Monday, December 9, 2013
As they brace for this winter's energy bills, Mainers can find some comfort in a changing climate: An analysis of annual National Weather Service data over the past 115 years confirms that our storied Maine winters just aren't as cold as they once were.
Compared to a century ago, the heating season has gotten milder.
Records show that temperature totals do rise and fall year to year, and that they move in cycles that span the decades. For instance: Winter cold was exceptional in the 1920s, while heating seasons in the 1950s were much warmer than average.
But a compilation of official temperature readings in Maine for more than a century clearly show a gradual winter warming trend, with the most profound change taking place in just the past decade. Since then, overall demand for heat during Maine's coldest months has been lower than during any similar period since at least 1896.
Past records can't predict the duration or intensity of today's record-setting warm stretch, nor do they indicate the reasons. But beyond the political and ideological debates over global warming and its causes and effects, the temperature trends show that change is taking place locally, and having an impact on everyone who heats a home or business in Maine.
The research was done by the Portland Press Herald with help from the government's National Climatic Data Center. The work was reviewed for accuracy and for comment by the National Weather Service in Gray and the state climatologist at the University of Maine in Orono.
The analysis compares heating degree-days, the standard index used by the weather service, as well as by oil dealers, to track how much heat is needed each day to maintain a certain indoor air temperature. The weather service compiles degree-days year round. But for this study, heating degree days were compared only between Oct. 1 and April 30, the typical period in which Maine homes need heat.
The results show that over the past 115 heating seasons, the average number of heating degree days has gradually declined, by roughly 3.5 percent. The rate of decline has been especially steep since 2004, tempered only by slightly above-average cold in 2009.
"The last 10 to 15 years are pretty striking," said George Jacobson, Maine's state climatologist. "There is decade variability, but the long-term trend is pretty unmistakable."
For instance: The heating degree-day data show that Maine experienced a half-dozen or so extremely cold winters prior to 1940. Since then, the state has had fewer prolonged periods of sub-zero temperatures, especially overnight. The shrinking ice pack in the Arctic Ocean appears to be behind the absence of super-cold air masses, Jacobson said.
There are exceptions to the warming trend, notably in 2003 and 2009. In January 2009, for example, Jacobson confirmed an all-time record low for Maine, observed in Aroostook County -- 50 below zero F.
While global climate change is an abstract and controversial concept for some people, warmer winters are a fact, not a theory, according to Steve Capriola, a meteorologist at the National Weather Service in Gray, who compiles climate data.
"This is the result, not the cause," he said of the heating degree-day trends. "Temperature change shows the climate is changing. This is a way people can see how it affects them."
The concept of heating degree-days is based on a calculation that when the daily mean temperature is lower than 65 degrees, most buildings require heat to maintain an inside temperature of 70 degrees.
The daily mean temperature is obtained by adding the maximum and minimum temperatures for the day and dividing the total by two. Each degree of mean temperature below 65 is one heating degree-day. If the high is 56 and the low 39, the mean is 48. Subtracting 48 from 65 results in 17 degree-days for the date.
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