation embodying the best of what we know about the underlying physics?(...)
Gray's grand answer to the riddle of global warming is evaporation, presumably modulated by changes in the THC.
Again, Gray simply doesn't seem to understand energy conservation. Evaporation does not create heat; it does not add any heat to the climate system or take it away. It is an energy transfer that moves heat from a moist surface (like the ocean's) into the atmosphere. That severely constrains what evaporation changes can do to climate. In contrast, changes in CO2 concentration affect the top of atmosphere radiation budget directly, and change the rate at which the whole climate system loses energy.
A more serious problem is that Gray doesn't even understand that the greenhouse effect works primarily through the effect of greenhouse gases on the top of atmosphere radiation budget, and only very indirectly through the surface budget (as explained in A busy week for water vapor). This compromises almost all of his analysis.
(...)
By the way, Gray discounts water vapor feedback, based on what seems to be a gut feeling on weather systems, plus some unspecified analysis of the NCEP reanalysis dataset (which is completely unsuitable for studying trends in mid tropospheric water vapor); more reliable satellite based studies (e.g. Soden's study described in A busy week for water vapor ) support a positive water vapor feedback, and even Lindzen seems to be no longer arguing against this feedback.
http://www.realclimate.org/index.php/archives/2006/04/gray-on-agw/#more-295
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