150,000 Americans will die from excessive heat by the end of this century if carbon pollution continues unabated.
"We think, if anything, (those estimates) are low," said the NRDC's Dan Lashof, explaining during a conference call with reporters that researchers didn't adjust for expected increases in population. But, he said, these stark numbers show "climate change has real life and death consequences -- one of which is that carbon pollution, which is continuing to increase our atmosphere, is going to continue to make climate change worse and increase the number of dangerously hot days each summer."
Thirty seven of the 40 cities studied would see increases in deaths. Larry Kalkstein, a professor of geography and regional studies at the University of Miami and co-author of the research, said they found a "regional coherence" in the heatwave effect. And perhaps counterintuitively, cities in the South appear to be spared the worst.
According to the NRDC, the three cities with the highest number of total estimated heat-related deaths through 2099 are Louisville, Kentucky (19,000 deaths); Detroit (17,900); and Cleveland (16,600).
"The Midwest is particularly hard hit," Kalkstein said, explaining that cities that experience sharp temperature fluctuations are more at risk than those cities with more constant temperatures, even if they're hot.
"Take a typical day in Washington or Philadelphia or New York, where most of the summer days are in the eighties," he said. "And then all of a sudden you get a hot streak, where the temperature goes up to a hundred degrees plus for a week -- and that is what causes the problem. The fact that people are not used to this high variable climate, that all of a sudden you have a 20-degree rise in maximum temperature...for this reason, many less people die of the heat in cities in the deep South."
But just pinning down how many heat-related deaths there are each year is more of an art than a science. "People tend to understate the threat of heat, and the medical examiners tend to under-report the number of people that die from heat," said Kalkstein.
The NRDC praised cities like Chicago, New York, and Philadelphia, which it said have upped their game in responding to heat-related emergencies and have adopted strategies like cooling centers, "heatlines", and organized programs where neighbors look in on local at-risk residents. Some cities also won't cut off power to residents who have failed to pay an electric bill during a heat-related emergency.
Case in point: New York City, with its eight million residents, sees an average of 184 heat-related deaths each summer. But just across the Hudson River in Newark, New Jersey -- population 280,000 -- that number is 56. "New York has one of the most aggressive local health departments in the country," Kalkstein said. "They're one of the few cities in the country where the Health Department calls the heat emergency, rather than the National Weather Service...they are doubling down on every effort to deal with heat."