WASHINGTON – At the National Arboretum, the white 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. And in Donna Izlar’s back yard in downtown Atlanta, the apricot tree has started blooming.

It’s not in your imagination. The 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. The Washington area is on track for its fourth-warmest year on record.

‘A WEIRD KIND OF FALL’

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.

The pattern is most pronounced in eastern Montana, northeastern Minnesota and parts of North Dakota, 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.

Both 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.

‘NOT CLOSE TO FREEZING’

Temperature anomalies happen for many reasons, and 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. But he added that the shifts in seasonality now on display are in line with the warming the United States has experienced in recent years.

“We’ve seen consistently in the last couple of decades more consistent warm episodes for the season than cold episodes,” Arndt said.

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 historic 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 Jan. 1,” he said. “They’re not close to freezing up.”

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 highbush blueberry, are blooming an average of 10 days earlier because of warmer temperatures.

POTENTIAL THREATS

Primack said it’s easier to detect these changes in the spring compared with 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, Primack added, New England lost almost its entire fall foliage season this year because there were few freezing temperatures in September and October and more rain than usual.

Rather than having the leaves turn color at the start of October, he said, they stayed green until late October, and “in a matter of days, all the trees went from having green leaves to having no leaves.”

While many have welcomed the balmy temperatures, they pose potential threats to both habitat and humans. Western bark beetles, whose reproductive cycles have sped up and numbers have increased because they don’t face the same cold winter temperatures as in the past, have ravaged pine trees out West.

University of Maine researcher William Livingston recently published a study showing that Cryptococcus fagisuga, a different bark beetle species, boomed during warm winters between 1999 and 2002 and has feasted on Maine’s beech trees ever since.

And in the Plains region, changes in vegetation pose a fire threat and can intensify allergies. The Minnesota Department of Natural Resources is requiring a permit to burn vegetation or trash in light of the state’s dry conditions; a recent wildfire burned 750 acres of forest close to the Hangaard State Wildlife Management Area.