As Arctic sea ice retreats as a result of rising regional air and sea temperatures, scientists are scrambling to understand the effects on Earth’s climate and weather at lower latitudes. An increasingly ice-free Arctic Ocean in summertime has profound implications—some good, some bad—for naval operations, fisheries, commercial navigation, and natural resources development.

Air temperatures above the Arctic Circle are warming at twice the global average. But the warming that occurred there in 2016 is unprecedented: Last year the average air temperature in some parts reached 6 °C above the 100-year average, said Jeremy Mathis, of NOAA’s Arctic Research Office. Sea-surface temperature in August 2016 was 5 °C above the 1982–2010 average in the Barents and Chukchi Seas and off the east and west coasts of Greenland. And where Arctic warming excursions had once been confined to the summer months, temperature anomalies have now propagated to the cold months: Average monthly...

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