By any measure, the Arctic has changed profoundly in the last 40 years, warming three times as fast as the global average, and losing half its summer sea ice, as well as billions of tons of land-based glacier ice.
And even though the Arctic only encompasses about 6 percent of the Earth's surface area, the warming there has kicked off climate chain reactions that are disrupting weather and climate patterns across the mid-latitudes of the Northern Hemisphere, including most major North American and European cities and agricultural areas. The abrupt and accelerating Arctic warming directly harms the communities, livelihoods and traditions of the 4 million people who live in the polar region.
Some scientists say a more frequently recurring cycle they refer to as "warm Arctic, cold continents," is a sign of that disruption. The pattern seemed to emerge as a global warming signal about 10 years ago, as researchers documented an increase of summer and winter extremes in parts of North America and Eurasia, including heat waves, killer blizzards, floods and cold snaps, occurring even as Arctic warming and ice loss accelerated.
A 2018 study in Nature looked at the link between Arctic warming and those extremes by comparing air pressure and temperature patterns with a winter severity index from a climate measurement network that stretched from the Pacific Northwest across the northern United States to Maine and down to Georgia.
The authors said their research showed that, from 1990 to 2016, severe winter weather became more frequent in the Eastern United States "when the Arctic warming trend is greatest and extends into the upper troposphere and lower stratosphere."
"In general we do see the tendency that, when the Arctic is very warm, you're displacing the cold air that is usually over the Arctic to somewhere else," said co-author Jennifer Francis, a climate researcher with the Woodwell Climate Research Center. "As the arctic warms faster we expect to see this more."
Still, there are geographic and seasonal nuances. And the patterns can also change depending on which slice of time is being studied. Pinpointing how Arctic warming affects other regions is a moving target, because the entire global climate system is swiftly moving into uncharted territory.
At the same time, Francis said, "You just can't lose 50 percent of sea ice extent and 70 percent of its volume without seeing an effect.
"We're seeing the Arctic heading off into a never-never land of conditions that we haven't seen before," she said. "As we go off into uncharted territories, you can't expect the models to see the extremes."
And There Are Caveats
Nobody is arguing that Arctic warming has no effect, said University of Exeter climate scientist James Screen, who has worked on several studies focusing on how the linkage manifests in different regions. But the climate models don't show the connection as strongly recorded observations, and it's important to remember that, overall, the entire planet is warming.
"One of the most robust effects of Arctic warming is the reduction of cold extremes," Screen said. "You get cold weather when wind is blowing from the north. Now those cold air outbreaks are less severe. You can see that effect in the observations and models. The bigger picture is, everything is going to get warmer, but the warming effects over continents might be slightly damped."
In a Nov. 16 correspondence in the journal Nature Climate Change, Screen suggested that evidence for mid-latitude impacts of Arctic warming has weakened since 2016.
"The short-term tendencies from the late 1980s through to early 2010s that fueled the initial speculation of Arctic influence have not continued over the past decade," Screen and his co-author, Russell Blackport, with the University of Exeter, wrote. "Arctic amplification and sea-ice loss have indeed continued. But predictions of a more negative Arctic Oscillation, wavier jet stream, colder winters in mid-latitudes or, more specifically, in Eurasia, and more frequent and/or widespread cold extremes have not become reality."
Screen added that "what we're saying, really, is if you take the trends from the 1970s to the present, the trend lines are flat and the models show a muted response.To reconcile that, you have to say the models are wrong. And there is research suggesting that models are underestimating the predictability of things like changes to atmospheric circulation."
The variability of the past few decades, however, doesn't wipe out the previously observed correlation, Screen said. Low sea ice in the Chukchi Sea, north of Alaska, seems to match up with severe winter conditions in Eastern North America, and low sea ice in the Barents and Kara Seas, north of Siberia, match with cold winters over Asia.
The evidence is strongest for a link between sea ice loss and East Asian winter cold, but less robust for North America. And it's also a chicken-and-egg question at some level, with question marks about whether the warming Arctic causes extreme cold; if it does, what the mechanisms are; and whether there are other overriding factors that cause both the Arctic warming and cold outbreaks over the continents.
"What we've seen to date suggests what we could see in the future, and we want to be sure that is in our models, going into projections that people are using for their decisions," he said, adding that is leading a new research consortium called ArctiCONNECT that was set up specifically to explore how Arctic warming will affect the United Kingdom and Europe. The goals are to understand how large the impacts will be, and when and where they might happen.
Thoughts About a Mechanism
Marilena Oltmanns, a climate researcher with the National Oceanography Centre in the United Kingdom, said that one weakness in the models might be that they can't accurately show a specific mechanism that makes the warming Arctic affect the mid-latitudes.
"But the fact that models don't show the mechanism doesn't mean it's not there," she said. "If we have a change in one part of the climate system, it has to affect the other parts, but it's hard to pin down the specific pathway."
One possible explanation is via the ocean, specifically the North Atlantic, where huge surface pools of fresh, cold water accumulating from melting Arctic ice perturb the exchange of heat between ocean and atmosphere. That can trigger cold outbreaks in the Northeast, as well as extreme rainfall and flooding in the UK and northwestern Europe.
Oltmanns demonstrated that connection in a June 2020 study published in Geophysical Research Letters, and said that, when such mechanisms are included in new climate models, the effects of Arctic warming on the mid-latitudes would be easier to see. But she also said that even the best current studies only look at a relatively short time frame of several decades, so it's still hard to separate global warming effects from natural climate cycles.
MIT climatologist Judah Cohen, who is director of seasonal forecasting with Atmospheric and Environmental Research, a weather and climate-risk consulting firm, mentioned the historic October 2020 North American cold wave as a possible example of the links between a warming Arctic and cold extremes in North America.
The cold snap came after months of record-setting heat in the Arctic, including the first-ever 100 degrees Fahrenheit temperature in the Arctic, recorded last June in Siberia.
Hundreds of monthly low temperature records were set, and the wave of polar air sent the national average temperature for the month plummeting to 1.77 degrees below average. Idaho reported its coldest October on record. Several other states, from Washington east to North Dakota and south to Oklahoma, experienced near record cold, and on Oct. 25, Potomac, Montana recorded minus 29 degrees Fahrenheit, the coldest October temperature ever recorded in the continental United States.
Cohen studies the link between Arctic warming and extremes of cold to make accurate seasonal forecasts for the winter that can help communities prepare for potentially unusual or unexpected conditions. He was a co-author of the 2018 paper with Jennifer Francis that documented the correlation and said the findings were statistically robust, but said it didn't prove a cause and effect.
He's convinced there is a connection, but fully understanding it will take more research and a longer period of observations. And Cohen acknowledged the geographical variations. From the 1980s through 2013, the warming Arctic correlated with cooling in Eurasia, but then the pattern flipped, with cooling in North America, he said.
"I'm not arguing that winters are getting colder, I'm saying winters are colder than the models predict, and I think that will continue," he said. "The models are constantly being updated, every winter and the divergence between their projections and the observations is striking." By contrast, he added, the model predictions for summers are nearly perfect.
Cohen said he considers complex atmospheric movements involving the polar vortex to be a key link between the warming Arctic and extreme cold events in North America and Eurasia. The polar vortex is a belt of winds around the Arctic that keeps cold air bottled up if it's tight, but spills frigid air masses southward when disrupted.
That disruption happens, he said, when the warm Arctic air works its way high into the upper atmosphere, where it crests like a wave to break through the vortex.
"It's getting increasingly difficult to get severe winter weather into the mid-latitudes without a polar vortex disruption," he said. "And amplified Arctic warming is favorable for disrupting the polar vortex."
Francis, of the Woodwell center, added that it's important to remember that the overall hypothesized impact of amplified Arctic warming is "to favor an increase in the persistence of weather conditions, including cold spells, heat waves, dry periods and storminess, all of which can be disruptive if they last long enough."
Those patterns, she said, can last a week to several weeks and they can flip suddenly: "The whiplash from a record-breaking heatwave to cold and snow that occurred in the western states (particularly Rockies) this fall was a great example."