Global climate change could alter life along the coast by changing the level of the sea, the frequency and intensity of storms, and water and air temperatures (IPCC 1990d). In the United States, only the first of these three potential impacts has been assessed in any detail.

Sea Level Rise

The IPCC estimates that under the “business as usual,” high-emission scenario of a doubling of CO2 emissions by the year 2100, sea level will rise on a global average of 3-10 mm (.1-.4 in.) per year, or a total of 30-110 cm (11.8-43.3 in.), with a best estimate of 65 cm (25.6 in.) by 2100 (IPCC 1990c).

The IPCC also emphasizes the need to address adaptive strategies to reduce vulnerability to sea level rise because of factors not related to climate change. In many parts of the United States and the world, natural systems that provide protection against the sea are being degraded by human development activities, such as mining for sand and coral, cutting mangroves, damming and confining the flows of rivers, and filling wetlands. In the United States, although many of these activities are being reduced through coastal zone management, many areas remain vulnerable to sea level rise because of high natural rates of erosion (e.g., New England) and subsidence due in part to human activities (e.g., Louisiana).

An acceleration in sea level rise could exacerbate current coastal problems, including shoreline erosion, loss of wetlands, flooding, and the salinization of water resources due to natural and human-induced causes. Given the scientific uncertainties of the rate of rise, there is no uniform perception of the risks at the federal or state government level. However, a number of scientific and technical studies have been carried out to identify areas at risk and to begin an assessment of the potential impacts.

The East and Gulf Coasts of the United States are

particularly vulnerable to sea level rise. Most of the wetlands and lowlands of the United States are found along the Gulf Coast and the Atlantic Coast south of the central part of New Jersey, although there is also a large, low wetland area around San Francisco Bay. Areas vulnerable to erosion and flooding are predominantly in the Southeast, while potential salinity problems are spread more evenly along the coast. In addition, 61 percent of the East Coast has low-lying shoreline geography consisting of unconsolidated sediments. Dominant land forms include estuaries (42 percent by length); barrier islands (18 percent); lagoons (15 percent); and rocky, embayed coasts (12 percent), with the highest concentrations of these in New England. Highrisk erosion- and flood-prone areas along the East Coast have been identified along parts of Cape Cod, Long Island, and the New Jersey barrier beaches, the North Carolina Outer Banks, the southern Delmarva Peninsula, and the Georgia-South Carolina coastline. Major port cities with low areas include Boston, New York, Charleston, and eastern Miami (Gornitz et al. 1992). Other areas of the United States are also vulnerable, including the Gulf Coast states, California, the Virgin Islands and Puerto Rico, Hawaii, and the U.S. trust territories. Some areas are less vulnerable, such as Alaska.

A study by the U.S. Federal Emergency Management Agency (FEMA 1991) estimated that from the midAtlantic to the Gulf of Mexico, approximately 8,900-10,400 square kilometers (23,000-27,000 sq. mi.) would be adversely affected by respective sea level rises of .3 and .9 meters (1 and 3 feet) by 2100, while the number of households in coastal floodplains is expected to increase from 5.1 to 6.6 million. About 110 million people now live in coastal areas. By the year 2010, the coastal population will have grown to more than 127 million. Some states, including Florida, will have increased by more than 200 percent (NOAA 1990). In addition, the full-risk premium rate for flood insurance would increase by approximately 200 percent with a 1meter (3.3-foot) rise in sea level (FEMA 1991). The study also found that the full-risk premium rate for

flood insurance would increase by approximately 200 percent with a 1-meter rise in sea level.

If shorelines are allowed to retreat under a 1-meter (3.3-foot) rise, coastal areas would become more vulnerable to flooding for four reasons: (1) a higher sea level provides a higher base for storm surges to build upon (a 1-meter rise in sea level would thus enable a fifteen-year storm to flood many areas that today are only flooded by a one-hundred-year storm) (Kana et al. 1984); (2) beach erosion would leave particular properties more vulnerable to storm waves; (3) higher water levels would increase flooding due to rainstorms by reducing coastal drainage (Titus et al. 1987); and (4) a rise in sea level would raise water tables.

Some coastal areas are protected with levees and seawalls, and would thus not necessarily experience nearterm inundation, erosion, or flooding. However, these structures were usually designed for current sea level and one-hundred-year storm events, and would not provide sufficient protection against future flooding and erosion from higher water levels (NRC 1987). In areas like New Orleans that are drained artificially, the increased need for pumping could exceed current capacities (Titus et al. 1987).

WETLANDS

The greatest estimated wetland losses would be in the Southeast, the location of 85 percent of all U.S. coastal wetlands. Without adaptive responses and assuming no formation of new wetlands, a 1-meter (3.3foot) rise could inundate 90–95 percent of wetlands in this region, with 40-50 percent of the losses occurring in Louisiana. By contrast, the Northeast and the West would each lose no more than 10 percent of their wetlands, if only currently developed areas are protected.

The dry land within 2 meters (6.6 feet) of high tide includes forests, farms, low parts of some port cities, communities that sank after they were built and are now protected with levees, and the bay sides of barrier islands. The low forests and farms are generally in the mid-Atlantic and Southeast and would provide potential areas for new wetland formation if no barriers to their migration exist.

BARRIER ISLANDS

Some of the most vulnerable areas are the recreational barrier islands and spits of the Atlantic and Gulf Coasts. A 1-meter (3.3-foot) rise in sea level would inundate much of this valuable land. However, erosion threatens the high parts of these islands, and is generally viewed as a more immediate problem than inundation. Thus, a 1-meter rise in sea level would generally cause beaches to erode 50-100 meters (165-330 feet)

from the Northeast to Maryland; 200 meters (660 feet) along the Carolinas; 100-1,000 meters (330–3,280 feet) along the Florida coast; and 200-400 meters (660–1,320 feet) along the California coast (Everts 1985; Kyper and Sorensen 1985; Kana et al. 1984; Bruun 1962; Wilcoxen 1986). Because most U.S. recreational beaches are less than 30 meters (100 feet) wide at high tide, even with a 30-centimeter (1-foot) rise in sea level, protecting the beach and dune systems would require some response.

SALTWATER INTRUSION

A rise in sea level would enable salt water to penetrate farther inland and upstream in rivers, bays, wetlands, and aquifers. This intrusion would affect some aquatic plants and animals, and would complicate human uses of water. Increased salinity has already been cited as a contributing factor to reduced oyster harvests in Chesapeake and Delaware Bays (Gunter 1974), and to the conversion of cypress swamps in Louisiana to open lakes (LWPP 1987). Moreover, New York, Philadelphia, and much of California's Central Valley get their water from areas that are just upstream of water that is saline during droughts. Farmers in central New Jersey, as well as the city of Camden, rely on the Potomac-Raritan-Maggothy aquifer, which could become salty if sea level rises. For example, a 50-cm (20in.) rise in sea level would enable the salt front in the Delaware River to migrate 10 km (6.2 mi.) upstream during droughts, threatening Philadelphia's freshwater intake, as well as the Potomac-Raritan-Maggothy aquifer. The Delaware River Commission's long-term plan for reservoir construction and operation has incorporated assumptions about sea level rise since 1980 (Hull and Titus 1986). The South Florida Water Management District also spends millions of dollars every year to prevent Miami's Biscayne Aquifer from becoming saline (Miller and Brock 1989). All of these areas would be vulnerable to sea level rise and climate change.

ADAPTIVE MEASURES

Measures for facilitating adaptation to a rise in sea level fall broadly into three categories:

• Protection (structural measures-seawalls, beach nourishment-to hold back the sea in vulnerable areas, and measures to protect or restore degraded or converted wetlands).

• Accommodation (allowing the sea to advance and adapting to it by raising the height of structures, etc.)

• Retreat (abandonment of the land and structures in vulnerable areas and resettlement of inhabitants) (Figure 22).

To date, most adaptive actions have been based on existing efforts to address impacts from activities not related to climate change, such as reducing damage to shoreline structures, controlling erosion, and mitigating flood hazards. Because these impacts could be exacerbated by an acceleration in sea level rise, attention to factors that are not related to climate change but that are associated with vulnerability to sea level rise is the first-order priority for adaptation measures (IPCC 1992c).

The United States has started to assess the impacts of sea level rise. Most assessments in the United States have concluded that low-lying cities would be protected with bulkheads, levees, and pumping systems, while in sparsely developed areas, shorelines would retreat naturally (NRC 1987). This conclusion has generally been based on the commonly accepted assumption that the cost of these structures would be far less than the value of urban areas being protected, but greater than the value of undeveloped land. Federal and state expenditures indicate a willingness to protect developed shorelines with groins, revetments, or sand nourishment. Decision making regarding adaptive strategies is addressed at many levels (federal, state, and local) and through various institutions and agencies. The U.S. Congress has passed several pieces of legislation with elements that address the issues of vulnerability and adaptive response.

The Coastal Zone Management Act of 1972 created an integrated, comprehensive state and federal cooperative program for coastal resource management and development. Section 303 of the 1990 amendments to the Act calls for states to manage coastal development to minimize the loss of life and property "in areas likely to be affected by or vulnerable to sea level rise" and provides federal grants (Section 309) for "preventing or Figure 22

significantly reducing threats to life and destruction of property... by anticipating and managing the effects of potential sea level rise. ..."

A 1990 analysis of alternative policy responses to accelerated sea level rise indicates that state coastal management agencies prefer "strategic retreat policies" for relatively undeveloped areas (Klarin et al. 1990). These include laws and regulations that put conditions on the use of property located in areas susceptible to erosion and flooding, restrictions on hard structural protection, protection of critical environmental areas, and restrictions on post-storm redevelopment. However, in many moderately developed areas, states have restrictions on bulkheads and other structural measures to protect wetlands and habitats (e.g., Washington, Massachusetts). However, actions of state and federal governments as a whole continue to show the preferred response in densely developed areas is protection, rather than retreat.

Several state and local authorities have policies that are partly responsive to the impacts of sea level rise, such as setbacks for development (a response to coastal erosion). Table 10 shows the results of a survey of 23 states concerning whether they recognize sea level rise as an issue and whether they have existing or new adaptive policies that partly respond to the associated impacts. Table 11 outlines a range of seven policy responses that address conditions similar to the impacts associated with sea level rise and examples of federal and state actions.

Congress has directed the Federal Emergency Management Agency to assess the changes that might be necessary to modify the Federal Flood Insurance Program, based on a .3-meter (1-foot) and a 1-meter (3-foot) rise in sea level; passed the Coastal Barrier Resources Act to eliminate federal subsidies to develop

Anticipatory measures for reducing wetland losses fall into the following categories:

1. Prevent development through regulation or land purchase.

2. Allow development, but put property owners on notice that they will have to abandon the property if and when sea level rises.

3. Allow development without any conditions, and abandon the land as sea level rises through purchases or regulatory agencies.

4. Enhance mitigation and restoration efforts.

Several states have adopted the first approach to a limited extent through the use of setbacks (Klarin et al. 1990). These setbacks generally preclude building closer than the thirty- or forty-year historical erosion rate but do not include projected sea level rise in these calculations. Figure 23 illustrates setback requirements adopted by North Carolina. The constitutionality of setback requirements would certainly be subject to question based on the distance inland, etc. (Fishman 1992). No state has shown an interest in buying all the land vulnerable to a 50-100-cm (20-40-in.) rise in sea level.

Although the second option would have the lowest social cost, only Maine thus far has adopted that approach (Titus 1991). While the third approach may work in states with a very strong public trust doctrine, such as California (Fishman 1992), governments will probably not force people to abandon their homes to allow wetland migration, even if the courts would allow it, unless there is substantial advance notice, as in the Maine approach (Titus 1991). Regarding the fourth approach, several coastal management agencies are undertaking wetland creation and reconstruction projects. In Louisiana, these projects are funded at $35 million a year.

Along the coast of Louisiana, adaptation to sea level rise is different from that of the rest of the United States. The Mississippi Delta is already losing 19 square kilometers (fifty square miles) of land a year because human activities have diminished the processes by which the delta once kept pace with sea level rise and natural subsidence. Many efforts are under way to correct the human modifications that are causing this rate of land loss, including a rediversion of freshwater flow and sediments; regulation of canal dredging; closing of unused canals; restrictions on new levels of construction; and regulation of freshwater flow to marshes. The currently planned measures are unlikely to delay the loss of the major parts of these wetlands if sea level rises even 60 cm (24 in.) (LWPP 1987).

Louisiana and other coastal states (including local governments) will continue to assess their vulnerability

that climate change could cause the extinction of some species and the expansions of others.

Over two-thirds of fish caught for human consumption, as well as many birds and terrestrial animals, depend on coastal marshes and swamps for part of their life cycle. Coastal wetlands can keep pace with a slow rate of sea level rise. This ability has enabled the area of intertidal wetlands to increase severalfold, with the slow rate of rise over the last few thousand years. However, if sea level rises too rapidly, the natural succession of the coastal ecology could be affected, leading to disruption in the life cycles of many species.

ADAPTIVE MEASURES

The reactions to a changing environment may have as much impact on users of aquatic resources as the change itself. Several measures are currently being taken to reduce vulnerability to climate changes.

• Determine the relationship of aquatic organisms to their environment and how the relationship changes with a changing climate. This work includes movement towards adoption of large marine ecosystems as the focus of research on biota in the oceans. Work in this area is being integrated domestically among various federal agencies and internationally through several scientific organizations.

• Strengthen fisheries management research, policies, and institutions. In fisheries management, large marine ecosystems increasingly are being considered as the management unit for the conservation and management of living marine resources (Sherman and Gold 1990). In addition, the Magnuson Fisheries Conservation and Management Act allows U.S. fisheries managers considerable flexibility in dealing with shifting fisheries production. To further improve its fisheries management, the United States is educating its fisheries managers and its fishing industry about the possible consequences of a changing climate. Finally, with respect to the impacts of international policies on fisheries and fisheries management, the United States has advocated the view that international treaties and agreements on fisheries and marine mammal conservation should consider the implications of the different population structures and distributions that would result from a changing climate.

Encourage cooperation among fishery, forestry, water, and other resource managers to ensure that the needs of aquatic resources are met. In many parts of the United States, fisheries and water managers routinely negotiate water flows in rivers to provide the most public benefit from scarce water resources for farmers, electric power users, and anadromous (or riverine) fish.