The unpredictable swings in temperature increasingly seen with climate change could push up the death rates in seniors in the United States, a study by Harvard University researchers suggests.
Scientists have long been concerned about the impact on the nation's health of more extreme heat, along with flooding and pollution pushed by climate change.
"Temperature variability emerges as a key feature in the potential impacts of climate change," says study author Liuhua Shi, a doctoral student in the department of environmental health at the university's T.H. Chan School of Public Health in Boston.
To gauge those impacts, Shi and her colleagues analyzed Medicare data on 2.7 million people age 65 and up in New England.
In the years studied, from 2000 to 2008, death rates rose when the average summer temperature increased significantly, the researchers report in Nature Climate Change.
"Many studies have reported associations between short-term temperature changes and increased daily deaths," Shi says. "However, there is little evidence to date on the long-term effect of temperature."
The scientists say they believe the reason for the increased risk of death in summer is the result of an increase in the variability of temperatures. Their study also found death rates dropped when winter temperatures rose, creating milder weather.
"Climate change may affect mortality rates by making seasonal weather more unpredictable, creating temperature conditions significantly different to those to which people have become acclimatized," Shi says.
The researchers acknowledge their study has some limitations; it was able to measure the death rate in seniors but could not identify individual cause of death.
It also only showed a possible association between climate and death rates, not a direct cause-and-effect relationship.
Still, other experts in the field say, the study should be looked at closely.
"This is clearly an important study," says Richard Keller, a University of Wisconsin-Madison professor of medical history and bioethics.
"It is valid and useful within certain limits," says Keller, who studies climate change's potential effects on health but was not involved in the Harvard research.
The Harvard scientists say they plan to expand their study to cover the whole of the United States.
"We would expect that people will respond differently to climate change in different climate zones," Shio says. "Hence, we plan to do a national study to examine the long-term effects of temperature on mortality in each climate zone."
