Because exactly those things you describe (as well as some other factors/events) were reckless, violent experimenting (ideological, military, technological). And now we're finding out. Like I'm not sure if you're aware but the world changed ridiculously fast during the 1900s and now the early 2000s compared to the rest of history.

easy, google

wet bulb temps not supportive of human life

everything else short of the cold war nuke apoc is lol and at least that had the option of not happening

we’re already in a positive feedback loop