For the past several years, one of the most hotly debated questions in climate science has been determining exactly how rapid Arctic warming, and associated sea ice loss, is affecting the weather thousands of miles away, in parts of the United States, Europe and Asia.
Numerous studies that have focused on Arctic warming and cold mid-latitude winters, in a configuration known as the "warm Arctic, cold continents" pattern, have generally concluded that sea ice loss at the top of the world is instigating a chain reaction throughout the atmosphere, altering the weather thousands of miles away. Such studies do this by looking at statistical patterns, finding strong correlations between Arctic warming and unusual mid-latitude weather, such as the polar vortex winter of 2013-2014.
Yet other studies using computer models and physical science data have been unable to match these results, instead finding that either flaws exist in the computer models or sea ice may not be having such a large influence beyond the Arctic itself.
Research published this week in Nature Climate Change combines observations over the past 40 years with results from climate modeling experiments. Scientists found that both sources of data show that reduced regional sea ice and cold winters coincide but that one does not necessarily cause the other.
The regions they examined included the Barents-Kara Sea and East Siberian-Chukchi seas, which have been implicated by previous studies in causing more Arctic air to flow into North America and parts of Asia during the winter.
This is not an esoteric battle being fought in scientific journals and academic conferences, though there is a fair share of back-and-forth occurring in both venues. Rather, it’s something that scientists are urgently trying to untangle given the swift pace of Arctic climate change. Right now, the Arctic is in the throes of an extraordinary melt season, with record low Arctic-wide ice extent, and no ice at all in Alaska waters as of early August.
Sea surface temperature departures from average across the region are so significant that it’s likely there will be an unusually late fall freeze up, particularly in the Barents and Chukchi seas. This could provide a crucial test case of the hypotheses put forward that what happens in the Arctic does not stay in the Arctic.
Outbreaks of extreme cold and heavy snows in the Mid-Atlantic and Northeast, for example, have been tied in part to Arctic ice loss. Some of this research relates to a well-publicized hypothesis that Arctic warming is leading to a "wavier" jet stream and more frequent stuck weather patterns. Other research has shown that losing ice in the summer increases fall snowfall in northern Siberia, which can affect air currents aloft by disrupting the stratospheric polar vortex, which can dislodge pieces of the vortex and direct them southward, into the United States and Eurasia.
For the new study, the scientists found that unusual atmospheric circulation patterns simultaneously drive cold mid-latitude winters and mild Arctic conditions, with sea ice loss having "a minimal influence" on severe mid-latitude winters.
Specifically, the new research, from atmospheric scientists in the United Kingdom and The Netherlands, finds that the same atmospheric circulation patterns that give rise to severe mid-latitude winter weather also serve to transport relatively mild air into parts of the Arctic, reducing ice cover.
In other words, the pattern bringing the snow and cold to places such as D.C., New York and parts of northeast Asia is also eating away at sea ice, rather than the other way around.
For the study, the researchers used different methods to tease out the causal factors, focusing closely on the question of which comes first, Arctic ice loss or mid-latitude cold.
Study co-author James Screen, a researcher at the University of Exeter, says the study relies on three main lines of evidence to conclude that cold mid-latitude winters are coincident with Arctic ice loss. The first is an examination of heat flow in the Arctic, second is the time sequence of events and third is climate model experiments using reduced ice cover.
"We found the direction of heat flow during cold events was predominantly from the atmosphere to the ocean, suggesting sea ice loss as a consequence not a driver," Screen said in an email. "We found cold events, and the circulation patterns that cause them, set up before the reduced sea ice, again pointing to sea ice loss as a consequence of circulation changes not a driver of them. Lastly, climate model experiments with reduced sea ice showed no circulation change or cooling."
Jennifer Francis, an atmospheric scientist at the Woods Hole Research Center, says the study doesn't convincingly rule out Arctic climate change as an instigator in mid-latitude cold outbreaks. Francis has published numerous studies on this topic.
"They confirm that recent colder winters in eastern North America and east Asia occur in sync with sea-ice loss in Arctic seas west of these areas, while convincingly showing that ice loss alone is not the main cause," Francis said via email.
"While this study helps unravel the knotted web connecting Arctic change and weather patterns, it does not address a variety of factors that could explain the recent warm Arctic/cold continent patterns. One is that sea ice is much thinner now, so it's more easily melted and blown northward by the wind. A warm, northward wind would have had little effect on the thick ice of only 20 years ago," said Francis, who was not involved in the new study. She pointed to increasing signs that sea ice loss can disrupt the polar vortex, which can disrupt weather patterns across the Northern Hemisphere during the winter.
"When it comes to connections between the Arctic and mid-latitude weather, both chickens and eggs are still fair game," she said.
Likewise, James Overland, an oceanographer with the National Oceanic and Atmospheric Administration who also was not involved in the new study, says regional sea ice loss may serve to reinforce wavy jet stream patterns, thereby helping to dislodge cold air from the Arctic and direct it into the Lower 48 states.
“Recent delay in freeze up of sea ice in early winter north of Alaska and in the Barents Sea can help reinforce, not cause, a wavy jet stream pattern,” he said via email.