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).
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).
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 Niņo 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.
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 32ēC 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.
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.
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):
Box 7-2. Air Pollution Problems of Large Cities in the Developing World-Update on the Case of Mexico City
The SAR notes the complex air pollution problems of Mexico City and expresses concern that increases in air stability episodes under climate change could exacerbate an already difficult situation. Although these problems remain severe, as noted below, Mexico City also shows that adaptation (mitigation of air pollution) is possible and effective.
Mexico City is located in an elevated inland valley (approximately 2,250 m above sea level) in central Mexico. The climate is subhumid tropical, tempered by the altitude. Industrial activity contributes 20% of the city's air pollution; 3 million motor vehicles that consume 17.3 million liters of gasoline and 5 million liters of diesel daily (in 1997) generate 75% of the pollution (Office of the Environment Annual Report, 1997). Typical anticyclonic weather prevailing during the dry season contributes to frequent thermal inversions that prevent dispersion of pollutants. Moreover, abundant insolation prevailing during the dry warm period (March to May) favors activation of precursors [mainly oxides of nitrogen (NOx) and hydrocarbons (HC)] to produce high levels of ozone. This also is the season for dust storms and blowing dust (Jáuregui, 1989). Although transport may generate 75% of the airborne pollutants by weight, estimates made in 1989 regarding transport's contribution to air pollution in terms of toxicity suggested its contribution was 42.4%. A considerable contribution also comes from vegetation and topsoil (12% of all airborne pollutants by weight in 1994) (see Connolly, 1999).
The Mexico City Ministry of the Environment subsequently found that ozone-one of the most serious threats to health-was still above acceptable levels on 300 days in 1999 but that some progress had been achieved. During the worst days from 1990 to 1992, pollutants hit emergency levels on as many as 177 days annually. Emergency levels occurred on 5 days in 1999. This achievement was considered to be a result of anti-pollution efforts by the local government. For example, in the early 1990s, lead was removed from gasoline sold in the Valley of Mexico; laws restricted the use of cars without catalytic converters to 4 weekdays; and inspections of factories to reduce pollution doubled in 1999, to 152. The average ozone reading fell from 197.6 to 144, where the air quality norm is 100 points of ozone-equivalent to exposure to 0.11 ppm for 1 hour (Mexico City Ministry of the Environment, 2000). This progress in improving air quality suggests that even huge developing world cities can begin to reduce pollution.
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.
Other reports in this collection