A recent study by a team at the Harvard School of Public Health has determined that global warming in U.S. cities during the summer causes an increase in mortality far greater than the increase experienced during the winter, prompting the need to find alternative ways of staying cool as temperatures rise.

The study, which was co-authored by Professor of Environmental Epidemiology Joel D. Schwartz and Department of Environmental Health research fellow Mercedes Medina-Ramon, unsurprisingly found that extreme heat during the summer months causes thousands of deaths every year.

Of note, however, was the fact that an equivalent change in temperature—a same-degree increase during summer and winter—had a much greater affect on summer mortality.

According to the study’s authors, the reason for this disparity was poor acclimatization—a person’s ability to adapt to changes in temperature—during the summer.

“Everyone has central heating, even in the South,” Schwartz said. “People have already become acclimated to the cold weather in their town, the right amount of central heating and clothing.”

In contrast, air conditioning in warm weather is a far more costly endeavor.

“In the summer, not everyone has air conditioning or access to air conditioning, so the impact of extreme hot weather is much larger,” Medina-Ramon said, “especially in more populated cities.”

Where air conditioning use was most prevalent, however, was actually where mortality was the lowest on average. Schwartz said that it was those “hotter places” where acclimatization was strongest, resulting in the fewest number of heat-related deaths.

“You can’t live in Houston or Orlando without air conditioning,” Schwartz said. “People are exposed a lot less to heat in other places. In Boston, everyone feels it. It’s 96 [degrees] in Atlanta and nobody feels it.”

Schwartz further explained that the amount of central air conditioning in homes only explains part of the difference in seasonal temperature-related mortality, and that the study, which examined data from 1989 to 2000, was unable to determine the other factors that contribute to the mortality disparity in the summer and winter.

Not helping matters is global warming, the authors said, which is increasing the rate at which temperatures are rising and as a result, the rate at which people are dying.

Ironically, it is air conditions that are a main contributor to global warming, prompting scientists to look for ways to mitigate the heat in the short-term without contributing to its even greater pitfalls in the long-term.

“We want to spread the use of air conditioning, but air conditioning is also part of the problem of climate change,” Medina-Ramon said. “We want to spread the use but stop the abuse. It’s not necessary to wear a jacket in the summer.”

The findings of the study were published yesterday in an online issue of the journal “Occupational and Environmental Medicine.”

The authors noted that the results are specific to the 50 U.S. cities they examined, and that the findings might be different in other parts of the world.

“That’s a U.S. result,” Schwartz said. “If you go to places where they don’t have heating, that’s a different story. This might not hold everywhere in the world.”

Additionally, the study showed different results depending on race, hinting at some of the underlying social disparities within the U.S.

“The effect in blacks is substantially greater than the effect in whites, which again speaks to the importance of how much of a acclimatization there is in the kind of weather that’s going on in your cities,” Schwartz said. “People in worse socioeconomic positions may not be able to acclimatize as well. There’s certainly a racial disparity, and that’s a serious issue also.”

—Staff writer Malcom A. Glenn can be reached at mglenn@fas.harvard.edu.