Memories of the record-breaking cold and snowstorms of last winter, including Snowpocalypse, Snowmaggedon and Snoverkill, have probably faded, as eventually will be true of memories of the record-breaking heat this summer. Unless, that is, the extreme winter and summer weather prove to be just a preview of conditions that become the new normal as a consequence of global climate change. Should that prove the case, it might be hard to forget "the good ole days" when the extremes of winter/summer of 2009/2010 were the exception, not the rule.
How likely is that we're entering an age where extremes in weather become the new normal? Personally, I believe the weight of evidence viewed objectively points in that general direction, although there is much uncertainty about details of how, when, where and in what phenomena and parameters extremes will appear and have consequence in human affairs.
In regard to the snowstorms that affected the D.C. metro region and other mid-Atlantic and northeast coastal regions, a new study by a team of scientists at Columbia University's Lamont-Doherty Earth Observatory argues that global warming was not involved. Rather, the study finds the anomalous winter was primarily the result of convergence of an exceptionally strong El Nino and unusually strong negative phase of North Atlantic Oscillation (NAO).
El Nino typically brings wet weather to the southeastern U.S. and enhances the odds for an East Coast storm track, while the negative phase of the NAO corresponds to an atmospheric circulation pattern which brings cold air from Arctic regions into eastern North America. What might be the weather/climate understatement of the year, the authors conclude when these patterns coincide, "end result: more snow".
I'm fully on board with contention that man-made global warming or, more generally, global climate change is not responsible for the past winter's cold and snow. It's certainly possible that, as the IPCC report indicates, increased atmospheric moisture in a warming world generally speaking could enhance precipitation and thereby snowfall amounts where it remains cold enough for snow.
In that context it's possible to argue that the cold and winter snows were consistent with expectations with global warming. But that's a far cry from evidence that necessarily led to, or even tilted the odds toward the major 2009/2010 snow events. Indeed there is nothing to indicate that the convergence in the specific manifestations of El Nino and NAO were anything more than coincidental.
The authors correctly disabuse the irresponsible proclamation of hard-core deniers that the cold and snow last winter is evidence against human caused global warming, not withstanding the fact that globally the winter was significantly above normal. Scientists agree that no single storm or series of storms in any given season affecting any given region are evidence for or against climate change.
Unfortunately for proponents of measures to reduce greenhouse gas emissions, snow buried the "seats of power" in Washington enabling use by some to undermine the political case for global warming and support for meaningful action. Ironically perhaps, the excessive heat this summer apparently has not increased support.
The authors justifiably note, but do not explain that El Nino and negative NAO were not necessarily the only reasons for the past winter cold and snow.
In a previous post, I indicated that the December snowstorm (Snowpocalypse) occurred before the nominal response to El Nino on the Pacific jet and probably sub tropical jet and storm tracks had kicked in. The subtropical jet often acts as a conveyor belt of moisture feeding storms that develop in the southern U.S. which sometimes head northeast towards us independently of El Nino. Additionally, the authors did not consider the likely critical influence of the third ingredient I mentioned, the Pacific North American (PNA) teleconnection - which can help lock in cold air over the East - in setting the stage (and enhancing predictability) for the snowstorms.
In a discussion of the technical study, the authors conclude:
In summary, what happened this past winter is just another example of the kind of seasonal climate anomaly that can arise from purely natural variability of the atmosphere-ocean system. There seems no evidence that it had anything to do with climate change and should not be exploited to make arguments, one way or another, about the reality of that (which we do not doubt) or how to tackle it.
In its press materials, Columbia University quotes Rutgers University climate scientist David Robinson, who says: "When the public experiences abnormal weather, they want to know what's causing it. This paper explains what happened, and why global warming was not really involved. It helps build credibility in climate science."
Mission accomplished? What do you think?
The views expressed here are the author's alone and do not represent any position of the Washington Post, its news staff or the Capital Weather Gang.
Causes for Mid-Atlantic Snowstorms 2009/10 (NOAA Fact Sheet, Word Document)
Forensic Meteorology Solves the Mystery of Record Snows (NOAA Climate Watch Magazine)
Research says big snow storms not inconsistent with -- and may be ampliflied by -- a warming planet (Joe Romm, Climate Progress)
New peer reviewed paper refutes claims of blizzards of last winter being driven by "global warming" (Anthony Watts, WattsUpWithThat Blog)
Expert: Blizzards "consistent with" climate change (Andrew Freedman, Capital Weather Gang with Jeff Masters, Weather Underground)