Decadal changes in summer mortality in the United States

Extreme weather is demonstrably linked to excess human mortality, particularly during summer heat waves. Concerns arising from anthopogenic climate change have lead some researchers to predict future mortality increases above historic levels. However, most weather/mortality studies do not directly consider temporal changes in weather mortality relationships. The goal is of this analysis is to examine if summer weather/human mortality relationships have changed significantly in United States cities over the past four decades.

Daily mortality files include data on every person who has died in the United States over 25 non-consecutive years from 1964–1994. These mortality data are organized by Metropolitan Statistical Area, coupled with hourly weather observations, and then standardized to account for demographic changes and to simplify comparisons both between cities and within a given city over time. Data are further subdivided by age, race, and cause of death categories.

Plots of daily mortality vs. apparent temperature on the previous day demonstrate that winter standardized mortality exceeds summer mortality but that extreme high temperatures are associated with greater excess mortality than winter cold extremes. Mortality responses vary regionally—whereas excess summer mortality is found in most northern U.S. cities there is little or no mortality response in southern cities.

Examination of mortality by decade indicates that standardized mortalilty has declined over time in all cities. However, the incremental mortality response to high temperatures was greater in the 1960s than in the 1990s. These mortality declines can be accounted for by changes in a few cause-of-death categories. In some cities, the response of minorities differs from that of the general population.

These results suggest that human and infrastructural adaptations (acclimitization, increased access to air conditioning, public health initiatives, etc.) have reduced the weather sensitivity of the population. These trends should be factored into forecasts of future mortality that might arise from climate changes.