Decadal changes in summer mortality in U.S. cities.

Recent studies suggest that anthropogenic climate warming will result in higher heat-related mortality rates in U.S. cities than have been observed in the past. However, most of these analyses assume that weather-mortality relationships have not changed over time. We examine decadal-scale changes in relationships between human mortality and hot, humid weather for 28 U.S. cities with populations greater than one million. Twenty-nine years of daily total mortality rates, age-standardized to account for underlying demographic changes, are related to afternoon apparent temperatures ( T(a)) and organized by decade for each city. Threshold T(a) values, or the T(a) at and above which mortality is significantly elevated, are calculated for each city, and the mortality rates on days when the threshold T(a) was exceeded are compared across decades. On days with high T(a), mortality rates were lower in the 1980s and 1990s than in the 1960s and 1970s in a majority of the cities. Regionally, northeastern and northern interior cities continue to exhibit elevated, albeit reduced, death rates on warm, humid days in the 1980s and 1990s, while most southern cities do not. The overall decadal decline in mortality in most cities is probably because of adaptations: increased use of air conditioning, improved health care, and heightened public awareness of the biophysical impacts of heat exposure. This finding of a more muted mortality response of the U.S. populace to high T(a) values over time raises doubts about the validity of projections of future U.S. mortality increases linked to potential greenhouse warming.