Try living in Goa in the monsoon season. Or even rewatch Apocalypse Now or basically any Vietnam war movie.
I assure you, even 36C/100% is survivable.
I assure you, even 36C/100% is survivable.
Survivable is a pretty low bar.
Also, "survivable but kills babies, the elderly, and the sick in much greater numbers than they die today" doesn't have the same shine as survivable for everybody...
An extended period of 36C/100% is going to pose serious danger for most people
The last to die will be the people who are most harmful to the environment on a per-capita basis. Before then, you’ll have socioeconomic disruption in warm countries, creating masses of people fleeing to cooler climates, creating political openings for xenophobes and unsavory characters throughout the western world who will take it as a key goal to preserve our right to pollute, now return to step 1, cycle repeats and amplifies.
There is no resistance to chemistry. Your proteins stop folding properly above a certain temperature. Others irreversibly change shape, like an egg’s albumen solidifying. Your body expends extraordinary effort to keep its temperature in a narrow range, one far from the outside temperature, for good reason.
When? Why?
Heat kills becauses it unfolds proteins. If that happens in your heart, you get a heart attack--that's the proximate cause of death. The ultimate cause remains, however, unless we're talking about charring or vaporising heat, your proteins misfolding.
The other common way to die from heat is dehydration, which kills by altering your electrolyte imbalance which causes arrythmia. But that can be countered through hydration. (Until the proteins in your gut start misfolding.)
Side note: I didn't say you die because your proteins stop folding. I said you die because your proteins stop folding properly.
> less resistance you have, the earlier you die
The resistance comes in the form of heat-shock proteins [1], which "enable excess heat to be stored temporarily" [2]. Above 40°C, your HSR proteins start misfolding. There is no resistance nor acclimation beyond that limit. Our chemistry doesn't permit it.
Variations exist in the 40 to 45°C regime in terms of tolerance duration, but that's irrelevant when we're talking about climate. Above 40°C / 104° F wet-bulb temperature, everyone without access to cooling will eventually die. You can't build resistance to it because that literally involves evolving proteins which, to my knowledge, don't exist in mammals.