Thanks for the reference, that is interesting. I think that's what I was looking for in a theory of why things will be bad: it's not the temperature change itself (a few degrees is fine), or the atmospheric CO2 levels (still far less than 1%), or even the water levels (we can move, if painful economically), but the release of H2S, which kicks off a chain reaction of extinction, which may impact food supply etc.

I hardly know enough to comment on whether that is likely. But I think it's a much better argument about why the Earth will be uninhabitable for us specifically.

(That said, it doesn't really matter if the earth eventually becomes uninhabitable or not, we want to avoid the cost of having to move everyone for even minor sea level rises.)