Now that the winter of 2006-07 is officially in the books it is clear that this was a winter of extremes. December and January both featured monthly anomalies 5-6 degrees above normal while February was around 7 degrees below normal (the official climate division values are not in yet). Now, as you would expect, there are already stories out there in cyberspace debating whether or not this anomalous winter was influenced by Global Warming. In fact, Elliot Abrams of AccuWeather recently wrote in his blog about Philadelphia…
February was more than 16 degrees colder than January, a stunning turnaround. Some researchers have suggested that with global warming we will see more extremes. Was this part of that, or would it have happened anyway? What’s the best way to answer that question?
Here’s how this researcher would do it.
First, here’s some backstory…By mid-January, some websites were talking about the “year without a winter” and the U.K. climate office stated that 2007 would be the warmest year ever. This topic was actually the first ever post for this blog back in January. Then February came along and as predicted here, winter came back with a vengeance, with most locations from the Midwest to New England having one of the coldest and snowiest winters on record.
So how does one determine whether or not this is related to global warming? The first step is to go to the NCDC climate division website where you can download monthly values of temperature, precipitation and drought for all 344 climate divisions of the United States. The next step is to choose your state and climate division of choice. If you want to see a map of the climate divisions, click here. I chose climate division 2 of KY since that includes both Bowling Green and Louisville. After getting a list of the monthly temperature anomalies from 1895-2007, I then calculated how often a winter month (DJF) had an anomaly of either >5 or < -5 during the course of a winter, since 5 degrees is roughly equal to one standard deviation of the monthly time series.
During the 112 years in the time series, I found that there have been 58 winters with one month <5 or <-5, 12 winters where it happened during two months, and a grand total of 6 winters where all three months had anomalies <5 or <-5. Now for this to be strongly related to global warming, it would make sense if the six winters with all three months of <5 or +5 but only two months with an anomaly <-5. In addition, winters from 1997-2006, which represent only 9% of the historical record, include 6 of the 20 warmest winters since 1895, a frequency that is three times greater than random chance. This pattern of warming across Kentucky during winter since roughly 1995 fits with the observed warming trend around the globe.