Climate Change 2001: Working Group II: Impacts, Adaptation and Vulnerability

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7.2.2.3. Urban Settlements

Urban settlements feature many of the same impacts of climate change as other settlements—such as air and water pollution, flooding, or consequences of increasingly viable disease vectors. These impacts may take unusual or extremely costly forms in urban areas—for example, flooding that results not from river flooding but from overwhelmed urban storm drains and sewers during extreme rainfall events (which may become more common in the future). Urban settlements also experience the consequences of accommodating migrant populations, the unique aspects of urban heat islands (which affect human health and energy demand), and some of the more severe aspects of air and water pollution. To some extent, the effects of climate change anywhere in the world are integrated through world markets, social and political changes, and migration. Many of these social and economic effects appear in the world’s cities, including some of the world’s largest (Rosenzweig and Solecki, 2000).

7.2.2.3.1. Migration

The SAR discussed at some length the potential impact of population movement in response to environmental impact and attractions of large urban areas. Human populations show significant tendencies to adapt to interannual variability of climate via migration, although migration may be the last of a complex set of coping strategies (Meze-Hausken, 2000). For example, decreases in crop and rice yields as a result of a prolonged dry season under ENSO conditions in Indonesia causes farmers to leave the villages to work in the surrounding cities. Population subsequently recovers (Yoshino, 1996b; Yoshino et al., 1997). In some cases, immigration is more permanent and does not involve large urban areas. For example, after three successive typhoons hit Tau Island in American Samoa in 1987, 1990, and 1991, about one-third of the population abandoned their homes and moved to Pago Pago on Tutuila Island, putting more population pressure on the limited economic opportunities and services of that island (Meehl, 1996).

A school of thought based on observations of several ethnic conflicts in the developing world suggests that environmental degradation, loss of access to resources, and resulting human migration (including “environmental refugees”) in some circumstances could become a source of political and even military conflict (see Chapters 10 and 11). The result is possible, but the many intervening and contributory causes of intergroup and intragroup conflicts allow only low confidence in predictions of increases in such conflicts as a result of climate change, even where environmental resources are scarce and threatened (e.g., Wolf, 1998).

7.2.2.3.2. Human health

The impact of climate change on human heath in settlements is a complicated mechanism that involves the interaction of physical attributes of settlements and precursors for direct effects of heat stress, vector-borne diseases such as malaria, and enteric diseases such as cholera (see Chapter 9). These impacts are common to all types of settlements, including traditional ones. The 1997–98 El Niño event provided a way to derive and test process models for the impacts of climate change. In Latin America, for example, outbreaks of malaria and Dengue fever appeared to be related to anomalously high nighttime minimum temperatures (Epstein et al., 1998). Settlements provide disease vectors and organisms with habitat in the form of standing water, garbage dumps, and space sheltered from the elements. Flooding can flush organisms into settlements’ clean water supplies, causing disease outbreaks. Heavy rainfall in normally dry areas leads to rapid increases in rodent populations, in turn leading to increases in rodent-borne diseases such as hantavirus (Glass et al., 2000). Cholera-harboring marine plankton blooms also can be triggered by riverine flooding, which provides extra nutrients to the coastal environment (Colwell, 1996). Extremely dry conditions reduce the quantities and quality of water available for sanitary and drinking purposes, which also can trigger cholera and diarrheal outbreaks. Historically, this has been a problem on various Pacific islands during drought. Poverty, crowding, and poor sanitation in settlements add to these problems, as reported in the SAR.

7.2.2.3.3. Heat islands and human health

Warming of urban air increases in intensity and area as cities grow (Oke, 1982). This growing “heat island” tends to aggravate the risk of more frequent heat waves, as well as their impacts. Research indicates that variability in summer nighttime minimum temperature (temperatures above 32ºC at night)—combined with lack of acclimatization, high humidity, and poorly ventilated and insulated housing stock—may be the most important factor in urban heat deaths (Chestnut et al., 1998). Elderly people, the very young, people in ill health, and poor people are most likely to be affected (see Chapter 9 for these and other health effects). Because climate change is expected to raise nighttime minimum temperatures more than daytime highs, urban heat islands would be a significant health concern in the largest human settlements.

Conversely, during the rainy season (except in a few cities, such as Cairo, where practically no rain occurs) the heat island enhances the intensity and frequency of rain showers (Changnon, 1992; Jáuregui and Romales, 1996), leading to higher risk of street flooding or mudslides where the urban poor live. Moreover, warmer and drier climates may aggravate air pollution seasonally because of wind erosion of bare soil areas in cities with semi-arid or arid climates (e.g., Mexico City, Beijing, Delhi, and cities located in sub-Saharan Africa). Blowing dust and high summer temperatures are likely to increase the incidence of heat stroke, respiratory illness, and transmission of disease by deposition of airborne bacteria in lungs and on food.

In warmer and drier climates, local minimum temperatures tend to be higher, which tends to attenuate the intensity (and depth) of temperature inversions formed by nocturnal radiation cooling and reduce the risk of poor air quality. However—and especially in large cities located in valleys (e.g., Mexico City, Santiago, Beijing, Delhi)—this attenuation effect could be compensated by a higher rate of radiation cooling to an air layer with less moisture content, aggravating the air pollution situation. Elevated subsidence inversions such as those on the descending branch of semi-permanent anticyclones and limiting vertical dispersion of pollutants in cities such as Saõ Paulo, Los Angeles, or Tijuana are less likely to significantly change their thermal structure in a warmer world.

7.2.2.3.4. Water pollution

Despite massive investment in water treatment in the developed world and increasingly in the developing world over the past century, many settlements throughout the world (especially in rural areas) still are without adequate water treatment (UNCHS, 1999). In the case of drought, reduced water availability could force people to use polluted water sources in settlements at the same time that reduced flow rates reduce the rate of dilution of water contaminants. In the opposite case, flooding frequently damages water treatment works and floods wells, pit latrines and septic tanks, and agricultural and waste disposal areas and sometimes simply overwhelms treatment systems, contaminating water supplies.

7.2.2.3.5. Air pollution

Air pollution is a serious problem in many cities of the world, even under the current climate. The following issues emerge from a review of developing country cities that are members of the 69 urban agglomerations with population of more than 3 million in 1990 (UNEP, 1992):

• Population trends have not yet stabilized, which means cities will continue to extend their urban area (and urban heat island).
• Motor vehicles now constitute the main source of pollutants in most cities of the industrialized world. Although the rate of growth in the number of vehicles in some settlements in the developing world has been very rapid (Simon, 1996), cities in developing countries (with exceptions) exhibit greater variety in air pollution sources. This depends on the level of motorization and the level, density, and type of industry present. Cities in Latin America, for example, tend to have high vehicle densities and high vehicle-to-total pollution loads. The major sources of air pollution in Delhi are 2.8 million vehicles, thermal power plants, industries, and domestic fuel combustion. The problem has been compounded by unplanned development, inadequate public transport, poor road conditions, lack of traffic management, inadequate vehicle/engine maintenance, use of old vehicles, and poor fuel quality. Many developing countries’ vehicle fleets tend to be older and poorly maintained (and not easily replaced because of the low incomes of their owners)—a factor that will increase the significance of motor vehicles as a pollution source (UNEP, 1992).
• Cities with seasonally warm, calm air and sunny weather with high traffic densities tend to be especially prone to the net formation of ozone and other photochemical oxidants, although it is not yet clear whether such conditions will be more or less prevalent under climate change. Volatile organic carbons (VOCs) from biogenic and anthropogenic sources such as automobiles increase at high temperatures, and thermal decomposition of peroxyacetyl nitrate (PAN) also increases (Samson et al., 1989).
• Atmospheric and air-shed modeling exercises suggest that higher temperatures and more stable air episodes under global warming of 4ºC could lead to 1–20% increases in peak urban ozone concentrations in the United States (Morris et al., 1989; Penner et al., 1989) and increased violation of clean air standards (Morris et al., 1995). See Chapter 9 for the health effects of elevated ozone levels.

Box 7-2 shows that adaptive measures can be effective in reducing many of the precursors to adverse air quality under current climate. These measures also would help in the context of unfavorable atmospheric conditions as a result of climate change.