Published in Nature Reviews Earth and Environment, the study shows that if worldwide warming reaches 2oC above the preindustrial baseline, the land area too scorching for adults aged 18 to 60 to maintain a healthy core temperature will likely expand to 6% of Earth's surface. This represents a tripling of the area that currently exceeds such limits, and approximates the size of the United States. Under those conditions, the researchers also foresee that around 35% of global landmass will breach dangerous heat thresholds for people over 60.
Scientists note that 2024 was the first full year to register a global mean temperature more than 1.5oC above preindustrial levels. If the climate keeps warming at its present pace, 2oC may well be reached by the middle to latter part of this century.
Dr Tom Matthews, lead author and Senior Lecturer in Environmental Geography at King's College London said: "Our findings show the potentially deadly consequences if global warming reaches 2C. 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 ".
To conduct their analysis, the research group synthesized a broad range of scientific data that associates physical climate factors with heat-related mortality risk. This included the notion of both "uncompensable" thresholds, where core body temperature starts rising uncontrollably, and "unsurvivable" thresholds, where the body can reach 42oC in six hours.
Between 1994 and 2023, about 2% of the globe's land area surpassed uncompensable heat tolerances for under-60 adults. Meanwhile, these thresholds were exceeded across more than 20% of Earth's land surface for individuals older than 60. Although older adults have occasionally encountered unsurvivable conditions, the paper notes that younger demographics have, so far, not seen them.
If warming of 4-5oC above preindustrial levels were to take hold, older adults could confront uncompensable heat in approximately 60% of land areas during extreme weather spikes. Unsurvivable heat would then start threatening younger cohorts, particularly in the hottest parts of the subtropics.
Regions like Saharan Africa and South Asia stand at the forefront of crossing these life-threatening heat boundaries. With further warming, more populations in those locales will need strategies to adapt or relocate, given the elevated risk levels.
Dr Matthews emphasizes that evaluating the potential magnitude of future heat extremes and their dire outcomes is essential for recognizing the profound consequences of failing to curb climate change. He adds that effective adaptation planning must target communities under greatest threat, using a proactive approach to protect human health.
"What our review really shows very clearly is that, particularly for higher levels of warming such as 4C above the pre-industrial average, the health impacts of extreme heat could be extremely bad," he said.
"At around 4C 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".
Extreme heat is already a serious hazard, having caused over 260,000 fatalities in major heatwaves since 2000. Among the most devastating events of this century are the 2003 and 2022 European heatwaves, which led to around 72,000 and 62,000 deaths, respectively, and the 2010 Russian heatwave, which claimed approximately 56,000 lives.
This project also involved Dr Josh Foster from King's College London, along with collaborators at the University of California, Stanford University, NASA Goddard Institute for Space Studies, Columbia University, and Boston University.
Research Report:Earth's most extreme heat events and mortality impacts under climate warming
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