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Other types of islands also may be vulnerable to sea-level rise. In the Chesapeake Bay, several islands populated by a traditional subculture of fishermen are likely to be entirely submerged (Toll et al., 1997). The coast of Prince Edward Island, except for some parts along the Northumberland Strait, is highly erodible because of its bedrock cliffs, sandy barriers, coastal dunes, salt marshes, and intertidal flats. The heart of the island's tourist industry, along the Gulf of St. Lawrence, is likely to experience increased beach erosion, which would threaten shorefront buildings.

Rising sea level would increase flooding and storm damage. Regional climate change could offset or amplify these effects, depending on whether river flows and storm severity increase or decrease.

Changing climate generally is increasing the vulnerability of coastal areas to flooding both because higher sea level raises the flood level from a storm of a given severity and because rainstorms are becoming more severe in many areas. It also is possible that hurricanes could become more intense, thus producing greater storm surges; IPCC (1996) concluded, however, that the science currently is inadequate to state whether or not this is likely. Existing assessments in coastal areas generally focus on the impact of rising sea level.

Because higher sea level provides a higher base for storm surges, a 1-m rise in sea level (for example) would enable a 15-year storm to flood many areas that today are flooded only by a 100-year storm (Kana et al., 1984; Leatherman, 1984). Many coastal areas currently are protected with levees and seawalls. Because these structures have been designed for current sea level, however, higher storm surges might overtop seawalls, and erosion could undermine them from below (National Research Council, 1987). In areas that are drained artificially, such as New Orleans, the increased need for pumping could exceed current pumping capacity (Titus et al., 1987).

The U.S. Federal Emergency Management Agency (FEMA, 1991) has examined the nationwide implications of rising sea level for the National Flood Insurance Program. The study estimated that rises in sea level of 30 cm and 90 cm would increase the size of the 100-year floodplain in the United States from 51,000 km² (19,500 mi₂) in 1990 to 60,000 km² and 70,000 km² (23,000 mi₂ and 27,000 mi₂), respectively. Assuming that current development trends continue, flood damages incurred by a representative property subject to sea-level rise are projected to increase by 36-58% for a 30-cm rise and 102-200% for a 90-cm rise.

Because of its higher elevations, the Canadian coastal zone is less vulnerable to flooding than the U.S. coast. Nevertheless, flooding appears to be a more serious risk to Canada than the loss of land from erosion or inundation. Some communities (e.g., Placentia, Newfoundland) already are vulnerable to flooding during high astronomical tides and storm surges, sometimes exacerbated by high runoff. In Charlottetown, Prince Edward Island, some of the highest-value property in the downtown core and significant parts of the sewage system would experience increased flooding with a 50- to 100-cm rise in sea level. According to Clague (1989), a rise of a few tens of cm would result in flooding of some waterfront homes and port facilities during severe storms in British Columbia, forcing additional expenditures on pumping.

Coastal flooding also is exacerbated by increasing rainfall intensity. Along tidal rivers and in extremely flat areas, floods can be caused by storm surges from the sea or by river surges. Washington, D.C., and nearby Alexandria, Virginia, were flooded twice by Hurricane Fran in 1996: first by a storm surge in the Chesapeake Bay and lower Potomac River, then three days later by the river surge associated with intense precipitation over the upper Potomac River's watershed. Higher sea level and more intense precipitation could combine synergistically to increase flood levels by more than the rise in sea level alone in much of coastal Louisiana and Florida, as well as in inland port cities along major rivers (such as Portland and Philadelphia). The direct effect of higher sea level also could be exacerbated throughout the coastal zone if hurricanes or northeasters become more severe-a possibility that has been suggested but not established (IPCC 1996, WG I).

Rising sea level would increase salinities of estuaries and aquifers, which could impair water supplies, ecosystems, and coastal farmland. As with flooding, regional climate change could offset or amplify these effects, depending on whether river flows increase or decrease.

Rising sea level also enables saltwater to penetrate farther inland and upstream in rivers, bays, wetlands, and aquifers; saltwater intrusion would harm some aquatic plants and animals and threaten human uses of water. Increased drought severity, where it occurs, would further elevate salinity. Increased salinity already has been cited as a factor contributing to reduced oyster harvests in Delaware Bay (Gunter, 1974) and the Chesapeake Bay and as a reason that cypress swamps in Louisiana are becoming open lakes (Louisiana Wetland Protection Panel, 1987).

Higher salinity can impair both surface and groundwater supplies. New York, Philadelphia, and much of California's Central Valley get their water from portions of rivers that are slightly upstream from the point at which the water is salty during droughts. If saltwater is able to reach farther upstream in the future, the existing intakes would draw salty water during droughts.

The aquifers that are most vulnerable to rising sea level are those that are recharged in areas that currently are fresh but could become salty in the future. Residents of Camden and farmers in central New Jersey rely on the Potomac-Raritan- Magothy aquifer, which is recharged by a portion of the Delaware River that is rarely salty even during severe droughts today but would be salty more frequently if sea level were to rise 50-100 cm or droughts were to become more severe (Hull and Titus, 1986). Miami's Biscayne aquifer is similarly vulnerable; the South Florida Water Management District already spends millions of dollars each year to prevent the aquifer from becoming salty (Miller et al., 1992).

A second class of vulnerable aquifers consists of those in barrier islands and other low areas with water tables close to the surface, which could lose their freshwater lens entirely (see IPCC 1990, WG II, Figure 6.3; also Chapter 9 in this report).

Finally, rising sea level tends to make some agricultural lands too saline for cultivation. In areas where shorefront lands are cultivated, the seaward boundary for cultivation often is the point where saltwater from ground and surface waters penetrates inland far enough to prevent crops from growing. As sea level rises, this boundary penetrates inland-often rendering farmland too salty for cultivation long before inundation converts the land to coastal marsh (see, e.g., Toll, 1997).

Coastal areas in the Arctic and extreme north Atlantic and Pacific are less vulnerable, except where sea ice and/or permafrost currently is present at the shoreline.

Sea-level rise and storm surges along the tundra coastline of Alaska and Canada are likely to cause erosion, flooding, and inundation through mechanisms similar to those for other parts of the North American coast. Several additional factors, notably sea-ice effects and coastal permafrost degradation, also will come into play. Projected changes in sea ice include a 35% decrease in winter ice thickness, along with significant retreat of the southern limit of sea ice and complete absence of summer sea ice among the Arctic Islands (Maxwell and Barrie, 1989). These decreases in the period and extent of sea-ice cover will result in larger ocean fetches and greater wave attack on the coastal zone (Lewis, 1974), with attendant erosion. Subsequent modeling suggested that the wave energy during the open-water season may increase wave heights by 16-40% (McGillivray et al., 1993).

Rates of erosion of permafrost also can be expected to increase. The Alaska and Yukon coasts already experience significant erosion during the annual thaw. According to Lewellen (1970), erosion rates in the mid-1960s and early 1970s ranged from a few decimeters to as much as 10 m per year. Maximum erosion occurred in areas where permafrost contained considerable pore, wedge, or massive ice (Lewis, 1974) or where the permafrost shoreline was exposed to the sea (Lewellen, 1970).

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