New research from an international group of scientists has revealed that continued global warming will result in 6% of the Earth becoming too hot for humans to safely inhabit.
Between 1994 and 2023, thermal tolerances—the combination of temperature and humidity—above which the human body can’t cope were breached for about 2% of the global land area for healthy adults (18 to 60). This new research approximately triples the “overheated land” to 6% of the Earth—an area almost the size of the United States.
For the study, published in Nature Reviews Earth and Environment, researchers drew together findings to link physical climate science with heat mortality risk. There are two types of risks: uncompensable, in which a human’s core body temperature rises uncontrollably, and unsurvivable, where the body’s core temperature increases to 42°C within 6 hours.
Last year was the first calendar year with a global mean temperature of more than 1.5°C above the pre-industrial average. While uncompensable thresholds have previously been passed for all ages, unsurvivable heat thresholds have only been exceeded briefly for older adults in the hottest regions on Earth recently. According to the new study, if global warming reaches 2°C—which will happen by mid- to late-century at current rates—even younger adults will be at risk.
“Unsurvivable heat thresholds 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,” said Tom Matthews, lead author and professor in environmental geography at King’s College London.
Under these conditions, Matthews and his co-authors warn that the area of land where the over 60s will be at risk will increase to about 35%.
If warming levels reach 4-5°C above preindustrial, older adults could experience uncompensable heat across around 60% of the Earth’s surface during extreme events.
The study results suggest certain regions are more at risk of crossing the critical uncompensable and unsurvivable thresholds, with people in Saharan Africa and South Asia.
“What our review shows very clearly is that the health impacts of extreme heat could be extremely bad,” said Matthews. “At around 4°C of warming above preindustrial levels, uncompensable heat for adults would affect about 40% of the global land area, with only the high latitudes, and the cooler regions of the mid-latitudes, remaining unaffected.”
Since 2000, there have been more than 260,000 heat-related fatalities. The three deadliest heat events of the 21st Century collectively caused nearly 200,000 deaths, including about 72,000 across Europe in 2003, another 62,000 in 2022, and 56,000 in the Russian heatwave of 2010.
“Anticipating the magnitude of future heat extremes and their worst-case impacts is critical to understanding the costs of failing to mitigate climate change,” concluded Matthews. “It is also crucial for targeting adaptation efforts at those communities most in need.”