Good point. I'm not an expert on global warming, but I guess crop failures are much more likely in southern areas? Recently, I read the science fiction book The Water Knife by Paolo Bacigalupi. This book takes place in a future US, on a planet where there has been lots of global warming. In this future US, there are border controls between the states, to stop people fleeing from the drought. Not sure how likely this is to happen, but it's great book anyway.