Earlier quoted context omitted.
I understand we are at risk of large populations being displaced. But you seem to be under the impression we are facing a human extinction event, is that something backed up by science?
I think that’s a risk on the right tail, although not a very likely one. But even the modal outcomes — 2 or 3 K of warming relative to the preindustrial baseline — will likely kill hundreds of millions of humans and lead to the extinction of countless nonhuman plant and animal species. I’m comfortable saying that we should do whatever possible to mitigate that outcome instead of focusing on creating Martian bunkers f…
Once we are dealing with scenarios where hundreds of millions of humans are dying, it becomes harder to rule out the possibility of some desperate political or military leader resorting to using nuclear weapons. From there it is a small step to full nuclear war and the deaths of billions of humans.
Perhaps the "optimistic" scenarios are ones in which the hundreds of millions of humans that die are located far away from any nuclear armed countries, or at least those countries aren't destabilised by the economic and migration consequences of those deaths. I can, unfortunately, imagine militaries being routinely used this century for wiping out the remaining civilian populations of failed states.