Barber and his colleagues explain in a recent paper in Geophysical Review Letters, the analysis of what the satellites were seeing was wrong. Some of what satellites identified as thick, melt-resistant multiyear ice turned out to be, in Barber’s words, “full of holes, like Swiss cheese. We haven’t seen this sort of thing before.”

In 2008 and 2009, scientists feared the record-thin Arctic ice cover might melt away.

Exactly when a catastrophic melt might occur, however, is unpredictable. The long-term rise in global temperature as a result of greenhouse-gas emissions is overlaid with natural, year-to-year variability in all sorts of interconnected oceanic and atmospheric cycles that slow down warming down or speed it up temporarily. But because these variations tend to be cyclical, the “perfect storm” of conditions that caused the record 2007 melting — a situation Stroeve calls “unusual, but not unprecedented” — will probably return at some point. If they do, the Arctic could be primed for major, even irreversible, changes.

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