You may have heard of “wet bulb temperature,” a combined measure of heat and humidity that tells us how effectively our bodies can cool themselves. Normally, evaporating sweat from our skin is enough to keep us at safe temperatures. But when humidity climbs high enough, sweat stops evaporating — and without external cooling, the human body shuts down.

In other words, the wet bulb temperature is more revealing than the ambient temperature. The old benchmark for human survivability was a wet-bulb temperature of 35 degrees Celsius, equivalent to 95 degrees Fahrenheit at 100 percent humidity. These conditions had been observed for brief periods, but never long enough to cause mass casualties on their own — or so we thought.

Horrifyingly, though, a new study of six extreme heatwaves published in the journal Nature Communications argues that wet bulb temperatures aren’t actually the relevant threshold for mass heat death. Instead, the real kill-zone is both cooler and less humid than anybody thought.

“My first thought was ‘oh sh*t’ — I really didn’t expect to see that, especially when you zoom in to individual cities,” Sarah Perkins-Kirkpatrick, the study’s lead author and professor of climate science at the Australian National University told the Guardian.

Perkins-Kirkpatrick’s team applied a new model based on human physiology called HEAT-Lim to six heatwaves: Saudi Arabia in 2024; Bangkok in 2024; Phoenix in 2023; Mount Isa in 2019; Karachi in 2015; and Seville in 2003. Except for the 2019 heatwave in Australia, each event was associated with thousands of deaths due to cardiovascular- or respiratory-related events that went unattributed to heat. The previous wet bulb model largely accounted for temperature only, leading to what the paper argues is a systematic undercount of just how lethal each heatwave was.

“We have often defined heatwaves by temperature alone and partly that has been because of the data that we had,” Perkins-Kirkpatrick told the Guardian. “But using this model of how the body functions, it is a much better way to understand how these events can be deadly.”

By updating the model to factor in the properties of the human body, researchers found that all six heatwaves contained periods of time which would have been unsurvivable for elderly people in direct Sun.

Two of them were particularly grim. During periods of the Phoenix heatwave of 2023 and the Karachi heatwave of 2015, no amount of shade would have been enough to save people over the age of 65. The Pakistan event got so hot that people aged 18 to 35 faced unsurvivable conditions in full Sun — as evidenced by the heavy death toll that ultimately exceeded 2,000.

As such, Perkins-Kirkpatrick posed a grim question to the Guardian: “If it’s already happening now, then what does a future that is two or three degrees warmer hold?”

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