the intermediate waters of the Northern Hemisphere Pacific were more ventilated than they are now (Behl and Kennett, 1996).

The threshold for deep-water formation cannot be thought of apart from the general circulation of the world ocean, because the density required for North Atlantic surface waters to sink is determined in relation to the “prevailing” deep-water density of the rest of the ocean. This density is determined globally and is intimately linked to processes in the Southern Ocean. Furthermore, the density of surface waters in the North Atlantic is not determined by purely local processes, in as much as the North Atlantic salinity is affected by mixing with transported subtropical Atlantic waters, whose salinity in turn is affected by tropical winds, which can systematically transport moisture out of the Atlantic basin. The freshwater balance of the Atlantic is further affected by melting of glaciers, transport of freshwater by sea ice, and land-surface processes that determine runoff patterns.

Several ocean heat-transport mechanisms other than the THC are in operation today. In particular, wind-driven ocean circulation dominates ocean heat transport in the North Pacific (Bryden et al., 1991) and the Indian Ocean (Lee and Marotzke, 1997). Results from relatively simple models suggest that in the absence of a vigorous THC, wind-driven heat and salt transports increase, making up at least in part for the loss of the THC as a transport agent (Marotzke, 1990; Winton and Sarachik, 1993).

Cryosphere

Land glaciers and sea ice enter into abrupt change mechanisms in many ways. The accumulation of ice on land and the associated ice-albedo feedback are probably too slow to be involved in abrupt climate change. However, because a glacier that is frozen to its substrate can surge if the basal temperature of the ice is raised to the melting point, glacial discharge and decomposition can be rapid (MacAyeal, 1993a,b; Alley and MacAyeal, 1994). Ice-sheet surging certainly would affect sea level, as noted in Chapter 4 with regard to the West Antarctic ice sheet. Surging also may affect the atmospheric flow pattern by changing the elevation of parts of the continental ice sheets (Roe and Lindzen, 2001). Furthermore, rapid glacial discharge can release armadas of icebergs into the ocean, which serve as an important indicator of abrupt climate change; increases in ice-rafted debris are the defining feature of Heinrich events (Broecker, 1994). More importantly, glacial discharge abruptly increases the delivery of freshwater to the ocean (to the North Atlantic, in the case of Heinrich events), with the po-



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