Feb 22, 2017

Giant crack in Antarctic ice shelf spotlights advances in glaciology

Jeff Tollfeson
Nature News & Comment

...The speed at which glaciers connected to Larsen A and B flowed to the sea increased — by up to a factor of eight — after those ice shelves disintegrated, says Eric Rignot, a glaciologist at the University of California, Irvine. “Some of [the glaciers] have slowed down a little bit, but they are still flowing five times faster than before,” he notes. Khazendar and his colleagues have also found that two glaciers flowing into Larsen B started to accelerate before its collapse, as the ice shelf weakened.


..Larsen C, which covers 50,000 square kilometres with ice up to 350 metres thick


The glaciers that flow into Larsen C contain enough water to raise the global sea level by about a centimetre — and they are likely to flow faster to the ocean in the absence of an ice shelf. In comparison, global sea levels are rising by about 3 millimetres a year, and a recent study estimated that one-third of that comes from ice loss in Antarctica and Greenland

Satellite images show that Larsen C has been receding since the 1980s, and radar measurements suggest that its ice is also thinning, Rignot says. Scientists have also seen meltwater ponds forming on the ice shelf’s surface 4; the same sort of ponds probably hastened the disintegration of Larsen B by carving holes in the ice and expanding cracks.

The ice sheet is protected, to some degree, from rapid collapse by favourable seafloor geometry. A pair of underwater ridges that surround Larsen C create friction that slows the flow of ice to the ocean.

Still, the parallels with the decline of Larsen B are striking, says Adrian Luckman, a glaciologist at Swansea University, UK, who heads a team that has monitored the Larsen C ice crack for several years. Larsen B experienced a major iceberg-calving event in 1995, followed by gradual retreat and then complete collapse seven years later. Larsen C may follow a similar pattern, he says, although it’s not clear how soon collapse might follow the imminent calving event.