The same factors that can contribute to accelerated sea level rise over relatively short periods of time could also potentially lead to other abrupt climate changes or "climate surprises" (see Chapter 6). For example, if the Greenland ice sheet were to shrink substantially in a short period of time, freshwater delivery to key deep-water formation regions of the North Atlantic could alter the ocean structure and influence its circulation. Normally, the surface waters of the North Atlantic release large amounts of heat to the atmosphere, thereby becoming sufficiently dense to sink and return southward, making room to be replaced with more warm water from the south. This meridional overturning circulation is important for the oceanic redistribution of heat from the tropics to the Northern Hemisphere; it is confined to the North Atlantic because of its higher salinity and thus greater density than the North Pacific (Haupt and Seidov, 2007).
Compelling evidence has been assembled indicating that rapid freshwater discharges to the North Atlantic due to the breaking of ice dams and drainage of meltwater lakes during the termination of the ice ages caused abrupt circulation changes in the oceans, with significant impacts on regional climate (Boyle and Keigwin, 1987; Lehman and Keigwin, 1992; McManus et al., 2004). The paleoclimate record indicates that the strong meltwater pulses diluted the surface waters of the North Atlantic and rendered them too buoyant to sink, thus shutting down the meridional overturning circulation for centuries at a time (Alley et al., 2003; Broecker, 1987; NRC, 2002a). These shutdowns of the overturning circulation were associated with a dramatic cooling of European climate and also influenced global weather patterns (Vellinga and Wood, 2002). Whether human-caused warming will cause similar abrupt climate changes in the future is an important topic for research (Rahmstorf, 1995) . A freshening of the surface waters of the North Atlantic over the past 50 years has been well documented (Boyer et al., 2005; Curry et al., 2003; Dickson et al., 2002; Levitus, 1989) but it is unclear if climate change will ultimately lead to a gradual slowing or even an acceleration of the meridional overturning circulation ( as discussed above). Many models suggest that some slowing of the meridional overturning circulation will result from the ice melting and increased Arctic river discharges that are already taking place, but these models have poor representation of oceanic mixing processes and coastal freshwater discharges. Thus, while the risk of these and other possible abrupt changes in climate should be taken seriously, much work remains to develop confident projections of future ocean circulation changes resulting from the ongoing freshening of the North Atlantic.
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