A third of the planet could become inhospitable to vulnerable populations if rising temperatures increase more than 2°C above preindustrial levels, according to new research by an international team of scientists.
Even young, healthy individuals would not be immune to extreme heat, the study warns, adding that an area equivalent to the size of the United States could become too hot for many to maintain safe body temperatures.
The new findings underscore the urgent need to mitigate climate change and protect those most at risk.
Climate Change and Aging
“Our findings show the potentially deadly consequences if global warming reaches 2°C,” said lead author Dr. Tom Matthews, Senior Lecturer in Environmental Geography at King’s College London. “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.
While many Americans associate retirement with warmer climates like Florida, individuals over 60 actually have greater difficulty coping with extreme heat compared to adults aged 20-60. The study found that the portion of the planet deemed unsafe for older adults would increase by 35% if temperatures rise by 2°C. Meanwhile, the regions experiencing temperatures dangerous even for young, healthy individuals would triple.
Matthews said the team’s findings represent “a step-change in heat-mortality risk.”
Unsurvivable Consequences
The urgency of these findings is reinforced by recent climate trends. In 2023, Earth briefly surpassed 1.5°C above preindustrial levels, and projections indicate the 2°C threshold could be reached between the middle and end of this century.
The study integrated existing climate models with health data to assess rising temperatures against known heat mortality risks. Medical scientists have identified two key danger thresholds. The first, known as uncompensable heat, occurs when the body can no longer regulate its core temperature, leading to a dangerous rise. The second, called unsurvivable heat, happens when core body temperature reaches 42°C within six hours, causing fatal heatstroke.
These risks are closely linked to the concept of human thermal tolerance, the temperature and humidity levels beyond which humans cannot survive.
Between 1994 and 2023, only 2% of the planet exceeded the thermal tolerance threshold for people under 60, while 20% did so for those over 60. During that period, uncompensable heat levels were recorded for all age groups, but unsurvivable conditions occurred only for older adults—and only briefly.
Rising Temperatures and A Harsh Future
If climate change leads to temperature increases of 4-5°C above preindustrial levels, vast portions of the globe would become uninhabitable. Under these extreme scenarios, 60% of the planet would surpass the uncompensable heat threshold for older adults, and even healthy young adults would face unsurvivable heat in some subtropical regions. The most affected areas would include Saharan Africa and South Asia.
“What our review really shows very clearly is that, particularly for higher levels of warming such as 4°C above the preindustrial average, the health impacts of extreme heat could be extremely bad,” Matthews 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.”
Climate Change Impacts Are Already Here
Extreme heat has already claimed over 260,000 lives since the year 2000. Nearly 200,000 of these deaths occurred during just three major heat waves. In 2003, 72,000 people died in Europe. In 2010, Russia saw 56,000 heat-related deaths. More recently, in 2022, another 62,000 people in Europe succumbed to extreme heat.
“Interdisciplinary work is vital to improving our understanding of unprecedented heat’s deadly potential and how it can be reduced,” Matthews said.
“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 concluded.
The paper “Earth’s Most Extreme Heat Events and Mortality Impacts Under Climate Warming” appeared on February 4, 2025 in Nature Reviews Earth & Environment.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.