Purdue University Professor Matthew Huber and University of New South Wales Professor Steven Sherwood say they've calculated the highest tolerable "wet-bulb" temperature and found it could be exceeded for the first time in human history if greenhouse gas emissions are not abated.
Wet-bulb temperature is the equivalent of what is felt when wet skin is exposed to moving air.
Humans and most mammals experience potentially lethal heat stress levels at a wet-bulb temperature above 95 degrees Fahrenheit sustained six hours or more, Huber said.
"Although areas of the world regularly see temperatures above 100 degrees, really high wet-bulb temperatures are rare," Huber said, explaining the hottest areas normally have low humidity, allowing the body to be cooled by perspiration.
But he said a warming of 21-degrees would put half of the world's population in an uninhabitable environment.
"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 Earth could see such temperatures again," Huber said. "If we consider these worst-case scenarios early enough, perhaps we can do something to address the risk through mitigation or new technological advancements that will allow us to adapt."
The study appears in the Proceedings of the National Academy of Sciences.
Copyright 2010 United Press International.