Recent estimates of future sea-level rise based on climate model output (Wigley and Raper, 1992) suggest an increase in global eustatic sea-level of between 15 and 95 cm by 2100, with a "best estimate" of 50 cm (I.P.C.C., 1995). This is more than double the rate of eustatic rise for the past century (Douglas, 1997; Peltier and Jiang, 1997). Thus, sea-level rise will have a large, sustained impact on coastal evolution at the societally-important decadal time scale. For example, Zhang et al. (1997) showed that sea-level rise over the past 80 years at two locations on the U.S. East Coast contributed directly to significant increases in the amount of time the coast is subjected to extreme storm surges. From 1910-1920, the coast near Atlantic City, New Jersey was exposed to anomalously high water levels from extreme storms less than 200 hours per year, whereas during the early 1990's the coast was exposed to high water from storms of the same magnitude 700 to 1200 hours per year. Interestingly, the authors found that although storm surge varied a great deal on annual to decadal scales, there was no long-term trend showing increases in storm intensity or frequency that might account for the increasing anomalously high water levels. Zhang et al. (1997) concluded that the increase in storm surge exposure of the coast was due to sea-level rise of about 30 cm over the 80-year period. This finding suggests that the historical record of sea-level change can be combined with other variables (e.g., elevation, geomorphology, and wave characteristics) to assess the relative coastal vulnerability to future sea-level change.
The prediction of coastal evolution is not straightforward. There is no standard methodology, and even the kinds of data required to make such predictions are the subject of much scientific debate. A number of predictive approaches have been used (National Research Council, 1990), including:
Each of these approaches, however, has its shortcomings or can be shown to be invalid for certain applications (National Research Council, 1990). Similarly, the types of input data required vary widely, and for a given approach (e.g. sediment budget), existing data may be indeterminate or may simply not exist (Klein and Nicholls, 1999). Furthermore, human manipulation of the coast in the form of beach nourishment, construction of seawalls, groins, and jetties, as well as coastal development itself, may dictate federal, state and local priorities for coastal management without proper regard for geologic processes. Thus, the long-term decision to renourish or otherwise engineer a coastline may be the primary determining factor in how that coastal segment evolves.