Sea-ice algae from 6,000 years ago lower Arctic winter ice projections by 1 million square kilometers

Climate models disagree with each other most in winter ^[2], so the team needed an independent check on them. It found one in ocean sediment.

Algae that live in sea ice make the biomarker IP25 ^[5]. Read alongside markers from open-water organisms, the levels in a core give an estimate of how much ice covered the water ^[7]. The cores date to the mid-Holocene, when shifts in Earth's orbit made the Arctic warmer and cut ice cover ^[6]. Each of the 27 models had simulated that period, and the weighting rewards the ones that match the sediment record ^[8].

The test of whether the weights do anything useful came on the recent past. "When we apply the model weights to historical simulations, we completely eliminate the positive bias," said lead author Crystal Fu, of Cambridge's Scott Polar Research Institute ^[10].

The result depends on the assumption that weights fitted to mid-Holocene warming apply to modern warming. Mid-Holocene warmth came from orbital shifts ^[6], and the Arctic warmed then for different reasons than it does now ^[13]. The authors argue the weights still apply because the same physical processes govern sea ice across different climates ^[8]. "It doesn't matter if the warming is natural variation or caused by human activity: if it warms, sea ice is going to melt," Fu said ^[12]. We think that is a fair statement about how ice responds to heat. Whether a model's winter errors 6,000 years ago match its errors in a future climate is a separate question, and the weighting depends on the answer.

The 1 million square kilometer figure measures ice area, and the report does not translate it into weather. Phys.org's report says winter ice loss has been linked to colder winters and heavier snowfall in the U.K., northern Europe and North America ^[3]. The authors say the results could have implications for winter conditions in the U.K. and across Europe ^[14]. North America appears only in that background link ^[3]. The study's own downstream result concerns precipitation: the researchers say they found hints that snowfall over southeast Greenland and rainfall in the Mediterranean are underestimated ^[15].

The report also does not give the baseline extent, the emissions scenario, the time horizon of the 1 million square kilometers, or the range the 85% is measured against ^[1][11].