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Coastal Zones
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Sea level is rising more rapidly along the U.S. coast than worldwide. Studies by EPA and others  have estimated that along the Gulf and Atlantic coasts, a one foot (30 cm) rise in sea level is likely by 2050. In the next century, a two foot rise is most likely, but a four foot rise is possible; and sea level will probably continue to rise for several centuries, even if global temperatures were to stop rising a few decades hence (see Sea Level in the Climate System section).
 Rising sea level inundates wetlands and other low-lying lands, erodes beaches, intensifies flooding, and increases the salinity of rivers, bays, and groundwater tables. Some of these effects may be further compounded by other effects of changing climate. Measures that people take to protect private property from rising sea level may have adverse effects on the environment and on public uses of beaches and waterways. Federal, state, and local governments are already starting to take measures to prepare for the consequences of rising sea level.
Coastal marshes and swamps are particularly vulnerable to rising sea level because they are mostly within a few feet of sea level. As the sea rises, the outer boundary of these wetlands will erode, and new wetlands will form inland as previously dry areas are flooded by the higher water levels. The amount of newly created wetlands, however, will be much smaller than the area of wetlands that are lost. The amount of dryland within a few feet above the wetlands is much less than the area of wetlands that would be lost if sea level rises a few feet. Moreover, developed areas will often be protected with bulkheads, dikes, and other structures that keep new wetlands from forming inland. See Environmental Management 15:39-58 (1991).
Nationwide, a two foot rise in sea level could eliminate 17-43 percent of US wetlands, even if no additional bulkheads or dikes are erected, with more than half of the loss taking place in Louisiana alone. See Coastal Management 19:171-204 (1991).
The dry land within a few feet above high tide includes forests, farms, low parts of some port cities, communities that sank after they were built and are now protected with levees, parts of deltas, and the bay sides of barrier islands. The low forests and farms are mostly in the Mid-Atlantic and Southeast. Major port cities with low areas include Boston, New York, Charleston, Miami, and New Orleans. The average elevation of New Orleans is about 2 meters below sea level, and parts of Texas City, San Jose, and Long Beach, California are about one meter below sea level.
Nationwide, about 5000 square miles of dry land are within two feet of high tide, 4000 of which are currently undeveloped. All of this land would be inundated by rising sea level, unless additional dikes and bulkheads are constructed. Including both the wetlands and dry land that would be lost to the sea, a two foot rise in sea level would eliminate approximately 10,000 square miles of land, an area equal to the combined size of Massachusetts and Delaware. See Coastal Management 19:199-201 (1991).
Some of the most economically important vulnerable areas are the recreational resorts on the "coastal barriers" of the Atlantic and Gulf coasts. See Coastal Management 18:65-90 (1991). In many cases, the ocean-front block of these islands is 5 to 10 feet above high tide; but the bay sides are often less than two feet above high water and regularly flooded. Erosion threatens the high ocean sides of these densely developed islands and is generally viewed as a more immediate problem than inundation of their low bay sides. Many ocean shores are currently eroding 1 to 4 feet per year. Coastal engineers generally estimate that a 1 foot rise in sea level will cause beaches to erode 0.5 to 1 feet from New England to Maryland, 2 feet along the Carolinas, 1 to 10 feet along the Florida coast, and 2 to 4 feet along the California coast. Because many US recreational beaches are less than 100 feet wide at high tide, even a 1 foot rise in sea level would threaten homes in these areas. A study by the Federal Emergency Management Agency estimated that about 25 percent of all buildings within 500 feet of the U.S. coastline will be taken by erosion in the next 60 years.
Along sandy beaches, like wetland shores, property owners often erect structures to halt erosion. Although these sea walls protect property, they can eliminate beaches, particularly bay beaches, which are usually less than 10 feet wide. Beaches are used for fishing, recreation, transportation, and landing small crafts, in addition to their environmental importance. In most states, the beach below mean high water is owned by the public. To protect these public assets, several states have adopted policies to ensure that beaches, dunes, or wetlands are able to migrate inland as sea level rises. Some states prohibit new houses in areas likely to be eroded in the next 30-60 years. Concerned about the need to protect property rights, Maine, South Carolina, and Texas have implemented some version of "rolling easements," in which people are allowed to build, but only on the condition that they will remove the structure if and when it is threatened by an advancing shoreline.
 Changing climate also increases the vulnerability of coastal areas to flooding. A higher sea level raises the flood level from a storm of a given severity. A 3-foot rise in sea level (for example) would enable a 15-year storm to flood many areas that today are only flooded by a 100-year storm. A 1991 report by the Federal Emergency Management Agency estimated that a one foot rise in sea level would increase the size of the 100-year floodplain in the US from 19,500 square miles in 1990 to 23,000 square miles, and increase flood damages (and hence flood insurance rates) by 36-58 percent.
Coastal flooding is also 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. 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 port cities along major tidal rivers, such as Alexandria (VA), Portland (OR), Philadelphia, and Washington (DC). If hurricanes were to be come more severe or frequent, flooding would further increase; but that possibility has not been demonstrated.
Finally, rising sea level tends to increase the salinity of both surface water and ground water. 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 salt water is able to reach farther upstream in the future, then the existing intakes would draw salty water during droughts (see river salinity section of 1985 report by Delaware River Basin Commission and EPA). Higher estuarine salinity has also been cited as a cause of declining oyster harvests in Chesapeake and Delaware Bays, and as a cause for wetland loss in Louisiana, Florida, and Maryland.
 The class of aquifers that are most vulnerable to rising sea level are those that are recharged in areas that are currently fresh but which 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. This part of the river and thus the aquifer would become salty more frequently if sea level rises a few feet or if droughts become more severe (see aquifer section of report by Delaware River Basin Commission and EPA). A second class of vulnerable aquifers are the shallow coastal aquifers found on small islands. Freshwater is lighter than salt water, and hence the fresh water "lens" floats on top of the salty groundwater. In some types of aquifers, for every inch the sea rises, the freshwater lens loses 40 inches of depth. Although few communities in the United States obtain their water from these types of aquifers, important parts of such coral atoll nations as the Maldives and Vanu Atu could lose their primary water supplies with even a two foot rise in sea level.
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