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National Assessment of
Rise in Sea Level
Sea-level rise is one of the more certain consequences of climate change and, because it affects the land/ocean interface, it will have significant impacts on coastal areas. Although global sea level rose between 4 and 8 inches during the 20th century, climate change is projected to accelerate the rate of rise by 2 to 4 times that rate because warming global temperatures will cause expansion of ocean waters and accelerated melting of glaciers and ice sheets.
Rising sea levels could encroach on low-lying regions of the Southeast, some of which include highly populated areas. As a result of mainly non-climatic influences, this region lost more than 32,000 acres of coastal salt marsh between 1985 and 1995. Although some of this loss was due to sea-level rise, the primary contributing factors included human development, ground water pumping, and, in some places, natural subsidence (sinking) as coastal soils compact. In addition to sea-level rise, changes in climate could lead to more frequent and/or intense storms. When combined with rising sea levels, the result could be damaging floods from more destructive storm waves. Beyond the economic and social affects of such damage, sea-level rise is likely to result in increased erosion, flooding of low-lying areas, intrusion of salt water, wetland destruction, and negative impacts on biology and ecology. All of these potential impacts, and strategies that could prevent or mitigate them, are explored in the following sections.
The rate of sea-level rise in a particular region depends not only on the rate of rise of global sea level but also depends on the rate of local land settling or uplift. Rates of rise for the past century along the Atlantic and Gulf coasts range, for example, from 3.5 inches per century in Boston, Massachusetts, to 37 to 38 inches per century on the Mississippi Delta (much of this is due to land subsidence). According to the Environmental Protection Agency (EPA), historic rates of sea-level rise include 9 inches per century in Miami and 14-17 inches per century in Hampton Roads, Virginia.
Regionally variable sea-level rise will continue. Results from a study sponsored by EPA suggests that, by the year 2100, there is a 50% chance of a 20-inch sea-level rise at Miami Beach; a 25-inch sea-level rise for Charleston, South Carolina; and a 55-inch sea-level rise for Grand Isle, Louisiana. According to another study, about 3,500 square miles of coastal lands in this region would be at risk from a 20-inch sea-level rise by 2100. About half of that land is presently dry land and the other half wetlands.
The effects of the regional differences in present and projected sea-level rise have much to do with the characteristics of coastal land and whether that land is a rocky shore, a sandy beach, a barrier beach, a sea cliff, a salt marsh, or contains offshore shoals. Each of these land types reacts differently to sea-level rise and to wind-generated waves and tides that erode, transport, and deposit coastal sediment. Depending on the land type and geographical location, then, accelerated sea-level rise could have severe physical impacts in some areas and very limited effects in other areas. For example, sandy beaches and unconsolidated cliffs would be most subject to erosion. Although erosion rates are difficult to predict, the “Bruun rule -- suggests that a one-foot rise in sea level would erode a 100- to 200-foot swath of the South Carolina coastline and, depending on location, 100 to 1,000 feet off the Florida coastline.
As sea level rises, the vulnerability to flooding of coastal areas from any given size storm would increase. Higher sea level provides a raised base from which storm waves can sweep inland, creating increased risk to life and property. Flooding would especially affect low-lying sheltered shores not subject to erosion. For example, occasional flooding is already suspected of having caused sabal palm losses in Pasco, Hernando, Citrus, Levy, and Dixie counties on Florida's Gulf Coast.
Another result of rising sea level is that saltwater, vital to the health of an estuary, would push upstream unless freshwater flow increases. This could cause a shift of marine ecosystems upriver. Because these ecosystems often are adapted to specific tidal ranges, however, species and whole ecosystems of the Southeast could be lost if the upstream conditions are not suitable for such migration or the species are unable to migrate along with the sea-level rise. Such shifts could also result in crowding-out species that are now in those locations, if there isn't suitable “upland -- or upstream habitat to which they could move.
If, however, protective structures are created along the shoreline to prevent erosion or submergence, or if coastal ecosystems are otherwise unsuitable (up against a cliff), then intertidal habitat shifts that might have occurred with sea-level rise would be prevented. Preserving intertidal habitats (e.g., mudflats and marshes) is also a significant concern. Sea-level rise is also likely to damage the ability of existing wetlands to serve as adequate nursery grounds for many species of coastal and anadromous fish -- species, such as salmon and shad that swim up-river to spawn. The wetlands and other intertidal areas are also important feeding grounds for many migrating waterfowl. An important aspect of wetlands is submerged aquatic vegetation. This vegetation, which includes wild celery, coontail, hydrilla in freshwater, and shoalgrass in salt water, show increased photosynthetic activity with increased levels of carbon dioxide. However, under these conditions wild celery and shoalgrass also have exhibited increased carbon-to-nitrogen ratios in their plant tissue, which renders them a poorer quality forage for wintering waterfowl.
Saltwater intrusion also would convert some coastal freshwater wetlands to salt marshes and increase the salinity in others. A third of the Everglades, for example, has an elevation of less than 12 inches above present sea level. Rising seas, resulting in saltwater intrusion, would harm delicate ecological communities and degrade this habitat for many species. A rise in sea level poses a critical threat to coastal wetlands, as they are among the most productive ecosystems in the world. Wetlands at the land/ocean margin provide many direct benefits to humans, including habitat for commercially valuable fisheries and wildlife; storm protection; improved water quality through sediment, nutrient, and pollution removal; recreation opportunities; and aesthetic value. Localized soil compaction, impacts from human activities, sea-level rise, and other factors have already contributed to rates of coastal wetland loss in south Louisiana that exceed roughly 40 square miles per year.
According to the EPA, many wetlands would not be able to build up their marsh surface fast enough to keep up with even a half-foot rise in sea level over the next century. As discussed previously, if the upstream zone is not developed or blocked off, certain wetlands would be able to migrate upland as the sea level rises and create new wetlands. However, some wetlands would surely become submerged. For example, a U.S. Geological Survey study of the St. Mark's National Wildlife Refuge in northwest Florida indicates that under projected figures for sea-level rise, major portions of the coastal wetlands studied will be permanently inundated over the next century.
In addition to damage to or shifts in existing ecosystems, the Southeast's groundwater would be affected as rising sea levels, over use of fresh groundwater, or a combination contribute to episodes of saltwater intrusion. Brackish (salty) water intrusion affects drinking water as it infiltrates aquifers supplying local communities.
Moreover, a rise in sea level would increase the exposure of many communities to storm waves. This poses a significant threat to low-lying urban areas. At particular risk would be many areas of southern Louisiana and Florida -- the highest point in Florida is only 53 feet above sea level, and the Florida Keys are all less than 10 feet above sea level. In addition, the low-lying barrier islands off the North Carolina coast already are highly vulnerable to the damages associated with storm waves, a phenomenon that will experience increasing stress as sea level rises.
Some property values will likely be reduced as the shoreline changes. Some property owners could lose homes, other buildings, and property with the long-term effects of sea-level rise even in the absence of storm wave impacts. New Orleans, for example, is already well below sea level and is kept dry by a series of levees and pumps. The Homa tribe in southern Louisiana, along with many communities along the various waterways and bayous of the region, is also in danger of having its homeland lost to erosion, and being in danger of flooding and inundation. Moreover, properties not lost to sea-level rise would still be at risk from the effects of hurricanes and the increased potential for storm wave damage.
A final social effect of a rise in sea level would be the harm wrought to ecosystems valued by residents and tourists to the Southeast. Florida's Everglades are a national treasure appreciated by residents and tourists alike. In addition, coastal recreational opportunities, available at the vast expanse of beaches in the Southeast, could be compromised by the effects of sea-level rise. Moreover, a rising sea level could jeopardize recreational fishing and fowling through the loss of coastal wetlands and associated habitat or poor-quality forage.
Increases in storm waves associated with catastrophic storms, combined with growth in population and building in low-lying coastal areas, have increased insurance claims in all coastal regions over the past decade. Over the past 20 years, this region has been the location of more than half of the nation's most costly weather-related disasters. Hurricanes, floods, droughts, tornadoes, and ice storms have resulted in over $87.7 billion in damages in this region. Six billion dollars and two hundred deaths are attributable to the heat wave and drought of 1998 alone; $6 billion is attributable to ice storms and $2.3 billion to tornadoes. With an increasing rate of sea-level rise and the possibility that precipitation in the form of heavy and extreme events will continue, this region has the potential for even greater damage to and loss of infrastructure to sea-level rise and its associated effects in the future.
Damage to coastal wetlands from sea-level rise could also harm the Southeast region's coastal fisheries. The major portion of the nation's commercial fishery catch is estuarine-dependent, and the Carolinian-South Atlantic fishery is no exception: 94% of its species require just the right mix of salt and freshwater found in estuaries. This fishery was valued at $169 million in 1990. Not all of the fishery would be lost and many warmer water fish may move into this location resulting in additional catches.
Three strategies for the built infrastructure are available to mitigate the effects of sea-level rise: to retreat, to accommodate, or to protect. In addition, all three coping strategies could be strengthened by education efforts that help to reduce the vulnerability to potential coastal changes. For example, all stakeholders could be educated about the risks of building in hazard-prone areas and likely changes in storm frequency, intensity, and sea level. There are very few options that are practical that could be applied to protection of the coastal natural resources of this region.
A strategy of retreat could include the following actions:
Accommodation could entail the following step:
A strategy designed to protect coastal lands could include efforts to:
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