Cracks show in the dry bed of the Stevens Creek Reservoir in Cupertino, Calif. (AP Photo/Marcio Jose Sanchez, File)

Vastly more Americans will be exposed to dangerous heat waves in future decades because of a combination of rising temperatures and rapid population growth in the South and West, scientists warned in a study published Monday.

The risk of exposure to extreme heat could be as much as six times higher for the average U.S. citizens by the year 2070, compared with levels experienced in the last century, researchers at the National Center for Atmospheric Research and the City University of New York found. The projected change carries significant implications for Americans’ health, as extreme heat kills more people than any other weather-related event, the study’s authors report in the journal Nature Climate Change.

“Both population change and climate change matter,” said co-author Brian O’Neill, an NCAR scientist and expert on modeling impacts of climate change. “If you want to know how heat waves will affect health in the future, you have to consider both.”

Scientists have long warned about the growing risk of weather extremes — from superstorms to killer heat waves — as the planet warms. But many previous studies failed to consider how population changes could amplify the effects of climate change. In the new study, the researchers sought to create a more accurate measure of the impact of future heat waves by creating a computer model that shows where Americans will likely be living a half-century from now.

Current trends point to continued rapid growth in the Sun-belt cities such as Atlanta, Dallas, Phoenix and Tampa — cities that also are likely to see many more days of 95-degrees-plus temperatures in the future because of global warming, the authors said. The computer model predicted that average yearly exposure to extremely high temperatures would rise from 2.3 billion person-days — the average during the years from 1971 to 2000 — to between 10 billion and 15 billion person-days during the years from 2041 to 2070. The study did not try to estimate the number of future heat-related illnesses and deaths.

“We show that heat exposure will go up,” O’Neill said, “but we don’t know how many of the people exposed will or won’t have air conditioners or easy access to public health centers, for example.”

Still, the results are a reminder of the potentially enormous societal costs related to climate change, even in a wealthy country such as the United States, the researchers said.


“It is how people experience these extremes that will ultimately shape the broader public perception of climate change,” said CUNY post-doctorate researcher Bryan Jones, the study’s lead author.