Umm, they seemed to be doing OK overall when I was there a couple of years ago, despite the horrible drought, terrible floods, and Hurricane Yasi.
Don't buy real estate in Florida. New York will be mostly OK ("The average elevation of Manhattan is reported to be 29 meters (97 feet"); I can't find a mean elevation for LA, but city hall is 300 feet above sea level so unless you buy beachfront property you'll be OK.
With global warming, we're talking changes that happen over a hundred or three (or more) years. I may be wrong (I often am), but I'd be the big worry 100 years from now will not be global warming, but something else. Maybe running out of Unobtanium for our flying cars....