Arctic Winter Sea Ice Could Be Far Worse Than Models Predict

A new study published in Nature Geoscience has used evidence from a warm period approximately 6,000 years ago to test — and find wanting — 27 climate models. Researchers extracted chemical biomarkers produced by algae and preserved in Arctic ocean sediment cores deposited during the mid-Holocene, when slow changes in Earth's orbit altered the distribution of sunlight and reduced sea-ice cover in the Arctic. Those biomarker records allowed the team to reconstruct actual winter sea-ice conditions at the time. When they compared those reconstructions with what the 27 models had simulated for the same period, the models consistently overestimated how much winter ice was present. The implication is significant: if the models are too optimistic about past conditions, their projections for the future may be as well. By using the past reconstructions to adjust the relative weight of each model, the researchers calculated that Arctic winter sea ice could shrink by roughly 621,000 square miles — around 1 million square kilometres — more than previously projected. While the summer retreat of Arctic sea ice has long been the most visible sign of climate change, winter ice matters too: it controls how much heat escapes from the ocean into the atmosphere, with effects that reach far beyond the polar region. Co-author Marco Aquino-López, a researcher at the Centro de Investigación en Matematicas in Mexico, said in a Cambridge University statement: "By using evidence from the past, we can now produce more reliable maps of where Arctic winter sea ice is likely to disappear."