I don't think civilization ends if temperatures rise dramatically. A lot of existing agricultural land gets destroyed, but some currently unusable/unproductive areas that are too cold become viable. So the regions will shift. Painful but not insurmountable if it happens over a 50 year time span. But even if there wasn't such a compensatory mechanism, modern problem solving abilities will find a way - yes really. Look at the problems already solved. Nuclear reactors, solar power, vertical farming, genetically enhanced crops, alternative food sources will be engineered if the need arises. We can really stop saying that we know for sure society will end. I don't know how poor people will be affected, and yes there woll be winnners and losers as always during massive disruptive change - but hardly the end of human civilization.