subsiding because of distant glacial rebound or subsurface fluid withdrawal due to water, oil, or gas extraction. Regional variation in sea level rise rates can also stem from changes in the Earth’s rate of spin as water is redistributed from the poles as high-latitude ice melts. Several studies indicate that sea level rise will be particularly problematic for both coasts of the United States as a result of the altered global mass distribution; they may experience 20 percent greater sea level rise than the global average (Bamber et al., 2009; Mitrovica et al., 2001). Differing spatial patterns in sea level trends have already been observed with satellite altimetry (Wunsch et al., 2007).

Changes in the intensity of ocean currents could also produce regional variations in sea level rise. For example, Yin et al. (2009) suggest that a warming-induced slowdown of the Atlantic meridional overturning circulation would contribute to a 6- to 8-inch (15- to 20-centimeter) additional rise in sea level for New York and Boston. However, such changes in the ocean circulation are highly uncertain, since they depend on poorly known parameterizations of vertical mixing in ocean models. Other studies suggest that an intensification, rather than a slowdown, of the overturning circulation with global warming is possible (Huang, 1999; Nilsson et al., 2003), in which case sea levels would fall on the U.S. east coast. A critical factor needed to resolve these disparate projections is a better understanding of vertical mixing processes in the ocean, which are sensitive to changing stratification and govern the absorption of heat by the ocean at all latitudes.

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