As much as we know a mean warming climate is out there, it has always been difficult to quantify a connection between that kind of climate change and an individual weather event or shorter term pattern. Some consequences of a connection with a phenomenon in the longer term, such as rising sea levels, are easily proven and irrefutable. The northern migration of certain plant varieties from Northern New England into Quebec and New Brunswick, and the lengthening of growing seasons are also associated with the warming.
On the topic of extremes during the winter season in the northern hemisphere, the picture is much fuzzier.
Research by Judah Cohen in Cambridge, Mass., has suggested the longer melt season for the Arctic Ocean — inextricably tied to a warming climate -- leads to more evaporation in the early autumn which, in turn, may lead to more mid-autumn snowfall over a large portion of Siberia in some years. And THAT occurrence of excessive October snow in Siberia has been tied to the formation of a mountainous dome of arctic high pressure over Siberia which, Cohen’s work suggests, can force the polar jet stream north over Alaska and western North America, allowing it to dive to the south over eastern North America. When there is a western ridge and an eastern trough over North America, it generally gets pretty darn cold over eastern North America.
This past October, above average snowfall occurred in Siberia. Note the large positive anomaly for snow over most of Siberia.
After early December, the western ridge/eastern trough took over for much of the remainder of the month and strengthened in early January. It got cold; then it got REALLY cold for almost two weeks. So, it might be tempting to tie the diminished ice cover on the Arctic Ocean, which is related to warming, with the fairly extreme cold we’ve just experienced.
But there are some important caveats to the Cohen body of work. Some years he has had great success in making a superior winter outlook compared to the National Weather Service’s Climate Prediction Center/CPC. This winter has been such an example. Cohen’s outlook later in the autumn beat the pants off CPC’s outlook. This is the way CPC was seeing probabilities in November 2017. Note the temperature probabilities outlook in the lower left for December-February, issued Nov. 19.
Here is Judah Cohen’s outlook via Mashable from his private firm, AER, also issued in November.
Up to this point, it may appear it’s been an open and shut case for Cohen’s theory. That was also the case in the “Snowmaggedon” winter for Baltimore, Washington and Philadelphia in 2009-2010. That winter, Baltimore actually got a little more snow than Buffalo did, which has never happened before or since on record. Cohen had predicted the cold, stormy eastern trough to dominate, tied to the previous October Siberian snow connection.
All this said, Cohen has not always been able to nail down the winter outlook as well as he did for this season. There have been a few cases in which other oscillating patterns not known to be related to a warming climate have taken a dominant role as one of the many variables we look at and possibly canceled out some of the impacts of heavy October Siberian snowfall. The relative thaw which will be developing for much of the next two to three weeks will be related to one such oscillation going into what amounts to a warmer phase for much of the lower 48, especially the east. It’s called the Madden-Julian Oscillation.
Back to the original topic: the recent cold wave. A quick study just published finds no evidence of the fingerprints of humanity on that cold. In general, there will be cold waves most winters, and the majority of those cold waves have been and will be less extreme than in the past. But we can still get mighty cold without a demonstrable tie to human activity.
Climate Central, a climate communications group, performed what’s called a rapid attribution study on the extraordinary cold wave around the start of the new year and found no evidence of a human tie-in. In general, there are fewer cold waves and less extreme cold waves in recent decades, and that does match well with climate model predictions. But this most recent cold wave demonstrates, despite ongoing warming in the global mean, you can still get a real doozy.
And every time that happens, lame jokes predictably come out of the woodwork mocking global warming.