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.

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?”