Much of the Arctic Ocean is covered by perennial sea ice, but in marginal regions near North America and Eurasia, sea ice melts during summer and reaches a minimum cover each year in September. During the past 20 years, satellite and instrumental measurements have shown that average sea-ice thickness and extent have decreased. Some of the most notable changes have occurred off the coasts of western and northern Alaska in the Bering and Chukchi Seas. 

New research is underway to improve our understanding of how sea ice varied over long time periods and whether recent trends are human-induced or represent one extreme of natural climate variability. 

Instrumental records extend back only a few decades, so they are insufficient to understand natural variability in sea ice, ocean temperature, circulation and ecosystems, or the impacts of a warming climate on the Arctic. Scientists from USGS CLU R&D collaborated with an international team of scientists to reconstruct a history of Arctic sea ice for the past 600,000 years. They used data on past sea ice conditions preserved in a sediment core from Mendeleev Ridge in the western Arctic Ocean. 

The results indicate that perennial sea ice developed about 350,000 years ago during a major global climate transition. Since then, the Arctic has experienced large glacial-interglacial climate oscillations with highly variable sea ice cover during the warm interglacial periods. Notable periods of seasonally sea-ice free conditions occurred at 5,000-10,000, 125,000 and 400,000 years ago. These results demonstrate that Arctic sea ice is extremely sensitive to climate changes and that Arctic Ocean marine ecosystems have experienced repeated short and long-term climatic cycles. 

The paper, published in Quaternary Science Reviews, is available at https://pubs.er.usgs.gov/publication/70157131

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