Global climate change is likely to be accompanied by an increase in the frequency and intensity of heat waves, as well as warmer summers and milder winters (see Table 3-10). The impact of extreme summer heat on human health may be exacerbated by increases in humidity (Gaffen and Ross, 1998; Gawith et al., 1999).

Daily numbers of deaths increase during very hot weather in temperate regions (Kunst et al., 1993; Ando, 1998a,b). For example, in 1995, a heat wave in Chicago caused 514 heat-related deaths (12 per 100,000 population) (Whitman et al., 1997), and a heat wave in London caused a 15% increase in all-cause mortality (Rooney et al., 1998). Excess mortality during heat waves is greatest in the elderly and people with preexisting illness (Sartor et al., 1995; Semenza et al., 1996; Kilbourne, 1997; Ando et al., 1998a,b). Much of this excess mortality from heat waves is related to cardiovascular, cerebrovascular, and respiratory disease. The mortality impact of a heat wave is uncertain in terms of the amount of life lost; a proportion of deaths occur in susceptible persons who were likely to have died in the near future. Nevertheless, there is a high level of certainty that an increase in the frequency and intensity of heat waves would increase the numbers of additional deaths from hot weather. Heat waves also are associated with nonfatal impacts such as heat stroke and heat exhaustion (Faunt et al., 1995; Semenza et al., 1999).

Heat waves have a much bigger health impact in cities than in surrounding suburban and rural areas (Kilbourne, 1997; Rooney et al., 1998). Urban areas typically experience higher—and nocturnally sustained—temperatures because of the "heat island" effect (Oke, 1987; Quattrochi et al., 2000). Air pollution also is typically higher in urban areas, and elevated pollution levels often accompany heat waves (Piver et al., 1999) (see also Section 9.6.1.2 and Chapter 8).

The threshold temperature for increases in heat-related mortality depends on the local climate and is higher in warmer locations. A study based on data from several European regions suggests that regions with hotter summers do not have significantly different annual heat-related mortality compared to cold regions (Keatinge et al., 2000). However, in the United States, cities with colder climates are more sensitive to hot weather (Chestnut et al., 1998). Populations will acclimatize to warmer climates via a range of behavioral, physiological, and technological adaptations. Acclimatization will reduce the impacts of future increases in heat waves, but it is not known to what extent. Initial physiological acclimatization to hot environments can occur over a few days, but complete acclimatization may take several years (Zeisberger et al., 1994).

Weather-health studies have used a variety of derived indices—for example, the air mass-based synoptic approach (Kalkstein and Tan, 1995) and perceived temperature (Jendritzky et al., 2000). Kalkstein and Greene (1997) estimated future excess mortality under climate change in U.S. cities. Excess summer mortality attributable to climate change, assuming acclimatization, was estimated to be 500-1,000 for New York and 100-250 for Detroit by 2050, for example. Because this is an isolated study, based on a particular method of treating meteorological conditions, the chapter team assigned a medium level of certainty to this result.

The impact of climate change on mortality from thermal stress in developing country cities may be significant. Populations in developing countries (e.g., in Mexico City, New Delhi, Jakarta) may be especially vulnerable because they lack the resources to adapt to heat waves. However, most of the published research refers to urban populations in developed countries; there has been relatively little research in other populations.

In many temperate countries, there is clear seasonal variation in mortality (Sakamoto-Momiyama, 1977; Khaw, 1995; Laake and Sverre, 1996); death rates during the winter season are 10-25% higher than those in the summer. Several studies indicate that decreases in winter mortality may be greater than increases in summer mortality under climate change (Langford and Bentham, 1995; Martens, 1997; Guest et al., 1999). One study estimates a decrease in annual cold-related deaths of 20,000 in the UK by the 2050s (a reduction of 25%) (Donaldson et al., 2001). However, one study estimates that increases in heat-related deaths will be greater than decreases in cold-related death in the United States by a factor of three (Kalkstein and Greene, 1997).

Annual outbreaks of winter diseases such as influenza, which have a large effect on winter mortality rates, are not strongly associated with monthly winter temperatures (Langford and Bentham, 1995). Social and behavioral adaptations to cold play an important role in preventing winter deaths in high-latitude countries (Donaldson et al., 1998). Sensitivity to cold weather (i.e., the percentage increase in mortality per 1ºC change) is greater in warmer regions (e.g., Athens, southern United States) than in colder regions (e.g., south Finland, northern United States) (Eurowinter Group, 1997). One possible reason for this difference may be failure to wear suitable winter clothing. In North America, an increase in mortality is associated with snowfall and blizzards (Glass and Zack, 1979; Spitalnic et al., 1996; Gorjanc et al., 1999) and severe ice storms (Munich Re, 1999).

The extent of winter-associated mortality that is directly attributable to stressful weather therefore is difficult to determine and currently is being debated in the literature. Limited evidence indicates that, in at least some temperate countries, reduced winter deaths would outnumber increased summer deaths. The net impact on mortality rates will vary between populations. The implications of climate change for nonfatal outcomes is not clear because there is very little literature relating cold weather to health outcomes.

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