IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group II: Impacts, Adaptation and Vulnerability Human settlement

Climate change is almost certain to affect human settlements, large and small, in a variety of significant ways. Settlements are important because they are where most of the world’s population live, often in concentrations that imply vulnerabilities to location-specific events and processes and, like industry and certain other sectors of concern, they are distinctive in the presence of physical capital (buildings, infrastructures) that may be slow to change.

Beyond the general perspectives of TAR (see Section 7.1.4), a growing number of case studies of larger settlements indicate that climate change is likely to increase heat stress in summers while reducing cold-weather stresses in winter. It is likely to change precipitation patterns and water availability, to lead to rising sea levels in coastal locations, and to increase risks of extreme weather events, such as severe storms and flooding, although some kinds of extreme events could decrease, such as blizzards and ice storms (see city references below; Klein et al., 2003; London Climate Change Partnership, 2004; Sherbinin et al., 2006).

Extreme weather events associated with climate change pose particular challenges to human settlements, because assets and populations in both developed and developing countries are increasingly located in coastal areas, slopes, ravines and other risk-prone regions (Freeman and Warner, 2001; Bigio, 2003; UN-Habitat, 2003). The population in the near-coastal zone (i.e., within 100 m elevation and 100 km distance of the coast) has been calculated at between 600 million and 1.2 billion; 10% to 23% of the world’s population (Adger et al., 2005b; McGranahan et al., 2006). Globally, coastal populations are expected to increase rapidly, while coastal settlements are at increased risk of climate change-influenced sea-level rise (Chapter 6). Informal settlements within urban areas of developing-country cities are especially vulnerable, as they tend to be built on hazardous sites and to be susceptible to floods, landslides and other climate-related disasters (Cross, 2001; UN-Habitat, 2003).

Several recent assessments have considered vulnerabilities of rapidly growing and/or large urban areas to climate change. Examples include cities in the developed and developing world such as Hamilton City, New Zealand (Jollands et al., 2005), London (London Climate Change Partnership, 2004; Holman et al., 2005), New York (Rosenzweig and Solecki, 2001a, b), Boston (Kirshen et al., 2007), Mumbai, Rio de Janeiro, Shanghai (Sherbinin et al., 2006), Krakow (Twardosz, 1996), Caracas (Sanderson, 2000), Cochin (ORNL/CUSAT, 2003), Greater Santa Fe (Clichevsky, 2003), Mexico City, Sao Paolo, Manila, Tokyo (Wisner, 2003), and Seattle (Office of Seattle Auditor, 2005).

Climate change is likely to interact with and possibly exacerbate ongoing environmental change and environmental pressures in settlements. In areas such as the Gulf Coast of the United States, for example, land subsidence is expected to add to apparent sea-level rise. For New York City, sea-level rise will accelerate the inundation of coastal wetlands, threaten vital infrastructure and water supplies, augment summertime energy demand, and affect public health (Rosenzweig and Solecki, 2001a; Knowlton et al., 2004; Kinney et al., 2006). Significant costs of coastal and riverine flooding are possible in the Boston metropolitan area (Kirshen et al., 2006). Climate change, a city’s building conditions, and poor sanitation and waste treatment could coalesce to affect the local quality of life and economic activity of such cities as Mumbai, Rio de Janeiro and Shanghai (Sherbinin et al., 2006). In addition, for cities that play leading roles in regional or global economies, such as New York, effects could be felt at the national and international scales via disruptions of business activities linked to other places (Solecki and Rosenzweig, 2007).

Sea-level rise could raise a wide range of issues in coastal areas. Studies in the New York City metropolitan area have projected that climate-change impacts associated with expectations that sea level will rise, could reduce the return period of the flood associated with the 100-year storm to 19 to 68 years on average, by the 2050s, and to 4 to 60 years by the 2080s (Rosenzweig and Solecki, 2001a), jeopardising low-lying buildings and transportation systems. Similar impacts are expected in the eastern Caribbean, Mumbai, Rio de Janeiro and Shanghai, where coastal infrastructure, population and economic activities could be vulnerable to sea-level rise (Lewsey et al., 2004; Sherbinin et al., 2006). Due to a long coastline and extensive low-lying coastal areas, projected sea-level rise in Estonia and the Baltic Sea region could endanger natural ecosystems, cover beach areas high in recreational value, and cause environmental contamination (Kont et al., 2003).

Another body of evidence suggests that human settlements, coastal and otherwise, are affected by climate change-related shifts in precipitation. Concerns include increased flooding potential from more sizeable rain events (Shepherd et al., 2002). Conversely, as suggested by the TAR, any change in climate that reduces precipitation and impairs underground water resource replenishment would be a very serious concern for some human settlements, particularly in arid and semi-arid areas (Rhode, 1999), in settlements with human-induced water scarcity (Romero Lankao, 2006), and in regions dependent on melted snowpack and glaciers (Chapter 1, Box 1.1; Chapter 12, Section 12.4.3; Chapter 13, Section 13.6.2).

A wider range of health implications of climate change also can affect settlements. For example, besides heat stress and respiratory distress from air quality, changes in temperature, precipitation and/or humidity affect environments for water- and vector-borne diseases and create conditions for disease outbreaks (see Chapters 4 and 8). Projections of climate-change impacts in New York City show significant increases in respiratory-related diseases and hospitalisation (Rosenzweig and Solecki, 2001a).

With growing urbanisation and development of modern industry, air quality and haze have become more salient issues in urban areas. Many cities in the world, especially in developing countries, are experiencing air pollution problems, such as Buenos Aires, London, Chongqing, Lanzhou, Mexico City and São Paulo. How climate change might interact with these problems is not clear as a general rule, although temperature increases would be expected to aggravate ozone pollution in many cities (e.g., Molina and Molina, 2002; Kinney et al., 2006). A study evaluating the effects of changing global climate on regional ozone of 15 cities in the U.S. finds, for instance, that average summertime daily maximum ozone concentrations could increase by 2.7 parts per billion (ppb) for a 5-year span in the 2020s and 4.2 ppb for a 5-year span in the 2050s. As a result, more people (especially the elderly and young) might be forced to restrict outdoor activities (NRDC, 2004).

Another issue is urban heat island (UHI) effects: higher temperatures occur in urban areas than in outlying rural areas because of diurnal cycles of absorption and later re-radiation of solar energy and (to a much lesser extent) heat generation from built/paved physical structures. The causes of UHI are complex, as is the interaction between atmospheric processes at different scales (Oke, 1982). UHI can affect the climatic comfort of the urban population, potentially related to health, labour productivity and leisure activities; there are also economic effects, such as the additional cost of climate control within buildings, and environmental effects, such as the formation of smog in cities and the degradation of green spaces. Even such small coastal towns as Aveiro in Portugal have been shown to create a heat island (Pinho and Orgaz, 2000). Rosenzweig et al. (2005) found that climate change based on downscaled general circulation model (GCM) projections would exacerbate the New York City UHI by increasing baseline temperatures and reducing local wind speeds.

In sum, settlements are vulnerable to impacts that can be exacerbated by direct climate changes (e.g., severe storms and associated coastal and riverine flooding, especially when combined with sea-level rise, snow storms and freezes, and fire). Yet climate change is not the only stress on human settlements, but rather it coalesces with other stresses, such as scarcity of water or governance structures that are inadequate even in the absence of climate change (Feng et al., 2006; Sherbinin et al., 2006; Solecki and Rosenzweig, 2007). Such phenomena as unmet resource requirements, congestion, poverty, political and economic inequity, and insecurity can be serious enough in some settlements (UN-Habitat, 2003) that any significant additional stress could be the trigger for serious disruptive events and impacts. Other stresses may include institutional and jurisdictional fragmentation, limited revenue streams for public-sector roles, and inflexible patterns of land use (UNISDR, 2004). These types of stress do not take the same form in every city and community, nor are they equally severe everywhere. Many of the places where people live across the world are under pressure from some combination of continuing growth, pervasive inequity, jurisdictional fragmentation, fiscal strains and aging infrastructure (UN-Habitat, 2003).