Some of the world's worst recent heatwaves were lethal even without temperatures hitting what had been considered the upper limit of human tolerance, a new study finds. Researchers revisited six major heat events across the world over two decades—including in Phoenix in 2023—and calculated not just temperature and humidity but how the human body functions under those conditions, per the Guardian. They found "non-survivable" six-hour stretches for people over 65 outdoors in full sun in all six locations, even though none reached the previously assumed human ceiling: six hours at a 95-degree Fahrenheit (35-degree Celsius) wet-bulb temperature.
Wet-bulb temperature represents the lowest temperature achievable through evaporative cooling. Sweating and the evaporation of sweat is how the body keeps cool, but sweat production declines with age and evaporation is reduced when humidity and extreme heat combine. Essentially, "your core temperature keeps rising, even if you're resting in the shade with water," per the Times of India. During the 2023 event in Phoenix and the 2015 heatwave in Larkana, Pakistan, certain periods were deemed deadly for older people even in shade; in Larkana, one window was found potentially fatal for adults aged 18 to 35 in full sun.
The study, published in Nature Communications, concludes "deadly conditions have already placed hundreds of millions of people at grave risk." Lead author Sarah Perkins-Kirkpatrick of Australian National University said the findings show conventional heatwave metrics, focused on air temperature alone, have understated the danger as climate-driven heat events grow longer and more intense. "If it's already happening now, then what does a future that is two or three degrees warmer hold?" she asks, per the Guardian.
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