Climate Change: The Crazy, Mixed-Up Cold Part of Warming

Get ready to thank global warming for the cold winter of 2010.

While sitting in a plane last week, I couldn’t help but overhear the gentlemen in the seat behind me carrying on an hour-long, quite voluble rant about the state of the world. Eventually they came around to the subject of climate change, and their talk went something like this: “Global warming? A bunch of bunk. With all that cold and snow last winter, it’s global cooling, not global warming.”

I wanted to turn around and point out that:

  1. A seasonal event does not constitute a climatic trend; and
  2. While it may have been cold in northern Europe and in parts of the United States last winter, on a global scale, the winter was actually the warmest on record.

But I held my tongue.

If I were witness to a similar conversation now, I would have to my hold my tongue on a third point: those cold temperatures and record snowfalls were not all that unusual and are actually consistent with there being a global warming trend.

That new insight comes courtesy of two recent reports: the Arctic Report Card prepared by the Arctic Council (discussed here yesterday) and a paper published last week in the journal Geophysical Research Letters by Julien Cattiaux of the French Laboratoire des Sciences du Climat et de l’Environnement and colleagues that focused on the anomalously cold temperatures in northern Europe during the winter of 2010.

What Caused the Cold Snowy Winter of 2010 in Parts of the U.S.

The climatological 850 mb geopotential height (lower atmosphere) field for December, over the period 1968-1996. Low heights over the Arctic are representative of the polar vortex of westerly winds. Data are from the NCEP–NCAR Reanalysis through the NOAA/Earth Systems Research Laboratory, generated online at www.cdc.noaa.gov. (Source: Arctic Report Card)

 

The observed 850 mb geopotential height field in December 2009. Note the near reversal of the pattern with the maximum heights now residing over the Arctic. Air streamlines follow the height contours, showing a connection between the Beaufort Sea region and the eastern United States and that the westerly flow into Europe is strongly displaced to the south. The normal region of the Icelandic Low is at a near maximum under these conditions. (Source: Arctic Report Card)

 

The 850 mb geopotential height field for February 2010. Again note the anomalous height maxima over the Arctic, the southward displacement of streamline into Europe and the northerly streamline into eastern Asia. Again the Icelandic Low location has a High. (Source: Arctic Report Card)

Normally during the winter, a strong low-pressure system, known as the polar vortex, sits over the Arctic. At the same time, the mid-latitudes tend to be covered by high pressures. This brings about strong westerly winds over the northern United States and Europe (known as the jet stream), which in turn tend to seal the region off from intrusions of cold polar air.

But last winter was punctuated by several episodes wherein these conditions were reversed: the typical Arctic low-pressure system was replaced by a high-pressure system (see graphs at side), and the mid-latitudes had relatively low pressure. This, the authors of the Arctic Report Card show, created instability in the jet stream and, in turn, sent strong episodic transports of cold polar air to points south, including to the United States, Europe, and eastern China. Hence the anomalously cold, snowy winter of 2010.

Cattiaux and co-authors take this analysis further.

They examine the reversal of the highs and lows that occurred last winter in terms of the climatic seesaw, known more technically as the Arctic Oscillation. When this seesaw is in its so-called negative phase, high pressures are found over the Arctic and low pressures over the mid-latitudes. During such conditions, the folks in the northern United States and Europe are generally treated to the kind of winter they had last year — cold and snowy.

If Not For Global Warming, Cold Europe Would Have Been Colder Last Winter

Analyzing meteorological records since 1949, Cattiaux et al found that last winter’s negative phase of the Arctic Oscillation was one of the strongest on record. But when compared to the cold temperatures in northern Europe during similar negative phases, the temperatures were unremarkable, even relatively warm.

For example, they found that the average of the 2010 wintertime temperatures over all European stations was about 1.3 degrees Celsius (2.3 degrees Fahrenheit) colder than the 1949–2010 mean, making it the 13th coldest winter in Europe since 1949. By comparison, the winter of 1963 (which the authors found to have similar intensities in terms of the Arctic Oscillation’s negative phase) had an average temperature that was 4 degrees Celsius (7.2 degrees Fahrenheit) below the mean.

Of all the European stations analyzed by Cattiaux et al, only a few in southern Europe had temperatures during the winter of 2010 that were colder than the wintertime temperatures observed in previous years when the flow regime was similar.

Cattiaux et al conclude that “the winter 2010 thus provides a consistent picture of a regional cold event mitigated by long-term climate warming.”

The implication: if it were not for global warming, the winter of 2010 would have been colder in Europe.

And Yet There’s a Kicker

Global warming may have actually helped cause the cold winter of 2010. Low sea-ice extent is often correlated with neutral or negative phases of the Arctic Oscillation, and so this relationship suggests that the loss of sea ice caused by global warming may drive episodes of colder, snowier weather in the mid-latitudes like that experienced last winter. It’s a crazy, mixed-up world where climate change may bring colder, snowier wi
nters that it also moderates.

Small solace to the folks who had to dig out of those record snowstorms last winter.

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