What is officially considered “dangerous heat” in the coming decades will likely hit much of the world at least three times more often as climate change worsens, according to a new study. In many of Earth’s rich mid-latitudes, sudden temperature and humidity spikes that seem 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, according to a study Monday in the journal Nature Climate Change. By 2100, this violent heat index could persist for most of the summer for places like the US Southeast, the study’s author said. And it’s much worse for the sticky tropics. The study said a heat index considered “extremely dangerous” where the sensation-like heat index exceeds 124 degrees (51 degrees Celsius) – now a rare occurrence – would likely hit a tropical belt that includes India one to four weeks a year per century end. “So that’s kind of scary 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 levels of heat very regularly. So something that has almost never happened in the past will go into something that happens every year.” Zeppetello and his colleagues used more than 1,000 computer simulations to examine the likelihood of two different levels of high heat — heat indices of 103 degrees (39.4 Celsius) and above 124 degrees (51 Celsius), which are dangerous and extremely dangerous limits according to the US National Weather Service. They calculated for the years 2050 and 2100 and compared this with how often this heat occurred each year around the world from 1979 to 1998. The study found a threefold to tenfold increase in heat of 103 degrees in mid-latitudes even under the unlikely best-case scenario of global warming limited to just 3.6 degrees (2 degrees Celsius) since pre-industrial times — less stringent than two international targets. There is only a 5% chance that the warming will be so low and so rare, according to the study. More likely, according to the study, is that 103-degree heat will steam the tropics “on most days of any typical year” by 2100. Chicago reached the 103-degree heat index level only four times from 1979 to 1998. But the study’s most likely scenario shows Chicago reaching that hot and sticky mark 11 times a year by the end of the century. Heat waves are one of the four new horsemen of apocalyptic climate change, along with rising sea levels, water scarcity and changes to the overall ecosystem, said Zeppetello, who did much of the research in Washington state during the during the 2021 heat wave caused by overheating. broke records and killed thousands. “Unfortunately, the dire predictions shown in this study are credible,” climatologist 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 bleak future, with deadly heat waves in Europe, China, northwestern North America, India, the south-central US, the United Kingdom, central Siberia and even New England. Already hot places will become uninhabitable as heat indices exceed dangerous limits, affecting humans and ecosystems alike. Areas where extreme heat is now rare will also suffer increasingly as infrastructure and living things do not adapt properly to the overwhelming heat.” The study focuses on the heat index, and that’s smart because it’s not just the heat but the combination with humidity that damages health, said Harvard School of Public Health professor Dr. Rene Salas, who is an emergency physician. “As the heat index rises, it becomes more and more difficult to cool our bodies,” Salas, who was not part of the research team, said in an email. “Heatstroke is a potentially fatal form of heat illness that occurs when the body’s temperature rises to dangerous levels.” The study relies on mathematical probabilities rather than other climate research that looks at what happens at various levels of carbon pollution. Because of this, University of Pennsylvania climate scientist Michael Mann is more skeptical of this research. It also doesn’t take into account landmark US climate legislation signed by President Joe Biden earlier this month or new Australian efforts, he said. “The obstacles at this point are political, and no statistical methods, no matter how powerful or sophisticated, can predict whether we will muster the political will to overcome them,” Mann said in an email. “But there is reason for cautious optimism.”
