BOX 2.1 VULNERABILITY OF U.S. COASTAL INFRASTRUCTURE

Thirty nine percent of the population lives in coastal shoreline counties. This population grew by 39 percent between 1970 and 2010, and is projected to grow by 8.3 percent by 2020. The population density of coastal counties is 446 people per sq mile, which is over 4 times that of inland counties.

Just under half of the annual GDP of the United States is generated in coastal shoreline counties, an annual contribution that was $6.6 trillion in 2011. If counted as their own country, these counties would rank as the world’s third largest economy, after the United States and China. Some portions of these counties are well above sea level and not vulnerable to flooding (e.g., Cadillac Mountain, Maine, in Acadia National Park, at 470 m). But, the interconnected nature of roads and other infrastructure within political divisions mean that sea-level rise would cause problems even for the higher parts of these counties. The following statistics, from NOAA’s State of the Coast,a highlight the wealth and infrastructure at risk from rising seas:

  • $6.6 trillion: Contribution to GDP of the coastal shoreline counties, just under half of US GDP in 2011.b
  • 51 million: Total number of jobs in the coastal shoreline counties of the US in 2011.c
  • $2.8 trillion: Wages paid out to employees working at establishments in the coastal shoreline counties in 2011.d
  • 3: Global GDP rank (behind the United States and China) of the coastal shoreline counties, if considered an individual country.e
  • 39%: Percent of the nation’s total population that lived in coastal watershed counties in 2010 (less than 20 percent of the total land area excluding Alaska).f
  • 34.8 million: Increase in US coastal watershed county population from 1970 to 2010 (or a 39 percent increase).g
  • 446 persons/mi2: Average population density of the coastal watershed counties (excluding Alaska). Inland density averages 61 persons per square mile.h
  • 37 persons/mi2: Expected increase in US coastal watershed county population by 2020 (or an 8.3 percent increase).i

Projections of sea-level rise remain notably uncertain even if the increase in greenhouse gases is specified accurately, but many recently published estimates include within their range of possibilities a rise of 1m by the end of this century (reviewed by Moore et al., 2013). For lowlying metropolitan areas, such as Miami and San Francisco, such a rise could lead to significant flooding (Figure A) (NRC, 2012e; Strauss et al., 2012; Tebaldi et al., 2012). In many cases, such areas would be difficult to defend by dikes and dams, and such a large sea level rise would require responses ranging from potentially large and expensive engineering projects to partial or nearcomplete abandonment of now-valuable areas as critical infrastructure such as sewer systems, gas lines, and roads are disrupted, perhaps crossing tipping points for adaptation (Kwadijk et al., 2010). Miami was founded little more than one century ago, and could face the possibility of sea level rise high enough to potentially threaten the city’s critical infrastructure in another century (Strauss et al., 2013). In terms of modern expectations for the lifetime of a city’s infrastructure, this is abrupt. If sometime in the coming centuries sea level should rise 20 to 25 m, as suggested



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