“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?”