CIESIN Thematic Guides

The Degree to which Mitigating Factors May Modify the Effects of Global Warming on Mortality

Cultural and physiological adaptations over time may alter the relationship between global warming and human mortality. Attempts to estimate potential increases in mortality, therefore, should take these factors into account. Several studies evaluate humans' physical ability to adapt to extreme heat episodes. Those studies that consider social adaptations focus almost exclusively on the effect of air conditioning. Because nearly all research has been for western populations, even less is known about the ability of people in developing countries to adapt.

The issue of acclimatization is addressed by White and Hertz-Picciotto (1985) in the section "Human Health" of the Department of Energy's report Characterization of Information Requirements for Studies of CO2 Effects and by Kalkstein and Valimont (1987) in the chapter "Climate Effects on Human Health" of the Environmental Protection Agency's (EPA) monograph Potential Effects of Future Climate Changes. Both cite Gover's 1938 study "Mortality During Periods of Excessive Temperature," indicating that after an initial heat wave, subsequent episodes of equal intensity cause fewer heat-related deaths. Two possible explanations for this phenomenon are that the pool of susceptible individuals has been significantly reduced or that survivors of the first heat wave have become acclimated to the higher temperatures. Kalkstein and Valimont (1986) also raise the issue of geographic variations in response to global warming. In "An Evaluation of Summer Discomfort in the United States," these authors apply the concept of a geographically relative index, the weather stress index, which takes into account the effects of acclimatization on human mortality.

In "The Impact of CO2 and Trace Gas-induced Climate Changes upon Human Mortality," part of the 1989 EPA report The Potential Effects of Global Climate Change on the United States, Kalkstein generates two summary tables, one for summer and one for winter, describing total mortality by city for the doubled CO2 scenario with and without acclimation. With no acclimation to higher temperatures, heat-related deaths could increase sevenfold by the year 2060 for the 15 U.S. cities considered. Because some acclimation is likely, Kalkstein adopts an analog approach where each of the study sites are matched to existing cities that have climates similar to what theirs is estimated to be in the future. Kalkstein warns, however, that there may be differences in the demographic composition of these cities that could have significant effects on heat-related mortality.

In the 1982 article "Risk Factors for Heatstroke," Kilbourne et al. identify a number of factors associated with heat stroke, including alcoholism, living on higher floors of buildings, and use of tranquilizers. Modifying factors include the use of air conditioning, frequent exercise (presumably not during the heat wave), consumption of large quantities of fluids, and a well-shaded residence. Their study demonstrates a direct inverse relationship between daily hours of air-conditioner use and heat-related deaths during high-temperature-periods.

The effect of air-conditioning on heat-related mortality has been the focus of several studies. In "Weather and Human Mortality," Kalkstein and Davis (1989) briefly discuss two studies that consider heat-related mortality in urban areas over several decades, during which air conditioning became widely used. They note that cultural interventions appear to have a minimal impact on heat-related mortality. One study indicates that air conditioning may decrease excess mortality only during initial heat waves, with no apparent effect later in the summer. Kalkstein and Davis take into account age and race, and still indicate a reduction in excess deaths as summer progresses. In addition, the authors find differential heat-related effects in northern and southern cities.

Marmor (1978) reports that short-term spikes in mortality are frequently associated with heat waves in "Heat Wave Mortality in Nursing Homes." He presents the results of a study comparing deaths at nursing homes with and without air conditioning during periods of high temperatures. Marmor also cites several occurrences of dramatic increases in deaths, including over 1,000 excess deaths after a 14-day heat wave in New York City in 1966. Although Marmor's much-quoted 1975 study indicates that the association between heat waves and mortality had not changed over a 20-year period despite widespread adoption of air conditioning, the author suggests this might be due to the inaccessibility of air conditioning to the most heat-sensitive groups of the population, particularly the elderly.

Kalkstein's article "Health and Climate Change," part of a 1993 series on global warming that appeared in The Lancet, emphasizes the ongoing controversy over factors that could alter the impact of climate change on mortality. He discusses several key factors: air conditioning, air pollution, acclimatization or reduced risk pool, demographic characteristics, and health care services. Although most of the discussion relates to the impact of global warming on the United States, Kalkstein also refers to the particular vulnerability of urban areas in developing countries to temperature increases.