About the last cold Chicago winter

From the Bulletin of the American Meteorological Society:

The winter of 2013/2014 had unusual weather in many parts of the world. Here we analyse the cold extremes that were widely reported in North America and the lack of cold extremes in western Europe. We perform a statistical analysis of cold extremes at two representative stations in these areas: Chicago, Illinois and De Bilt, the Netherlands. This shows that the lowest minimum temperature of the winter was not very unusual in Chicago, even in the current warmer climate. Around 1950 it would have been completely normal. The same holds for multi-day cold periods. Only the whole winter temperature was unusual, with a return time larger than 25 years. In the Netherlands the opposite holds: the absence of any cold waves was highly unusual even now, and would have been extremely improbable half-way through the previous century. These results are representative of other stations in the regions. The difference is due to the skewness of the temperature distribution. In both locations, cold extremes are more likely than equally large warm extremes in winter. Severe cold outbreaks and cold winters, like the winter of 2013/2014 in the Great Lakes area, are therefore not evidence against global warming: they will keep on occurring, even if they become less frequent. The absence of cold weather as observed in the Netherlands is a strong signal of a warming trend, as this would have been statistically extremely improbable in the 1950s.

Capsule summary: The winter of 2013/2014 was notable in central North America for its persistent cold, the cold waves were not unusual. The absence of cold waves in Europe was statistically more extreme.