Billions will face conditions so hot and humid that their bodies can't naturally cool if global temperatures rise by 1ºC or more from current levels, research warns.
The study, published in the Proceedings of the National Academy of Sciences, highlights the severe health risks of a planet warming beyond 1.5ºC above preindustrial levels.
Lead author Dr Daniel Vecellio told The National: “Being exposed to this heat for an hour or two for a few days every year is something that can be adapted to and does not make a place 'too hot for humans'.
“Experiencing this heat for 6-7 hours a day for multiple days or weeks throughout the year is something that will have to result in major behavioural or technological adaptive measures”.
Since the Industrial Revolution began, global temperatures have risen by approximately 1ºC.
The research team simulated global temperature rises between 1.5ºC and 4ºC, identifying regions where rising heat and humidity could surpass human tolerances.
According to previous Penn State research, young, healthy individuals can tolerate wet-bulb temperatures up to about 31ºC at 100 per cent humidity.
However, tolerance varies depending on factors like exertion, wind speed and solar radiation.
Dr Vecellio told The National: “These thresholds have been rarely exceeded in the past and, when they are, are usually done for only an hour or two”.
A 2ºC temperature increase could mean that regions like Pakistan and India’s Indus River Valley, eastern China, and sub-Saharan Africa experience unbearable heat levels regularly.
These high-humidity heatwaves can be particularly deadly, especially in areas without widespread access to cooling solutions like air conditioning.
Dr Vecellio cited the unexpected 2021 Oregon heatwave, stressing that while models predict trends, they can't always foresee specific catastrophic events.
His past research with Prof W. Larry Kenney, co-author of the study, investigated human limits to heat, humidity, and physical exertion.
This foundational work was essential for the current predictions, with co-author Prof Matthew Huber noting its importance in understanding the impacts of climate change on human tolerance.
Graduate student Qinqin Kong, who collaborated in the study, emphasised that the world's climate adaptation strategies, usually focusing solely on temperature, need to address the heightened risks of humid heat.
Older populations, for instance, face higher health risks at lower heat and humidity thresholds.
Dr Vecellio told The National: “The most important thing is to drastically reduce our global fossil fuel consumption, reaching net-zero emissions quickly, and building resilience through public health measures and smaller-scale heat reduction measures”.
If ignored, many regions, especially in low-to-middle income nations, will face catastrophic conditions. Al Hudaydah, Yemen, for instance, could see over 300 days annually surpassing human heat tolerance if global temperatures rise by 4ºC.
“The worst heat stress will occur in regions that are not wealthy … As a result, billions of poor people will suffer, and many could die,” Prof Huber said.
Cop28, a conference for world leaders and other stakeholders to discuss and address climate change, will take place in Dubai in November.