Coastal regions play an integral role in the United States, serving as home to over half of the U.S. population, providing recreational opportunities to many, and supplying numerous valuable ecological services. At the same time, these areas are constantly evolving and face a wide range of natural and human-induced stresses, including erosion, storms, and pressures from development and recreational uses.

Current scientific research shows that climate change will lead to substantial sea-level rise along much of the U.S. coastline. Sea levels have already risen between 10 and 25 cm over the last century. Global warming will accelerate these rates, with sea levels projected to rise by 50 cm by 2100.

“Sea-Level Rise and Global Climate Change” is the fourth in a series of reports examining the potential impacts of climate change on the U.S. environment and society. This report finds that the vulnerability of a coastal area to sea-level rise varies according to the physical characteristics of the coastline, the population size and amount of development, and the responsiveness of land-use and infrastructure planning at the local level. The authors conclude the following:

• Low-lying developed areas in the Gulf Coast, the South, and the mid-Atlantic regions are especially at risk from sea-level rise.
• The rapid growth of coastal areas in the last few decades has resulted in larger populations and more valuable coastal property being at risk from sea-level rise. This growth, which is expected to continue, brings with it a greater likelihood of increased property damage in coastal areas.
• The major physical impacts of a rise in sea level include erosion of beaches, inundation of deltas as well as flooding and loss of many marshes and wetlands. Increased salinity will likely become a problem in coastal aquifers and estuarine systems as a result of saltwater intrusion.
• Although there is some uncertainty about the effect of climate change on storms and hurricanes, increases in the intensity or frequency or changes in the paths of these storms could increase storm damage in coastal areas. Damage to and loss of coastal areas would jeopardize the economic and ecological amenities provided by coastal wetlands and marshes, including flood control, critical ecological habitat, and water purification.

Damages and economic losses could be reduced if local decision-makers understand the potential impacts of sea-level rise and use this information for planning.

The authors and the Pew Center gratefully acknowledge Dr. Donald Boesch and Dr. George Parsons for their review of previous drafts of this report. In addition, the Pew Center would like to thank Joel Smith and Brian Hurd of Stratus Consulting for their management and oversight of this Environmental Impacts Series.

Executive Summary

Climate change is likely to accelerate the historical rise in sea level through warming of oceans and melting of ice, which in turn will affect coastal development, wetland resources, and recreation along the U.S. coast. The impacts of sea-level rise will occur in coastal areas that are continually evolving and already face a wide range of natural and human-induced stresses, including erosion, storms, land subsidence, wetland loss, and environmental degradation from recreation and development pressures. Responses to sea-level rise at the national, state, and local level must therefore reflect an understanding of the complex interactions of human and ecological systems in coastal areas. In this report, we review the state of understanding of the impacts of sea-level rise on U.S. coasts.

Impact assessment for sea-level rise requires careful assessment of local conditions, the magnitude and uncertainties of global sea-level rise, and the costs and feasibility of response options. Important local conditions include coastal topography, geology, and economic and demographic factors. The areas in the United States most vulnerable to sea-level rise are in the mid-Atlantic and south-Atlantic states (because of their low-lying topography, high economic value, and relatively high storm frequency) and along the Gulf Coast (because of low-lying topography and rapid land subsidence). Parts of New England are also at risk, particularly coastal islands in southern New England. The West Coast is generally at lower risk, with the exception of San Francisco Bay and Puget Sound.

Existing threats to coasts that may be increased by climate change include: gradual sea-level rise, catastrophic sea-level rise (i.e., Antarctic ice sheet melt), and changes in storm frequency or intensity. Impact assessments have focused on the first threat, with some consideration in recent studies of the effect of storms on development and redevelopment patterns in the coastal zone. Although evidence of Antarctic ice sheet melt exists in the geologic record, recent evaluations suggest the probability of this event occurring in the next century is very low. Research on the effects of climate change on storm frequency and intensity is active, but currently inconclusive.

The Intergovernmental Panel on Climate Change (IPCC) concludes that increases in global temperatures over the next century could accelerate the historical rate of global mean sea-level rise from 1 to 2.5 millimeters per year to about 5 mm/yr (50 cm/century), with an uncertainty range of 2 to 9 mm/yr (IPCC, 1996a). More recent work using new greenhouse gas emissions scenarios shows a slightly higher rate of sea-level rise (Wigley, 1999).

The impacts of sea-level rise will vary by location and depend on a range of biophysical characteristics and socioeconomic factors, including human response. The primary impacts of sea-level rise are physical changes to the environment. These changes, in turn, affect human uses of the coast such as tourism, settlement, shipping, commercial and recreational fishing, agriculture, and wildlife viewing. The most serious physical impacts of gradual sea-level rise on coastal lowlands are (1) inundation and displacement of wetlands and lowlands; (2) coastal erosion; (3) increased vulnerability to coastal storm damage and flooding; and (4) salinization of surface water and groundwater.

National assessments suggest that a one-meter rise in global sea levels could have significant impacts, including the inundation of about 35,000 square kilometers (km2) (13,000 square miles (mi2)) of land, divided about equally between wetlands and upland. In addition, the 100-year coastal flood plain could increase by 38 percent, or at least 18,000 km 2 (7,000 mi2). Estimates of land inundated if global sea levels rise 0.5 meter are closer to 24,000 km 2 (9,000 mi2). Major coastal cities such as New Orleans, Miami, New York, and Washington, DC, will have to upgrade flood defenses and drainage systems or risk adverse consequences.

Three options have been proposed to respond to coastal threats: planned retreat, accommodation, and protection. Impact and adaptation assessments evaluate where these responses might be implemented and then calculate the costs of implementation and the damages to resources that are not protected. Generally, property losses or the costs to protect property dominate the existing impact estimates for the United States. The implications of lost wetlands, which are not reflected in most current impact estimates, could also be significant. Based on a review of the existing literature, estimates of the cumulative impacts of a 50-cm sea-level rise by 2100 on coastal property range from about $20 billion to about $150 billion. Estimates at the low end of the range reflect modeling of the most economically efficient adaptation to sea-level rise. Those estimates at the high end reflect assessments of vulnerability or protection costs, and assume that all currently developed vulnerable areas will be protected, regardless of costs.

Although these cumulative costs are a relatively small percentage of total property values in the coastal zone, these aggregate estimates do not reflect the potentially large effects on coastal wetlands and, perhaps more important, provide no information on the distribution of impacts. Depending on the policy options chosen to respond to sea-level rise, the impact of rising seas could fall disproportionately on a small number of people or communities in the most vulnerable areas.

In many cases, the impacts of sea-level rise could be mitigated by forward looking state or local land-use policies. The major challenges of future impact assessments include improving their comprehensiveness and accuracy and making their results more accessible and useful to state and local decision-makers who are most able to prepare coastal areas to respond to the threat of sea-level rise.