Warning: Deadlier Heat Waves Ahead

In the second week of July 1995, a high-pressure air mass stalled over the Midwestern U.S. In the cities and countryside below, the temperature began to climb. On July 12, the temperature reached 98 degrees in downtown Chicago. The following day, it hit a searing 106, with a nighttime low of 84. Over the next two days, the high stayed around 100.

In the city’s poorer neighborhoods, elderly people who either had no air conditioning or could not afford to use it suffered, and some died.

“Subsequent studies have estimated that more than 700 excess deaths occurred in the hottest part of that summer,” says Roger Peng, PhD, MS, author of a new analysis suggesting that deadly hot spells in cities like Chicago could get much hotter and deadlier before this century is out.

Peng, an associate professor in Biostatistics, took an interest in future heat wave mortality after reading a climate-modeling paper in Science in 2004, which predicted worse and more frequent heat waves for two exemplar cities, Chicago and Paris, as human-caused climate change increased. “I thought that was a very interesting paper, and wondered about the implications in terms of public health,” Peng says.

Peng had previously done analyses linking air pollution indices to increases in mortality—increases that show up as statistically significant moves above the normal background death rate.

“I thought, why don’t we use the same tools to estimate excess mortality risk from past heat waves, then use climate models to project those estimates into the future? There haven’t been many studies showing how climate change will affect human health,” he says.

Peng worked with several co-authors, including Claudia Tebaldi, who is a climate statistician at the University of British Columbia and senior author of the 2004 Science paper. They calculated excess hot-weather mortality for the Chicago area in the final two decades of the 21st century, under several climate scenarios. “Naturally, our excess mortality estimates varied with these [scenarios],” says Peng. “But under every scenario, excess mortality was greater than what one would expect from population growth alone. In fact, for some climate projections, we could expect the mortality equivalent of more than one 1995-type heat wave every year.”

The study, which appeared online in December in Environmental Health Perspectives, estimated that Chicago summers in the years 2081-2100 would claim between 166 and 2,217 excess deaths annually, depending on the climate scenario and model used.

The analysis didn’t take into account a number of factors, including the possible mortality offset from warmer winters. But Peng observes that “climate change in general is thought to be pushing the weather to greater extremes, so hotter summers could end up being only part of the climate-change health problem.”