Changes in longitudes
If it wasn't for the warm Gulf Stream Current, Europe wouldn't be able to support the industry, agriculture and population it currently has.
Ironically, if the Stream should shut down due to global warming-provoked changes in the salinity or temperatures of the Arctic Ocean, Europe could be thrown into another Ice Age.
How the interruption of the Gulf Stream would affect weather (either temperature or precipitation) in N America is anybody's guess. But I think it would be safe to say the effects would be dramatic.