I mean billions of deaths and civilization collapse is probably at the extreme end but still imaginable.
If poor people and poor countries are screwed then that will lead to major disruption in rich countries as we are reliant on them and potentially a large and dangerous climate refugee situation. Economic collapse in rich countries isn't off the table either.
Then there are questions like how much of the current status quo has to change for "our civilization" to have collapsed?
It would have been far cheaper to spend trillions to mitigate and prevent, but that would have required foresight from politicians and voters.
The Texas cold snap and German floods are examples of what the rich world will be facing.
The Indian Ocean tsunami killing hundreds of thousands is an example of what the poor will be facing.
Extinction is what many non human species are facing.
But billions of deaths or civilization collapse is what some are predicting, which is nonsense.