Across much of America and especially in the normally chilly north, the country went through the winter months without, well, winter.
In parka strongholds Burlington, Vermont, and Portland, Maine, the thermometer never plunged below zero. The state of Minnesota called the last three months "the lost winter," warmer than its infamous "year without a winter" in 1877-1878. Michigan, where mosquitos were biting in February, offered disaster loans to businesses hit by a lack of snow. The Great Lakes set records for low winter ice, with Erie and Ontario "essentially ice-free."
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For a wide swath of the country from Colorado to New Jersey, and Texas to the Carolinas, spring leaves are arriving three to four weeks earlier than the 1991-2020 average, according to the National Phenology Network, which tracks the timing of plants, insects and other natural signs of the seasons.
"Long-term warming combined with El Nino conspired to make winter not show up in the U.S. this year," said Yale Climate Connections meteorologist Jeff Masters, who co-founded the private firm Weather Underground. Masters said he was bitten by a mosquito in Michigan this year, which he called crazy.
On Friday, the National Oceanic and Atmospheric Administration confirmed that the winter of 2023-2024 was the warmest in nearly 130 years of record-keeping for the United States. The Lower 48 states averaged 37.6 degrees (3.1 degrees Celsius), which is 5.4 degrees (3 degrees Celsius) above average.
This is just the latest in a drumbeat of broken temperature records, national and global, that scientists say is mostly from human-caused climate change from the burning of coal, oil and gas.
And it was the warmest U.S. winter by a wide margin. The past three months were 0.82 degrees (0.46 degrees Celsius) warmer than the previous record set eight years ago, which "is a pretty good leap above the previous record," said Karin Gleason, chief of monitoring at NOAA's National Centers for Environmental Information.
Last month was only the third-warmest February on record. But Iowa blew past its warmest February by 2 degrees, while parts of Minnesota were 20 degrees warmer than average for all of February, Gleason said.
On Feb. 11, Great Lakes ice cover hit a February record low of 2.7%.
A strong ridge of high pressure kept the eastern United States warm and dry, while California kept getting hit with atmospheric rivers, she said.
The European climate agency Copernicus earlier this week said it was the warmest winter globally, mostly due to climate change with an added boost from a natural El Nino, which alters weather worldwide and provides extra heat.
In the past 45 years, winter has warmed faster in the United States than it has worldwide, with the Lower 48 states' winters now averaging 2.2 degrees (1.2 degree Celsius) warmer than in 1980, according to an analysis of NOAA data by The Associated Press.
That's probably because land warms faster than ocean with much of the United States as land and most of the globe as ocean, Gleason said.
While it is still getting warmer in the United States, since 2000 the rate of extra warming has slowed a bit, NOAA data shows. Winter weather expert Judah Cohen of Atmospheric Environmental Research, a commercial firm outside Boston, blames Arctic Amplification, which is how climate change has made the Arctic warm three to four times more than the rest of the globe and seems to shift weather patterns further south.
As the Arctic warms faster, the jet stream — which moves weather systems across the Earth — wobbles and weakens. That means the cold air trapped at the top of the planet, called the polar vortex, escapes its normal confines and drifts elsewhere, bringing short plunges of frigid air that temporarily counteracts the overall warming trend in places, Cohen said.
That happened briefly in January when winter "just made a cameo appearance in the Lower 48," Cohen said. But most of the time this year, when the polar vortex went wandering it hit Europe or Asia with bursts of icy air, not the United States, so there wasn't an offsetting effect on winter American temperatures, he said.
Boston never even got a sniff of single digit temperatures this year, with a winter low of 14 degrees, a record for lack of deep cold.
And snow? Forget about it, at least in the east and north.
In Fort Kent, in far northern Maine, lack of snow canceled an annual dog sled race. The town had had 46.8 inches (119 cm) of snow this year as of last week, a bit more than half as much as usual, the National Weather Service said.
Snow cover in the United States in February was the second lowest on record and third lowest in December, with only January above normal, according to the Rutgers Snow Lab.
There's consequences to warm winters, said Theresa Crimmins, director of the National Phenology Network.
"Warm winters can also lead to earlier, longer, and more abundant pest seasons, because populations weren’t knocked back by cold," Crimmins said in an email. "As well, the allergy season can be worse — starting earlier, lasting longer, and resulting in more pollen in the air."
Because it's warmer, trees and flowers may bloom early. Washington's cherry blossoms are predicted to peak about two weeks earlier than they did in 2013. Early blossoming can screw up intricate timing with pollinators and birds.
"Many of the birds that migrate south for the winter use day length as a cue to come north in the spring," Crimmins said. "In years like this one, where plant and insect activity is cued to start much earlier than usual, the birds can miss out on peak food availability by arriving too late. "
But there is some good news for California with atmospheric rivers and snowstorms likely to rebuild snowpacks and fill in reservoirs that had been dangerously low until a couple years ago, Gleason said.
Winter weather expert Cohen, who is based outside of Boston, joked that the U.S. no longer has four seasons: "We have two seasons. We have summer and we have November."
