Temperature swings can kill, and not just with heat waves, researchers reported Monday.
In a study published in the journal Nature Climate Change, researchers found that in New England, more people died in years with warmer-than-average summers, while warmer-than-usual winters reduced the numbers of deaths. But warmer winters did not quite offset warmer summers.
"A rise in summer mean temperature of 1 degree C (just under 2 degrees F) was associated with a 1 percent higher death rate, whereas an increase in winter mean temperature corresponded to a 0.6 percent decrease in mortality," Joel Schwartz, a professor of environmental health at the Harvard Chan School of Public Health and colleagues wrote.
The researchers studied just under 3 million people 65 and older.
"We followed everyone on Medicare in New England and we looked at annual deaths and the average temperature in the winter and summer," Schwartz said.
The team checked peoples' dates of death against a separate database of temperature readings for each day by zip code from 2000 to 2008. They calculated if each year was warmer or colder than average for the zip code and checked to see if death rates were different in those years.
They were. Hotter-than-average summers increased the number of deaths. Warmer-than-average winters lowered it. The study adds to a growing body of evidence that some of the changes caused by the gradual warming of the climate are having real-time effects on people.
But the researchers also found that it's not so much a steady rise or fall in temperature that kills, but the up-and-down variation that takes temperatures out of their average range.
"Variability mattered," Schwartz said. "We don't acclimate to temperature very fast," he added. "If the day-to-day variability within the season was higher, then more people died."
This fits in with other studies that show, for instance, that people who exercise entirely in the heat become accustomed to it and can eventually push themselves more in the heat, but if they get the occasional break from the heat they don't become acclimated, Schwartz and colleagues said.
How can heat kill people if they don't immediately succumb to heat stroke? It can raise blood pressure and worsen cholesterol levels, for one thing, Schwartz said.
Climate experts agree that the world's overall temperature is headed upwards, but this doesn't mean every place is getting steadily and predictably warmer. Climate change is making the weather more variable -- causing hotter summers, colder winters in some places, and more severe storms.
President Barack Obama says the changes are hazardous to the health of Americans.
It's no surprise to learn that excessively hot summer days kill people -- deaths are common during heat waves. But the researchers say it's important to study less obvious variability over time. "This work provides an important example of how temperature may affect human health in a temperate climate region," they concluded.