Forsythias are one of the first flowering plants to bloom in the spring — often signaling the change of season.
Lawrence Connolly, a resident of Boston for 61 years, first spotted flowering forsythia in his backyard in West Roxbury last week, after the temperature had surpassed 60 degrees for two days in a row.
“I’ve never noted Forsythia blooming in December before, I did think it unusual,” he wrote in a message. Others in New England also have observed the blooming flower.
Nicknamed the Easter tree, the woody shrubs first produce bright yellow bell-shaped flowers before their leaves emerge. They retain their leaves until late fall and go dormant in winter until milder weather returns in March or April.
“Those are plants that should be putting on their flowers in the spring, after winter, not before,” said Theresa Crimmins, the director of the USA National Phenology Network and a research professor at the University of Arizona.
December temperatures are traditionally near or below freezing in the northern and central United States, but much of the country has been hit with repeated warm spells — temperatures up to 30 to 40 degrees above normal.
Forecasts show temperatures reaching the 70s in the Midwest on Christmas Eve and the 60s in the Mid-South and Mid-Atlantic on Christmas Day.
This unusual winter warmth isn’t a one-off, though.
A recent analysis by Climate Central shows that winter in the United States is warming faster than any other season. Since 1970, average winter temperatures have increased one degree or more in every state, while 70 percent have seen increases of at least three degrees.
States experiencing the fastest winter warming are in the Northeast and the Great Lakes region.
Other studies have shown the length of winter season shrinking globally as well. From 1952 to 2011, winter shrank by at least 2.1 days per decade on average. By 2100, winter could be less than two months and could start a half-month later.
“What it really boils down to is those kinds of consequences hit us either economically or from a recreation or lifestyle perspective,” Crimmins said. “When crops are more vulnerable, we can’t get plants and food crops that we expect to get anymore.”
Crimmins mentioned the occurrence of false springs, or when temperatures increase much earlier than expected for an extended period, typically in late winter or early spring. Certain plants respond to the increase in temperature and sometimes bloom, but only to be hit with a detrimental frost that can wipe out the plant for the season.
“If those flower buds get hit by frost, they don’t regenerate that spring, and they don’t regenerate for the whole year,” Crimmins said. “There’s been in recent years some really devastating impacts where we’ve had early warmth, followed by frosts and then total loss of crops.”
Changes in the blooms of fruits and plants can affect other links in the food chain. For instance, many migratory birds travel north according to the movement of the sun. If plants bloom earlier or insects move because higher temperatures occur earlier, the birds may arrive when most of their food is no longer abundant.
Crimmins said the higher temperatures can affect hibernation behavior in mammals as well as insects that overwintered as eggs or larvae in the soil. It also affects frogs and turtles that have buried themselves in mud.
Warmer winters also don’t suppress disease-carrying insects as effectively as colder winters. Mosquitoes and ticks can move to new regions that were too cold to live previously, spreading diseases more widely.
“The biological parts of the [Earth] system don’t look at the calendar,” Crimmins said. “They are responding to what’s happening in temperature and the moisture availability for the most part.”
Despite the warming winter trend, some places still will experience extreme or record cold. In February, polar air spilled south to produce a fatal freeze in Texas. Additionally, the Dakotas and Montana have experienced some recent winters colder than were observed in the 1970s.
Across the eastern United States, Climate Central found that cold weather still will occur in the coming decades, although cold snaps have become shorter and less frequent recently.
“Though we’re seeing warmth now, and though we’re seeing some anomalous plant responses, it’s too early to know if it’s going to be a totally off-the-rails spring — with no winter at all,” Crimmins said.