Seasonal sea-ice extent, at least in some areas of the Northern Hemisphere, is retreating. This information, coupled with projections of warming, suggests that current barriers to gene flow among marine mammal stocks in the Arctic may change dramatically in the next 50 years. Although this shift may not result in a reduction in abundance at the species level, it could very well change the population structure of many species of Arctic whales and seals, which will greatly affect their management.
Coastal wetlands and beaches may be eliminated in some areas by rising sea level. As a result, marine mammal calving and pupping beaches may disappear from areas where there are no alternatives. Affected marine mammals could include, for example, all of the temperate and tropical seals and sea lions, coastal whales and dolphins, and manatees in estuarine habitats.
Six species of sea turtles (all of which are listed as endangered or threatened under the U.S. Endangered Species Act) regularly spend all or part of their lives off North American coasts and in U.S. territorial waters of the Caribbean Sea and Pacific Ocean. The loss of nesting beaches that would result from the combination of coastal development and projected sea-level rise is a threat to all marine turtle species.
Rising sea level is gradually inundating wetlands and lowlands; eroding beaches; exacerbating coastal flooding; threatening coastal structures; raising water tables; and increasing the salinity of rivers, bays, and aquifers (Barth and Titus, 1984). The areas most vulnerable to rising seas are found along the Gulf of Mexico and the Atlantic Ocean south of Cape Cod. Although there also are large low areas around San Francisco Bay and the Fraser delta (British Columbia), most of the Pacific coast is less vulnerable than the Atlantic and Gulf coasts. Because of a combination of rocky shores, lower rates of sea-level rise, higher elevations, and less shorefront development, most of the Canadian coast is much less vulnerable to the direct effects of rising sea level (Shaw et al., 1994) than the low, sandy and muddy shores of the United States.
This section focuses primarily on the impacts of sea-level rise, which is the most thoroughly studied effect of global warming on coastal zones. Nevertheless, global climate change also is expected to alter coastal hydrology, the frequency and severity of severe storms, and sea-ice cover. Moreover, the impacts of regional climate change on inland areas also will affect coastal zones-particularly the estuaries into which most of the continent drains.
The implications of rising sea level are well understood, in part because sea level has been rising relative to the land along most of the coast of North America (and falling in a few areas) for thousands of years. For the most part, the relative rise and fall of sea level has been caused by adjustments of the earth's crust to the glacial mass that was removed from the land surfaces after the end of the last ice age (Grant, 1975). Change in the volume of water in oceans was also of importance. Water locked up in ice caps during ice ages lowered the volume of water in oceans, thus lowering sea level. The changes discussed here have occurred over geologic time (Holocene Epoch-last 10,000 years). The land is rising (i.e., relative sea level is falling) in the northern areas that had been covered by the ice sheet; land is subsiding in nearby areas that were not covered by the glaciers, such as the Canadian maritime provinces and U.S. middle Atlantic states.
A 50-cm rise in sea level would inundate approximately 50% of North American coastal wetlands in the next century; many beaches would be squeezed between advancing seas and engineering structures, particularly along estuarine shores.
Coastal marshes and swamps generally are found between the highest tide of the year and mean sea level. Coastal wetlands provide important habitat and nourishment for a large number of birds and fish found in coastal areas. Wetlands generally have been able to keep pace with the historic rate of sea-level rise (Kaye and Barghoorn, 1964). As a result, the area of dry land just above wetlands is less than the area of wetlands. If sea level rises more rapidly than wetlands can accrete, however, there will be a substantial net loss of wetlands (Titus, 1986; Park et al., 1989). Because the current rate of sea-level rise is greater than the rate that prevailed over the past several thousand years (IPCC 1996, WG I), some areas-such as Blackwater National Wildlife Refuge (NWR) along the Chesapeake Bay-are already experiencing large losses of coastal wetlands (Kearney and Stevenson, 1985). Blackwater NWR is also a victim of herbivory by an introduced rodent, which makes interpretation of the role of sea-level rise difficult.
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