On a recent day in Denver, snow fell just eight hours after the temperature had hit 83 degrees. Such “weather whiplash,” which stands to become more common, adds to the risks of climate change. One counterintuitive argument, though, suggests that climate change may, at the same time, prolong human life by lowering the peaks in death rates that occur in winter months. Unfortunately, the evidence suggests this benefit will be slim, if it occurs at all.
Mortality rates are indeed much higher during winter — about 10-20% higher across the world than they are at other times of year. In European cities, a 1 degree Celsius decline in temperature has been correlated with a 1.4% rise in daily natural deaths.
Most of this increase is due to respiratory and cardiovascular diseases, though deaths from falls and related injuries also spike from December to March. However, suicides, contrary to urban mythology, are not part of the phenomenon; they are highest during the spring (even in sunlight-starved Nordic countries).
Another surprising finding is that mortality rises for people in winter regardless of income.
So it’s easy to see the logic in believing warmer winters would lower death rates. Thomas Gale Moore of Stanford University’s Hoover Institution boldly predicts that, in the U.S., “a 2.5°C increase in temperatures would cut deaths nationwide from 37,000 to 41,000, approximately the number of people who die annually on our highways.”
What could be wrong with this argument? First, there are concerns about methodology. A statistical estimation based on the number of days requiring special heating produces much smaller results than one based on a broader definition of winter days. (And how should one treat that day in Denver, which was both extremely hot and extremely cold?) In addition, some studies suggest there are problems with blaming excess mortality on cold weather alone, and not other factors that change during the winter.
Weather cold enough to cause hypothermia may be the most easily understandable channel for the rise in mortality, but hypothermia accounts for only a very small share of winter deaths. If cold weather isn’t associated with much increase in mortality through that direct channel, how much confidence should we have that it’s responsible for other winter-related deaths? Warmer weather might not break much of the link between winter and death.
Second, even if the effect is causal, it appears to be larger in warmer climates than in colder ones. This “excess winter mortality paradox” means the effect is about twice as large in the U.K. and Ireland than it is in Denmark and Finland. And this may be because the colder environments are better adapted to handling even extreme cold days. If climate change gives us more weather whiplash, extremely cold days may increasingly occur in areas that are unprepared for them, attenuating any benefit from higher average winter temperatures.
Finally, the winter death effect seems to be gradually declining in importance. (It’s recently risen a bit in the U.K., but even there the overall trend is downward.) In other words, any benefit from warmer winters is becoming smaller over time.
The essential truth here is that climate change is only partly about higher average temperatures. The most important concerns involve weather volatility, and feedback effects that become unstoppable. Ultimately, the winter death effect may not decline materially with further climate change. In any case, it’s no reason to be complacent about the broader risks. -Bloomberg