There are multiple pressures on human settlements that interact with climate change. The discussion in this chapter shows that these other effects are more important in the short run; climate is a potential player in the long run. For example, urban population in the least-developed countries currently is growing at about 5% yr^-1, compared with 0.3% yr^-1 in highly industrialized countries.¹ Providing for this rapidly urbanizing population will be much higher on most countries� agendas than longer term issues with climate change.

Other environmental problems will tend to interact with climate change, adversely affecting human settlements. For example, 25�90% of domestic energy supply in the developing world is met by biomass resources, especially in small urban centers (Barnes et al., 1998). In some countries, 11�20% of all deforestation may be attributable to charcoal production (Ribot, 1993), much of it to meet urban needs. If biomass growth is slowed via climate change effects, the impacts on biomass may be compounded.

Deforestation and cultivation of marginal lands can compound the effects of extreme events. For example, floods resulting from Hurricane Mitch, though not caused by climate change, illustrate the fact that poor watershed management can contribute to flooding and landslides�which, in turn, causes loss of life and destroys infrastructure and the means of livelihood. Mitch cost Honduras 80% and Nicaragua 49% of one year�s GDP (FAO, 1999). Poor watershed management and technical failures has contributed to loss of life in landslides in Sri Lanka, Peru, Brazil, several European countries, and the United States (Katupotha, 1994)

Urban water resources already are in extremely short supply in 19 Middle Eastern and African countries (IPCC, 1998) and in cities in many parts of the world, where as many as 60% of poorer residents may not have access to reliable water supplies (Foronda, 1998). Poor urban water management may be responsible for losses through leakage of 20�50% in cities in the developing world and even in some cities in the industrialized world (WRI, 1996). If climate change makes water more scarce�by increasing demand (even in regions that currently are not particularly short of water, such as Great Britain�see Arnell, 1998) or by reducing supply (reduced surface runoff, exacerbation of water quality problems as a result of warmer temperatures and reduced flows, or salinization of coastal aquifers resulting from sea-level rise)�water supply problems would be exacerbated.

Liquid waste disposal is a significant problem in urban areas as diverse as Chimbote, Peru (Foronda, 1998); Buenos Aires, Argentina (Pirez, 1998); Cotonou, Benin (Dedehouanou, 1998); and Chicago, USA (Changnon and Glantz, 1996). There are two ways in which climate could interact with this problem: reductions in supplies of water with which wastes are diluted and the impact of more severe flooding episodes that overtop sewer systems and treatment plants (Walsh and Pittock, 1998). Where most inhabitants rely on pit latrines and wells, flooding spreads excreta from pit latrines everywhere and contaminates wells (Boko, 1991, 1993, 1994). Land-use changes associated with urbanization also have reduced the absorptive capacity of many river basins, increasing the ratio of runoff to precipitation and making flooding more likely (Changnon and Demissie, 1996).

Large urban areas, especially in the developed world, depend on extended �linkage systems� for their viability (Timmerman and White, 1997; Rosenzweig and Solecki, 2000). They depend on imports from the local area, region, nation, and even the world for everything from raw material and food, to product and waste exports and communications. These linkage systems often are vulnerable to severe storms, floods, and other severe weather events. Management, redundancy, and robustness of these interlocking systems is a top priority for developed world settlements especially, but increasingly so for the developing world (Timmerman and White, 1997).

Key climate-related sensitivities in urban areas of the world include water supply and the effects of extreme events (primarily flooding) on infrastructure in river floodplains and coastal zones. These areas should be considered sensitive to climate change. To the extent that sensitive settlements coincide with conditions of poverty and lack of technical infrastructure, these settlements also will be particularly vulnerable to climate change.

Because of their role as centers for administration and commerce, urban areas integrate all of the environmental effects that visit a society and to some extent buffer their human occupants from natural environmental fluctuations. However, these urban areas still may be affected by several stresses that interact with each other in a nonlinear fashion (Rosenzweig and Solecki, 2000; Wilbanks and Wilkinson, 2001). Whatever cash economy there is in a country tends to reside in its biggest cities, and trade routes also focus on these areas. These are two very important coping mechanisms. Thus, for example, climate-related food shortages are more likely to be experienced in urban areas as an increase in migrants from the countryside or loss of business in agriculture-related business rather than as famine per se.

Once populations are housed in urban settlements, there are other potential interactions among climate effects that lead to nonlinear impacts on them. Flooding events that are beyond the designed capacity of settlement infrastructure are a case in point, especially in cases in which systems already may be degraded. Urban flooding can overwhelm sewage treatment systems, thereby increasing the risk of disease at the same time that water treatment systems are compromised, health services are disrupted, disease vector species are driven into close contact with people, and people are exposed to the elements because of lost housing. Outbreaks of epidemics are always a risk under such circumstances, whereas the breakdown of any single one of the affected systems might be merely inconvenient. Although all of these effects and mechanisms are well understood, however, it is not possible to predict impacts quantitatively at this time.

Table 7-3 shows the primary synergistic effects between climate-related factors that may affect human settlements and the primary types of settlements or industries affected. Each cell identifies a synergistic effect between the climate impact featured in the row and another effect shown in the column. For example, climate-related impacts of flooding, landslides, and fire are compounded when they occur in settlements that also might be crowded by migration. Likewise, flooding in particular exacerbates water pollution and human health impacts and probably would compound problems in obtaining drinking water and transportation. In addition, the agricultural base and energy supplies could be affected in regions that already are water-deficient.

Air and water pollution effects of climate change would be worse if the health of human populations already is compromised (e.g., asthma attacks may be more severe or prolonged in a weakened population). Charlot-Valdieu et al. (1999) argue for reducing some stresses in a multiple-stress context to handle other stresses in a more sustainable manner.

Access to energy, clean water, sanitation, and selected other resources is essential to maintain human settlements and the health of the populations within. Flooding, landslides, or fire resulting from extreme weather could compound the shortage of resources by destroying critical infrastructure (floods in Honduras in 1998 and Mozambique in 2000 are examples of the phenomenon under current climate) and, in the case of water, polluting the sources. Similarly, water pollution reduces the effective water supply by making some sources unusable.

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