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70s in November? What fall warming means for Maine
By Annie Ropeik
Last Saturday was the warmest it has been in Augusta in any November since record-keeping began — 76 degrees Fahrenheit, like a Maine beach day two weeks from Thanksgiving, or like a typical November high in parts of California or Texas. Portland also set a November record of 75 on Saturday. After a quick dip into what felt like real fall around Election Day, much of the state was back into the upper 60s at the end of the week, about 20 degrees above normal.
Maine is one of many places that claims the adage, "If you don't like the weather, wait five minutes." We're used to some variability – but climate change is increasing it, supercharging our weather with energy as humans' fossil fuel emissions trap heat in the atmosphere and warm the oceans. And here, as we saw this week, these new extremes are mostly tending toward warm.
Alix Contosta, a research assistant professor at the University of New Hampshire, studies the effects of winter warming. I've talked to her before about unseasonable warmth in, say, January, or the effects of increasing freeze-thaw cycles in early spring. But what about fall warming? I asked Contosta and other scientists what people should be thinking about during weather events like this.
"In a nutshell, we have seen temperatures this warm before at this time of year," Contosta wrote to me. "Yet late autumn 'heat waves' are rare. … The implications for ecosystems are not well-understood as we are experiencing novel conditions without much historical precedent."
Sean Birkel, Maine's state climatologist and a research assistant professor at the University of Maine, explained some of what’s going on when temperatures spike into the 70s in early November. In an interview this week, Birkel said a hallmark of climate change in our region is a lengthening summer season that extends further into historical fall, shrinking down the coldest stretches of winter.
Birkel used his state climate data dashboard to show me how this week's warm weather stacks up to this trend. This data shows Maine has gotten 3-4 degrees Fahrenheit warmer on average year-round since 1901. The change is particularly pronounced in shoulder months like September and November. And all of Maine's warmest falls on record have been in the past 10 years.
On a chart showing daily temperatures and norms for this year in Bangor, Birkel pointed out how the recent high temperatures all pushed solidly into above-normal territory. One warm day on its own may have had less of an effect on cold-adapted ecosystems than a series like we saw this week, he said.
He underlined overnight temperatures as a key climate signal — more humidity during the day traps more heat that doesn't burn off at night and sticks around.
"It's this overall increase in the accumulation of heat that the natural systems are responding to," Birkel said.
One reason for the change lies in Arctic sea ice. This northern sea freezes and partially thaws throughout the year as seasons change. Its frozen area is at its smallest in September before refreezing begins heading into winter. It’s that refreezing process that drives Maine winters, Birkel said – as new ice forms, the air moving over it chills and heads down to us, bringing on the cold seasons.
The warming climate means slower ice formation and more melted area to cover, causing a longer, more unsettled autumn wait before winter really sets in and leaving openings for warm spikes like this week's.
Birkel said sea ice has been at a "new low normal" for about 15 years. And right now, the North Atlantic and Gulf of Maine happen to be experiencing a heat wave, pushing into record territory on the heels of another record-warm autumn in 2021, as the Gulf of Maine Research Institute reported.
"The climate system is always trying to reach a new equilibrium," Birkel said. "Changes in the Arctic are linked to the changes in the oceans, and ultimately, we feel that here in Maine… by these changes in the weather."
There’s a lot of variability in the way this affects Northern ecosystems, the timing of seasonal changes and the way those changes are expressed in nature, which is known as phenology. Jay Wason, an assistant professor in UMaine's School of Forest Resources, said trees in general are primarily tuned to day length, not temperature, to trigger their transitions to winter. But he said later falls and warmer winters do have ripple effects for the timing of forests' spring leaf-out.
"Plants can sense the amount of cold weather they have experienced and only initiate spring phenology after they have experienced enough cold," Wason said. "So, warmer winters may actually delay spring phenology for some of our native trees and more southerly species with lower chilling requirements may be better able to take advantage of warm springs."
Plus, he said, warm fall weather is a photosynthetic boon to invasive species like buckthorn. Birkel also pointed out how disease-carrying ticks are getting a boost to their life cycles and survival rates from warmer falls and winters.
UNH's Contosta offered another hypothesis, though she said she said she'd need to test it in data to prove it: "I am guessing that soil microbial activity has increased dramatically during the recent fall warming event," she wrote. "The leaves have largely fallen from the trees, and all this fresh litter is like a giant buffet to decomposer organisms living on top of or within the soil," especially in warm, moist conditions like this week's, she said.
"Normally soil nutrient and carbon cycling would slow down at this time of year as temperatures cool. I hypothesize that over the past week, we might see an opposite phenomenon, where soil microorganisms are super active, and rates of soil carbon and nutrient cycling are elevated," she said. "This is carbon that might have been stored in soil but is now in the atmosphere where it can act as a greenhouse gas."
These are just a few examples of how a short spell of unseasonable warmth, or a few degrees of warming over a long period, can have destabilizing ripple effects throughout Maine's delicate ecosystems — to say nothing of vital aspects like foliage, hibernation, migration, agriculture or the water cycle, or effects on human systems like energy usage, heating, infrastructure. These increasing spikes in the weather that Maine is built around add up, baking in changes that may persist even after, or if, people start to do more to lower emissions.
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