What's causing Europe's cold snap?

One of the things that makes it difficult to understand weather, climate, and long-term climate changes is the fact that, when something noticeable happens, there's a good chance it's being caused by more than one thing. So, when you look at a weather phenomenon and ask, "Is this being caused by anthropogenic climate change?", there's several (technically correct) ways that question could be answered.

Take, for instance, the recent cold snap in Europe that's killed more than 300 people and dropped snow as far south as Libya. As Andrew Freedman explains on Climate Central, this particular bit of weather weirdness is being caused by natural variations in the air currents over the Arctic:

The Arctic Oscillation, or AO, is is a climate index that describes the characteristics of the atmospheric circulation over the Arctic, and a related index describes the circulation over the North Atlantic. Depending on whether it's in a "positive" or "negative" phase, the Arctic Oscillation can bring warmer or cooler than average wintertime conditions to the U.S. and Europe.

Right now the Arctic Oscillation is in a negative phase, which tends to favor colder than average weather in Europe and the U.S. Scientists don't fully understand what causes the Arctic Oscillation to switch from one phase to the other, which limits their ability to forecast these changes ahead of time beyond a week in advance.

But (and, ladies and gentlemen, this is a great big but) scientists have noted that the Arctic Oscillation has been behaving more strangely than usual for the last decade. In fact, Freedman points out that several record-breaking positive and negative oscillations have coincided with extreme weather events you probably took note of: December 2009's Snowpocalypse, February 2010's Snowmageddon, and April 2011's massive outbreak of tornadoes (which, thankfully, doesn't have a cutesy name associated with it).

And this is where the lines between "naturally occurring" and "anthropogenically caused" get blurred. Because this record-breaking decade of Arctic Oscillations has coincided with a record-breaking decade in loss of Arctic sea ice and there's good reason to suspect that the two might be related.

... in recent years there have been studies examining how the global warming-related loss of Arctic sea ice might affect winter weather patterns in the northern hemisphere. Some of this research shows that sea ice loss may favor winters with predominately negative phases of the Arctic Oscillation. One potential result of global warming, referred to as the "Arctic Paradox," is that sea ice loss can help warm the Arctic during the winter, while setting in motion a chain reaction of events that make winters colder than they otherwise would be in Europe and the U.S.

This actually gets even more complicated, because it also appears that AO can affect the amount of sea ice that melts in a given year, which can, in turn, affect what happens with the AO. For more information, check out:
An explainer from The National Snow and Ice Data Center
— A NASA explainer from a couple of years ago that talks about the relationships between climate change, AO, and cold weather.

Also, just so we're clear, the AO is not the same thing as the climate systems that could drive "abrupt climate change"—a possible scenario that served as the basis for the highly fictional movie "The Day After Tomorrow". You can read more on that at the Weather Underground blog.