The great Greenland meltdown

From a helicopter clattering over Greenland’s interior on a bright July day, the ice sheet below tells a tale of disintegration. Long, roughly parallel cracks score the surface, formed by water and pressure; impossibly blue lakes of meltwater fill depressions; and veiny networks of azure streams meander west, flowing to the edge of the sheet and eventually out to sea.

In Greenland, the great melt is on. The decline of Greenland’s ice sheet is a familiar story, but until recently, massive calving glaciers that carry ice from the interior and crumble into the sea got most of the attention. Between 2000 and 2008, such “dynamic” changes accounted for about as much mass loss as surface melting and shifts in snowfall. But the balance tipped dramatically between 2011 and 2014, when satellite data and modeling suggested that 70% of the annual 269 billion tons of snow and ice shed by Greenland was lost through surface melt, not calving.

The accelerating surface melt has doubled Greenland’s contribution to global sea level rise since 1992–2011, to 0.74 mm per year. “Nobody expected the ice sheet to lose so much mass so quickly,” says geophysicist Isabella Velicogna of the University of California, Irvine. “Things are happening a lot faster than we expected.”

It’s urgent to figure out why, and how the melting might evolve in the future, because Greenland holds the equivalent of more than 7 m of sea level rise in its thick mantle of ice. Glaciologists were already fully occupied trying to track and forecast the surge in glacial calving. Now, they are striving to understand the complex feedbacks that are speeding up surface melting.

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