reliability of present-day electricity supply from hydropower and traditional generation sources, such as coal, gas, and nuclear, that require cooling water. {5.5}

Human Health

Heat-related illness and deaths occur as a direct result of sustained, elevated levels of extreme temperatures during heat waves, which are projected to increase with increasing temperatures. The frequency and severity of heat waves in Europe and North America are projected to increase under climate change. Under a 2ºC increase in global mean temperature, for example, the average number of days per year with maximum temperatures exceeding 38ºC or 100ºF across much of the south and central United States could increase by a factor of 3 relative to the 1961-1990 average. Under a 3.5ºC increase, the number of days is projected to increase by 5 to nearly 10 times. Some adaptation is inevitable as populations become accustomed to permanently different conditions. However, most research indicates that the public health impacts of climate change are likely to increase with temperature extremes; and new research highlights the potential that heat stress may impose hard limits on the inhabitability of some land areas under global temperature changes on the order of 10ºC or more. {5.5}

Climate change is likely to affect the geographic spread and transmission efficiency of illnesses and disease carried by hosts and vectors, but complexity precludes any quantitative estimates of the relationship between incidence of a given disease and temperature change. Confounding factors—involving viral, bacterial, plant, and animal physiology, as well as sensitivity to changes in climate extremes, including precipitation intensity and temperature variability—challenge attempts to resolve the influence of temperature on observed trends in disease incidence. Most recent projections suggest that the ranges of malaria and other diseases may shift, but increases in some areas will likely be accompanied by decreases in others. {5.5}

Climate change may exacerbate existing stressors, such as air pollution, water contamination, and pollen production. Warmer temperatures increase production of ground-level ozone, which affects respiratory health. For a given level of ozone precursor emissions, background ozone levels and days with high ozone pollution levels above a defined safety standard (or “ozone exceedances”) are projected to increase across much of the United States. Where heavy precipitation increases, risk of water contamination could also increase. Shifts in growing season, mean temperatures, and atmospheric

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