Another day, another ludicrous Greenland meltdown scare story.
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 ice sheet and eventually out to sea.
The scientists flying over the world’s largest thawing chunk of ice have selected a particularly auspicious summer to be studying the melt. The edges of Greenland’s 1.7-million-km2 ice sheet regularly melt in summer, even in years when the ice sheet as a whole grows because of snowfall in its higher, colder center. But in 2016, the melting started early and spread inland fast. By April, 12% of the ice sheet’s surface was melting; in an average year the melt doesn’t reach 10% until June. And just before the scientists’ journey, a violent river of meltwater, one of hundreds coursing out from the ice sheet, swept away a sensor, bolted to a bridge to measure the water’s turbidity. It was the second time in 4 years such a device had fallen victim to the liquid fury of the glaciers. “I’ve been doing these trips for years, but I’ve never seen so much water,” the helicopter pilot told the researchers.
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.”
They begin with the early melt last April, but forgot to say that was due to a warm and wet weather front from the Atlantic. NSIDC explained that similar events have happened in the past, and after a few days the melt had stopped.
Over the year as a whole, the accumulated surface mass balance of the Greenland ice sheet was close to the long term average. This year, of course, it is running well above average.
Top: The total daily contribution to the surface mass balance from the entire ice sheet (blue line, Gt/day). Bottom: The accumulated surface mass balance from September 1st to now (blue line, Gt) and the season 2011-12 (red) which had very high summer melt in Greenland. For comparison, the mean curve from the period 1990-2013 is shown (dark grey). The same calendar day in each of the 24 years (in the period 1990-2013) will have its own value. These differences from year to year are illustrated by the light grey band. For each calendar day, however, the lowest and highest values of the 24 years have been left out.
Meanwhile, temperatures last year in Nuuk, close to where the study took place, were the lowest since the 1993.
Apart from the anomalously mild year of 2010, temperatures since 2000 have been similar to the 1930s and 40s.