It’s no surprise that a sudden summer heat wave can kill the elderly; it’s a serious public health hazard that will only grow as the world warms. But will more old folks survive milder winters, balancing out the loss of life in the summers?

A new study suggests not. A rise of 1°C in mean summer temperatures killed 1% more people, whereas that same rise in mean winter temperatures saved a mere 0.6%, according to an analysis of death records for nearly 3 million people 65 years and older living in New England from 2000 to 2008. Not only that, but sudden swings in temperature—another phenomenon that could increase along with climate change in some regions—were found to be even worse killers, in either winter or summer.

“People do physically adapt,” says study author Joel Schwartz, an environmental epidemiologist at Harvard University. “But if [temperature] bounces back and forth, we don’t.”

Many studies have looked at the immediate effect on death rates in the days following a heat wave. But it is difficult to say whether some of these people, with compromised cardiovascular or respiratory systems, would have died anyway. Few studies look at long-term effects or compare the harm of summer heat waves with the benefits of warmer winters.



So Schwartz and his colleagues calculated average summer and winter temperatures in New England zip codes for 8 years, and tracked the deaths of elderly people living within them, based on Medicare data. In addition to finding that milder winters do not make up for hotter summers, the team discovered that sudden shifts in temperature—jumping from a cold winter day to a warm winter day and back again, for example—was a worse killer than summer heat waves, in either winter or summer. Schwartz says the killing power of jumpy temperature swings is greater than that of AIDS, and comparable to diseases such as liver cancer, which kills about 25,000 people in the United States each year. The team reports its results online today in Nature Climate Change.

The work highlights “a major public health issue,” says Jonathan Patz, a public health researcher at the University of Wisconsin, Madison, who not involved with the research. “I think it’s a very important study.”

As for why temperature swings are so deadly, Schwartz says that it has do to with not giving a person enough time for their cardiovascular or respiratory systems to adapt. Looking ahead, he wants see if the same trends hold up in other regions, starting with the southeast of the United States, where people may be more acclimatized to warmer temperatures. He also wants to study temperatures and death rates in Europe, where air-conditioning is less prevalent.