Ice in Bering Sea Hits Lowest Level in 5,500 Years

A view of St. Matthew Island in the Bering Sea. Isotope analysis of a peat sample from the island allowed researchers to determine sea ice extent in the region for the past 5,500 years. (Credit: NOAA)

(CN) — Climate scientists have found that the Bering Sea experienced lower winter sea ice coverage in 2018 than at any other point in the last 5,500 years.

“In light of the increased amount of concentration of CO2 that has been building up in the atmosphere over the decades, it would be expected that that warming would have an impact on the ice extent,” said lead study author Miriam Jones, U.S. Geological Survey researcher and University of Alaska Fairbanks geologist. “And we now have shown that it has.”

The Bering Sea, located in the portion of the Northern Pacific Ocean that cleaves between Eurasia and North America, accumulates ice over the winter before it all melts by summertime.

Scientists believed the Bering Sea’s winter ice coverage to be relatively stable compared to the nearby Arctic Ocean’s summer sea ice, which has diminished with the rise of greenhouse gas emissions in recent decades. But findings published Wednesday in the journal Science Advances turn this notion on its head.

Jones’ team collected a core of peat cellulose – which contained plant matter dating as far back as 5,500 years ago, according to radiocarbon dating methods – from the remote St. Matthew Island, located in the Bering Sea between the Alaskan and Russian coasts.

“Peat accumulates anywhere where the ground remains saturated. It’s a type of wetlands. As the plants that are living on the surface die, they don’t fully decompose, so you still have a record of those plants, and they accumulate vertically through time,” Jones said in an interview. “What we did is look at the cellulose from those plant remains.”

Jones couldn’t go out to the island herself, so she taught colleague Dave Klein, a University of Alaska Fairbanks wildlife ecologist who was traveling to the island to survey its birds and foxes, how to collect the peat core.

“The cellulose has oxygen in it, and the source of that oxygen, ultimately, is water. The source of the water in the peatlands, ultimately, we found to be precipitation-sourced,” Jones said. “We were able to make this extrapolation about what the atmospheric conditions were doing over that period based on the peat oxygen isotopes.”

Co-author Max Berkelhammer, a climate scientist at the University of Illinois at Chicago, ran an isotope-enabled general circulation model to compare the ratios of oxygen-18 isotopes to oxygen-16 isotopes at different sections of the peat core. This provided a window into the past, allowing the scientists to see how water from different sources made its way to the island’s peatlands.

Isotopically “heavier” precipitation periods originated from the North Pacific Ocean, Jones explained, while isotopically “lighter” periods — marked by an abundance of oxygen-16 compared to oxygen-18 — originated from the Arctic and signal seasons with more ice.

“It turns out the biggest anomaly — differences from the average — that we saw was the result of dominant wind directions. When the wind was primarily coming out of the north, they had a very different isotopic signature than when the wind was coming out of the south,” Jones said.

She added, “We were able to confirm a really strong relationship between dominant wind direction and sea ice extent. The isotopes were lighter when the winds were coming down out of the north, out of the Arctic, and that also corresponded with a greater extent of sea ice, and the opposite was true during years of lower sea ice extent.”

This corroboration is confirmed by data from satellites that have been monitoring the Bering Sea’s ice coverage since 1979.

“The Bering Sea ice zig-zags interannually between higher years and lower years of sea ice, but there’s not been a real trend observed. There’s not this long-term decline,” Jones said, unlike the rapid decline of Arctic summer ice.

So the dip in wintertime Bering Sea ice extent two years ago was exceptional, Jones added.

“2018 was … remarkably lower than the mean, and it was definitely more than twice as low as any previous low-extent year,” she said. “It also retreated really early. The Bering Sea often still has ice, historically, in May. But there was hardly any sea ice left in the Bering Sea, in 2018, by the end of February. It was dramatic: people were raising alarm bells, of course people who live and rely on that sea ice for harvest and things like that were severely alarmed.”

Jones added that while the group did not record the Bering Sea’s ice extent last year, 2019’s coverage seemed very similar to what they observed in 2018.

It may be that the North Pacific Ocean responds easily to small changes in warming factors, suggesting that the loss of sea ice might be lagging decades behind changes in atmospheric greenhouse gas concentrations.

“There could be a lag of a response in the Bering Sea ice extent. What that suggests is that whatever decrease we’re observing now is actually a product of the buildup of greenhouse gas emissions of previous decades,” Jones said. “The decline or loss of sea ice in the Bering Sea could be, potentially, locked in already.”

Declining ice coverage can accelerate warming, as a “dark” Bering Sea would absorb more heat from solar radiation than a sea covered with more white, reflective ice sheets.

“As the Bering Sea becomes more free of ice, how does that then increase heating in the Arctic Ocean basin? We suspect it’s likely going to contribute to an acceleration of the loss of ice,” Jones said when asked about where the group’s findings are guiding her next research enquiries.

She added that it would take a separate research project, and distinct climate science methods, to determine whether the Bering Sea ice extent is lagging behind changes in the concentration of greenhouse gases in the atmosphere, and by how long it may be lagging.

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