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.


  1. As people love to say on Slashdot : “Correlation  is not causation.” Of course it’s not very exiting to have an article that says : “Is the recent weather because of global warming ? – We don’t know. Maybe ?”

  2. The wording of this article seems to read that if the AO is causing it to be colder in Europe, it should simultaneously be colder in North America.  But I live in Minnesota and we basically haven’t had winter in the Midwest!  I think we’ve had 5 inches of snow and we’ve only had a low below zero once (usually we spend an entire week with lows at -20 and highs below zero).  So what’s that about?  Is this weird even by their reckoning or is this explanation just poorly-worded?

    1. The warm North American winter is supposedly being caused by an unusually stable jet stream (IIRC?).  I think the idea is that “everything else being equal” the AO would cause a colder winter in North America but as usual, not everything else is equal.

    2. kthugha, yeah, Michigan’s winter has been pitiful too. I keep crossing my fingers, saying, “It’s only February, it’s only February!” but no luck yet.

  3. Why is it cold in Europe just now?  Well, it might be climate change, the AGW magic carbon pixie believers are shouting about being caused by greenhouse gases and the climate change atheists say it’s nothing to do with greenhouse gases at all.

    Me? I think it’s cold because it’s February.

  4. Fascinating article. I will do my best to use the word anthropogenically in a sentence at some point today. I’m also not ashamed to admit that I giggled a little bit when I read the phrase “and, ladies and gentlemen, this is a great big but”.

  5. What’s causing Europe’s cold snap? They stole it from America, damn them!  Down here the daffodils are already out and the white blooms on the dogwoods have already come and fallen.  That’s almost three months ahead of schedule.  I want my winter back!  (To be fair, I expect many Europeans would be happy to hand it over if they could.)

    1. Actually, we haven’t had that much of a winter, at least not here in Central Europe. 

      Sure, it was cold during the last two weeks, but not unusually cold. I actually expect to get a hefty refund on gas. 

      But most of December and January was more or less mild and we’ve had barely any snow.

  6. And behind the AO/sea ice feedback loop?  Another feedback loop.  Weather is feedback loops all the way down. (Well, except solar insolation.)

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