WASHINGTON -- Unusually mild temperatures across several regions of the country in the past few months are disrupting the natural cycles that define the winter landscape.
What began as elevated temperatures at the start of fall in parts of the United States have become "dramatically" warmer around the Great Lakes and New England, according to Deke Arndt, chief of the Climate Monitoring Branch at the National Oceanic and Atmospheric Administration's National Climatic Data Center.
That, in turn, has created conditions where plants are blooming earlier and some birds are lingering before moving south.
"It's a weird kind of fall blending right into spring," said Scott Aker, head of horticulture at the National Arboretum, where petals of snowdrops, normally an early spring flower, have unfurled.
In Maine's Acadia National Park, lakes still have patches of open water instead of being frozen solid.
The pattern is most pronounced in eastern Montana, northeastern Minnesota, and parts of North Dakota, Mr. Arndt said, where December temperatures so far have averaged 10 degrees above normal.
But the mild weather extends to other Great Lakes states along with New England and the mid-Atlantic, with temperatures this month averaging between six and eight degrees above normal.
Just 19.6 percent of the continental United States is now covered in snow, according to the latest snow analysis by NOAA, compared with 50.3 percent this time last year.
Scientists and those who question dire global warming predictions emphasize that one warm season should not be interpreted as a broader sign of climate change.
"It's about long-term trends, and one year does not make a trend," said Doug Inkley, a senior scientist at the National Wildlife Federation in Reston, Va.
Temperature anomalies happen for many reasons, and Mr. Arndt said some of the mild weather stems from a persistent ridge of high pressure that has settled over the eastern third of the country, pumping up south winds in many areas.
He added that the shifts in seasonality now on display are in line with the warming the United States has experienced in recent years.
The decreasing frequency of cold snaps should not lead anyone to conclude that there is dramatic warming across the globe, said Patrick Michaels, a senior fellow in environmental studies at the libertarian Cato Institute.
Climate change is happening, he said, but not at the "magnitude" that some suggest.
Some researchers have detected warming trends in key habitats, but incomplete history records make it difficult to measure these changes with precision.
Abe Miller-Rushing, science coordinator for Acadia National Park, said it's clear that when the lakes freeze no longer comports with conventional wisdom.
"They typically freeze by January 1," he said. "They're not close to freezing up."
Mr. Miller-Rushing collaborated with Boston University biology professor Richard Primack to examine how the seasonality of plants and animals in Concord, Mass., has shifted since the 1850s, when naturalist Henry David Thoreau recorded their spring patterns with precision.
They found that plants, including the high bush blueberry, are blooming an average of 10 days earlier because of higher temperatures.
Mr. Primack said it's easier to detect these changes in the spring than in the fall, when a combination of temperature, precipitation, and day length governs plant behavior.
"There's a climate change signal, but it's much more complicated," he said.
Still, Mr. Primack added, New England lost almost its entire fall foliage season this year because September and October had fewer temperature readings of freezing or below and more rain than usual.
Rather than turning color at the start of October, he said, leaves stayed green until late October, and "in a matter of days, all the trees went from having green leaves to having no leaves."
And in the Plains region, changes in vegetation pose a fire threat and can intensify allergies.
Scientists have found that a roughly one-month delay in the onset of the first frost in the Central Plains allows more time for ragweed pollen production, which has extended the region's allergy season.