You need to find what are the stable equilibriums with - or in the worst case without - a biosphere. +7C-8C, if it's the peak, is most probably survivable for the biosphere as a whole, because we know that this happened before (permian triassic boundary).

However, there's a very important distinction here: The permian triassic boundary conditions did not arise coming abruptly out of an ice age. An ice age after a warm period leads to lots of methane being stored, and it's reasonable to assume that the current methane storage is much larger than at the permian triassic (oh, did I mention that that was already an extinction event, probably the worst we know about?).

All in all it's not completely out of the question that an abrupt release of enough methane could lead to a Venus syndrome, and thus the complete sterilisation of the planet.