Pakistani youths cool off in a canal during a heat wave in June 2009, as
temperatures soared to 117 degrees in some parts of the country. Extreme
heat stress due to global warming could render much of the world
uninhabitable in upcoming centuries, according to new research.

A worst-case scenario of global warming, in which temperatures would
soar some 21 degrees, is that much of the world may simply become too
hot for humans to live in, according to new research published last week
in the Proceedings of the National Academy of Sciences.

"We found that ... a 21-degree warming would put half of the world's
population in an uninhabitable environment,"says study co-author Matthew
Huber of Purdue University.

While the Intergovernmental Panel on Climate Change found that the
result of business-as-usual warming would be 7 degrees by 2100, eventual
warming over several centuries of 25 degrees is feasible, says Huber.

The new research calculated the highest tolerable "wet-bulb" temperature
that humans can withstand.

"The wet-bulb limit is basically the point at which one would overheat
even if they were naked in the shade, soaking wet and standing in front
of a large fan," says study lead author Steven Sherwood of the
University of New South Wales in Sydney.

The researchers found that humans and most mammals experience a
potentially lethal level of heat stress at a "wet-bulb" temperature
above 95 degrees sustained for six hours or more.

(The more familiar air temperature is known as the "dry-bulb"
temperature; wet bulb temperatures can be used along with the dry bulb
temperature to calculate humidity.)

Researchers say that while wet-bulb temperatures of 95 degrees never
happen now, they would begin to occur with global-average warming of
about 12 degrees, calling the habitability of some regions into question.

"We show that even modest global warming could therefore expose large
fractions of the population to unprecedented heat stress, and that with
severe warming this would become intolerable," the authors write.

"If warmings of 10 degrees C (18 degrees F) were really to occur in next
three centuries, the area of land likely rendered uninhabitable by heat
stress would dwarf that affected by rising sea level. Heat stress thus
deserves more attention as a climate-change impact."

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