Why is it so cold if the Earth is warming? (video)
Dr. Jeff Masters from Weather Underground drops some science on those who think they can conclude that the planet isn't warming because "it's so cold this winter!" Interestingly, cold isn't the only abnormal weather pattern for North-America right now. There's also historic warm temperatures in Alaska, and the deepest drought in decades across the west...
You really have to see the graphics and listen to the interview - it's great - but the general idea is that sometimes the Jet Stream slows down and shifts South, allowing very cold arctic air to come down over most of Canada and the US. This is a regional phenomenon, more like shifting heat around on the planet to unusual places, rather than actually cooling it. Evidence of this is that while big parts of North-America are colder than usual, the warm air that is being pushed aside is changing the weather in opposite directions in places like Alaska and Washington State, as well as Russia and parts of Europe.
Via Peter Sinclair