Winter Season Mortality: Will Climate Warming Bring Benefits?

Abstract

Extreme heat events are associated with spikes in mortality, yet death rates are on average highest during the coldest months of the year. Under the assumption that most winter excess mortality is due to cold temperature, many previous studies have concluded that winter mortality will substantially decline in a warming climate. We analyzed whether and to what extent cold temperatures are associated with excess winter mortality across multiple cities and over multiple years within individual cities, using daily temperature and mortality data from 36 US cities (1985-2006) and 3 French cities (1971-2007). Comparing across cities, we found that excess winter mortality did not depend on seasonal temperature range, and was no lower in warmer vs. colder cities, suggesting that temperature is not a key driver of winter excess mortality. Using regression models within monthly strata, we found that variability in daily mortality within cities was not strongly influenced by winter temperature. Finally we found that inadequate control for seasonality in analyses of the effects of cold temperatures led to spuriously large assumed cold effects, and erroneous attribution of winter mortality to cold temperatures. Our findings suggest that reductions in cold-related mortality under warming climate may be much smaller than some have assumed. This should be of interest to researchers and policy makers concerned with projecting future health effects of climate change and developing relevant adaptation strategies.

Figure 1

Scatter plot of winter excess mortality (the ratio of Dec, Jan, and Feb deaths to Jun, Jul, Aug deaths) vs. seasonal temperature difference for the same months over 22 years in 39 cities. See supplemental file, Table 1 for a list of cities and input data.

Figure 2

Penalized spline functions fit to the relation between temperature and mortality in Paris and New York City (top), and Marseille and Miami (bottom). The black circles show the relationship when season is left uncontrolled. The red triangles show the relationship when season is controlled using a natural spline with 6 degrees of freedom per year, and the green crosses show the relation with 8 degrees of freedom per year of seasonal control. The French data are for 1971 to 2007 and the US data are for 1985 to 2006.

Figure 3

Penalized spline functions relating mortality with temperature on the day of death by calendar month in Paris and New York City (top) and Marseille and Miami (bottom). The Paris and Marseille data are for 1971 to 2007 and the NYC and Miami data are for 1985 to 2006.