Winters in New England are becoming decidedly less wintry.
Over the past century, the number of days below freezing across the northeastern United States has dropped by more than a day a decade, a new analysis finds. Bitter cold and days with snow have also waned, whereas “mud days”—when earth is bare and thawed—have gained ground.
“We’re losing the cold,” said Alix Contosta, an ecosystem ecologist at the University of New Hampshire in Durham. Contosta is the lead author of a new study in Ecological Applications reviewing winter meteorological data from Minnesota and Manitoba all the way to Maine and the Maritimes. The study found that both extreme cold days (days below -18°C, cold enough to kill forest insect pests) and frost days (where temperatures dip just below freezing) have declined significantly.
“The freezing point of water is one of the strongest physical thresholds in nature,” Contosta noted. “And when you lose that freezing point, that causes a lot of change.”
Among the potential changes are disruptions in the region’s water cycle, such as snow melting and streams flowing earlier in the year.
“That can affect the water balance of the ecosystem later on,” Contosta said. “It also can impact water resources in populated areas that rely on that spring flush for reservoir recharge.”
For the analysis, researchers collected daily temperature, total precipitation, snowfall, and snow depth data recorded at weather stations between 1 November and 31 May each year from 1917 to 2016. They found that the number of frost days declined by a median of 1.1 days per decade over that time. Extreme cold days dropped a median of 1.3 days per decade across the entire region, with declines greatest in New England and southeastern Canada, as well as the western Great Lakes area. Snow-covered days, meanwhile, fell around 2 days a decade in New Hampshire and the surrounding environs and nearly as much in the western part of the study area. But parts of the central region actually saw increases in snowy days.
The researchers also devised combined measures to assess events with potentially large impacts on soil, such as days with both bare ground and freezing temperatures.
“There’s an idea that in a warming world we may have colder soil during the winter because we are losing snow cover,” said Contosta. Without an insulating snow blanket, soils can freeze, damaging soil structure and leaching nutrients.
Muddy Days Ahead
The researchers were surprised to find that the number of bare-ground ice days did not change significantly.
Instead, researchers documented a rise in bare-ground thaw days—with above-freezing temperatures and no snow cover. The authors identified these days as mud days.
“In some ways less soil freezing might be a positive outcome,” Contosta said. “But there may be other outcomes that we don’t fully understand.…What does it mean for the ecosystem to have more of these mud days, especially between the end of winter and the onset of spring?”
The team was also surprised there was no significant increase in rain-on-snow days, which can cause floods. “That may also be because as we have warmer and warmer winters, there is simply less snow for rain to fall on,” Contosta said.
The study provides an important compilation of “data across a wide area of northeastern North America where winter is changing a lot,” said Peter Groffman, a microbial ecologist with the City University of New York Advanced Science Research Center who studies how climate change affects nitrogen cycling and greenhouse gas dynamics in soils. Groffman was not involved in the study.
The findings suggest that soils will be more affected by winter warming than freezing as the climate changes, Groffman said.
The researchers “confirmed across a broader landscape what a lot of us are seeing in our smaller areas—that on average we’re getting less snow,” said Douglas Burns, a research hydrologist with the U.S. Geological Survey’s New York Water Science Center who also was not involved in the study.
And if a mud day is “truly a muddy condition, then there’s the potential for sediment runoff,” Burns said. “That’s a water quality issue for sure.”
Contosta said she is motivated to study winter because “a lot of people see winter as a nuisance season, especially in the part of the world that I live in.” And research has tended to focus on the growing season, she added. “We know a lot less about what happens in winter. Winter is sort of the underdog here.”
Contosta is now investigating the ecosystem impacts of “weather whiplash,” such as extreme cold and snow followed abruptly by heavy rains or unseasonal heat.
“We’re not just getting overall warmer winters with less snow,” Contosta said of recent winters in the Northeast, “we’re also getting wackier winters with crazy weather.”
—Cheryl Katz (@ckatz99), Science Writer
13 December 2019: This article has corrected the temperature associated with extreme cold days.