One of the theories that a lot of people are arguing is that global warming may "turn off" the Gulf Stream entirely. If this happens, eastern North America, western Europe, and especially the British Isles, would get a lot colder, quite quickly, global warming or no global warming.
Another theory is that there'll be a lot more snow in northern latitudes as a result of the increased evaporation from the open water where the arctic ice used to be. Maybe more each winter than can melt in the following summmer. Which is how an ice age starts.
When Robert Frost (that's appropriate) spoke of not knowing whether the world would end in fire or in ice, I used to think he was just being, like, a poet.