This graphic shows areas that could be too hot for humans in one scenario (12 degrees C over pre-industrial levels, I think). The too-hot areas include much of Africa, the American Southeast, Australia, Brazil India, Oceana and eastern China.
From their abstract:
Despite the uncertainty in future climate-change impacts, it is often assumed that humans would be able to adapt to any possible warming. Here we argue that heat stress imposes a robust upper limit to such adaptation. Peak heat stress, quantified by the wet-bulb temperature TW, is surprisingly similar across diverse climates today. TW never exceeds 31 °C. Any exceedence of 35 °C for extended periods should induce hyperthermia in humans and other mammals, as dissipation of metabolic heat becomes impossible. While this never happens now, it would begin to occur with global-mean warming of about 7 °C, calling the habitability of some regions into question. With 11–12 °C warming, such regions would spread to encompass the majority of the human population as currently distributed. Eventual warmings of 12 °C are possible from fossil fuel burning. One implication is that recent estimates of the costs of unmitigated climate change are too low unless the range of possible warming can somehow be narrowed.Translation (mostly from Purdue press release):
The researchers 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 unabated.On the other hand, the 22nd century is a long time from now. Who knows what technology will be available to keep people and livestock comfortable even in lethal environments. But it could be a very different Earth.
"Although areas of the world regularly see temperatures above 100 degrees, really high wet-bulb temperatures are rare," co-author Huber said. "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."
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," Huber said. "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."
"We found that a warming of 12 degrees Fahrenheit would cause some areas of the world to surpass the wet-bulb temperature limit, and a 21-degree 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."
"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," lead author Sherwood said. "Although we are very unlikely to reach such temperatures this century, they could happen in the next."
[Update 2013-02-25 1800GMT:
Recent research by NOAA scientists suggests warming is already reducing capacity of people to work in many regions, and could have severe effects by 2050. "This planet will start experiencing heat stress that's unlike anything experienced today," says one of the authors of the study in Nature Climate Change. (Abstract here; article behind Nature paywall.) Here is a Reuters item about the research.]