Climate change is not the only reason why gators have moved north. http://upload.wikimedia.org/wikipedia/en/thumb/a/a2/Rangemapx.gif/210px-Rangemapx.gif
In fact, the climate has only warmed about a third of a Fahrenheit in the last 31 years. http://www.ssmi.com/data/msu/graphics/plots/MSU_AMSU_Channel_TLT_Trend_Map_v03_2.png
Damming rivers has created lakes that act as solar collectors and warm the cold snow melt that made up rivers. Nuclear plants that use lakes to cool their reactors make lakes artificially warm, and alligators really like it.http://www.jstor.org/stable/2425371?seq=3
However, do not expect to find them living in your sewers. http://www.snopes.com/critters/lurkers/gator.asp