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What’s considered officially “dangerous heat” in coming decades will likely hit much of the world at least three times more often as climate change worsens unless countries move faster to curb their greenhouse gas emissions, according to a new study.

In much of Earth’s wealthy mid-latitudes, spiking temperatures and humidity that feel like 103 degrees 39.4°C/103°F or higher—now an occasional summer shock—statistically should happen 20 to 50 times a year by mid-century, said a study published last week in the journal Communications Earth & Environment.

The study is based on mathematical probabilities instead of other climate research that looks at what happens at various carbon pollution levels, The Associated Press reports. Because of that, University of Pennsylvania climate scientist Michael Mann is more skeptical of this research. It also doesn’t take into account landmark U.S. climate legislation that President Joe Biden signed earlier this month or new efforts by Australia, he said.

“The obstacles at this point are political and no statistical methods, regardless of how powerful or sophisticated, can predict whether we will garner the political will to overcome them,” Mann said in an email. “But there is reason for cautious optimism.”

But without that action, the study says that by 2100, a brutal heat index may linger for most of the summer for places like the U.S. Southeast, the study’s author said.

And it’s far worse for the sticky tropics. The study said a heat index considered “extremely dangerous” where the feels-like heat index exceeds 51°C/124°F—now something that rarely happens—will likely strike a tropical belt that includes India one to four weeks a year by century’s end.

“So that’s kind of the scary thing about this,” said study author Lucas Zeppetello, a Harvard climate scientist. “That’s something where potentially billions of people are going to be exposed to extremely dangerous levels of heat very regularly. So something that’s gone from virtually never happening before will go to something that is happening every year.”

Zeppetello and colleagues used more than 1,000 computer simulations to look at the probabilities of two different levels of high heat—heat indexes of 39.4°C/103F and above 51°C/124°F, which are dangerous and extremely dangerous thresholds according to the U.S. National Weather Service. They calculated for the years 2050 and 2100 and compared that to how often the same heat happened each year across the world from 1979 to 1998.

The study found a three- to 10-fold increase in 39.4° heat in the mid-latitudes, even in the unlikely scenario of average global warming limited to only 2°C/3.6°F since pre-industrial times—the less stringent of two international goals.

There’s only a 5% chance for warming to be that low and that infrequent, the study found. What’s more likely, according to the study, is that the 39.4° heat will steam the tropics “during most days of each typical year” by 2100.

Chicago hit that 39.4° heat index level only four times from 1979 to 1998. But the study’s most likely scenario shows the city hitting that hot-and-sticky threshold 11 times a year by the end of the century.

Heat waves are one of the new four horsemen of apocalyptic climate change, along with sea level rise, water scarcity, and changes in the overall ecosystem, said Zeppetello, who did much of the research at the University of Washington during the warming-charged 2021 heat wave that shattered records and killed thousands.

“Sadly, the horrific predictions shown in this study are credible,” climate scientist Jennifer Francis of the Woodwell Climate Research Center, who was not part of the study team, said in an email. “The past two summers have provided a window into our steamy future, with lethal heat waves in Europe, China, northwestern North America, India, the south-central U.S., the UK, central Siberia, and even New England. Already hot places will become uninhabitable as heat indices exceed dangerous thresholds, affecting humans and ecosystems alike. Areas where extreme heat is now rare will also suffer increasingly, as infrastructure and living things are ill-adapted to the crushing heat.”

The study focuses on the heat index and that’s smart, because it’s not just heat but the combination with humidity that hurts health, said emergency room physician and Harvard School of Public Health professor Dr. Renee Salas.

“As the heat index rises, it becomes harder and harder to cool our bodies,” Salas, who wasn’t part of the research team, told AP in an email. “Heat stroke is a potentially deadly form of heat illness that occurs when body temperatures rise to dangerous levels.”

This Associated Press story was republished by The Canadian Press on August 25, 2022.