Yesterday evening, I stood at a bus stop in Minneapolis wearing no socks, no gloves, and no hat. The breeze was warm. The birds were singing. Clearly, something is deeply wrong here. In fact, 2012 has brought the warmest start to a January on record in the Twin Cities. We're also in the middle of a major drought, which, this time of year, means no snow on the ground.
All of that has consequences—just this morning Minnesota Public Radio was talking about the economic impact the drought has had on snowmobile-based tourism in this state. What everybody wants to know: Is this caused by climate change? Is this what it will be like next year, too?
That's really hard to say. Remember: The really solid stuff scientists can tell you about climate change comes from analysis of trends over decades—for instance, when you look at global temperature anomalies over 50 years and find that the last time the global mean monthly average was lower than the 20th century average was back in February 1985. That's because, while anthropogenic climate change exists, it's not the only thing influencing the local weather or the global climate. The climate system involves a lot of different phenomena, which act alone and together. We can see a pattern of warming that can be linked to rising carbon dioxide levels in the atmosphere. But there's other stuff going on, too, which affects year-to-year fluctuations within the decades-long pattern.
In this case, says Jeff Marsters on the Weather Underground blog, the abnormally high temperatures are related to oddities in the jet stream—air currents in Earth's atmosphere. And those oddities may, or may not, be the result of anthropogenic climate change.
The cause of this warm first half of winter is the most extreme configuration of the jet stream ever recorded, as measured by the North Atlantic Oscillation (NAO). The Arctic Oscillation (AO), and its close cousin, the North Atlantic Oscillation, are climate patterns in the Northern Hemisphere defined by fluctuations in the difference of sea-level pressure in the North Atlantic between the Icelandic Low and the Azores High. The AO and NAO have significant impacts on winter weather in North America and Europe--the AO and NAO affect the path, intensity, and shape of the jet stream, influencing where storms track and how strong these storms become. During December 2011, the NAO index was +2.52, which was the most extreme difference in pressure between Iceland and the Azores ever observed in December (records of the NAO go back to 1865.) The AO during December 2011 had its second most extreme December value on record, behind the equally unusual December of 2006. These positive AO/NAO conditions caused the Icelandic Low to draw a strong south-westerly flow of air over eastern North America, preventing Arctic air from plunging southward over the U.S. and Europe.
The December Arctic Oscillation index has fluctuated wildly over the past six years, with the two most extreme positive and two most extreme negative values on record. Unfortunately, we don't understand why the AO varies so much from winter to winter, nor why the AO has taken on such extreme configurations during four of the past six winters. Climate models are generally too crude to make skillful predictions on how human-caused climate change may be affecting the AO, or what might happen to the AO in the future. There is research linking an increase in solar activity and sunspots with the positive phase of the AO. Solar activity has increased sharply this winter compared to the past two winters, so perhaps we have seen a strong solar influence on the winter AO the past three winters. Arctic sea ice loss has been linked to the negative (cold) phase of the AO, like we observed the previous two winters. Those winters both had near-record low amounts of sunspot activity, so sea ice loss and low sunspot activity may have combined to bring a negative AO.
Image: Crazy (awesome) Minnesota Christmas weather, a Creative Commons Attribution (2.0) image from shilad's photostream. Please note the lack of snow, the fact that there is open water on Lake Harriet, the presence of ducks, and the lack of hat and gloves on that woman. This is not normal for Minnesota in December.