[jfengel]

Runaway greenhouse effects are global, not local. It would be caused by a self-reinforcing feedback where the warming releases more CO2, but that would quickly spread over the entire planet.

There are lots of possible causes for the extra warming in the Arctic in particular, but the high-level view is that ice is more reflective than water, so the increased area of water surface absorbs more heat. There's going to be a lot more to it than that, from the way water and air flow within and around nearby regions, but it was expected that the Arctic would warm faster than the rest of the planet.