Nationalism as a force for promoting environmental protection and responsible climate policies? Dream on! It is far more likely to degenerate into precisely the catastrophic runaway feedback loops you describe at first: vicious xenophobia in response to growing waves of desperate climate migrants fleeing from the torrid south to the (slightly) more habitable northern latitudes, coupled with the rise of moronic, climate-denying demagogues like Trump withdrawing from any and all forms of international cooperation on this or any other issue, and finally, bitter wars of competition for depleted resources, especially fresh water, arable land, and oil. So I see little to no hope for our collective future.