In September 1986, the World Meteorological Organization, the United Nations Environment Programme (UNEP) and the World Health Organization (WHO) sponsored an international symposium on climate and human health. Over 50 papers were presented by international experts in the field, covering topics such as the effects of meteorologic elements on human health, the evaluation of health data bases for bioclimatologic analyses, and the development of human biometeorologic classifications (WMO 1986).
Interest in climate/human health relationships has increased further during the late 1980s and into the 1990s, partly because of the availability of more complete data bases and because of the threat of a human-induced global warming, which is predicted to increase mean global temperatures by 2-5deg.C over the next century (Rind et al. 1989).
This paper focuses on the potential impacts of a large-scale climatic change on human health by examining three major issues. First, a discussion of recent global warming/human health research is offered, concentrating particularly on heat stress/human mortality relationships and the impact of warming on the possible spread of vector-borne infectious diseases. Second, the question of human acclimatization to warmer conditions is discussed. Finally, a plan developed at the U.S. Environmental Protection Agency (EPA) to evaluate the domestic and international ramifications of global warming on human health is described.
Possible Health Impacts of a Global Warming
Although weather has an impact on a variety of human ills, ranging from depression to heat stroke, it seems likely that the major impacts of a large-scale global warming will involve a potentially large increase in heat-related mortality and a shift in the ranges of various Vector-borne infectious diseases. In June 1989, EPA completed a report to Congress on the potential effects of a global climate change on the United States (U.S. EPA 1989). As part of this effort, the possible impacts of climate change on agriculture, forestry, human health, biodiversity, sea level rise, and water resources were described, with the human health aspect possibly containing the greatest uncertainties. However, it was suggested that enormous increases in heat-related mortality could occur and that the vectors of several important infectious diseases could move considerably northward.
Increases in Temperature-related Mortality
The human mortality project evaluated 15 large cities around the country in an attempt to determine if there would be a differential inter-regional response to a global warming (Kalkstein 1989). Current research suggests that weather presently affects mortality much more than might be expected; during unusual weather events, deaths from all causes can rise 50 percent above normal baseline levels (Driscoll 1971; Kalkstein and Davis 1989; Lye and Kamal 1977; Taesler 1986). It appears that weather has a differential regional impact on mortality in summer, and the strongest relationships are found in the northeastern and midwestern United States. Thus, the impact of heat on mortality is more profound where high temperatures occur irregularly, while in the southern United States, where heat is a relative constant, a much smaller impact is noted (Kalkstein and Davis 1989). In addition, the timing of hot weather has an additional impact on mortality. Extremely hot weather occurring early in the season appears to have a more devastating impact than similar weather occurring in August. These differential inter-regional and seasonal responses to weather imply that some degree of human acclimatization to stressful conditions is likely.
The impact of weather on mortality in winter appears to be much less than in summer. Research suggests that overcast, damp, and possibly snowy days may provide conditions for heightened mortality in winter, as people are forced indoors and in closer contact, creating an environment that increases the probability of microbial or viral infection (Richards and Marriott 1974). In addition, differential inter-regional and seasonal responses are not apparent.
With the use of algorithms developed from the historical weather-mortality relationships described earlier, estimates of present-day mortality attributed to weather were attempted (Table 1). Not surprisingly, cities in the northern and midwestern United States showed the greatest response in summer. These algorithms were then used to predict future trends in mortality attributed to global warming with the use of weather scenarios developed by the NASA Goddard Institute for Space Studies (GISS). If one assumes that people do not acclimatize to warmer weather, weather-induced mortality might increase sevenfold over present levels in the 15 evaluated cities by the year 2060 (Table 2). However, this total might be considered misleading, as some degree of acclimatization would be expected to occur. For this reason, analog cities were established for each of the 15 cities to account for full acclimatization. For example, use of the GISS scenario to predict New York City's summer weather for 2060 yields a weather regime approximating that of Kansas City, Missouri, today (Kalkstein and Davis 1989). Since Kansas City residents are fully acclimatized to this regime, the weather/mortality algorithm developed for Kansas City can be utilized for New York City to account for full acclimatization in 2060. It is important to note that the utilization of weather analogs to define human acclimatization ignores the possible difference in racial and socioeconomic composition between the evaluated city and its analog. It should not then be expected that the analog approach described here handles human acclimatization with any degree of precision.
Assuming full acclimatization, only a twofold increase in weather-induced mortality should be expected by 2060 (Table 2). In fact, estimates of future acclimatized mortality indicate that predicted warming might lessen weather-induced mortality in certain cities. However, the probability of full acclimatization occurring by the year 2060 is very low, even if the population can adapt to the increasingly stressful heat. It is improbable that the physical structure of the city would change sufficiently by the year 2060 to match the expected climate change. Thus, estimates of future weather-induced mortality assuming partial acclimatization is probably the most reasonable assumption (partial acclimatization is accounted for by computing mortality estimates exactly halfway between full and no acclimatization values). In this case, a best-guess estimate yields a fourfold increase in weather-induced mortality by the year 2060. This study also determined that any predicted changes in winter weather-induced mortality are too small to have a significant impact on these totals.
Changes in Distributions of Vector-borne Infectious Diseases
Research on the potential spread of vector-borne diseases is fraught with as many uncertainties as the heat stress/mortality findings. Using two weather-based models for simulation of the population dynamics of disease vectors, Haile (1989) attempted to assess the effects of climate change on vector-borne disease transmission in the United States. His first model simulates population dynamics of the American dog tick, Dermacentor variabilis, which is the primary vector of Rocky Mountain Spotted Fever. This model includes the effects of temperature and atmospheric moisture on the life processes of the tick. The second model simulates the population dynamics of Anopheles quadrimaculatus mosquitoes and the transmission of malaria between the insect vector and humans; weather variables included are temperature, humidity, and rainfall. The model simulates direct incidence of malaria, assuming that the human population is continuously exposed to mosquito bites.
The tick model results indicate that with the proposed climatic change scenarios, populations of the dog tick will disappear in certain southern locations such as Jacksonville, Florida, and San Antonio, Texas; as the predicted high temperatures and low humidities will be outside the range of tolerance. However, certain northern locations, such as Missoula, Montana, North Bay, Ontario, and Halifax, Nova Scotia, will become warmer and more humid, allowing for increased populations of the tick.
The results of the malaria transmission model showed little change from present conditions. Areas in Florida where A. quadrimaculatus already exists will continue to have relatively high populations, and small population increases are predicted farther north, especially in cooler areas of the Southeast such as Atlanta and Nashville.
Possibly more threatening are tropical and Third World vector-borne infectious diseases that might spread to other underdeveloped regions where public health facilities would be unable to deal with the problem. One such disease is trypanosomiasis (sleeping sickness), transmitted by the tsetse fly in areas of central Africa. This disease is of particular importance, as its presence may preclude human habitation from areas where wild animals act as a reservoir of the disease. Research has indicated that mortality rates of the tsetse fly are closely related to humidity and, to a lesser extent, temperature (Dobson and Carper 1988). Given a mean 2deg.C increase in temperature expected for sub-Saharan Africa if global warming occurs, it is possible that tsetse flies may become less common in western Africa and across portions of sub-Saharan central Africa. This decrease, however, is expected to be offset by a spread farther south in portions of eastern Africa having high densities of human and domestic animal populations.
The Issue of Human Acclimatization
Of course, no one knows for sure how humans may react to changes in climate, but numerous efforts have already been attempted to determine how adaptable the general population may be. Unfortunately, most of the acclimatization research suffers from two major shortcomings. First, much of the work is confined to the western world, and little is known about the potential ability of populations in the Third World to acclimatize. Second, most of the acclimatization research evaluates the physical abilities of humans to adapt, but little is known about societal responses. For example, how might the structure of a city change under various climate warming scenarios? What will be the impact on human migration? These cultural or social issues must be addressed in future research relating to human acclimatization.
Any assumptions on human acclimatization to climate change must be based on present-day responses of the population to shorter-term climatic stresses. One of the first observations was put forth by Gover (1938), who noted that excess mortality during a second heat wave in any year will be slight in comparison to excess mortality during the first, even if the second heat wave is unusually extreme. This is supported by more recent research that indicates that excess mortality during hot summers diminishes as the season progresses (Kalkstein and Davis 1989). Two possible explanations for this phenomenon are possible. First, the weak and susceptible members of the population die in the early heat waves of summer, thus lowering the population of susceptible people who might have died during subsequent heat waves. Second, those who survive early heat waves become behaviorally or physically acclimatized and, hence, deal more effectively with later heat waves (Marmor 1975). Acclimatization is implied only within the second explanation, which our research lends to support. If the pool of susceptibles were lessened after a severe heat episode, it would be expected that the baseline of daily deaths would dip shortly after the episode and then would return to typical levels. This is not apparent in any of our research, as the baseline for daily mortality is unchanged after a severe heat event (Kalkstein in press). Thus, behavioral or physical acclimatization appears to be the more reasonable explanation.
Geographical acclimatization also appears to be an important aspect of human response to hot weather. Kalkstein and Davis (1989) note that mortality increases dramatically during heat waves in northern cities, but no mortality increase is observed in southern cities, even under the hottest conditions. Further support for geographic acclimatization is provided by Rotton (1983), who suggests that people moving from a cool to a subtropical climate will adapt rather quickly, often within 2 weeks.
Unfortunately, many meteorologists have ignored the possible impacts of acclimatization, and their use of climatic indices implies that they believe that the role of acclimatization has been overstated by many researchers. The widespread use of the wind-chill index for winter and the temperature-humidity index for summer in the meteorologic community indicates minimal recognition of acclimatization as a major aspect of human response to weather. Both indices are based on absolute values only. A temperature of 93deg.F with a relative humidity of 43 percent yields the same temperature-humidity index value whether it occurs in New Orleans or Duluth. The hot weather indices most widely accepted by the National Weather Service are all absolute, and they include the temperature humidity index, the heat index, humiture, humidex, the discomfort index, and apparent temperature (Quayle and Doehring 1981; Steadman 1984; Weiss 1983; Winterling 1979). The only geographically relative index that has been published, the weather stress index, is only beginning to be utilized to evaluate the impacts of climate on human well-being (Kalkstein and Valimont 1986).
The only social adjustment that has been evaluated in some detail is the impact of increased air-conditioning accessibility on heat-related mortality. Kilbourne et al. (1982), in an attempt to identify factors relating to heat stroke, found a strong negative relationship between daily hours of home air-conditioning and heat-related mortality. However, Ellis and Nelson (1978) and Kalkstein and Davis (1989) have noted that during the past 30 years, mortality during heat waves has not changed significantly despite the increased use of air-conditioning.
It is quite apparent that insufficient knowledge exists about the role of human acclimatization in coping with a climate change. Research involving present-day or short-term acclimatization is contradictory and inconclusive. The only study to date that attempts to estimate global-warming-induced acclimatized mortality (using the analog city approach described earlier) is primitive, and it ignores virtually all social or cultural issues. In addition, recent research in the medical community on the role of heat shock proteins, which synthesize in the body as a response to environmental stresses such as temperature change (Born et al. 1990), complicate the acclimatization issue even further. There is no doubt that the possible role of human acclimatization in climate change studies must be evaluated in much greater detail.
The EPA Global Warming/Health Initiative
In response to these weather/health uncertainties, and with the possibility of a major global warming occurring over the next century, the EPA has proposed a new global warming/health initiative with three major goals: to evaluate with greater precision the potential domestic and international impacts of a global warming, to draw upon the expertise of numerous climate/health specialists at the national and international level, and to coordinate the efforts of all participants for maximum efficiency (U.S. EPA 1990). The program will address three major areas of uncertainty:
The global warming/heat stress mortality project will expand beyond the previous domestic research described earlier by attempting to identify specific weather-related causes of death and by assessing more precisely the role of extreme weather events on mortality. A new synoptic climatological approach, which evaluates weather situations rather than individual weather elements, will be employed to determine weather/mortality relationships in a more realistic and holistic fashion (Kalkstein in press). In addition, new estimates of future mortality under various global warming scenarios will be developed with an increased consideration of human acclimatization. This research will also contain an international component and will identify several larger areas in developing countries to determine possible heat stress-related mortality in these regions. An EPA-sponsored study is already underway in China (investigating organization: Department of Atmospheric Sciences, Zhongshan University) to determine the impact of global warming on mortality in Guangzhou and Shanghai. In addition, global warming/mortality studies are also proposed for Canada, Spain, Japan, and possibly the Soviet Union. This research will be directed by the Center for Climatic Research at the University of Delaware.
The second study involves an evaluation of specific infectious diseases and their vectors that appear to have the potential to spread if the globe warms. At a June 1990 WHO/NASA/UNEP workshop held in Baton Rouge, Louisiana, on remote sensing for surveillance of vector-borne diseases, much discussion concentrated on this issue, which many epidemiologists and climatologists fear might be one of the most pressing health problems for the developing world in the 21st century. This study will distinguish those infectious vector-borne diseases most suitable for evaluation and will identify three to five endemic developing countries that have enumerated population groups suitable for analysis to develop present-day climate/disease relationships. If one assumes these relationships can be established, estimates of future changes in the range of these infectious diseases will be attempted based on the climatic limiting factors of the vectors and the pathogens. In addition, some attempt will be made to estimate future numbers of people who might be afflicted with the disease(s) in question using global warming scenarios. Finally, an attempt will be made to evaluate the possible simultaneous warming-induced habitat alterations that might eliminate or include the vectors irrespective of climate. This research will be directed through the Tropical Medicine and International Research Program, National Institute of Allergy and Infectious Diseases, National Institutes of Health.
The human acclimatization study will concentrate on the possible social and cultural adjustments expected to occur in a warmer world. This includes possible demographic alterations that could take place because of migration and other factors attributed to global warming. The main objective is to improve upon the simplistic climatic analog approach used in previous studies by including social factors that were previously ignored when attempting to account for acclimatization. This research will most likely be coordinated by the Institute of Medicine at the National Academy of Sciences.
The global warming/health initiative represents the first large-scale coordinated effort to evaluate the impacts of a long-term climate change on human health and well-being. A number of agencies will assume an advisory or support role in this initiative, which will originate from EPA's Office of Policy Analysis, Climate Change Division. For example, Environment Canada's Canadian Climate Center is funding a heat stress/mortality study for Canada, and similar studies for Spain (investigating organization: Institut Municipal d'Investigacio Medica in Barcelona) and Japan (investigating organizayion: Japan National Institute for Environmental Studies) are also being contemplated. The United Nations Environment Program and World Meteorological Organization will also participate in an advisory role, especially for the vector-borne infectious disease studies. EPA's Office of Research and Development will provide additional monetary and advisory support.
It is probable that results from the various case studies that comprise this initiative will lead to a series of policy options to mitigate potential health problems attributed to global warming. This unique interdisciplinary program comprising climatologists, epidemiologists, and the policy community should provide the first in-depth evaluation of what may lie ahead if global warming proceeds as expected.
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