As noted above, sea level rise will not be uniform across the globe. Regional variations in the rate of sea level rise occur for a number of reasons. Some coasts are still adjusting to the disappearance of glaciers—the weight of glacial ice pushed them down, and they are still rising in response to the loss of ice. In other regions, coasts may be 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.
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