Cost and Effect of Sea Level Rise

Previous studies suggest that the expected global warming from the greenhouse effect could raise sea level 2 to 7 feet in the next century. I will present the first nationwide assessment of the primary impacts of such a rise on the United States: (1) the cost of protecting ocean resort communities by pumping sand onto beaches and gradually raising barrier islands in place; (2) the cost of protecting developed areas along sheltered waters through the use of levees (dikes) and bulkheads; and (3) the loss of coastal wetlands and undeveloped lowlands. The total cost for a one meter rise would be $270-475 billion, ignoring future development.

It is estimated that if no measures are taken to hold back the sea, a one meter rise in sea level would inundate 14,000 square miles, with wet and dry land each accounting for about half the loss. The 600-700 square miles of densely developed coastal lowlands could be protected for approximately one to two thousand dollars per year for a typical coastal lot. Given high coastal property values, holding back the sea would probably be cost-effective.

The environmental consequences of doing so, however, may not be acceptable. Although the most common engineering solution for protecting the ocean coast–pumping sand–would allow us to keep our beaches, levees and bulkheads along sheltered waters would gradually eliminate most of the nation’s wetland shorelines. To ensure the long-term survival of coastal wetlands, federal and state environmental agencies should begin to lay the groundwork for a gradual abandonment of coastal lowlands as sea level rises.

The level of the oceans has always fluctuated with changes in global temperatures. During ice ages when global temperatures were 9^oF lower than today, much of the ocean’s water was tied up in glaciers and sea level was often over three hundred feet lower than today. On the other hand, during the last interglacial period (100,000 years ago) when temperatures were about  2^oF warmer, sea level was approximately 20 feet higher than today. Global sea level trends have generally been estimated by combining the trends at tidal stations around the world. These records suggest that during the last century, worldwide sea level has risen 4 to 10 in, much of which has been attributed to the global warming of the last century.

A rise in sea level would inundate wetlands and lowlands, accelerate coastal erosion, exacerbate coastal flooding, threaten coastal structures, raise water tables, and increase the salinity of rivers, bays, and aquifers. 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 area around San Francisco Bay. Similarly, the areas vulnerable to erosion and flooding are also predominantly in the southeast, while potential salinity problems are spread more evenly throughout the coast.

The dry land within seven feet of high tide include 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. Major port cities with low areas include Boston, New York, Charleston, Miami, and New Orleans; the latter averages about two meters below sea level, and parts of Texas City, San Jose, and Long Beach California are about one meter below sea level.

Studies suggest that a one meter rise in sea level would generally cause beaches to erode 50-100 meters from the Northeast to Maryland; 200 meters along the Carolinas; 100-1000 meters along the Florida coast; and 200-400 meters along the California coast. Because most U.S. recreational beaches are less than 30 meters (100 feet) wide at high tide, even a thirty centimeter (one foot) rise in sea level would require a response.

It is estimated that shoreline retreat from a one meter rise in sea level would cost the United States 270 to 475 billion dollars. Fourteen thousand square miles of land could be lost from a one meter rise, with wet and dry land each accounting for about half the loss. For a few hundred billion dollars, six to seven hundred square miles of currently developed land could be protected, but the loss of coastal wetlands would be that much greater.

The fact that it may be cost effective to protect property does not necessarily imply that it would be in the interest of society to do so. We must also consider the loss of natural shorelines and coastal wetlands that would result. The results suggest that up to a point, the objectives of protecting wetlands and coastal property may be compatible. Abandoning developed areas would increase the area of surviving wetlands by only 5 to 10 percent–but at great cost. By contrast, limiting coastal protection to areas that are already densely developed (and allowing currently undeveloped areas to flood) would increase the area of surviving coastal wetlands by 40 to 100 percent, depending on how much the sea rises.

The results are consistent with the hypothesis of a 1987 study by the National Academy of Engineering that shore protection will be cost-effective for most developed areas. From the perspective of civil engineers, that study concluded that little action is necessary today because shore protection structures can be erected rapidly compared with the rate of sea level rise. However, the speed with which communities could build these structures is small comfort to the birds and fish whose habitat would be destroyed by doing so.

Sea level rise is an urgent issue for coastal environmental planners for the very reason that it lacks urgency for directors of public works. If environmentalists do not lay the necessary groundwork today to institutionalize a gradual abandonment of the coastal plain as sea level rises, the public will almost certainly call upon engineers to protect their homes in the years to come.

With thanks to James G. Titus, Richard A. Park, Stephen P. Leatherman, J. Richard Weggel, Michael S. Greene, Paul W. Mausel,Scott Brown, Cary Gaunt, Manjit Trehan, and Gary Yohe for their research and articles.

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