Winter sea ice extent in the Bering Sea is the lowest it has been for the past 5500 years and will soon be gone completely, according to a study of how it has changed in recent millennia.
“We are essentially locked into a complete loss of winter sea ice in the Bering Sea,” says Miriam Jones, who began the research while at the University of Alaska Fairbanks and is now at the US Geological Survey in Virginia.
The Bering Sea borders the Arctic Ocean, which almost completely freezes over in winter. More and more of its ice has been melting each summer due to global warming, leading to more extreme weather around the northern hemisphere.
Winter sea ice levels can also vary where the Arctic Ocean joins with other bodies of water. In the Bering Sea, located between Alaska and Russia, levels of sea ice in winter have been relatively stable in recent decades. However, they plummeted to half the usual extent recorded over the past 40 years in 2018 and reached similar levels in 2019 too, shocking researchers.
“In those two years in particular, it was very alarming because it was unprecedented,” says Jones.
Her team has been studying oxygen isotopes in peat cores taken from St Matthew Island in the middle of the Bering Sea. Unexpectedly, they found a very strong correlation between the sea’s ice levels in winter and the oxygen isotope ratio in the peat over the past 40 years, the period for which sea ice records exist.
This is because the oxygen isotope ratio reflects the prevailing wind direction – and so where the precipitation falling on the island evaporated – and the wind direction also affects sea ice extent, says Jones.
Since the oldest peat in the cores is 5500 years old, the team was able to infer winter sea ice levels in the Bering Sea going back this far.
The peat cores suggest that there has been a slow fall in winter ice over this time. The long-term decline is due to the wobble in Earth’s orbit, says Jones, which has resulted in the region getting slightly more sunshine during winters.
However, the cores also indicate a correlation between the Bering Sea’s winter ice levels and carbon dioxide levels in the atmosphere, with changes in sea ice lagging several decades behind the CO2 changes. The implication is that CO2 levels are already high enough to cause the complete loss of all the winter ice within decades, with knock-on effects for the Arctic Ocean.
The loss of winter sea ice is bad for local communities, says Jones. Subsistence hunters depend on it, and changes in fish stocks caused by ice loss will affect the region’s large fishing industry. What’s more, coasts will be battered by higher waves.
It will also have broader consequences, speeding up the loss of summer sea ice in the Arctic and so ultimately affecting the entire northern hemisphere.
Journal reference: Science Advances, DOI: 10.1126/sciadv.aaz9588
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