Climate Change 2001:
Working Group II: Impacts, Adaptation and Vulnerability
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15.2.5. Human Settlements and Infrastructure

Large metropolitan centers and industrial areas are particularly vulnerable to global environmental change (Schmandt and Clarkson, 1992). Large cities are considered to be areas of high risk because warming could lead to problems such as heat stress, water scarcity, and intense rainfall. Other potential impacts vary with location. In Canada and the northern United States, for example, people in larger cities are expected to experience fewer periods of extreme winter cold (Born, 1996). Many coastal communities will be affected by rising sea levels and increased risk of storm surge, but the impacts will differ because of variations in local and regional factors (Nicholls and Mimura, 1998). Most people in North America live in land that now is considered coastal and subject to coastal weather extremes. This sector of the urban population is growing faster than the population as a whole—a trend that is expected to continue (Boesch et al., 2000). Indeed, "25 percent of the buildings within 500 feet of U.S. coastlines are predicted to fall victim to erosion in the next six decades" (Associated Press, 2000). Cities that are vulnerable to regular flooding may experience changes in the timing, frequency, and severity of this hazard (Weijers and Vellinga, 1995). In addition, the risk of increased periods of drought will be a challenge, particularly for communities that already are struggling to cope with water management issues. Overall, climate change should reduce vulnerability to some hazards such as cold waves but increase it with respect to others such as sustained periods of extreme heat. Demographic Pressures

The number of people in North America increased from 83 million in 1901 to 301 million in 1998 (Statistics Canada, 1999a; U.S. Census Bureau, 1999b). The number living in large urban communities of more than 750,000 people increased over this period from 6 million to more than 140 million. A higher share of the population in North America lives in large urban centers (45% in 1995) than in any other region of the world (UNDP, 1999). These large population centers may be vulnerable to climate change.

Aging of the population is another important demographic trend in North America. Europe and Japan currently are adapting to an aging of the population that is just emerging in North America. These demographic trends also appear in native communities with individuals living to older ages: Elderly people who have lived away from reservations may return home for retirement. An older population typically is more vulnerable to climate extremes (McMichael, 1997). Infrastructure Investments in Adaptation

Many systems have the potential to be impacted negatively by climate change, including drainage and water systems, roads and bridges, and mass transit (Miller, 1989). Communities can reduce their vulnerability and increase their resilience to adverse impacts from climate change through investments in adaptive infrastructure (Bruce et al., 1999a).

Across North America, however, government spending on public infrastructure has been declining for some time, measured as a share of economic activity. Almost 3.5% of GDP was spent on a broad range of infrastructure projects in the early 1960s, compared to less than 2% in the 1990s (Statistics Canada, 1999b; USBEA, 1999).

Sustained lower spending has increased society's vulnerability to some hazards. The American Society of Civil Engineers, for example, has warned that many dams in the United States have exceeded their intended lifespan (Plate and Duckstein, 1998). More than 9,000 regulated dams have been identified as being at high risk of failing, and there may be significant loss of life and property from future failures.

Highways, bridges, culverts, residences, commercial structures, schools, hospitals, airports, coastal ports, drainage systems, communications cables, transmission lines, and other infrastructure have been built on the basis of historical climate experience (Bruce et al., 1999a). Similarly, land-use practices and building codes have been developed to provide effective protection from the existing climate. Emerging knowledge about future climate pressures was not available when most investment decisions were made. This includes, for example, recent research into actions that can be taken to protect underground transit systems from the increased risk of intense rainfall and subsequent flooding (Liebig, 1997). Furthermore, building codes have been modified frequently over the years to reflect emerging information about safer construction techniques, but these changes have not been applied to existing structures.

Coastal communities have developed a variety of systems to manage exposure to erosion, flooding, and other hazards affected by rising sea levels, but some of these systems have not been maintained—increasing the difficulty of putting in place enhancements to address future risk. Similarly, consideration of investments in larger diameter storm sewers in communities that expect more periods of intense rainfall may be affected by the age of existing systems (Fowler and Hennessey, 1995; Trenberth, 1998). Urban expansion and population growth further complicate decisionmaking with respect to infrastructure investments.

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