US Mortality Likely To Rise As Climate Changes

Estimates of percent change in mortality associated with 2 consecutive days of extreme heat. Largest effect in Chicago. Credit: Mercedes Medina-Ramon, Harvard School of Public Health

Tuesday, July 3rd, 2007--

A new study examines how global warming will impact death rates in the U.S. Looking only at temperature, Mercedes Medina-Ramon and Joel Schwartz, researchers from the Harvard School of Public Health, estimate that mortality will increase across the U.S. because of global warming. But some cities may be hit harder than others.

According to the Intergovernmental Panel on Climate Change, temperatures on the planet by somewhere between 1.7 and 5 degrees C and will change the frequency of extreme weather events by increasing extremely hot days and decreasing extremely cold days. This study looks at the likely effects of these climate changes.

Previous studies have already shown that extreme weather--periods of particularly hot or cold weather for a region--are correlated with increased mortality. Medina-Ramon and Schwartz went on to ask whether the decrease in death associated with fewer frigid days would compensate for the increase in mortality associated with more hot days. Using eleven years of mortality and weather data from fifty US cities, the researchers found that in the past, mortality increased by 1.59 percent after two days of extreme cold; whereas in extreme hot weather, mortality increased by 5.74 percent.

The study, published in the journal Occupational and Environmental Medicine, found that the while cold-weather mortality will likely drop with fewer extreme cold events, this change will not offset the increase in mortality associated with the increase of extreme hot weather. Overall, the researchers conclude, more people will die because of new hot weather events associated with global warming than will live because of fewer cold weather events.

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The data showed that heat-driven mortality was not uniform across the country. “For heat, the worst effects were in cities where the summers were not as hot,” Medina-Ramon says. This may be related to the presence of air-conditioning: cities that usually have hot summers have more air-conditioners. “Where air-conditioning is very common, the effects are not as strong.” Density was also positively correlated with greater increases in mortality rates.

According to the analysis, cities like Chicago, Jersey City, Detroit, and Philadelphia had the highest increases in mortality associated with extreme hot weather. Medina-Ramon says that may mean that these cities could see the greatest increase in mortality due to global warming.

The correlation with air conditioning also indicates that the poorest people could be hit the hardest by increasing temperatures associated with global warming, Medina-Ramon says. “People that aren’t as rich are at greater risk,” she says.

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The study does not factor in the effects of particulate matter and pollution, however, which may be responsible for some of the mortality increases.

While the death rates associated hot days were not uniform across cities, cold weather produced similar effects everywhere. Medina-Ramon says that this uniformity in cold weather effects suggests that humans in the U.S. are acclimatized to the cold, no matter where they live: “We are pretty much adapted to the cold.” This may be because central heating is more common than central air—people have ways to heat their homes even in warm climates.

If humans have adapted to the cold with central heating, will installing more air-conditioners be the key to preventing deaths in fair-weather cities? Medina-Ramon says: “Yes, that would help mitigate the effects of heat, but it that would also contribute to increased global warming.”

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--Flora Lichtman