People born in the 1960s may be the last human beings who will get to live out their full actuarial life expectancies.”Climate change now represents a near- to mid-term existential threat” to humanity, warns a recent policy paper by an Australian think tank. Civilization, scientists say, could collapse by 2050. Some people may survive. Not many.Some dismiss such purveyors of apocalyptic prognoses as hysterics. To the contrary, they’re Pollyannas. Every previous “worst-case scenario” prediction for the climate has turned out to have understated the gravity of the situation.