With a sweltering heat wave in Massachusetts not far behind us, new research is finding that periods of heat and humidity so great that humans cannot survive without sources of cooling are likely coming much sooner than previously believed.
Hundreds of millions of people worldwide could be regularly exposed, including in parts of the southern United States, it concludes.
“It’s going to implicate huge swaths of humanity. Billions of people are going to have to deal with this on an annual basis,” said Christopher Schwalm, a senior scientist and the Risk Program Director at the Woodwell Climate Research Center, who co-authored the study, published earlier this month in the journal Science Advances.
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That kind of heat — defined roughly as six hours or more of 95 degrees and 100 percent humidity — would be deadly to a person without access to cooling, the authors write. At that point, even lying in the shade would not be safe. For comparison, the hottest day in Boston during the recent heatwave, Sept. 7, registered a high of 93 degrees and 76 percent relative humidity.
Earlier work predicted that waves of such extreme heat would happen only with extreme warming — what is expected closer to the end of the century, assuming the planet had warmed at least 9 degrees F (5 degrees C) above preindustrial temperatures by then.
The latest study dramatically moves up that timeline, finding that more than a quarter of the weather stations around the world could expect to experience lethal heat at least once a decade at 3.6 degrees F (2 degrees C) of warming — a threshold the planet will likely hit by the middle of the century.
By then, much of India and major regions of Africa and South America, as well as the southeastern United States, could expect lethal heat on a near-annual basis. The northeastern United States could begin seeing this heat with 4.5 degrees F (2.5 degrees C) of warming, which the planet is currently on track to pass before the end of the century. Already, the planet has warmed roughly 2 degrees F.
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The shifting timeline of the impacts of climate change has been somewhat of a theme this summer, as scientists have noted that extreme impacts of climate change, like intense heat waves, wildfires, flooding, and increased ocean temperatures, have far exceeded expectations.
“Climate models are inherently conservative,” Schwalm said, noting that a growing body of science has informed newer climate models that are able to more accurately project the impacts of warming.
The lethal heat days described in the recent article are defined by the wet bulb temperature — a measure that combines heat and humidity. And while it roughly equates to 95 degrees F and 100 percent humidity, the point at which wet bulb conditions become lethal can also be hit at lower humidity levels but higher temperatures.
The authors note that these kind of events have happened in some parts of the world already, mostly in high heat and humidity areas like the Persian Gulf, Red Sea, and the Indo-Gangetic Plain, just south of the Himalayas, or places that may have low humidity but extremely high heat, like West Australia and the Sonoran Desert. In those cases, there have not been high numbers of deaths caused by the extreme heat, likely because over time people there have developed physical attributes that allow them to tolerate higher heat, they wrote.
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But as the climate gets warmer, more of the planet will be exposed to these kinds of dangerous heat events, including large numbers of people who have not adapted.
“Communities are often only prepared for extreme temperature events within the bounds of past experience, and populations only acclimatized to the present climate,” the authors wrote. Should one of these events happen in a place not accustomed to extreme heat, “the number of excess deaths that result could markedly exceed the impacts of past local extremes.”
New England is not among the parts of the world expected to be hit most severely by lethal heat, but even so, the region is already having to address more health-threatening heat, and that will only increase in the future.
“Boston is a city that’s been worried about cold — we worry about snowstorms, and we are really well prepared for extreme cold,” said Amruta Nori-Sarma, a professor of environmental health at Boston University, who was not a part of the study. “We need to start doing more to prepare people for extreme heat emergencies, and make sure that our populations are resilient as summertime temperatures become hotter and hotter.”
That includes things like establishing more cooling centers, planting trees, and preserving green spaces that can keep heat islands at bay, and making sure air conditioners are accessible and affordable for low-income residents.
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“The take-home messages are clear — we must educate our communities about how to limit the heat stress on our bodies,” said Gaurab Basu, director of education and policy at Harvard Chan C-CHANGE, who was not involved with the study. “And above all, we must end the use of fossil fuels which is stoking the warming of our planet.”
Sabrina Shankman can be reached at sabrina.shankman@globe.com. Follow her @shankman.