Thursday, September 3, 2020

Bering Sea winter ice shrank to its lowest level in 5,500 years

Sea ice in the Bering Sea, on the southern margin of the Arctic Ocean, dwindled to its smallest wintertime expanse in 5,500 years in 2018, new data show.  

Summertime sea ice loss due to climate change has captured headlines, but winter ice in the region has also shown recent signs of decline. In both February 2018 and February 2019, the extent was 60 to 70 percent lower than the average February-to-May extent from 1979 to 2017. However, researchers thought that those declines might be linked to unusual short-term atmospheric conditions.

Instead, the new study suggests that human-caused climate change is also helping to shrink Bering Sea ice during the winter. The findings, by geologist Miriam Jones of the U.S. Geological Survey in Reston, Va., and colleagues, were published September 2 in Science Advances.

Jones and her team collected cores of peat from St. Matthew Island, a remote spot in the Bering Sea west of Alaska. Within the peat — packed remains of partially decomposed plants — oxygen-bearing organic compounds called cellulose contain clues to the climate history of the region.

Rain falling on the island contains two different isotopes, or forms, of oxygen: oxygen-18 and oxygen-16. The relative values of those isotopes in the rainfall change depending on atmospheric conditions, and as plants take up that oxygen from the air, they record those changes. By analyzing the amounts of those isotopes in the cellulose over time, the team was able to track changes in precipitation and atmospheric circulation going back 5,500 years.

Then, the team established the link between this oxygen isotope record and sea ice extent.

Bering Sea ice is known to be directly tied to shifts in wind direction. So the researchers created a computer simulation that included climate conditions from 1979 to 2018, oxygen isotope values from cellulose during that time and satellite observations of sea ice. When winds were strongly blowing from the south, and there was less sea ice, the relative amount of oxygen-18 increased. When winds from the north dominated, and there was more sea ice, there was less oxygen-18 in the cellulose.

Next, the researchers used the oxygen isotopes in the peat to track the waxing and waning of the region’s sea ice over thousands of years. Most of the area’s rainfall occurs in winter and spring, so those oxygen isotopes are indicative of conditions between February and May, rather than summer. The peat cellulose oxygen-18 values recorded in winter 2018 were the highest, and the sea ice extent the smallest, in the last 5,500 years, the team found. 

Climate change due to CO2 and other climate-warming gases has already taken a visible toll on summertime sea ice in and around the Arctic; within 10 to 15 years, the region may be ice-free during the warmer months. Arctic sea ice in September 2019 tied for second-lowest on record with 2007 and 2016; first place still goes to 2012. The loss of that ice is not just a bellwether for climate change in the Arctic, but is also speeding up the rate of warming in the region, a process called Arctic amplification. And the missing summer ice is also triggering a cascade of changes through Arctic ecosystems, including within the Bering Sea.

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