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4 hours ago
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Extreme heat is already creating “non-survivable” conditions for humans in heatwaves that have killed thousands and likely many more, according to new research that warns people are more susceptible to rising temperatures than first thought.
Scientists re-examined six extreme heatwaves between 2003 and 2024 and found that when temperature, humidity and the body’s ability to stay cool were accounted for, all were potentially deadly for older people.
The absolute limit for humans to survive had been assumed to be a six-hour exposure to a wet bulb temperature of 35C – a measure that accounts for temperature and humidity but has rarely been observed on the planet at that level.
Heatwaves in Mecca (Saudi Arabia, 2024), Bangkok (Thailand, 2024), Phoenix (United States, 2023), Mount Isa (Australia, 2019), Larkana (Pakistan, 2015) and Seville (Spain, 2003) had seen thousands of deaths despite none approaching that wet bulb limit, the research found.
But when scientists applied a new model of human survivability that takes into account the body’s ability to function and stay cool depending on age, they found all six events had seen non-survivable periods for older people who could not find shade.
Prof Sarah Perkins-Kirkpatrick, the study’s lead author at the Australian National University, said the results were shocking.
“My first thought was ‘oh shit’ – I really didn’t expect to see that, especially when you zoom in to individual cities,” she said.
“If it’s already happening now, then what does a future that is two or three degrees warmer hold?”
Climate studies have found that heatwaves are lasting longer and delivering higher temperatures around the world.
Perkins-Kirkpatrick, a climate scientist and global expert on extreme heat, said: “We have often defined heatwaves by temperature alone and partly that has been because of the data that we had.
“But using this model of how the body functions, it is a much better way to understand how these events can be deadly.”
The study only looked for periods where deadly heatstroke was likely if a person was exposed for six hours and also assumed people were entering the heat with normal core body temperature.
All six heatwaves included periods that would not have been survivable for people aged over 65 remaining outside in full sun.
The Larkana and Phoenix heatwaves included periods that were not survivable for older people even if they had found shade.
The Larkana heatwave included a non-survivable period that would have been deadly for people aged 18-35 in full sun.
The research, published in the journal Nature Communications, suggested deaths from heat, particularly in developing and densely populated areas, were “undoubtedly and seriously underreported”, Perkins-Kirkpatrick and colleagues wrote.
The results showed, they wrote, that “deadly conditions have already placed hundreds of millions of people at grave risk”.
Perkins-Kirkpatrick said before the development of the model, scientists had tended to rely on statistical analysis to understand the likely number of deaths related to extreme heat.
Prof Ollie Jay, a co-author of the study and the director of the University of Sydney’s Heat and Health Research Centre, said: “Conditions that threaten human life are already here and the risk moving forward is almost certainly much greater than we previously thought.”
The only way for the human body on its own to keep its core temperature within safe limits is by sweating, and for that sweat to be able to evaporate.
But when high temperatures and humidity combine, this reduces the evaporation of sweat and if left unchecked, can cause heatstroke.
Jay said older people were particularly vulnerable because their ability to sweat is reduced, particularly for people over 75.
Prof Steve Sherwood, a climate scientist at the University of New South Wales, carried out early research that helped establish theoretical human temperature limits.
“The newer work more accurately establishes where the true limits are,” he said.
“The fact we are so close to physiological limits means that mitigating higher temperatures is essential to humans still being able to live and thrive in the hottest and most humid places, including the top end of Australia and much of the tropics, but especially in India and the Middle East.
“Much of the world’s population lives in these places. Global warming will really hurt if it goes too much further just due to the extreme temperatures and humidity, even if we manage rainfall, aridity, storm and sea level changes.”
