Hadley circulation, which leads to some temperature change in the northern half of the tropics, and more important precipitation changes.

There have been a number of simulations of the atmospheric response to THC shutdown or to directly imposed North Atlantic sea-surface temperature perturbations. Fawcett et al. (1997) eliminated Nordic sea oceanic heat transport in the GENESIS model and found localized reductions in surface air temperature of as much as 24°C. Even with such an extreme cooling, the temperature perturbation is very localized. It amounts to only 2.8°C over the summit of Greenland versus about 8°C observed (Severinghaus et al., 1998) and 2°C or less over much of Europe, underestimating some changes based on proxy records. As summarized in Ágústsdóttir et al. (1999) and Fawcett et al. (1997), these experiments matched many aspects of changes reconstructed from proxy records (including changes in seasonality of precipitation in central Greenland and other such details) but generally underestimated the magnitude of reconstructed changes except close to the North Atlantic. It is difficult to determine how the oceanic heat transport involved in these experiments compares with the heat carried by the real THC.

Manabe and Stouffer (1988, 2000) carried out experiments with a coupled atmosphere-ocean model in which the THC was largely suppressed by a massive artificial injection of freshwater. They found a more moderate Atlantic surface cooling of 6°C with cooling of 1-4°C over Europe and the greatest coolings confined primarily to coastal areas. The atmospheric-response experiments of Hostetler et al. (1999), forced by imposed sea-surface temperature patterns, yield a quite different impression, as the North Atlantic cooling leads to temperature reductions in excess of 4°C over all of Europe and much of Asia. The general locality of atmospheric response in those simulations does not by any means disprove the THC theory of the Younger Dryas and Dansgaard/Oeschger events. Current atmospheric models might be missing some crucial physical feedbacks that allow the real atmosphere to exhibit such large and widespread responses to THC changes. This is an unsettling possibility, in that it suggests that models could also fail to anticipate the threat of surprising and abrupt changes, which might occur in connection with global warming, as indicated in Plate 7 and discussed extensively in Chapter 4.

Ocean dynamics could help to extend northern extratropical influences to the tropics and to the Southern Hemisphere, and indeed ocean circulation experiments show some global changes in response to North Atlantic freshwater pulses. A full treatment of the role of the oceans must be carried

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