Huge swathes of the world could become too hot for human beings to survive, if current worst-case scientific projections of global warming turn out to be true.
Researchers from Purdue University and the University of New South Wales, Australia, have for the first time calculated the highest 'wet-bulb' temperature that people can tolerate - and have found that it could be exceeded for the first time in human history under reasonable worst-case climate change scenarios.
Wet-bulb temperature includes temperature and atmospheric humidity, and is equivalent to what is felt when wet skin is exposed to moving air.
The researchers calculated that humans will experience a potentially lethal level of heat stress at wet-bulb temperature above 95 degrees, sustained for six hours or more.
"Although areas of the world regularly see temperatures above 100 degrees, really high wet-bulb temperatures are rare," said Matthew Huber, professor of earth and atmospheric sciences.
"This is because the hottest areas normally have low humidity, like the 'dry heat' referred to in Arizona. When it is dry, we are able to cool our bodies through perspiration and can remain fairly comfortable."
While the Intergovernmental Panel on Climate Change estimates the world will warm by seven degrees Farenheit by 2100, eventual warming of 25 degrees is feasible, said Huber.
"We found that a warming of 12 degrees Fahrenheit would cause some areas of the world to surpass the wet-bulb temperature limit, and a 21-degree warming would put half of the world's population in an uninhabitable environment," Huber said.
"These temperatures haven't been seen during the existence of hominids, but they did occur about 50 million years ago, and it is a legitimate possibility that the Earth could see such temperatures again."