To have one bad winter may be considered a misfortune, to have two on the run could be construed as a pattern. In fact, what we are experiencing now is well within the bounds of natural variability, even in a globally warmer world.
There are undoubtedly many other complications that have combined to give us a bitterly cold December. The first is a "blocking high" pressure in the North Atlantic that has sent our mild westerlies, and surprisingly balmy weather, to Greenland.
Another is the possibility that we are entering a cooler phase of the climatic oscillation of the North Atlantic, when the usual pattern of low pressure over Greenland and high pressure over the Azores breaks down.
A final complication is what's happening in the tropical Pacific Ocean, where the "La Nina" sea current is entering a 20-year high of activity. However, the work by Vladimir Petoukhov makes a remarkable prediction based on the loss of Arctic sea ice. We could be seeing more colder winters than normal precisely because the Arctic is getting warmer.