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 Ni�o 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 32�C 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; J�uregui 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 Sa� 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 4�C 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.
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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 (J�uregui, 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.
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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.
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