When ice failed to cover much of the eastern Bering Sea between Alaska and Russia in early 2018, oceanographer James Overland chalked it up to a freak chance. Then, it happened again this year, with late-winter sea ice falling to some of the lowest levels seen in at least 4 decades.
Now, scientists are studying whether this is the meteorological equivalent of drawing the ace of spades twice in a row, or another sign of the systemic changes sweeping the Arctic as a result of climate change. "I'm not ruling out that we really have a new regime over the Bering Sea," says Overland, who works at the National Oceanic and Atmospheric Administration's (NOAA's) Pacific Marine Environmental Laboratory (PMEL) in Seattle, Washington.
A lasting shift could dramatically transform a region with some of the nation's most valuable fisheries and indigenous communities whose way of life relies on ice. Already scientists have documented changes in algae as well as zooplankton, fish, and seabird populations. The shifts are "a bit of a warning sign that these things can happen rather quickly," says Robert Foy, the Juneau-based science and research director for NOAA's Alaska Fisheries Science Center, which is headquartered in Seattle.
In March of most years, ice extends from the Bering Strait southward almost to the Aleutian Islands, covering an expanse greater than the state of Texas. But in the past two winters, ice cover bordering Alaska peaked at the lowest levels seen since routine satellite monitoring began in 1978. In 2018, the coverage was 50% of the average high. This year it was 65%.
The decline came as a surprise. Some computer climate models had suggested the northern Bering Sea would remain covered in winter ice for decades to come. Instead, unexpected wind shifts and rising water temperatures could be accelerating ice loss. Historically, cold winter winds blowing from the north drove ice south from the sea's northern reaches, says Phyllis Stabeno, a physical oceanographer at PMEL. But in each of the past four winters—and especially the past two—winter winds have instead blown from the south for a month or more, bringing warmer air and pushing against the ice, slowing it's southward advance. "We're beginning to scratch our head and say, ‘Is it random? … Or has something shifted?’" Stabeno says.
One possibility is that a warming climate is changing the polar jet stream—the powerful river of air that flows around the Arctic from west to east—by making it more "wavy," apt to meander north and south. A wavier jet stream has been blamed for shifts in the "polar vortex" that brings icy weather to the northeastern United States, and it might also favor warm, southerly winds over the Bering Sea, Overland says. But other scientists have argued the data don't yet point to clear connections between climate change and the jet stream.
The ice retreat could also reflect an unexpected warming of the Bering Sea, says Seth Danielson, an oceanographer at the University of Alaska in Fairbanks who tracks Bering Sea temperatures. In early 2018, temperatures in the northern Bering Sea were as much as 2°C higher than normal, and Danielson expects the data to show the warm trend continued this past winter. He suspects a troubling feedback: Low ice coverage in 2017 would have allowed open water to absorb more heat, paving the way for even more ice loss in later years. He estimates that every additional degree delays the start of ice formation by 3 weeks. "It doesn't take too many degrees of warming to severely cut down the number of days of ice," he says.
Regardless of whether they prove lasting, the ice changes are already rippling through the ecosystem. Sea ice is home to the algae that underpin much of the food web in the northern Bering. The algae bloom in 2018 was small, as was the number of zooplankton feeding on it, according to NOAA. The small forage fish that feed on the zooplankton in the sea's northern reaches were scarce as well, and people in the region reported an unusual die-off of common murres, a seabird that feeds on these fish.
A massive "cold pool" of water in the central and northern Bering normally confines commercially valuable fish such as walleyed pollock and Pacific cod to the southern part of the sea. In 2018, the pool was the smallest ever seen, presumably because of the lack of winter ice. Research ships caught fewer fish than normal in the usual fishing grounds. With the cold pool virtually gone, it appears many of the fish had sped north.
If such changes persist, they could scramble one the most lucrative U.S. fisheries. Boats in the southeastern Bering Sea haul up a cornucopia of seafood: king crab, halibut, turbot, and cod. The pollock fishery—the region's largest—alone produced $1.3 billion worth of fish in 2017. If the fishery shifts north, existing fish-processing factories could be stranded too far from the fishing grounds to be economical, Foy notes.
Native Alaskans living at the edges of the Bering Sea, meanwhile, already know what less ice can mean. Thick sea ice typically serves as a road to reach winter crabbing grounds and to hunt walrus and other marine mammals. But recent changes have made travel more treacherous and hunting more sporadic. Around Little Diomede Island, which sits in the Bering Strait, ice has formed later and less regularly in the past 6 years, says Opik Ahkinga, environmental coordinator for the native village of Diomede. In February 2018, fierce winds from the south pounded the island, something she hadn't witnessed before. That year she made her first holes in the ice for crabbing in late March. But just 2 weeks later, the ice broke away, she wrote in an email. "Hearts were broken."