There are a large number of things that could kill everyone, but I think the top threat will always be ourselves. It's very easy to imagine some resource war de

There are a large number of things that could kill everyone, but I think the top threat will always be ourselves. It's very easy to imagine some resource war degenerating into a nuclear war. A few self-induced crises like that on top of the climate crisis would be enough to finish the survivors (who might have made it if the only danger were the environment). Think of the world population's size and global distribution as hedges against disaster: the fewer people who live, in fewer habitable places, the more likely it is that some disaster will affect everyone remaining and leave no one unaffected. We have fewer rolls of the dice, and there may come a day where they're all snake eyes. This why many are excited about the idea of "making humanity an interplanetary species", as I once was until I realized how hard it would be to make a working biosphere anywhere else, given how bad we are at maintaining one that already works. If we don't figure out how to save this biosphere, we won't have enough time to make more.