Dangerous heat is said to be three times more common in the future

What’s officially considered “dangerous heat” in the coming decades is likely to hit much of the world at least three times more often as climate change worsens, according to a new study.

Across much of the world’s affluent mid-latitudes, temperatures and humidity that feel like 103 degrees (39.4 degrees Celsius) or higher — now an occasional summer shock — should statistically occur 20 to 50 times a year by mid-century, one said Study Monday in the journal Communications Earth & Environment.

By 2100, this brutal heat index could last for most of the summer for places like the US southeast, the study author said.

And it’s a lot worse for the sticky tropics. The study says that a heat index considered “extremely dangerous” where the felt heat index exceeds 124 degrees (51 degrees Celsius) — which is now rare — is likely to hit a tropical belt that burns India one to four weeks a year Year includes end of century.

“So that’s the scary thing about it,” said study author Lucas Zeppetello, a Harvard climate scientist. “This is something where potentially billions of people will be exposed to extremely dangerous heat on a very regular basis. So something that has practically never happened before will become something that happens every year.”

Zeppetello and colleagues used more than 1,000 computer simulations to examine the probabilities of two distinct levels of high heat — heat indices of 103 degrees (39.4 degrees Celsius) and above 124 degrees (51 degrees Celsius), which are dangerous and extremely dangerous thresholds nationally US Weather Service. They calculated for the years 2050 and 2100 and compared that to how often that heat occurred around the world each year from 1979 to 1998.

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The study found a three- to ten-fold increase in 103-degree mid-latitude heat, even in the unlikely best-case scenario of global warming limited to just 3.6 degrees (2 degrees Celsius) since pre-industrial times — the less strictly by two international goals.

The study found that there was only a 5% chance that warming would be so small and rare. More likely, the study says 103-degree heat will dampen the tropics “on most days of any typical year” by 2100.

Chicago only reached that 103-degree heat index four times from 1979 to 1998. However, the study’s most likely scenario shows Chicago reaching that heat threshold 11 times a year by the end of the century.

Heatwaves are one of the new four horsemen of apocalyptic climate change, along with sea-level rise, water scarcity and changes across the ecosystem, said Zeppetello, who conducted much of the research at the University of Washington State during the warming-stressed heat wave of 2021 , which broke records and killed thousands.

“Unfortunately, the dire predictions presented in this study are credible,” Woodwell Climate Research Center climate scientist Jennifer Francis, who was not part of the study team, said in an email. “The past two summers have given us a glimpse of our hot future, with deadly heat waves in Europe, China, northwestern North America, India, the central-southern US, the UK, central Siberia and even New England. Already hot places become uninhabitable when heat indices exceed dangerous thresholds, impacting people and ecosystems alike. Areas where extreme heat is rare today will also increasingly suffer as infrastructure and living beings are ill-adapted to the stifling heat.”

The study focuses on the heat index, and that’s smart because it’s not just heat, it’s the combination with humidity that’s bad for health, said Dr. Renee Salas, a professor at Harvard School of Public Health who is an emergency room physician.

“As the heat index goes up, it becomes increasingly difficult to cool our bodies,” Salas, who was not part of the research team, said in an email. “Heat stroke is a potentially fatal form of heat illness that occurs when body temperature rises to dangerous levels.”

The study is based on mathematical probabilities and not other climate research that looks at what happens at different levels of carbon pollution. For this reason, climate scientist Michael Mann of the University of Pennsylvania is more skeptical about this research. It also doesn’t take into account the landmark US climate legislation signed into law by President Joe Biden earlier this month, or new efforts by Australia, he said.

“The obstacles at this point are political, not statistical, no matter how powerful or sophisticated they can be in predicting whether we will muster the political will to overcome them,” Mann said in an email. “But there is cause for cautious optimism.”

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Follow Seth Borenstein on Twitter at @borenbears

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