Masses of cold and warm air move around, both North-South and East-West, and these movements are affected both by cyclical patterns (such as the Arctic Oscillation) and long-term trends (like global warming). NASA explains:
NASA climatologist Bill Patzert of the Jet Propulsion Laboratory explains what happened: "A couple of weeks ago, Mother Nature did an about face. The tight polar vortex that had bottled up the cold arctic air in the beginning of winter suddenly weakened. Cold air swept out of Siberia and invaded Europe and the Far East."
The "tight polar vortex" is caused by the Arctic Oscillation (AO), a see-sawing pressure difference between the Arctic and lower latitudes. When the pressure difference is high, a whirlpool of air forms around the North Pole. That's what happened earlier this winter: the whirlpool was more forceful, corralling the cold air and keeping it nearer the pole.
Now the vortex is weakening. With "the AO Index going negative," as an expert or weather-nerd might put it, cold air escapes from that whirlpool and heads southward, resulting in the killing extremes now plaguing the other half of the planet.
However, even the breakdown of the vortex cannot completely account for the severity of the winter Europe is suddenly experiencing. As strange as it sounds, some climatologists, among them Judah Cohen of Atmospheric and Environmental Research in Massachusetts, attribute the unusual cold to global warming. Cohen contends that since sea ice is being melted by warmer temperatures in the Arctic, more moisture is available for the atmosphere to pick up – and drop as snow. As a result, Siberian snow cover has increased, and this snow cover has a cooling effect which reaches East Asia and Europe.
"Cohen's research is cutting edge and could bring important improvements to forecasting climate and weather over North America and Europe," says Patzert. "Cohen and others are on the threshold of understanding of how climate change affects the behavior of the Arctic Oscillation."
To discern whether the earth is warming or not, you have to look at the entire planet, over a few decades, not just one region in one year. Globally, the 2010-2011 winter has been warmer. According to NOAA, January 2012 was the 4th warmest January in recorded history.
For related bites and links to further science on the "Warm Arctic/Cold Continent" phenomena, use left sidebar to search for keyword (or tag) "cold winter."
Update 2-18-12: More new research confirms that melting Arctic ice can cause cold winters in Europe.
Update 3-9-12: NOAA reports that 2011-12 was the 4th warmest winter ever in the contiguous U.S.