Extreme heat is pushing parts of Earth beyond human survival 

Rising global temperatures may render extensive regions of the planet too hot for human survival over the coming decades, according to a new study led by King’s College London.

The research shows that if worldwide warming surpasses 2°C above pre-industrial levels, the proportion of land experiencing unsurvivable heat will triple, spanning an area nearly as large as the United States.

These extremely high temperatures would endanger healthy young adults (aged 18–60), pushing their bodies past the point where they can regulate internal heat – even with adequate shade and hydration.

Moreover, older adults (60+) would be at high risk across nearly 35% of the global land surface.

Heat thresholds for human survival

Study lead author Tom Matthews is a senior lecturer in Environmental Geography at King’s College London.

“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,” said Matthews. 

“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.”

With 2023 recorded as the first full year averaging more than 1.5°C above pre-industrial norms, many scientists caution that 2°C could be reached by the middle to latter half of this century if present trends remain unchecked.

The rise of unsurvivable heat

The researchers connect physical climate science to human heat-mortality risks by focusing on two key thresholds:

• Uncompensable heat – A critical point where the human body cannot effectively release heat, resulting in a perilous increase in core temperature.
• Unsurvivable heat – The point at which core body temperature rises to 42°C within six hours, invariably resulting in fatal outcomes.

Between 1994 and 2023, about 2% of Earth’s land area exceeded the uncompensable threshold for healthy adults under 60. By contrast, over 20% of global land already reached those levels for older adults during periods of intense heat.

Deaths from extreme heat

According to the experts, since 2000, more than 260,000 people have died from extreme heat during major heatwaves, a finding highlighting the scale of heat-related dangers. 

The most lethal incidents – Europe’s 2003 and 2022 heatwaves, along with the 2010 Russian heatwave – together were responsible for nearly 200,000 fatalities.

Up to now, unsurvivable heat has appeared only briefly in Earth’s hottest locales, but experts predict a significant expansion of these extreme temperatures if global warming intensifies.

Severe consequences of global warming

Although 2°C of warming carries serious risks, the study also indicates that a 4–5°C rise could be catastrophic. Older adults may face uncompensable heat over 60% of Earth’s land during extreme hot spells, while younger adults could encounter unsurvivable heat in the most sweltering subtropical zones.

Moreover, at 4°C above pre-industrial levels, around 40% of the globe would regularly endure uncompensable heat, restricting tolerable conditions largely to high-latitude and cooler mid-latitude areas.

Such projections place regions like Saharan Africa, South Asia, and parts of the Middle East in particular jeopardy, where even limited outdoor exposure might become life-threatening.

“At around 4°C of warming above pre-industrial 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,” Matthews explained.

Urgency for climate adaptation 

Recognizing these thresholds is pivotal for evaluating the potential human toll of escalating temperatures and for guiding adaptation strategies. 

While rapid cuts in greenhouse gas emissions remain essential, the study emphasizes the need to protect vulnerable groups through measures such as:

• Revamping urban design to mitigate heat via increased greenery and reflective surfaces.
• Boosting cooling infrastructure, from widespread air conditioning to designated cooling centers.
• Ensuring sufficient water and resource availability in heat-prone regions.

“Interdisciplinary work is vital to improving our understanding of unprecedented heat’s deadly potential and how it can be reduced,” noted 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.”

Extreme heat and human survival

Ultimately, the report clarifies that rising global temperatures are a direct menace to human survival, not merely an environmental concern. If 2°C of warming is reached, massive portions of the planet may become too hot for younger adults engaging in normal outdoor activities. 

At 4°C, almost half of the world could confront periods of inescapable heat, rendering these regions effectively uninhabitable.

The findings underscore an urgent call for global mitigation efforts as well as strategic climate adaptation. 

Without bold intervention to curb emissions and shield populations from extreme heat, the deadly consequences of climate change could become an enduring reality – one that fundamentally alters how and where people can live safely on our planet.

The study is published in the journal Nature Reviews Earth and Environment.

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