Climate change will affect human settlements against a very dynamic background of other environmental and socioeconomic factors. Human settlements are expected to be among the sectors that could be most easily adapted to climate change, given appropriate planning and foresight and appropriate technical, institutional, and political capacity. This chapter covers the same general topic areas as Chapters 11 and 12 of the Second Assessment Report (SAR); however, this chapter analyzes a wider variety of settlement types, provides a specific assessment of uncertainty and confidence in findings and adaptive capacity in human settlements, and places much of the discussion in a context of development, sustainability, and equity (DSE) (see Munasinghe, 2000). Energy and industry are treated as part of settlements. Figure 7-1 characterizes the conclusions of this chapter on two dimensions: scientific support (evidence available in the literature to support the finding) and consensus in that literature. Both scales are described more fully in Box 1-1 of Chapter 1 and in Moss and Schneider (2000).

Major Effects on Human Settlements

Infrastructure would have increased vulnerability to urban flooding and landslides (established but incomplete). Detailed modeling of rainfall event frequency and intensity in the context of global warming has been linked to increased intensity and frequency of urban flooding, with considerable damage to infrastructure (see Chapters 4, 8, and 10�15). Although not definitive for any part of the world, the model-based analysis is plausible and demonstrates that flooding could be an increased threat for riverine settlements under climate change. More predictable is loss of snow pack in many regions, combined with more winter flooding. Landslides are a current threat in many hilly areas and could be more so with more intense rainfall events.

Figure 7-1: Human settlements impacts, categorized by state of scientific knowledge.

Tropical cyclones would be more destructive under climate change (established but incomplete). Close behind floods and landslides are tropical cyclones (hurricanes or typhoons), which could have higher peak intensity in a warmer world with warmer oceans (see Chapters 3 and 8, and TAR WGI Chapter 10). Tropical cyclones combine the effects of heavy rainfall, high winds, storm surge, and sea-level rise in coastal areas and can be disruptive far inland but are not as universally distributed as floods and landslides.

Water supplies for human settlements would be vulnerable to increased warming, dryness, and flooding (established but incomplete). There is reasonable consensus among experts that settlements in regions of the world that already are water-deficient (e.g., much of north Africa, the Middle East, southwest Asia, portions of western North America, and some Pacific islands) would face still higher demands for water with a warmer climate, with no obvious low-cost ways in which to obtain increased physical supplies. Observations on current water supply balances tend to back up this conclusion (see Chapters 4 and 10�13). However, theory and model output, though consistent with this view, are too weak quantitatively to offer much support, especially for urban areas. Repeated flooding also could create water quality problems in other areas.

Fire danger in settlements could increase with climate change (speculative for resource-dependent settlements; established but incomplete for infrastructure). Examples include forested and wildland/urban fringes in boreal regions (e.g., Canada, Alaska, Russia) and in Mediterranean climates in both hemispheres (e.g., California, southern Spain and France, and Australia) that could be affected (see Chapters 11, 12, 13, and 15). Although general circulation model (GCM)-projected summer climate in many regions looks similar to the hot, dry �fire weather� in many warm years of recent memory and economic activity in forests sometimes is restricted to reduce fire danger, impacts on the resource base have not been demonstrated, research has not shown what future fuel loadings would be, and it is unclear whether future economic activity and settlement infrastructure would be more vulnerable to fire.

Hail and windstorm could cause more damage to settlements (speculative). Although there is potential for more (and more severe) extreme weather episodes in a warmer atmosphere, modeling and data have not demonstrated a higher incidence of storms or of more severe storms (see Chapters 3, 8, 12, and 15).

Agroindustry and artisanal fisheries are sensitive to and in many cases vulnerable to climate change (well-established overall; competing explanations in specific regions). This conclusion dates back to the First Assessment Report (FAR). Additional studies and analysis conducted in the past 10 years have modified the details of the conclusion but have not overturned it. As described in Chapter 5, agriculture itself is sensitive to climate change. In some cases, yields may be reduced by as much as several tens of percent as a result of hotter weather, greater evaporation, and lower precipitation in mid-continental growing regions in particular. However, other regions may benefit, with higher yields possible. Impacts on agricultural processors and suppliers would tend to follow the impacts on agriculture itself. Changes in ocean conditions from El Ni�o episodes have demonstrated that changes such as ocean warming have substantial impacts on the locations and types of species available for fisheries, especially artisanal fisheries, but other regions could benefit (see Chapters 5, 6, and 10�17).

Heat waves would have more serious effects on human health and productivity (competing explanations). The impact of heat waves is most severe on the weakest parts of the populations (old, chronically ill, very young) that are not acclimated, but effects on future overall death rates are less clear (see Chapters 9, 11, 13, 14, and 15). Because anthropogenic warming is projected to be greater at night than during the day, it would deprive sufferers of nighttime relief. Projections for several temperate climates show increased risk of severe heat waves (Chapter 3). As the weather becomes very warm, economic productivity of unprotected and outdoor populations declines.

Sea-level rise increases the cost/vulnerability of infrastructure and coastal resource-based industry (well-established for infrastructure; established but incomplete for resources). Although the amount of sea-level rise to be expected as a result of global warming by any given date and in any given location is uncertain, some studies are beginning to discuss likely ranges and probability distributions (e.g., Titus and Narayanan, 1995). The sensitivity of human infrastructure in coastal zones to given levels of sea-level rise is backed by theory, model results, and data on current rates of increase. In addition, several industries�such as tourism and recreation (the principal industry in many island economies)�are dependent on coastal resources (see Chapters 6, 8, and 10�17). Effective types of adaptive responses also are known in some circumstances, but vulnerability with adaptation is difficult to assess because the capacity and will to respond are uncertain or in doubt in many instances.

Energy demand in some locations is sensitive, and parts of the supply system are vulnerable (well-established). Modeling, theory, data, and expert opinion all say that warming of 1�5�C would considerably reduce the amount of energy that would be needed to heat buildings at mid- and high latitudes and altitudes, whereas cooling energy use would increase (see Chapters 10�15 and 17). The net overall impact on energy use would depend on local circumstances. If temperature increases take place primarily at night and during winter months, heating demand would be smaller and the increase in demand for energy for cooling and irrigation would be somewhat smaller than otherwise. Future climate is expected to include more intense rainfall events (which would require more conservative water storage strategies to prevent flood damage), greater probability of water deficits (less hydroelectric production), and less precipitation falling as snow (less water available during warm months) (see Chapter 4). All three factors point to less (or, at least, less flexible) hydroelectric capacity at current powerhouses. Reduced flows in rivers and higher temperatures reduce the capabilities of thermal electric generation, and high temperatures may reduce transmission capabilities as well.

There will be increased air and water pollution impacts (competing explanations). Climate change could contribute to water pollution problems in human settlements through drought or flooding, although not by simple increases in flow (which offers more dilution for pollutants). If droughts and floods become more frequent (see Chapter 3), so would instances of poor water quality (see Chapters 4, 10�15, and 17). Air pollution could be exacerbated if climate change alters the stability of air sheds and permits greater buildup of atmospheric pollutants (see Chapters 10�15). However, the outcomes remain largely theoretical, unsupported by data or modeling.

Infrastructure in permafrost regions is vulnerable to warming (well-established). Data from circumpolar regions and model results suggest that permafrost areas would see some melting of permafrost. Permafrost melting is a threat to infrastructure in these regions because of increased landslides and loss of foundation stability for structures, as well as increased damage from freeze-thaw cycles, among other impacts. In addition, melting permafrost is thought to be a source of methane (CH₄) and carbon dioxide (CO₂) gases (see Chapters 15 and 16).

Heat island effects could increase heat stress, increase summer energy demand, and reduce winter energy demand (competing explanations). As discussed in Chapters 3 and 9, heat waves may increase in frequency and severity in a warmer world, leading directly to increases in mortality among sensitive populations that are not acclimated. Heat island effects exacerbate the oppressive effects of heat waves by increasing temperatures experienced in the summer by up to several �C; at the same time, increased demand for air conditioning increases the demand for electricity and the severity of the heat island itself through thermal electric production. Winter energy use for heating would be reduced by the same phenomenon (see Chapters 11, 13, 14, and 15). Effects in specific regions are far less clear.

Other Observations

Local capacity is critical to successful adaptation (well-established). Adaptation means local tuning of settlements to a changing environment, not just warmer temperatures. Urban experts are unanimous that successful environmental adaptation cannot occur without locally based, technically and institutionally competent, and politically supported leadership. Local adaptive capacity generally is strongly correlated with the wealth, human capital, and institutional strength of the settlement. In addition, capacity depends in part on the settlement�s access to national resources. Attempts to impose environmental solutions on settlements from the international or national level frequently have been maladapted to local circumstances. The most effective sustainable solutions are strongly supported and often developed locally, with technical assistance and institutional support from higher level bodies (see Chapters 10, 11, 14, 17, and 18).

Nonclimate effects are likely to be more important than climate change (competing explanations). The effects of climate change would occur against a background of other socioeconomic and environmental change that is itself very uncertain and complex (see Chapter 3). Model results, the current rate of environmental change, and economic theory all suggest that climate would be a relatively small additional uncertainty for most human settlements. Climate change in isolation also is unlikely to be as important a factor for DSE effects as other aspects of development, such as economic and technological change. In combination with other stresses from other processes such as population growth, however, climate change is likely to exacerbate total stresses in a multi-stress context. Particularly important could be effects of climate change on equity because relatively advantaged parts of global and local societies are likely to have better coping capacities than less advantaged parts.

Managing growth to ensure that it is sustainable and equitably distributed currently is a greater problem for most countries than the impacts of climate change. However, some experts are not in agreement on this point for the future, pointing out that the economic models do not show climate feedback to the economy and that climate effects are so uncertain that they could well dominate in some regions, especially by the end of the 21st century.

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