A hot topic of conversation this past winter was how mild and snowless this winter has been, especially compared to the past two winters along the Eastern Seaboard. And the mild winter certainly had people asking: is this what we can now expect from global warming?
In the coming decades when more and more winters are mild, much milder than the winters of the twentieth century, will we look back and say: winter 2012 was the winter we first experienced a regime shift to winters dominated by the impact of increased greenhouse gases. Or as Jim Hansen wrote in a recent New York Times Op-Ed, is it game over for the climate and we have seen the future? In my opinion this past winter was not exceptional in demonstrating that global warming is the major factor in influencing our weather; and while it is tempting to conclude that we should only expect warmer and warmer winters in the years to come, the data doesn’t support this yet.
The January through March period was the warmest such period observed for the contiguous United States. March was exceptionally warm beating the previous record by half a degree F; it was the second greatest departure from normal ever observed for one month across the US and every state recorded at least one daily record warm temperature. Such extreme and expansive warmth begs the question” is this outside the realm of normal or natural variability, and therefore is the climate is being forced by human activity?
Former US Speaker of the House Tip O’Neill said that “all politics is local” and human perception of climate is most influenced by the local weather one experiences. But when it comes to understanding the climate and its variations, we are behooved to take a global approach; we need to average or integrate over space and time.
Unfortunately the reality is that our climate data is too limited in coverage in both space and time and the error bars in both the data and models are too large for us to quantify what is the true envelope of natural variability. These errors and data limitations are magnified when going from the global to the regional and certainly the local scale. Therefore, the best and most accurate conclusions are drawn from integrating over as much time and space available.
As it turns out, globally January through March 2012 was the coolest such period since 1996. So I would argue, as crazy as it may seem to all my neighbors in and around Boston, that winter 2011 was a more compelling example that global warming is real and present than this winter, since globally temperatures were warmer by a relatively large margin. Sure in 2011 it seemed like a glacier enveloped Boston while this winter it seemed as if we were geographically transported to Atlanta, but we need to take a broader view in relating weather events or even isolated climate anomalies to longer term variations such as global warming. So while many East Coast residents enjoyed spring weather for much of the winter, Alaskans endured record cold and snow this winter as did many in Europe and Asia. So for many people across the Northern Hemisphere the winter of 2012 is not what they envision for future winters in a significantly warmer world.
In conclusion, my answer to the question of whether the mild winter was related to global warming is: “maybe”. That may be a disappointing answer but in my opinion, given the limitations to our knowledge, the data and the models, an answer of “yes and no” is indefensible. Ironically, if you were to answer “yes, global warming is impacting our winters” then the winter of 2011 is more supportive of your argument and if your answer is “no” then this past winter is more compelling. Try explaining that to your neighbors with a straight face.