Nuclear superpowers are among the least likely countries to actually collapse from climate damage.
US isn't Syria, and it's Syria that's at risk.
First world countries like France can absorb a +30% spike to food prices. Countries where the same food price spike would come with a major death toll don't have the tools to kick off WW3.
And you think a second, much larger Syrian refugee migration will have 0 impact on France?
Nothing happens in a vacuum. Everything and everyone is connected.