Imagine the force required to make governments do their share to prevent >2 degree shift.
There is a cost to compliance. There is a point where the damage is so incalcuable that you are compelled to force compliance on all other governments.
I'm pretty sure a nuclear winter does not reduce global temperature other than the short term (~10 years).
IIRC the hopi prophecies have one such outcome where all humans die crawling in the dark with no food due to no agriculture possible without photosynthesis.
There is a cost to compliance. There is a point where the damage is so incalcuable that you are compelled to force compliance on all other governments.