During the first half of the 20th century, the Arctic sea-ice cover was thought to be in a near-steady seasonal cycle, reaching an area of roughly 15 million km2 each March and retreating to 7 million km2 each September. Ice thick enough to survive the melt season, termed perennial or multiyear ice (MYI), adds to the ice cover. A large fraction of MYI typically remained in the Arctic Basin for several years and grew to an equilibrium thickness of about 3.5 m—melting half a meter at the surface from June through August and growing by about half a meter at the bottom from October through March. In the late 1970s, MYI occupied more than two-thirds of the surface area of the Arctic Basin, with first-year ice (FYI) covering the remaining one-third. FYI is the thinner, seasonal ice that fills cracks in the ice cover and grows on the...

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