The Arctic Ocean’s floating sea-ice cover waxes and wanes with the seasons. The icecap grows in the fall when the hours of sunlight shorten and intense cold sets in. When long summer days return, ice floes melt or are driven by wind and ocean currents into the North Atlantic Ocean. A quarter century ago, the coverage ranged from about 7 million to 16 million km2 between late summer and the following March.

Since 1978, when satellites began routinely monitoring the Arctic, the extent covered by perennial ice—that which survives the summer melt—has declined by close to 10% per decade, at least until 2007. In September of that year, the summer ice extent plummeted to a record low 4.2 million km2, down 23% from a previous record low in 2005. The perennial ice lost in those two years alone covered an area almost twice the size of Texas....

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