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Advancing the Science of Climate Change
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.
IMPACTS OF SEA LEVEL RISE AND OTHER CLIMATECHANGES ON COASTAL ENVIRONMENTS
Coastal areas are among the most densely populated regions of the United States, and around the world. In 2003, 53 percent of the U.S. population lived in (1) counties with at least 15 percent of its total land area located within the nation’s coastal watershed or (2) a county with a portion of its land that accounts for at least 15 percent of a coastal cataloging unit1 bordering the ocean and the Great Lakes, and 23 of the 25 most densely populated counties in 2003 were coastal counties (Crossett et al., 2004). Considering only coastal counties that border the ocean or contain flood zones with at least a 1 percent chance every year of experiencing flooding from coastal storms and being impacted by wave action, the coastal population, excluding the Great Lakes counties, was 85,640,000, or 30 percent of the total U.S.
The National Oceanic and Atmospheric Administration defines a coastal cataloging unit as ‘‘a drainage basin that falls entirely within or straddles an Estuarine Drainage Area or Coastal Drainage Area” (Crowell et al., 2007).