the see-saw relation between Greenland and Antarctic temperatures on millennial scales (Blunier and Brook, 2001; see also Plate 2), which suggests that reduction in heat transport to the north allowed that heat to remain in the south.

Those and other considerations focus attention on changes in the THC as one cause of abrupt climate change. However, additional processes presumably were active in the past abrupt changes exemplified by the Younger Dryas, as indicated by the difficulty of fully explaining the paleoclimatic data on the basis of the single mechanism of North Atlantic THC changes. Therefore, the ocean’s role in climate is developed more fully in the following.

Water has enormous heat capacity—oceans typically store 10-100 times more heat than equivalent land surfaces over seasonal time scales, and the solar input to the ocean surface for a year would warm the upper kilometer only 1 degree—so the oceans exert a profound influence on climate through their ability to transport heat from one location to another and their ability to sequester heat away from the surface. The deep ocean is a worldwide repository of extremely cold water from the polar regions. If much of this water were brought to the surface in temperate or tropical regions, it could cause substantial cooling that, although transient, could last for centuries. It is not easy to bring cold water to the surface against a stable gradient, though, and this can happen only in special circumstances. Such localized change could, however, have a wider impact through atmospheric teleconnections. Fluctuations in ocean heat transport can also affect climate; for example, an increase in equator-to-pole heat transport would warm the polar regions (melting ice) and cool the tropics.

The implications of fluctuation in heat transport by the Atlantic THC have received particular attention, especially as a mediator of Younger Dryas and Dansgaard/Oeschger abrupt change. Deep water forms only in the North Atlantic and around the periphery of Antarctica, where extremely cold, dense waters occur. There is no deep-water formation in the North Pacific, because the salinity is too low to allow high enough density to drive deep convection, despite the low temperatures. By analogy, change in the freshwater balance of the North Atlantic, which might be caused by glacial discharge or warming of the planet through increases in carbon dioxide, potentially can act as a trigger to turn the THC on or off. In contrast, it is not thought that future climate change could turn on deep-water formation in the North Pacific, although there is evidence that at times during ice ages



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