Sea ice has emerged as the canary in the coal mine of climate change. Its summer extent in the Arctic has decreased by about 50% over the past decade, and the Arctic Ocean has undergone a regime shift from a cover of thick multiyear ice to a largely seasonal and much thinner ice cover. The recent loss is unprecedented in the periods of satellite and historical records of sea ice, and it also appears to be unique in paleo reconstructions spanning more than a thousand years. A “perfect storm” of warmer atmospheric and oceanic forcing, together with a boost from natural variability of wind forcing in some years, drove the change. However, the reduction of ice coverage is not apparent in some sub-Arctic regions during the winter, nor has it occurred in the Antarctic region.

Signals of a response to the loss of sea ice are emerging in the ocean and the atmosphere. Ocean heat storage during the ice-free season not only contributes to a later freeze-up than in the past, but it also reduces the thickness to which first-year ice can grow. The vulnerability of this thinner ice to rapid spring melt is a manifestation of the ice-albedo-temperature feedback that has long been postulated as a contributor to polar amplification of climate change. More notably for middle latitudes, the loss of sea ice appears to be triggering a reduction of the large-scale westerlies that characterize atmospheric circulation in middle and subpolar latitudes. This response is consistent with increased persistence of departures from normal temperature, precipitation, and extreme weather during autumn and winter in heavily populated areas of the Northern Hemisphere.

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