Impacts Of Sea Level Rise And Other Climate Changes On Coastal Environments

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Coastal areas are among the most densely populated regions of the United States, and around the world. In 2003, 53 percent of the U.S. population lived in (1) counties with at least 15 percent of its total land area located within the nation's coastal watershed or (2) a county with a portion of its land that accounts for at least 15 percent of a coastal cataloging unit1 bordering the ocean and the Great Lakes, and 23 of the 25 most densely populated counties in 2003 were coastal counties (Cros-set et al., 2005). Considering only coastal counties that border the ocean or contain flood zones with at least a 1 percent chance every year of experiencing flooding from coastal storms and being impacted by wave action, the coastal population, excluding the Great Lakes counties, was 85,640,000, or 30 percent of the total U.S.

1 The National Oceanic and Atmospheric Administration defines a coastal cataloging unit as "a drainage basin that falls entirely within or straddles an Estuarine Drainage Area or Coastal Drainage Area" (Crowell et al., 2007).

population in the 2000 Census (Crowell et al., 2007).2 Such population concentration and growth are accompanied by a high degree of development and use of coastal resources for economic purposes, including industrial activities, transportation, trade, resource extraction, fisheries, tourism, and recreation. They also imply significant investments in infrastructure to support these human activities (see Chapters 12 and 13).

While humans have always made use of coastal resources and areas, permanent settlements with high levels of investment and infrastructure are a relatively recent phenomenon, as prehistoric peoples and even early settlers of the United States did not have the technology to protect themselves against storms. The modern concentration of people, human activities, development, and infrastructure is taking place in one of the most dynamic environments on Earth, where land, ocean, and climate are constantly changing. This interaction between a highly variable natural environment and the growing pressures from human use and development produces multiple stresses that make coastal areas particularly vulnerable to additional impacts from climate change.

The IPCC (Nicholls et al., 2007), the recent Global Climate Change Impacts on the United States report by the U.S. Global Change Research Program (USGCRP, 2009a), and other studies have documented that a growing number of well-studied coastal areas are already experiencing the effects of rising sea levels and related changes in climate. Physical damage and economic losses from coastal storms and related flooding, erosion, and cliff failures in highly developed regions are increasing; coastal wetlands, hemmed in by human development and deprived of river-borne sediment supplies, are being lost at an increasing rate; the frequency of coral bleaching and mortality events is increasing (see Chapter 9); water quality is declining from the combined impacts of effluent, higher water temperatures, and changes in runoff; saltwater is increasingly intruding into coastal groundwater resources (see Chapter 8); and coastal ecosystems are almost exclusively negatively impacted by the combination of all these climatic changes and human pressures, undermining fisheries, tourism, and long-term sustain-ability of coastal areas (Nicholls et al., 2007).

Increases in average sea level magnify the impacts of extreme events on coastal landscapes. Relatively small changes in average sea level can have dramatic impacts on storm surge elevation and on the inland extent and frequency of flooding events, depending on coastal topography and the existence of protective structures such as

2 The selection of the most appropriate demographic data set for evaluating vulnerability to sea level rise (or any other impact of climate change) depends on the focus, scale, and purpose of the study (see, e.g., Crowell et al., 2010).

seawalls, levees, and dikes (e.g., Kirshen et al., 2008). For example, analyses for San Francisco Bay indicated that increases in average sea level as small as 1 foot (0.3 meter) would lead to floods as high as today's 100-year floods (that is, a flood that could be expected to occur once every one hundred years under current climate conditions) every 10 years (Field et al., 1999). Interestingly, in a number of locations along the U.S. coastline, average higher high water (the higher of the two high waters of any tidal day) is rising faster than average sea level, for reasons not yet fully understood; this increases the risk of extensive coastal flooding even more than the rise in average sea levels would suggest (Flick et al., 1999, 2003). In general, the direct losses of coastal habitat and built environments from gradual sea level rise can be greatly amplified by the far larger impacts of flooding, erosion, and wind damage caused by extreme events (Adams and Inman, 2009; Flick, 1998; Nicholls and Tol, 2006; Nicholls et al., 1999; Pendleton et al., 2009; Sallenger et al., 2002; Zhang et al., 2004).

The economic impacts of climate change and sea level rise on coastal areas are probably the second most frequently studied economic impacts in the United States after those on agriculture. Since the first study of this sort in 1980 (Schneider and Chen, 1980), economic impact assessment methodologies have become increasingly sophisticated, though they remain partial and subject to the commonly cited challenges of cost-benefit analyses (see Chapter 17). Analysts have examined the damage potential of gradual sea level rise on taxable real estate in coastal areas subject to inundation; expected impacts of extreme events (floods) on land loss, housing structures, property values, and building contents, as well as integrated impact analyses of combined sea level rise and extreme events; the wider impacts of sea level rise on economies dependent on coastal areas; and the cost of various response options (e.g., seawalls and other hard structures to prevent inundation or erosion loss, beach nourishment requirements as higher sea levels increase the rate of coastal erosion and sediment movement, and relocation or retreat from the shoreline; e.g., Bosello et al., 2007; Nicholls et al., 2007; Yohe et al., 1999). Simple conclusions about the nationwide magnitude of economic impacts cannot be drawn from these studies as metrics, modeling approaches, sea level rise projections, inclusions of coastal storms, and assumptions about human responses (e.g., the type and level of protection) vary considerably. The U.S. National Assessment's coastal sector assessment (Boesch et al., 2000) estimated the cumulative cost of an 18-inch (46-centimeter) sea level rise by 2100 at between $20 and $200 billion, and a 3-foot (roughly 1-meter) sea level rise produced roughly double this figure. The wide range of estimates illustrates the considerable uncertainties involved in the underlying assumptions and calculations. Consistent approaches across U.S. coastal regions would provide much improved understanding of the economic threats.

Steady progress is being made toward more interdisciplinary, integrated analyses of the impacts of sea level rise and other climate and climate-related changes on coastal areas (see Box 4.2). However, most analyses to date still have not assessed economic impacts on culturally or historically important sites, or on coastal infrastructure such as wastewater treatment plants, water supply (drinking water treatment and desalination facilities), utilities (natural gas, electricity, and telephone lines), roads, airports, harbors, and other transportation infrastructure although there are some notable exceptions for certain U.S. locations (e.g., Heberger et al., 2009; Larsen et al., 2008; NRC, 2008g). Impacts on nonmarket values such as the loss of natural habitat have been equally challenging to assess and therefore are still often omitted from economic impact assessments. Coastal ecosystems such as dunes, wetlands, seagrass beds, and mangroves provide numerous ecosystem goods and services, ranging from nursery habitat for certain fish and shellfish to habitat for bird, mammal, and reptilian species, including some endangered ones; protective or buffering services for coastal development against the impacts of storms; water filtering and flood retention; and the aesthetic, cultural, and economic value of beaches and coastal environments for recreation, tourism, and simple enjoyment (for detailed reviews of this literature see Darwin and Tol, 2001; Nicholls et al., 2007; West and Dowlatabadi, 1999).

As climate continues to change and sea level continues to rise through the twenty-first century, these physical, ecological, and socioeconomic impacts on coastal areas are expected to increase and intensify. Moreover, they can be expected to be exacerbated by continued growth in human pressures on coastal areas. Even if sea level rise were to remain in the conservative range projected by the IPCC (0.6 to 1.9 feet [0.18 to 0.59 meters])—not considering potentially much larger increases due to rapid decay of the Greenland or West Antarctic ice sheets—tens of millions of people worldwide would still become vulnerable to flooding due to sea level rise over the next 50 years (Nicholls, 2004; Nicholls and Tol, 2006). This is especially true in densely populated, low-lying areas with limited ability to erect or establish protective measures. In the United States, the high end of the conservative IPCC estimate would result in the loss of a large portion of the nation's remaining coastal wetlands. The impact on the east and Gulf coasts of the United States of 3.3 feet (1 meter) of sea level rise, which is well within the range of more recent projections for the 21st century (e.g., Pfeffer et al., 2008; Vermeer and Rahmstorf, 2009), is shown in pink in Figure 7.7. Also shown, in red, is the effect of 19.8 feet (6 meters) of sea level rise, which could occur over the next several centuries if warming were to continue unabated.

municipality

) % area susceptible @ 6-meter rise o % area susceptible @ 1-meter rise

6-meter rise >1-meter rise

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