[newt_slowly]

When the combination of heat and humidity gets high enough (look up wet bulb temperature) then it means death for even fit young people, even when sitting in the shade with access to drinking water. We are thermodynamically incapable of surviving too high of web bulb temperatures.

Huge swaths of the globe will experience fatal web bulb temperatures multiple times a year by the end of the century. Very clearly this makes a region "unable to support humans".

[dodobirdlord]

A large fraction of the human population have been living in parts of the world where the external environment is lethal for months of the year for thousands of years. It’ll certainly be an interesting development when air conditioning becomes a matter of life and death the way heating in winter already is in much of the world, but air conditioning is also much less energy intensive than heating.

[newt_slowly]

I don't think it is reasonable to compare other hostile environments to one where humans are literally incapable of being outside for extended periods of time. Even in harsh cold climates, it is possible to wear suitable clothing, or build a fire. That is simple, resilient, low tech and low carbon. Requiring a structure with AC and power is a much larger challenge.

Furthermore, a lot of the areas that will suffer from fatal wet bulb temperatures do not have the wealth to provide AC to everyone. This will drive mass migrations of people on a scale we have never seen before, destabilizing neighbouring countries (or more likely, leading to mass murder).

Finally, this is another positive feedback loop: more AC required -> more energy required -> more severe global warning.