Heat is the primary weather-related cause of death in the United States. Increasing heat and humidity, at least partially related to anthropogenic climate change, suggest that a long-term increase in heat-related mortality could occur. We calculated the annual excess mortality on days when apparent temperatures--an index that combines air temperature and humidity--exceeded a threshold value for 28 major metropolitan areas in the United States from 1964 through 1998. Heat-related mortality rates declined significantly over time in 19 of the 28 cities. For the 28-city average, there were 41.0 +/- 4.8 (mean +/- SE) excess heat-related deaths per year (per standard million) in the 1960s and 1970s, 17.3 +/- 2.7 in the 1980s, and 10.5 +/- 2.0 in the 1990s. In the 1960s and 1970s, almost all study cities exhibited mortality significantly above normal on days with high apparent temperatures. During the 1980s, many cities, particularly those in the typically hot and humid southern United States, experienced no excess mortality. In the 1990s, this effect spread northward across interior cities. This systematic desensitization of the metropolitan populace to high heat and humidity over time can be attributed to a suite of technologic, infrastructural, and biophysical adaptations, including increased availability of air conditioning.

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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 weathermortality 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.

[1] Whereas there is a significant relationship between overall sea-surface temperature (SST) and tropical cyclone intensity, the relationship is much less clear in the upper range of SST normally associated with these storms. There, we find a step-like, rather than a continuous, influence of SST on cyclone strength, suggesting that there exists a SST threshold that must be exceeded before tropical cyclones develop into major hurricanes. Further, we show that the SST influence varies markedly over time, thereby indicating that other aspects of the tropical environment are also critically important for tropical cyclone intensification. These findings highlight the complex nature of hurricane development and weaken the notion of a simple cause-andeffect relationship between rising SST and stronger Atlantic hurricanes.

One hypothesized impact of climate warming is higher human mortality rates in the warm season, due to increasingly hot and/or humid conditions. On days with high afternoon apparent temperatures, we compared changes in daily mortality rates over 4 decades in 6 major metropolitan areas along a north-south transect in the eastern US. In the 3 southernmost cities examined, we found few significant weather-mortality relationships for any decade or demographic group. But in the 3 northern cities, we determined that statistically significant declines in population-adjusted mortality rates occurred between 1964 and 1994 for the total population and the white and elderly demographic subgroups during hot and humid weather. These statistically significant reductions in hot-weather mortality rates suggest that the populace in cities that were weather-sensitive in the 1960s and 1970s have become less impacted by extreme conditions over time because of improved medical care, increased access to air conditioning, and biophysical and infrastructural adaptations. This analysis counters the paradigm of increased heat-related mortality rates in the eastern US predicted to result from future climate warming.