NASA discovers new mode of ice loss in Greenland

NASA has discovered a new long pulse of ice loss in Greenland resulting from a warming climate, with implications for the future rate of sea level rise. The study found that during Greenland's hottest summers on record, 2010 and 2012, the ice in Rink Glacier on the island's west coast did not just melt faster than usual, it slid through the glacier's interior in a gigantic wave, like a warmed freezer pop sliding out of its plastic casing.

The wave persisted for four months, with ice from upstream continuing to move down to replace the missing mass for at least four more months, NASA said. This long pulse of mass loss, called a solitary wave, may increase the potential for sustained ice loss in Greenland as the climate continues to warm, with implications for the future rate of sea level rise. 

The study by scientists from NASA's Jet Propulsion Laboratory (JPL) was the first to precisely track a glacier's loss of mass from melting ice using the horizontal motion of a Global Positioning System (GPS) sensor. They used data from a single sensor in the Greenland GPS Network (GNET), sited on bedrock next to Rink Glacier.

Rink is one of Greenland's major outlets to the ocean, draining about 11 billion tonnes (gigatonnes) of ice per year in the early 2000s. In the intensely hot summer of 2012, however, it lost an additional 6.7 gigatonnes of mass in the form of a solitary wave. Previously observed melting processes can not explain that much mass loss. 

The wave moved through the flowing glacier during the months of June through September at a speed of about four kilometres a month for the first three months, increasing to 12 kilometres during September. The amount of mass in motion was 1.7 gigatonnes. Rink Glacier typically flows at a speed of a few kilometres a year. 

"We know for sure that the triggering mechanism was the surface melting of snow and ice, but we do not fully understand the complex array of processes that generate solitary waves," said JPL scientist Surendra Adhikari, who led the study. During the two summers when solitary waves occurred, the surface snowpack and ice of the huge basin in Greenland's interior behind Rink Glacier held more water than ever before.

In 2012, more than 95 per cent of the surface snow and ice was melting. Meltwater may create temporary lakes and rivers that quickly drain through the ice and flow to the ocean. "The water upstream probably had to carve new channels to drain. It was likely to be slow-moving and inefficient," said Erik Ivins from JPL. Once the water had formed pathways to the base of the glacier, the wave of intense loss began. The research was published in the journal Geophysical Research Letters.