A paper in the Proceedings of the National Academy of Science looks at the physiological adaptability of mammals to warming. It finds that even though temperatures have briefly been to 10 degrees C above the present, temps above 7 degrees C warmer than today would begin to see areas across the planet become uninhabitable to mammals because of heat stress. This must not be confused with the middle of the Sahara Desert being uninhabitable heat stress. That area is habitable. The habitability is poor, but it is inhabitable. What the authors are describing is the point when heat dissipation by the body at night can not overcome the heat gain in the daytime. Nighttime temperatures become too warm and a continuous heat stress sets in that results in death. This is as much a function of humidity as it is temperature. Most of us have likely heard of the great heat wave in Europe in 2003 where 37,000 people died. This heat wave occurred in places where they do not normally have air conditioning and nighttime heat was so great that old folks who were weak or stressed for some other reason, could not cool off enough, even in front of a fan, at night, to relieve the heat stress of the day. These people died because they could not cool down enough. What the authors are talking about are conditions where all humans, and virtually all mammals would succumb to these overheated conditions, not just old folks.
Today’s emissions are tracking along, or worse than, the worst-case IPCC scenario. the temperature increase at the end of the 21st century, following along with these emissions that are tracking along the worst case scenario, reach approximately 6.4 degrees C. warmer than today (the 1980 to 1999 average). This is an average global temperature increase of 11.5 degrees F. It is a couple of degrees less than the difference between the deepest cold of the ice ages and the interglacial warmth between the ice ages. These temperatures will make areas of the planet uninhabitable. mammals, including humans if they have to stay outdoors, in unairconditioned conditions for long periods or time. Humans could survive in air conditioning, but interruption of the air conditioning would be deadly.
For the next several hundred years though, temperature will continue to increase. The authors warn that at some point in the next century or two, our entire planet could, if we don’t act soon, become completely uninhabitable.