For what it's worth they can live under ground [1][2], even build cities but they would have had to start that project some time ago. Before someone says it, yeah not everywhere and its not for everyone but in enough places and for enough people that we can adapt to heat and at least survive as a civilization.
[1] - https://en.wikipedia.org/wiki/Coober_Pedy
[2] - https://www.youtube.com/watch?v=JsYmw6FtSIA Wyoming Trona mines