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 from 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.

However, a compilation of official temperature readings in Maine for more than a century clearly shows 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 about global warming and its causes and effects, the temperature trends show that change is taking place locally and affecting everyone who heats a home or business in Maine.

The research was done by the Maine Sunday Telegram 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. For this study, however, 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 gradually has declined, by roughly 3.5 percent. The rate of decline has been especially steep since 2004, tempered only by slightly greater-than-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 subzero temperatures, especially overnight. The shrinking ice pack in the Arctic Ocean appears to be behind the absence of supercold 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.

This information appears in the weather almanac of daily newspapers, among other places.

Degree-days vary greatly by location; but on a statewide basis, Maine now averages slightly more than 7,000 degree-days during the October-through-April heating season. That’s a decline of 246 average heating degree-days since 1896.

Homes weren’t heated with oil furnaces in 1896, but for comparison purposes, the decline in degree-days means a household that needed the equivalent of 1,000 gallons of heating oil a century ago would need 35 fewer gallons today. At $3.50 a gallon, for instance, that adds up to a $122 savings.


To plot this gradual drop in degree-days, the National Climatic Data Center’s Website was used to create a set of time series graphs. The most relevant view came from a series that focused on the seven-month heating season over the century and combined Maine’s three climate divisions — coastal, inland and northern.

The initial graph displayed wide annual temperature swings. To reveal patterns over time, a scientist at the data center applied a binomial filter, a standard statistical tool to smooth out short-period fluctuations.

Once filtered, the time series revealed temperature trends that move in wavelike cycles over 115 heating seasons, in a bumpy, undulating line. It shows extreme cold around 1900, followed by warmer temperatures later in the decade, and another cold spell that stretched into the early 1930s. Above-average cold around 1940 was followed by a very warm period in 1950.

Colder winters returned from around 1960 into the early 1990s. Since then, with a few exceptions, winters have been warmer than average. That trend was seen most clearly in a second, filtered time series covering the period from 1950 to 2011, which marked Maine’s second-warmest winter on record.

The binomial filter was calculated by Trevor Wallis, a physical scientist who works under contract for the data center. In interpreting the time series, Wallis cautioned that because the trend is inconsistent, it’s important not to “cherry-pick” data to reach conclusions. In his view, the 3.5 percent decline in heating degree-days isn’t statistically significant. The steep, record decline in the past decade is more striking, he said.

“But there’s nothing to say it won’t go up again,” Wallis added. “We’ve had some exceptional weather lately, but whether (the decline in degree-days) is going to smooth from now on, I wouldn’t comment.”


Wallis also noted that the number and location of weather observing stations over a century can influence degree-day collection data.

That variable is seen in Portland, where the National Weather Service moved its thermometers from downtown to the jetport, in 1940. Even so, the most-recent warming trend is clear in Portland, Capriola said, reflected in a falling number of heating degree-days since 1961. The latest study period, from 1981 to 2010, shows the biggest drop — 159 fewer degree days than the 6,586 recorded in the previous 30-year period.

The coming years will tell whether this warming trend will be reinforced, but it seems unlikely that, this winter, the state will see another heating season as mild as the last one.

The blog The Weather Centre is calling for colder-than-normal temperatures in the Northeast. The Maine-based Farmer’s Almanac also foresees snow and cold. AccuWeather predicts above-average snow in the Northeast and southern New England, although not Maine.

In its short-term outlook released Wednesday, the U.S. Energy Information Administration said households are expected to use more heating fuel this winter compared with last year, because temperatures are expected to be near normal in much of the country.

The agency forecast reflects a much colder winter east of the Rocky Mountains, with the number of heating degree-days in the Northeast expected to be 20 percent to 27 percent greater this winter than last winter, according to the National Oceanic and Atmospheric Administration.


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