The biggest risk is probably when a lot of people in a given place will want to move at the same time (and/or into the same destination), as it can cause political instability.
Politically I think water wars could be big depending on how much water is lost as a result of evaporation and polution, I think this could be more pronounced in Africa and the Middle east, but we could have issues between Mexico, AZ, California and the south western USA where it's more draught ridden. Forest fires will continue to get bigger and more frequent causing a change in ecosystems where forests may turn to baren deserts.
Lack of water also will cause farming to be tougher. I'm hopeful we eventually science the shit out of all this stuff, but then we could get an asteroid the next year that wipes us out anyway, no use really fearing but doesn't hurt to prepare as best we can.
