A large number of healthy people won't handle the heat if temperatures continue to increase into next century, predict researchers.
The study, which appears today in the Proceedings of the National Academy of Sciences, also suggests heat could affect more land mass than rising sea levels.
The human body maintains a constant core temperature of 37°C by giving off excess heat through the skin.
But, if the 'web-bulb' temperature of the air reaches 35°C, this heat dissipation stops causing the body to retain heat, resulting in heat stress.
Wet-bulb temperature is measured using a thermometer wrapped in a wet, porous material. It is typically less than 'dry-bulb' temperature and is used to calculate humidity.
Professor Steven Sherwood of University of New South Wales and Associate Professor Matthew Huber of Purdue University in Illinois, used climate models to predict where and when temperatures will increase to uncomfortable levels.
They found a global temperature increase of 7°C above pre-industrial levels would push temperatures in some regions above 35°C for extended periods, resulting in heat stress across the whole population.
Sherwood says while heat-related deaths among the elderly and young already occur, global warming will result in more of the population suffering.
"What we're talking about here is something a bit different - these limits apply to a healthy person," he says.
The study highlights a number of potential 'hot spots' in the future.
"The places that heat stress will be highest are places near sea level and at lower latitudes and that's where people live," says Sherwood.
"This includes Amazonia; most of China; India; Indonesia; pretty much all of South East Asia; eastern United States; northern Australia and parts of Africa."
Sherwood says populations faced with heat stress have two options - relocate to a cooler climate or rely on air conditioning.
"Right now we have air conditioning for comfort," he says. "Under these circumstances you would be using it for survival."
Impact for livestock
The study also mentions the impact higher temperatures will have on livestock.
"You wouldn't be able to raise the usual levels of livestock unless you have them living in giant air conditioned domes," Sherwood says
While the Earth has experienced similar levels of warming in the past, Sherwood says it is currently occurring on a much faster scale.
"The last warm period lasted for many millions of years and came on gradually," he says. "[Current warming] is not coming on gradually. Compared to other climates of this type it's more explosive."
Sherwood says a 7°C increase isn't likely to happen until next century, but he says it's important to understand the impact should it occur.
"When you're planning sensibly for anything you plan for the worst case scenario," he says.
"We're saying this is the worst scenario, we're not saying it's going to happen soon, but to ignore it seems foolhardy."
The researchers conclude further warming would have a more drastic impact.
"If warmings of 10°C were really to occur in [the] next three centuries, the area of land likely rendered uninhabitable by heat stress would dwarf that affects by rising sea level," they write.
The average global temperature has increased by 0.8°C since pre-industrial times. Some scientists and environmental groups are pushing for limits on human-produced greenhouse gas emissions to limit the increase to no more than 2°C.