In a gathering evening at the edge of the Green Mountains, steam rising from the Branon family farm's sugar house signals that it's syrup season again.
Inside the sugar house, Tim Branon, the eldest brother of an extended family that has made syrup for five generations, presides over the boiling process. It is now done largely in a gleaming stainless steel extractor made in nearby Rutland. It boils the sap, removing moisture and concentrating the maple sugar.
Branon, a man of few words but vast knowledge about the process he learned as a child, only opens the tap when it reaches the exact density of syrup.
This year, the Branon family will produce about 14,000 gallons of syrup. Tom Branon, Tim's brother, is in charge of the farm.
"You know," Tom said, "we really enjoy making the maple syrup."
Tom's wife, Cecile, an energetic woman with a constant smile, thinks about her four children, including Evan, this year's Vermont Maple King. She also thinks about the sense of community that develops when the sap is running.
"This business right here revolves around family," she said. "Friends, you know, people who stop in and feel that they're part of it."
But this year, there is a growing realization that global warming -- if that's what it is -- is beginning to take a toll on Vermont's signature product, and the farmers and their families who produce it.
Tom and Cecile spend almost every day tramping through the snow-covered hills on their farm, checking the system of vacuum lines snaking around their 50,000 trees. When the trees are tapped, the lines capture the sap and connect with thicker hoses that carry it to the sugar house.
"It's totally changed," said Tom. "In the '50s, '60s and even '70s we tapped by hand and used buckets, gathering by hand and using horse-drawn sleighs with wood fired evaporators."
But there have been bigger changes -- in the climate. This year, many farmers in Vermont are ending the syrup season in early March, a time when it used to begin.
"I can remember [when] we were first married that's when you started tapping," Cecil said. "Now, we're tapping in January.
Tom, wearing a plaid thermal vest, but no heavy jacket adds, "It seems, though, the springs come earlier and the winters are less harsh."
And that's not good news for making syrup. The sap flows best when there are warm days followed by below-freezing nights. The combination creates the pressure inside the maple trees that makes the sap flow.
According to the National Oceanographic and Atmospheric Administration, over the last generation, the average winter temperature has risen by almost three degrees. That has led some Vermont researchers to conclude the state is in the grip of a long-term warming trend. And it raises the prospect that the climate here will one day no longer sustain maple trees.
One projection has Vermont's climate becoming as warm as Virginia's. The climate to sustain maples would move two degrees north, from the 45th parallel that runs along Vermont's northern border, to the 47th parallel, in Canada.
In his lab at the University of Vermont's Proctor Maple Research Center, director Tim Perkins said that would be a death knell for the maple syrup industry in his state.
"It appears it's going to warm for the next several decades, or centuries," he said.
Already, added Perkins, the syrup season has shrunk by 10 per cent because of the warmer winters.
"Essentially, the change from winter to summer is happening faster, so the spring period, which is the critical time when maple syrup is made, is shorter," he said.
And the future is bleak.
"If that trend continues," said Perkins, who has studied maples for 20 years, "the maple season will get shorter and shorter, and yields will become less, and eventually, people will not make maple syrup because they won't be able to make money doing it."
The economic blow to tiny Vermont would be devastating. Maple syrup accounts for $200 million of the state's economy every year.
Walk into the Shelburne Country Store in Shelburne, Vt., and it's evident. The shelves are packed with syrup and maple candy.
As Joan Braun paid for a book all about maples, she considered the future without the trees or without syrup.
"Oh, it's the heart and soul of Vermont," she said. "It's like a religion, maple syrup."
In Burlington, an ancient neon sign heralds a place where they worship. Henry's Diner has been an institution in Burlington since the Depression and a purveyor of the syrup that has been part of Vermont even before there were Vermonters.
Tom and Naomi Randall tucked into plates of waffles, slathered in maple syrup. Naomi said it's simple: "Vermont is known for its maple syrup."
Added Tom:"It's a tradition, you know. It's been a long winter, and when the winter's over people go out in the woods and make maple syrup."
The Branons, who savor their life as well as the syrup, find it difficult to event think about a future without their trees.
"We'll probably both live through it," said Cecile. "But I hope our kids can. You know, we've got four sons and we want to be able to see them take over their business."
But Tim Perkins, who lives every day with the disturbing scientific reality, doesn't think the trend can be reversed.
"It would be very difficult, I think, at this point," he said.
"The loss would be really a very large blow to the psyche of the state," he added sadly. "It would affect a large number of sugar makers, obviously, but it would also impact the way that people see themselves as Vermonters. It's an emblem of the state."
ABC News' Wendy Brundige contributed to this report.