Arctic Winter Ice Could Shrink Much More Than Scientists Expected

Keypoints:

• A new study in Nature Geoscience suggests Arctic winter sea ice could shrink by about 1 million square kilometers more than earlier projections
• Researchers used ocean sediment clues from about 6,000 years ago
• They tested 27 climate models and found they tended to overestimate winter ice
• The method narrowed the uncertainty in projections by as much as 85%
• Arctic sea ice this September tied for the 10th lowest minimum on record

The Arctic is losing its summer ice, and everyone can see that. Winter ice is harder to predict, and it matters because it controls how much heat escapes from the ocean into the air. A new study suggests that much more of it could disappear than climate models have been telling us.

The researchers looked back about 6,000 years, to a warm stretch called the mid-Holocene. At that time, slow changes in Earth's orbit shifted the sunlight that reached the Arctic, and the sea ice shrank. Scientists can read that history in ocean mud. Tiny algae that live in sea ice leave behind a chemical marker called IP25, and it ends up preserved in sediment cores.

The team collected those marker measurements and compared them with simulations from 27 climate models of the same period. The results were clear. On average, the models predicted more winter sea ice than the evidence shows was really there. Once the researchers gave more weight to the models that matched the past best, they found the Arctic could lose about 1 million square kilometers more winter ice than earlier projections suggested. That is roughly 386,000 square miles. The approach also narrowed the range of uncertainty by as much as 85%.

You might ask whether a natural warm period can really tell us about today. Co-author Marco Aquino-López said using evidence from the past gives clearer maps of where winter ice is likely to vanish. Lead author Chaoyuan Fu has said it does not matter whether warming is natural or caused by people, because if it warms, sea ice melts.

The study was published in Nature Geoscience on October 16. The authors point out that it does not predict an exact date. What it does is make the forecasts more reliable.

The news fits what satellites are already showing. This year the Arctic's September minimum tied for the 10th lowest on record, and Antarctica's winter peak was the third lowest. Both poles are running low at the same time, which scientists are still working to explain.