If global warming continues, more parts of the planet will become too hot for humans over the coming decades, according to a study published in Nature Reviews Earth and Environment.
The team from King’s College London assessed mortality risk in hot conditions, including crossing ‘uncompensable’ (when the core temperature rises uncontrollably) and ‘unsurvivable’ thresholds (when the core temperature reaches 42°C within six hours).
Between 1994 and 2023, the temperature above which the human body can’t cope occurred in about 2% of land for adults under 60 and 20% for older adults, who are more vulnerable to heat.
If we fail to keep global warming below 2°C in the future, the area of our planet that will become too hot even for healthy humans would approximately triple to 6%. The area of land that will be unsuitable for those over 60 would also increase to about 35%. Last year, the global mean temperature was more than 1.5°C above the pre-industrial average, and at the current rates, it will be 2°C by the mid to late century. Certain regions are more at risk of crossing the critical uncompensable and unsurvivable thresholds, with people in Saharan Africa and South Asia most at risk.
“Our findings show the potentially deadly consequences if global warming reaches 2°C. 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 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 Dr Tom Matthews, lead author and Senior Lecturer in Environmental Geography at King’s College London.
Dr Matthews emphasised that anticipating the extend of future heat events is critical to understand the cost of failing to mitigate climate change. “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 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. Interdisciplinary work is vital to improving our understanding of unprecedented heat’s deadly potential and how it can be reduced. As more of the planet experiences outdoor conditions too hot for our physiology, it will be essential that people have reliable access to cooler environments to shelter from the heat”.
Matthews, T., Raymond, C., Foster, J. et al. Mortality impacts of the most extreme heat events. Nat Rev Earth Environ 6, 193–210 (2025). https://doi.org/10.1038/s43017-024-00635-w