"Unless the refugees overrun us all. Like they did in the 'ring of fire' years, back when Ur and Hammurabi were writing. A chain reaction of failed crops, refugee migrations and overloaded ecosystems culminated in Egypt being nearly destroyed by the 'sea people', a flotilla of desperate refugees. It only ended when Egypt armed everybody and defeated them (killed them all)."

https://www.technologyreview.com/s/613342/get-ready-for-tens-of-millions-of-climate-refugees/