Climate experts generally agree that, despite efforts aimed at reducing the emissions of greenhouse gases, temperatures are expected to continue to increase during the foreseeable future. Therefore, the ability of societies to adapt to real climate variations, as needed, becomes the paramount issue.

An example of significant adaptations can be seen in human mortality trends in the United States in summer. Although summer death rates are lower than in winter in all major U.S. cities, summer deaths increase significantly on hot and humid days and during heat waves, particularly in the Northeast and Midwest. In this study, we examine decadal-scale trends in daily mortality rates during both isolated hot events and heat waves.

Daily mortality counts are retrieved from National Center for Health Statistics archives for the 27 largest Metropolitan Statistical Areas (MSAs) in the United States. Daily counts are age-standardized to account for differing demographics and death rates over time and between cities. Weather data are gathered for an appropriate first-order weather station in each MSA. Apparent temperatures are computed using air temperature and humidity observations and used as the independent variable to define hot days and heat waves. All available data from 1964–present are organized into decadal groups to examine mortality trends in response to high apparent temperatures.

Preliminary results indicate that southern cities, where summer heat is common, show no evidence of a mortality response to either isolated hot days or heat waves. The greatest heat wave sensitivity is in northern and central U.S. cities. Over time, mortality rates in most heat-sensitive MSAs have generally declined given comparable heat wave durations and intensities. However, the small sample sizes and inherent variability of heat waves makes a direct comparison of heat waves difficult. There is evidence that early-season heat waves have a greater mortality impact than mid–late summer heat waves in many cities.

When hot days are examined independent of their occurrence within more extensive heat waves, statistically significant declining trends in heat wave sensitivity are evident in most heat-sensitive cities. Thus, fewer people died in the 1990s than in the 1960s and 1970s given comparable apparent temperatures.

Declining heat sensitivity of the population can be attributed at least partially to technological adaptations. Central air conditioning, improved health care, and engineering and architectural materials and techniques designed to diffuse heat have all had some impact. Increased public awareness and community action plans in response to extreme heat forecasts have mitigated against mortality, particularly in less affluent areas. The net impact of these adaptations is that formerly heat sensitive cities are now more similar to southern cities in their mortality response to heat. Although some cities still exhibit significantly elevated mortality when apparent temperatures are high, the U.S. metropolitan populace has, as a whole, become largely immune to deaths related to hot and humid weather.