Imma Perfetto
Cosmos science journalist
A new review has outlined the potentially deadly consequences of extreme heat if global warming reaches 2°C above the preindustrial average.
Climate change is increasing the frequency and magnitude of heat extremes worldwide, with cascading effects on human life – from crops, to infrastructure, and health.
According to the new research, the areas of the Earth that will reach temperatures too hot for healthy young humans to maintain a safe body temperature in is expected to triple to 6%. For people aged over 60 years old this will rise to about 35% of Earth’s landmass.
“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,” says Dr Tom Matthews, lecturer in environmental geography at King’s College London, UK, and lead author of the paper.
“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.”
Already the 3 deadliest heat events of the 21st century have collectively caused nearly 200,000 deaths, with mortality expected to be underreported in the global south.
Matthews and collaborators reviewed existing scientific literature to assess how heat thresholds have been breached in the past, and how this may change in a 0.5°C hotter world.
Beyond “uncompensable” heat thresholds, human core body temperatures rise uncontrollably. Beyond “unsurvivable” thresholds, core temperatures reach an almost certainly lethal 42°C within 6 hours.
These thresholds depend on age – as older people are more vulnerable to heat stress – and the combination of air temperature and relative humidity.
They found that from 1993-2023, roughly 2.2% experienced at least 1 hour of uncompensable heat, but 0.6% of Earth’s land surface experienced 6 hours of that level of heat for adults under 60.
“Most of these … heat events have occurred in the Persian/Arabian Gulf region and across the Indo-Gangetic Plain, with more isolated hotspots in tropical West Africa, the Amazon Basin, the southern USA and Mexico, Australia, and eastern China,” the authors write in their review.
However, the global average temperature exceeded 1.5°C above pre-industrial levels for the first time in 2024. Current predictions by Climate Action Tracker indicate we are set to reach 2°C by mid- to late-century.
“Uncompensable heat for older adults could be expected across [approximately] 35% of the land area at 2 °C above preindustrial levels,” the authors predict.
“Unsurvivable heat for this warming level tends to remain generally restricted to older adults, expanding slightly the areas already at risk.”
Matthews says that anticipating the magnitude of future heat extremes and their worst-case impacts is critical to understanding the costs 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 extremelybad,” says 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.
It is also crucial for targeting adaptation efforts at those communities most in need. People employed to work outside and doing manual labour, the socially isolated with low levels of mutual aid, and lower income communities lacking access to water and electricity, are particularly at risk of heat-related mortality.
“Interdisciplinary work is vital to improving our understanding of unprecedented heat’s deadly potential and how it can be reduced,” says Matthews.
“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.”
The research appears in Nature Reviews Earth & Environment.
