Also, I realize in a way I didn't answer rilla's inquiry

I don't know how long an extinction event along the lines of oceanic anoxia will take. Could be 50k years, could be 5 million, could be 2k. The feedbacks to achieve around 10+C warming and 1000PPM should be triggered within a few hundred years at most. I think they're already triggered, but if they're not they will definitely be triggered at 2C warming given the descriptions of what that would do. Regardless, a sociological understanding of this makes the feedbacks already triggered. If, however, the global chemical death scenario like anoxia would take a really really long time to fully bloom, there might be some geo-engineering solutions.

And as to the severity, I'm sure some humans will be able to survive somehow. Not all species died during GHG induced extinction events