As the world warms beyond 1.5掳C, large parts of the world will start to have heatwaves so extreme that healthy young people could die within several hours if they fail to find respite, a study has warned. This could result in mass deaths in places where people and buildings aren鈥檛 adapted to extreme heat and air conditioning is rare, says at the University of Oxford.
鈥淵ou could have a very extreme heatwave that departs from historical norms substantially, crosses this threshold and causes much more mortality than you would otherwise expect,鈥� he says. 鈥淲hat we [will] see, particularly in Europe and North America, is an enormous increase in the incidence of these heatwaves [as the world warms] between 1.5 and 2 degrees [C].鈥�
Global warming is already sparking more intense and more frequent heatwaves, so is causing large numbers of fatalities. It is estimated that there were in the summer of 2022, for instance. However, the vast majority of these were people aged over 65 who may have had existing health issues.
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Could global warming result in parts of the world getting so hot that even healthy young people die? at Purdue University, Indiana, and his colleagues set out to investigate this question in 2010.
Based on theory, they decided the limit of survivability is when the temperature measured by a thermometer covered in a wet cloth exceeds 35掳C (95掳F). This is the so-called wet-bulb temperature. It reflects the fact that humidity affects our ability to stay cool by sweating. At this wet-bulb reading we can no longer keep core body temperature in check naturally and it will rise to deadly levels if we don鈥檛 take action to stay cool in other ways.
At present, the wet bulb temperature very seldom exceeds 31掳C (88掳F) anywhere on Earth鈥檚 surface. Huber鈥檚 team concluded that large areas would only start to exceed the 35掳C wet bulb limit if the world warmed by more than 7掳C 鈥� which is thought highly unlikely.
However, recent studies suggest parts of the tropics could exceed this limit at lower levels of warming. What鈥檚 more, in practice most people couldn鈥檛 survive anything close to a wet-bulb temperature of 35掳C. 鈥淭he original 35-degree limit was meant always as an upper limit,鈥� says Huber.
Last year, at Pennsylvania State University and his colleagues tested 24 healthy young women and men to see how hot and humid it could get before their bodies were unable to stop their core temperature rising 鈥� the point at which heat is 鈥渘oncompensable鈥�. Continued exposure to these conditions for several hours can result in death.
The findings suggest reading, though other factors will affect this in reality. Because the volunteers weren鈥檛 acclimatised to heat and were doing everyday tasks during the tests, this should be seen as a lower limit with a 35掳C wet-bulb temperature being the upper limit, says Powis.
鈥淎nything between those two is very much in the danger zone,鈥� he says. 鈥淭here鈥檚 not just one threshold that is relevant to everyone. Different populations have different thresholds where there could suddenly be dramatic mortality outcomes.鈥�
Powis and his colleagues have now used data from weather stations and climate models to see where in the world such conditions may currently occur based on Vecellio鈥檚 31掳C wet-bulb findings, and how this will change at the world warms.
For instance, with 1掳C of global warming 鈥� a level already passed 鈥� only 3 per cent of weather stations in Europe are likely to pass Vecellio鈥檚 threshold more than once in 100 years. With 2掳C of warming, 25 per cent are likely to. In the US, 20 per cent of stations are likely to pass the threshold more than once in 100 years with 1掳C global warming, rising to 28 per cent for 2掳C.
鈥淪ometimes these human survivability limits are useful to understand the problem, but the reality is that we see a significant health burden on the population even at 鈥榤oderate鈥� temperatures,鈥� says at the University of Bristol, UK, who wasn鈥檛 involved in the study. 鈥淯sing a threshold-based temperature can be misleading, because even if it鈥檚 hot outside, it doesn鈥檛 mean that it鈥檚 hot inside.鈥�
鈥淚 would like to highlight that all heat-related impacts on human health and well-being are preventable,鈥� says at the University of Warwick in the UK. But with heatwaves becoming more frequent, more intense and more prolonged, urgent action is needed to prevent more heat-related deaths, she says.
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