New research theorizes that if global warming continues at its current pace, Earth's temperatures could exceed livable limits for humans in the future.

Scientists at Purdue University and the University of New South Wales calculated the highest possible "wet bulb" temperature that humans can withstand and found that this temperature could be exceeded in future climate scenarios.

"Wet bulb" temperature is meant to simulate what is felt by wet skin when it meets moving air, taking into account measures such as humidity. In order for a person's cooling process to work, the surrounding air must be cooler than the skin, which must be cooler than the person's core body temperature.

If "wet-bulb" temperature is warmer than the skin, metabolic heat cannot be released by the body, which can lead to heat stress.

Purdue professor of earth and atmospheric sciences Matthew Huber co-authored the paper which appears in the May 6 issue of the Proceedings of the National Academy of Sciences.

He says researchers have calculated humans can only handle a wet-bulb temperature of 35 C for about six hours before succumbing to a lethal level of heat stress.

"Although areas of the world regularly see temperatures above (37.7 C), really high wet-bulb temperatures are rare," Huber said in a news release. "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 researchers said while the Intergovernmental Panel on Climate Change estimates global warming to increase mean temperatures by about 3 degrees C, warming of up to 12 degrees C is possible.

"We found that a warming of (4 C) would cause some areas of the world to surpass the wet-bulb temperature limit, and a (12-degrees C) warming would put half of the world's population in an uninhabitable environment," Huber said.

"When it comes to evaluating the risk of carbon emissions, such worst-case scenarios need to be taken into account. It's the difference between a game of roulette and playing Russian roulette with a pistol. Sometimes the stakes are too high, even if there is only a small chance of losing."

Researchers said that whole countries would have to adapt wide-scale plans to battle severe heat stress under such an extreme scenario.

"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," Huber said.