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Have you ever wondered how hot the Earth would have to be before it got unlivable? Atmospheric scientist Matthew Huber used computer simulations to determine what would happen in a worst case scenario climate change scenario. 

If global temperatures rise just 21 degrees fahrenheit, the world will be cooked! His findings will be published in the Proceedings of the National Academy of Sciences:
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 wetted 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.
He added:
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 . . . 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.
[via Purdue University]