When the Copenhagen climate summit ended in disappointment and finger-pointing, we saw again just how difficult it would be to get the world's nations on board for an agreement to lower greenhouse emissions and slow global warming. This week brings another reminder of how far away we are from meaningful action: We can't even get past the difference between weather and climate. It's bitter cold this week, even for January.

Beijing had its coldest morning in almost 40 years and its biggest snowfall since 1951. Britain is suffering through its longest cold snap since 1981 [AP]

. The southern United States is in the grip of freezing weather; the Midwest has seen dangerously cold wind chills far below zero. Trying to stave off the inevitable "where's your global warming now" chants, the AP and other news sources rushed to run pieces trying to get across—one more time—that weather isn't climate. The chants came, inevitably. But despite pundits and columnists who try to conflate the two to take the same old swings at global warming, a single bout of cold weather—or hot, for that matter—doesn't actually say diddly squat about long-term climate patterns. However, if one can set aside for a moment climate politics as usual and this weather-is-climate misunderstanding, the short-term weather patterns at play in our current spell of frigidity are pretty interesting. Whatever happened to this year's El Niño, for instance?

Shorter-term, naturally variable patterns such as El Niño account for seasonal differences -- making one winter warmer or colder than another. But it takes a strong El Niño to dominate the pattern of a U.S. winter with unusually warm and dry conditions across the northern tier of the country, and cooler and wetter weather across the south, and the current El Niño is not strong [Discovery News]

. In addition, there's the curious case of the current Arctic Oscillation, which is rather out of sorts.

Essentially, air pressure is measured at various places across the Arctic and at the middle latitudes of the Northern Hemisphere - about 45 degrees north, roughly the latitude of Milan, Montreal or Vladivostok. The difference between the average readings for the two latitudes gives the state of the Arctic Oscillation index [BBC News]

. A positive reading means high pressure in the mid-latitudes and low pressure at the pole; a negative reading means it's the opposite. And what we see right now is an "extraordinary negative plunge" to levels not seen since at least 1950, Andy Revkin shows at his New York Times blog. What these

conditions mean is that cold air spills out of the Arctic down to mid-latitudes, which this time round includes much of Europe, tracts of the US and China [BBC News].

As you can see in the historical chart of the Arctic Oscillation, it's a pretty scattershot phenomenon. But it's an important one, which could help to explain why it's frigid in the continental United States but unseasonably nice in some far northern locales.

In 2001, after analyzing its impact on Northern Hemisphere winters, University of Washington researchers suggested that effects of the Arctic Oscillation on weather patterns "appear to be as far-reaching as those triggered by El Niño in the South Pacific" [Discovery News]

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Image: flickr / bsabarnowl