Gray and Muddy Thinking about Global Warming

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 This conception is at least 50 years out of date.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?

Note that Gray does not merely claim that THC changes are responsible for the observed hurricane cycles. He in fact claims that the entire 20th century warming signal is due to a slowdown of the THC, and that CO2 has nothing to do with it. He claims flatly and without supporting evidence that models cannot simulate the THC properly, neglecting the fact that the models employed in the IPCC reports yield a rather wide variety of different possible THC behaviors, and none of them, including ones known to have a sensitive THC, spontaneously generate a warming of the sort Gray claims. Insofar as we can follow Gray’s reasoning, he appears to think of the THC as burying heat in the deep ocean, as if the heat were some kind of solid nuclear waste. Thus, weak THC = less heat removal = warming, in Gray’s world view. In reality, everything known about the physics of the THC’s effect on climate suggests the opposite. For example, Vellinga and Wood find that, owing to certain nonlinearities like sea ice formation, a shutdown of the THC leads to a reduction in the Northern Hemisphere mean temperature, and very little multidecadal scale effect on the Southern Hemisphere mean temperature.

The other reasons Gray thinks that the THC could cause global warming are tied up with a number of additional misconceptions he has about the physics of climate.

Claim: Evaporation changes cause global warming, hurricane cycles, etc.

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.

Let’s start with an atmosphere that is in equilibrium, both at the surface and top-of-atmosphere. Now reduce the evaporation (you could do it by reducing the surface wind). The surface is now receiving more energy than it loses, so it will begin to warm. However, the atmosphere is no longer receiving all the energy it used to obtain from the surface as evaporative heat transfer; hence the atmosphere will begin to cool. This adjustment will continue until balance is restored. The precise way the adjustment is divvied up between atmospheric cooling and surface warming depends on details like the net atmospheric infrared opacity, boundary layer relative humidity,and so forth. However that all shakes out, the net result is nothing at all like the observed pattern of warming, in which both troposphere and surface warm up. This reasoning can be confirmed in the simplest radiative-convective model, of the sort introduced by Manabe and Strickler in the 1960’s.

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