Half-degree of warming triples area too hot for humans

King’s College London – An international group of scientists has revealed how continued global warming will lead to more parts of the planet becoming too hot for the human body over the coming decades.

The paper, published in Nature Reviews Earth and Environment, finds that the amount of landmass on our planet that would be too hot for even healthy young humans (18- to 60-year-olds) to keep a safe core body temperature will approximately triple to six percent — an area almost the size of the United States — if global warming reaches two degrees Celsius above the pre-industrial average.

Under these conditions, they also warn that the area of land where people over the age of 60 will be at risk will increase to about 35 per cent.

Last year was the first calendar year with a global mean temperature of more than 1.5 C above the pre-industrial average, and at current rates of warming, 2 C could be reached by mid- to late-century.

“Our findings show the potentially deadly consequences if global warming reaches 2 C,” said Tom Matthews, lead author and senior lecturer in Environmental Geography at King’s College London.

“Unsurvivable heat thresholds, which so far have only been exceeded briefly for older adults in the hottest regions on Earth, are likely to emerge even for younger adults. In such conditions, prolonged outdoor exposure — even for those if in the shade, subject to a strong breeze, and well hydrated — would be expected to cause lethal heatstroke. It represents a step-change in heat-mortality risk.”

For their assessment, the team drew together scientific findings to link physical climate science with heat mortality risk, including crossing ‘uncompensable’ and ‘unsurvivable’ thresholds. Uncompensable thresholds are when human core body temperature rises uncontrollably, while unsurvivable thresholds are when the body’s core temperature increases to 42 C within six hours.

Between 1994 to 2023, human thermal tolerances, the combination of temperature and humidity above which the human body can’t cope, were breached for about two per cent of the global land area for adults under 60. More than 20 per cent of the Earth’s land surface crossed this threshold for older adults, who are more vulnerable to heat stress.

While uncompensable thresholds have been passed for all ages, unsurvivable thresholds have so far only been passed briefly for older adults.

For 4 to 5 C above pre-industrial levels, older adults could experience uncompensable heat across around 60 per cent of the Earth’s surface during extreme events. At this level of warming, unsurvivable heat would also begin to emerge as a threat to younger adults in the hottest subtropical regions.

Certain regions are more at risk of crossing the critical uncompensable and unsurvivable thresholds, with people in Saharan Africa and South Asia most in the firing line.

Matthews said anticipating the magnitude of future heat extremes and their worst-case impacts is critical to understanding the costs of failing to mitigate climate change. It is also crucial for targeting adaptation efforts at those communities most in need.

“What our review really shows very clearly is that, particularly for higher levels of warming such as 4 C above the pre-industrial average, the health impacts of extreme heat could be extremely bad,” he said. “At around 4 C of warming above pre-industrial levels, uncompensable heat for adults would affect about 40 per cent of the global land area, with only the high latitudes, and the cooler regions of the mid-latitudes, remaining unaffected.”