Along almost the entire U.S. coast, sea level is rising[1]—and the rate of that rise is expected to accelerate in the coming decades.[2] Even today, rising sea level is inundating low-lying lands, eroding beaches, exacerbating coastal flooding, and increasing the salinity of estuaries and aquifers.[3]

Over the last several thousand years, shallow-water estuaries have gradually submerged tidal wetlands, which in turn have survived by migrating inland, onto low-lying coastal plains (see Figure 1). Barrier islands and other beach ecosystems have also remained intact by migrating inland. In areas with few if any people, floodplains and tidal ecosystems will probably continue to move inland as sea level rises. In developed areas, however, human activities will complicate—or perhaps prevent—this landward migration.

Communities can respond to sea level rise by any of three or four pathways (See Box 1):[4]

1. Shore Protection
1. Shoreline armoring. Protect land and buildings from erosion and flooding using dikes, seawalls, bulkheads, and other hard structures. Wetlands and beaches are eliminated as they are squeezed between the rising sea and the shoreline armoring.
2. Elevation of land surfaces. Elevate land and buildings as the sea rises. Efforts to protect oceanfront communities usually involve beach nourishment, which elevates the surface of the beach. In theory, the land surfaces of wetlands can also be elevated, though shore protection projects along wetland shores rarely do so.
2. Accommodation. Do not try to prevent tidal inundation, erosion, or flooding. But instead of moving people out of harm's way, develop coping strategies that enable continued human habitation in spite of the increased hazards. Wetlands and beaches migrate inland, though they may be impaired by the presence of homes on pilings.
3. Retreat. Allow wetlands, beaches, and other coastal habitats to migrate naturally as the sea encroaches inland; move people out of harm's way; and prevent new construction in vulnerable

Because accommodation would rarely be sustainable,[5] the fundamental question is: Which communities will be protected and where will people have to retreat?

Beach nourishment is common along developed ocean shores, and shoreline armoring is common along developed estuarine shores. Although retreat often occurs in undeveloped areas, it is uncommon along developed ocean beaches and very rare along developed estuarine shores. Shore protection is common because it generally costs less than what the protected property is worth. But protecting all developed lands from a rising sea would eventually eliminate tidal wetlands, destroy ocean habitat through dredging, expose millions of people to the hazards from living below sea level, and become economically unsustainable in many areas where it initially seemed successful.[6]

What can society do if individual communities and property owners are inclined to protect more land than would be in society's long-run interest? Logically, there are three ways to limit the portion of our coast eventually subject to shore protection:[7]

1. Setbacks. Prevent development of some lands vulnerable to sea level rise, either through regulation or by purchasing land (or development rights) from the current owners.
2. Rolling easements. Make no effort to restrict land use but prevent shore protection of some coastal lands either through regulation or by transferring any right to hold back the sea from owners inclined to do so to organizations that would not.
3. Laissez-faire. Make no effort to prevent either development or shore protection, but curtail government subsidies for both, and hope that eventually the forces of nature and economics will lead owners to allow their lands to be submerged.

Each way is appropriate in some circumstances.

Landowners tolerate setbacks as long as they can build somewhere on their property. Thus setbacks can be practical where parcels are large or the land is steep enough so that each lot can have a building site high enough to be safe for the next few centuries. But in most places with setbacks, development is only set back by at most a few hundred feet or enough to keep a home out of harm's way for a few decades.[8] In the United States , more than ten thousand square miles of land are within two meters above the sea.[9] The expectation of additional development is reflected in the high prices of undeveloped coastal lands. To prevent development of these lands would impose a great cost either on landowners unable to put their land to its most profitable use, or on governments and private parties who purchase or otherwise pay landowners to refrain from development. Buying most of the nation's undeveloped coastal lands seems unlikely and economically infeasible.

The laissez-faire approach is based on the assumption that investors are more likely to appropriately manage known risks if they bear all of the burdens of bad decisions and reap all of the rewards of good decisions. This approach can reduce eventual shore protection in places where government subsidies would otherwise fund shore protection or coastal development. The Coastal Barrier Resources Act[10] removed federal subsidies for certain barrier islands,[11] causing some to remain undeveloped and reducing the likelihood of shore protection for several that have been developed without the subsidies.[12] Some ocean beach communities have funded their own shore protection or would do so if federal and state subsidies were unavailable.[13] Other oceanfront communities are unlikely to be protected without public funds; so a laissez-faire approach would reduce the extent of beach nourishment along the ocean. But along estuaries, private landowners generally pay for shore protection. Therefore, laissez-faire is unlikely to provide much vacant land for a gradual upslope migration of wetlands and beaches along estuarine shores. Planners view shore protection as likely for at least 60 percent of the low land along the Atlantic coast if sea level rises three feet in the next century.[14] Many landowners will eventually decide to yield their lands to the sea, as shore protection costs escalate,[15] but only after interim shore protection have blocked the inland migration of wetlands and compromised use of the beach.