Will warmer winters mean fewer winter deaths?

As climate change threatens to alter ecosystems and in some places make the world a bit less aesthetically pleasing, it’s reasonable to wonder whether there are any silver linings to be found in a warming world. Here’s one: perhaps the new normal will involve fewer winter deaths. Extreme heat is certainly associated with mortality, but the cold is statistically worse. On average, death rates are actually highest during the coldest parts of the year, and some have speculated that those winter deaths could be reduced in a warming world.

For that to bear out, the relationship between winter and mortality would have to be causal. That is, it would have to be the temperature itself that leads to deaths, and not some other variable that’s associated both with winter and with human health. To find out which was the case, a group of French and American public health researchers led by Patrick L. Kinney, director of the Climate and Health Program at Columbia University’s Mailman School of Public Health, looked at daily temperature data and morality information for 36 US cities for the years 1985–2006 and for 3 French cities during 1971–2007.

If warmer climate was associated with a lower wintertime mortality rate, then they reasoned that they’d find evidence of a less severe spike in winter mortality for cities that don’t see extreme variations between summer and winter, or that have warmer winters overall.

In short, they found no evidence that the cold actually causes more people to die. Despite temperature variations from season to season that differed by as much as a factor of four, and average winter temperatures that varied from -5°C (23°F) in some cities to more than 20°C (68°F) in other cities, “a similar level of winter excess mortality was observed for all cities.” These are cities, the researchers note, that are widely different in terms of demographics, city design, and socio-cultural background, all of which could influence the amount of exposure people have, on average, to the outdoors during winter. It isn’t that more folks aren’t dying in the winter, necessarily, but if they are, it isn’t because of the temperature. So it’s unlikely that future global warming would diminish any excesses in winter mortality.

Even if cold does play a role in some deaths – the study did not separate deaths by different causes, or by age, sex, ethnicity, or socioeconomic status – the researchers conclude that it would be “limited.”

But if it isn’t temperature, than what is driving the apparent increase in winter deaths? Kinney and his colleagues highlight the possibility that it’s actually the flu. They point out that 40 years of American mortality statistics highlight a correlation between influenza mortality and season, and that the flu, which tends to go around in the winter months, also increases the risk of future cardiovascular events. “These studies imply that influenza infections may underlie much of the winter excess in general mortality risk,” they say. The researchers also point their collective finger at a handful of other possibilities: lack of exercise, increased time spent indoors, school schedules, holiday gatherings, and air moisture could all have consequences for public health. Or maybe it’s all the overeating and overdrinking we tend to do around the wintertime holidays. – Jason G. Goldman | 26 June 2015

Source: Kinney, P. L., Schwartz, J., Pascal, M., Petkova, E., Le Tertre, A., Medina, S., & Vautard, R. (2015). Winter season mortality: will climate warming bring benefits? Environmental Research Letters, 10(6), 064016.

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