Global climate change will have a wide range of health impacts. Overall, negative health impacts are anticipated to outweigh positive health impacts. Some health impacts would result from changes in the frequencies and intensities of extremes of heat and cold and of floods and droughts. Other health impacts would result from the impacts of climate change on ecological and social systems and would include changes in infectious disease occurrence, local food production and nutritional adequacy, and concentrations of local air pollutants and aeroallergens, as well as various health consequences of population displacement and economic disruption.

There is little published evidence that changes in population health status actually have occurred as yet in response to observed trends in climate over recent decades. A recurring difficulty in identifying such impacts is that the causation of most human health disorders is multifactorial and the "background" socioeconomic, demographic, and environmental context varies constantly. A further difficulty is foreseeing all of the likely types of future health effects, especially because for many of the anticipated future health impacts it may be inappropriate to extrapolate existing risk-function estimates to climatic-environmental conditions not previously encountered. Estimation of future health impacts also must take account of differences in vulnerability between populations and within populations over time.

Research since the Second Assessment Report (SAR) mainly has described the effect of climate variability, particularly daily and seasonal extremes, on health outcomes. Studies of health impacts associated with the El Niño-Southern Oscillation (ENSO) have identified interannual climate-health relationships for some epidemic diseases. The upward trend in worldwide numbers of people adversely affected by weather disasters has been characterized by peak impacts during El Niño events. Meanwhile, there has been an expanded effort to develop, test, and apply mathematical models for predicting various health outcomes in relation to climate scenarios. This mix of epidemiological studies and predictive modeling leads to the following conclusions.

An increase in the frequency or intensity of heat waves will increase the risk of mortality and morbidity, principally in older age groups and the urban poor (high confidence). The greatest increases in thermal stress are forecast for higher latitude (temperate) cities, especially in populations that have limited resources, such as access to air conditioning. The pattern of acclimatization to future climate regimes is difficult to estimate. Recent modeling of heat wave impacts in U.S. urban populations, allowing for acclimatization, suggests that several U.S. cities would experience, on average, several hundred extra deaths per summer. Poor urban populations in developing countries may be particularly vulnerable to the impacts of increased heat waves, but no equivalent predictions are available. Warmer winters and fewer cold spells, because of climate change, will decrease cold-related mortality in many temperate countries (high confidence). The reduction in winter deaths will vary between populations. Limited evidence indicates that, in at least some temperate countries, reduced winter deaths would outnumber increased summer deaths.

Any regional increases in climate extremes (storms, floods, cyclones, etc.) associated with climate change would cause physical damage, population displacement, and adverse effects on food production, freshwater availability and quality, and would increase the risks of infectious disease epidemics, particularly in developing countries (very high confidence/ well-established). Over recent years, several major climate-related disasters have had major adverse effects on human health—including floods in China, Mozambique, Bangladesh, and Europe; famine in Sudan; and Hurricane Mitch, which devastated Central America. Although these events cannot be confidently attributed to climate change, they indicate the susceptibility of vulnerable populations to the adverse effects of such events.

Climate change will cause some deterioration in air quality in many large urban areas, assuming that current emission levels continue (medium to high confidence). Increases in exposure to ozone and other air pollutants (e.g., radon, forest fire particulates) could increase known morbidity and mortality effects.

Vector-borne diseases are maintained in complex transmission cycles involving blood-feeding arthropod vectors (and usually reservoir hosts) that depend on specific ecological conditions for survival. These diseases are sensitive to climatic conditions, although response patterns vary between diseases. In areas with limited or deteriorating public health infrastructure, and where temperatures now or in the future are permissive of disease transmission, an increase in temperatures (along with adequate rainfall) will cause certain vector-borne diseases (including malaria, dengue, and leishmaniasis) to extend to higher altitudes (medium to high confidence) and higher latitudes (medium to low confidence). Higher temperatures, in combination with conducive patterns of rainfall and surface water, will prolong transmission seasons in some endemic locations (medium to high confidence). In other locations, climate change will decrease transmission via reductions in rainfall or temperatures that are too high for transmission (low to medium confidence). In all such situations, the actual health impacts of changes in potential infectious disease transmission will be strongly determined by the effectiveness of the public health system.

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