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In what could be called the worst scenarios for global warming, the temperatures could reach such heights that it would be impossible for humans to survive on the planet, say researchers from Purdue University and the University of New South Wales, Australia.
For the first time, researchers have calculated the highest tolerable "wet-bulb" temperature and found that this temperature could be exceeded for the first time in human history in future climate scenarios if greenhouse gas emissions continue at their current rate.
Wet-bulb temperature is equivalent to what is felt when wet skin is exposed to moving air.
It includes temperature and atmospheric humidity and is measured by covering a standard thermometer bulb with a wet cloth and fully ventilating it.
The researchers calculated that humans and most mammals, which have internal body temperatures near 98.6 degrees Fahrenheit, will experience a potentially lethal level of heat stress at wet-bulb temperature above 95 degrees sustained for six hours or more, said Dr. Matthew Huber, who co-authored the paper.
"Although areas of the world regularly see temperatures above 100 degrees, really high wet-bulb temperatures are rare. 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. The highest wet-bulb temperatures ever recorded were in places like Saudi Arabia near the coast where winds occasionally bring extremely hot, humid ocean air over hot land leading to unbearably stifling conditions, which fortunately are short-lived today," said Huber.
The study did not provide new evaluations of the likelihood of future climate scenarios, but explored the impacts of warming.
The challenges presented by the future climate scenarios are daunting in their scale and severity, he said.
"Whole countries would intermittently be subject to severe heat stress requiring large-scale adaptation efforts. One can imagine that such efforts, for example the wider adoption of air conditioning, would cause the power requirements to soar, and the affordability of such approaches is in question for much of the Third World that would bear the brunt of these impacts. In addition, the livestock on which we rely would still be exposed, and it would make any form of outside work hazardous," said Huber.
While the Intergovernmental Panel on Climate Change central estimates of business-as-usual warming by 2100 are seven degrees Fahrenheit, eventual warming of 25 degrees is feasible, he said.
Steven Sherwood, who is the paper''s lead author, said prolonged wet-bulb temperatures above 95 degrees would be intolerable after a matter of hours.
"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. Although we are very unlikely to reach such temperatures this century, they could happen in the next," said Sherwood.
The study will be published in the latest issue of the Proceedings of the National Academy of Sciences.
