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‘Extraordinarily warm’: winter is fastest-heating season in most of US | Climate crisis

American winters are rapidly warming and December 2021 was no exception.

In New York, last month’s average temperature was 43.8F (6.5C) – 4.7F above the 1991-to-2020 average according to a recent analysis by Climate Central. The American south had an especially warm December, with Shreveport, Louisiana (+13.4F), Dallas, Texas (+13.2F), and Memphis, Tennessee (+12.4F), all posting unusually high temperatures.

“Many places were just extraordinarily warm for this time of year,” said Andrew Pershing, director of climate science at non-profit Climate Central where he and his team document changing weather patterns and release their analysis each month. “Winter is the season when we don’t think about heat the way we do in July or August – this is a sign that we live on a planet that’s changing.”

Interactive map of temperature anomalies across US cities

Although we can’t draw conclusions from one month of data, experts say it is part of a broader trend in which winters are progressively getting hotter. 2021 was the fourth hottest year for the US on record and winter is the fastest-warming season in 38 out of 49 American states, excluding Hawaii, since 1970.

“These are the conditions that we expect to see more and more of,” Pershing said. While some might find milder winters enjoyable and more manageable, cold temperatures are crucial for biodiversity, water supply and farm yield.

Line chart of December temperature changes in the past 50 years

Warming winter months affect the migration of animals, like disease-carrying ticks that are increasingly found in areas that were once too cold for them to survive in.

“They’re emerging earlier in the spring, and they’re staying active later in the fall,” said Theresa Crimmins, the director of the USA National Phenology Network. She is a research professor at the University of Arizona focusing on the relationship between climate and biological life cycles. “That’s a longer period of time that they could potentially be interacting with humans and potentially biting and spreading diseases.”

Shifts in weather patterns also mean that the pollen allergy season is starting 20 days earlier and lasting more than a week longer in North America, compared with the allergy season in 1990. Rising temperatures are disrupting the freshwater supply in much of the western US, with states such as Nevada, Utah and Wyoming experiencing 15-30% decreases of snow-derived freshwater.

“There are a lot of plants that require cold conditions for the fruits to mature,” Pershing said. Apples, peaches, cherries and blueberries are among the many farm crops that need winter chilling hours in order to develop for the spring and summer harvest. With the increased winter temperatures, the chance for these fruits to be adequately cooled is causing premature bloom and quicker ageing.

“Warmer winters and springs are generally resulting in earlier springtime activity [in some plants],” said Crimmins, noting that fruit trees are all at risk of “false spring” – a phenomenon which triggers early flowering.

Although most of the US has seen increased temperatures last month, some cities, such as Los Angeles, Seattle and San Francisco, have seen colder than normal December temperatures.

“Even with climate change, we would expect in a typical month some places in the US to be cooler than normal,” Pershing said. “But this December what really stood out was this [region] right along the Pacific coast with locations that were below normal.”

That is probably attributable to La Niña, a climate pattern condition that occurs in the Pacific Ocean every two to seven years. This anomaly leads to cooler weather and increased snow and rain in the north-west and generally warmer, drier conditions in the south-east.

But despite this cool kick, the nation overall is experiencing an unusually warm winter.

“It’s actually quite remarkable – in general when you have a La Niña condition it’s like you’ve turned the air conditioner on,” Pershing said. “So the fact that we’re in the ballpark of the record warming year suggests that global warming is starting to really overpower some of these natural climate cycles.”

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