This chapter assesses how climatic changes and associated environmental and social changes are likely to affect human population health. Such an assessment necessarily takes account of the multivariate and interactive ecological framework within which population health and disease are determined. This ecological perspective recognizes that the foundations of long-term good health lie in the continued stability and functioning of the biosphere's natural systems—often referred to as "life-support systems."

Deliberate modification of these ecological and physical systems by human societies throughout history has conferred many social, economic, and public health benefits. However, it also has often created new risks to health, such as via mobilization of infectious agents, depletion of freshwater supplies, and reduced productivity of agroecosystems (Hunter et al., 1993; Gubler, 1996). Consider, for example, the chain of consequences from clearance of tropical forests. In the first instance, it typically leads to a warmer and drier local climate. The consequent drying of soil and loss of its organic structure predisposes the area to increased water runoff during heavy rainfall. This, in turn, can endanger human health via flooding, water contamination, impaired crop yields, and altered patterns of vector-borne infectious diseases. Meanwhile, forest clearance also contributes to the atmospheric buildup of carbon dioxide (CO₂) and hence to climate change and its health impacts.

Today, as the scale of human impact on the environment increases, a range of population health impacts can be expected from these large-scale changes in the Earth's life-support systems (Watson et al., 1998). That is the complex context within which actual and potential health impacts of global climate change must be assessed.

The IPCC Second Assessment Report (McMichael et al., 1996a) relied on the relatively limited scientific literature that had emerged during the late 1980s and early 1990s. Most published studies were on health impacts associated with climate variability (e.g., El Niño) and extreme events (natural disasters and heat waves). Predictive modeling of future health impacts was in an early developmental stage.

The SAR noted the many inherent uncertainties in forecasting the potential health impacts of climate change. This included recognition that various other changes in social, economic, demographic, technological, and health care circumstances would unfold over coming decades and that these developments would "condition" the impact of climatic and environmental changes on human health. However, such accompanying changes can be foreseen neither in detail nor far into the future.

The overall assessment was that the likely health impacts would be predominantly adverse. Reflecting the published literature, most of the specific assessments were nonquantitative and relied on expert judgment. They drew on reasoned extrapolations from knowledge of health hazards posed by extreme weather events, increases in temperature-dependent air pollution, summertime increases in certain types of food poisoning, and the spectrum of public health consequences associated with economic disruption and physical displacement of populations. It was noted that the projected effects of climate change on agricultural, animal, and fishery productivity could increase the prevalence of malnutrition and hunger in food-insecure regions experiencing productivity downturns.

For two of the anticipated health impacts, the published literature available by 1995 allowed a more quantitative approach. The relevant conclusions were as follows:

• An increase in the frequency or severity of heat waves would cause a short-term increase in (predominantly cardiorespiratory) deaths and illness. In some very large cities (e.g., Atlanta, Shanghai) by about 2050, this would result in up to several thousand extra heat-related deaths annually. This heat-related mortality increase would be offset by fewer cold-related deaths in milder winters, albeit to an extent that was not yet adequately estimated and likely to vary between populations.
• Climate-induced changes in the geographic distribution and biological behavior of vector organisms of vector-borne infectious diseases (e.g., malaria-transmitting mosquitoes) and infective parasites would alter—usually increase—the potential transmission of such diseases. For example, simulations with global/regional mathematical models indicated that, in the absence of demographic shifts, the proportion of the world's population living within the potential malaria transmission zone would increase from ~45% in the 1990s to ~60% by 2050. Some localized decreases in malaria transmissibility also may occur in response to climate change.

This is the last of the sector-impact chapters in this volume. This is appropriate because human population health is influenced by an extensive "upstream" range of environmental and social conditions. Indeed, over time, the level of health in a population reflects the quality of social and natural environments, material standards of living, and the robustness of the public health and health service infrastructure. Therefore, population health is an important integrating index of the effects of climate change on ecosystems, biological processes, physical environmental media, and the social-economic environment.

Two other points are important. First, the causation of most human diseases is complex and multifactorial. Second, there is great heterogeneity in the types of disease: acute and chronic; infectious and noninfectious; physical injury and mental health disorders. These two considerations explain some of the difficulties in fully understanding and quantifying the influences of climate on human health.

Profiles of health and disease vary greatly between regions and countries and over time. Currently, noncommunicable diseases (including mental health disorders) predominate in developed countries, with cardiovascular diseases and cancer accounting for more than half of all deaths. In poorer countries, infectious diseases (especially in childhood) remain important, even as noncommunicable diseases increase in urbanizing populations that are exposed to changes in lifestyle and environmental and occupational exposures. Globally, infectious diseases remain a major cause of human morbidity and are responsible for approximately one-third of all deaths (WHO, 1999a). Many of these water-, food-, and vector-borne infectious diseases are sensitive to climate.

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