> enormous die off of the world's population as large swathes of the planet become essentially uninhabitable at current densities

Can you provide any more details to go on for that? I can't find such predictions in an IPCC report but they're huge so any extra specifics would help.

I can't imagine a place being essentially uninhabitable. Las Vegas and Israel are both full of people, for instance. Almost everywhere is uninhabitable without shelter.

I'm pretty sure nobody is predicting existential risk to civilization. You really need a reference for that.