FIGURE 3.5 Model of surface air temperature averaged over an area spanning from southwestern Greenland to Iceland found in a 15,000-year control integration of a coupled atmosphere-ocean Global Circulation Model. The top shows the entire series, while the bottom highlights a 500-year period that contains an abrupt cooling event. (Hall and Stouffer, 2001).

enhanced the East Greenland Current, and brought freshwater into the northern North Atlantic. Only then did the THC respond with a weakening that amplified the initial perturbation. In this example, changes in the THC were a response to rather than a cause of the cooling event. Although the result is intriguing, its robustness is poorly understood; the lack of intermediate-size events during the long integration is a cause for concern.

Even if one accepts that the THC played a central role in past abrupt climate change, the above examples of recent studies underline how our understanding of past rapid changes is still limited. In particular, the question of what triggered abrupt climate change, such as the Younger Dryas, has not yet been answered, unless we assume that the climate system essentially exhibits spontaneous regime transitions of the THC. In the following, we attempt to break down that overarching question into smaller, more manageable topics. The problem contains some fundamental aspects, as epitomized by the two different hysteresis structures in Plate 6, indicating that the whole spectrum of approaches—theory, modeling, and observations—will be needed.