US Coastlines

Perhaps the chief worry in coastal areas is that sea-level rise will cause a loss of land—especially loss of valuable wetlands. According to the Environmental Protection Agency, "Coastal wetland ecosystems, such as salt marshes and mangroves (trees and shrubs that grow in salt-water habitats) are particularly vulnerable to rising sea level because they are generally within a few feet of sea level____Wetlands provide habitat for many species, play a key role in nutrient uptake, serve as the basis for many communities' economic livelihoods, provide recreational opportunities, and protect local areas from flooding."31 As sea levels rise, wetlands turn into ocean, and areas further inland become wetland. Researchers think that, in part because of human developments such as dikes and barriers, there may be far fewer wetlands created than there are wetlands destroyed. By 2080, "sea level rise could convert as much as 33 percent of the world's coastal wetlands to open water."32

Rising sea levels may also affect drinking water. Cities such as Philadelphia and New York obtain drinking water not far from parts of rivers that are salty during droughts. When sea levels rise, saltwater pushes further upstream and may contaminate water supplies. The water in underground natural aquifers, which store freshwater, may also turn saline. Such contamination could have a major effect on drinking water in the Florida Keys and elsewhere.

Rising seas could also increase the likelihood of damage from storms and flooding. Higher sea levels erode the shores that protect properties from severe weather. Rising seas also cause coastal areas to drain more slowly, because the rate at which water drains depends on the difference in elevation between the place that is draining and the place the water is flowing to. Thus, when sea levels rise, the difference in elevation is reduced, and drainage

CALIFORNIA POPULATION VULNERABLE TO A 100-YEAR FLOOD ALONG THE PACIFIC COAST, BY COUNTY

County

Current Risk

Risk with 1.4m Sea-Level Rise

Percent Increase

Del Norte

1,800

2,600

47%

Humboldt

3,700

7,800

110%

Los Angeles

3,700

14,000

270%

Marin

530

630

20%

Mendocino

530

650

22%

Monterey

11,000

14,000

36%

Orange

72,000

110,000

55%

San Diego

3,000

9,300

210%

San Francisco

4,800

6,500

35%

San Luis Obispo

670

1,300

98%

San Mateo

4,700

5,900

24%

Santa Barbara

3,400

6,700

94%

Santa Cruz

11,000

16,000

49%

Sonoma

580

700

21%

Ventura

7,300

16,000

120%

TOTAL

130,000

210,000

67%

Source: Matthew Heberger et al., Impacts of Sea-Level Rise on the California Coast, Oakland, CA: Pacific Institute, 2009, p. 42.

The Wetland Feedback Loop

Rising sea levels are only one of the threats facing wetlands. Humans often drain wetlands to create dry land for agriculture and for homes. Because of such development, in the last century more than 60 percent of wetlands worldwide were destroyed, and more than 90 percent in Europe. In addition, global warming itself has a powerful effect: as temperatures rise, some wetlands evaporate.

The reduction of wetlands in these ways can have a profound impact on global warming. In the first place, wetlands help soak up excess water during storms, and so might help mitigate increased flooding as sea levels rise. Even more important, the destruction of wetlands may itself contribute to climate change. This is because wetlands "hold massive stores of carbon—about 20 percent of all terrestrial carbon stocks," according to Eugene Turner of Louisiana State University. Wetlands cover only 6 percent of the earth's surface, but they hold as much carbon as is currently contained in the earth's entire atmosphere. Wetlands also contain large stores of methane, a greenhouse gas 21 times more potent than carbon. Destroying wetlands, therefore, releases large stores of greenhouse gases, contributing to global warming—which in turn causes temperatures and sea levels to rise, both of which contribute to the destruction of more wetlands.

For all these reasons, many researchers argue that preserving wetlands is a vital part of controlling global warming. After spending so much time and effort draining wetlands, humans now need to figure out how to preserve the remainder, and perhaps even restore some of what has been lost.

slows down. Slower drainage, in turn, contributes to flooding. As a result, storm surges from hurricanes, for example, could do serious damage to New York City, which is only 16 feet (5m) above mean sea level. Jianjun Yin, a climate modeler at Florida State University, believes that sea-level rise along the Atlantic Coast would be significantly higher than in other areas because of changes in ocean currents. Yin explains that "The northeast coast of the United States is among the most vulnerable regions to future changes in sea level and ocean circulation, especially when considering its population density and the potential socioeconomic consequences of such changes."33

North Carolina is another area that might be especially hard hit, since parts of the coast there are already sinking, potentially compounding rise in sea levels. "Tide-gauge readings in the mid-Atlantic indicate that relative sea level rise [the combination of rising waters and sinking land] was generally higher—by about a foot—than the global average during the 20th century." Some mid-Atlantic barrier islands could end up "destabilizing and breaking apart" as the waters rise. Louisiana, Florida, and other sections of the East Coast would also be threatened.34

California, too, is threatened by sea-level rise. The 4-foot (1.4m) increase in sea levels that many scientists expect within the next one or two centuries would "put 480,000 people at risk

A dike near the town of Wilnis, Netherlands collapsed on August 26, 2003, causing the evacuation of2,000 residents. A rise in sea levels could cause similar problems for many low-lying nations. Continental/AFP/Getty Images.

of a [hundredj-year flood [that is, of a flood expected to occur only once every 100 years] . . . In addition, the cost of replacing property at risk of coastal flooding under this sea-level rise scenario is estimated to be nearly $100 billion."35

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