One of the more important proposed impacts of future human-induced climate warming in the United States is increased human mortality rates in some cities. Recently published and ongoing research by our group has demonstrated that summer mortality rates have been declining over time per incremental increase in apparent temperature on hot and humid days.

Average mortality rates are higher in winter than in summer. Seasonal differences in long-term surface temperature trends which show almost twice as much warming in the cold season as in the warm season have lead some researchers to postulate that “global warming” will actually produce a net reduction in future mortality rates. This proposition, of course, depends upon the existence of a relationship between weather and mortality during winter, one similar to that observed in summer.

In this study, we examine the relationship between daily mortality rates in the winter season and daily weather events in 27 U.S. cities. Daily mortality totals are compiled from National Center for Health Statistics archives for 25 non-consecutive years from 1964–1994 and are age-adjusted relative to a standard population. Weather data are acquired from first-order surface observations for each city. Since the mortality response occurs after a given weather event, a variety of lags are explored for each city to determine the strongest relationship between daily mortality and weather.

Preliminary results show little evidence for a daily relationship between cold weather and elevated mortality. This is true when examined on both an absolute and relative basis (in terms of temperature departures from normal). Instead, winter seasonal mortality appears to be more influenced by episodes of influenza than by specific weather conditions. If weather has any influence on influenza outbreaks, it has not yet been uncovered.

The lack of a weather-mortality relationship in winter is in contrast to summer weather-mortality relationships, in which one-day lagged mortality increases significantly per incremental increase in afternoon apparent temperature.

When we compared mortality rates across the year, we find that while the slope of the overall relationship between daily mortality rates and daily temperatures is more negative in cities in cold climates than those in warm ones, the absolute differences between the maximum mortality rates of winter and the minimum mortality rates of summer are very consistent between northern and southern locations. This finding suggests that populations in warmer climates have wintertime mortality rates that are not appreciably different than the mortality rates in cities with much colder climates.

Our preliminary results suggest that changes in winter mortality rates are not linked in any obvious way to daily weather variations. This result is in contrast to the situation in summer, in which hot and humid days significantly raise the mortality rates on subsequent days. Mortality rates are higher in winter because of the prevalence of diseases like influenza which appears to be uncorrelated to daily weather fluctuations. The lack of significant relationships between daily mortality and cold conditions, and the lack of any significant difference between absolute winter and summer mortality rates in cities across the country, suggest that there will be little impact of a wintertime warming on future mortality rate reductions.