Process controlling Greenland ice sheet collapse discovered

Scientists have uncovered a process that can control the "calving" of glaciers -- when large chunks of ice collapse into the sea, forming icebergs like the one that sank the Titanic. The discovery, published in the journal Nature Communications, will help the scientific community better model future Greenland ice loss and sea level rise.

Glacier calving is one of the more dramatic aspects of climate change. Depending on the height of the glacier, calving can be akin to an ice structure the size of a tall skyscraper falling into the sea. "Iceberg calving has been challenging to model," said Professor Tim Dixon from the University of South Florida (USF) in the US.

"One of the big unknowns in future sea level rise is how fast Greenland falls apart, and iceberg calving is one of the least understood mechanisms," Dixon said. The researchers ventured to Greenland in the summer of 2016 to install a new radar system to better understand the process.

They wanted to monitor formations known as pro-glacial "melange", a combination of sea ice and icebergs in front of the glacier. The melange can be tightly packed in the long, narrow fjords that front many of Greenland's glaciers that meet the sea.

Scientists have long known that melange can impede glaciers as they move towards the sea, but they haven't had the data to fully understand the phenomenon. Dixon's team developed a new radar-based approach to precisely measure elevations of the melange in front of Jakobshavn Glacier, a major outlet glacier on Greenland's west side.

Using analytical techniques developed by USF PhD student Surui Xie, the scientists measured the height of the melange. The scientists found a thick melange wedge pressed up against the glacier in late spring and early summer.

During this period, no icebergs calved, the scientists observed. Once the wedge thinned and melted by mid-summer, calving began in earnest. "On the surface, this melange is a subtle thing -- it appears almost flat -- but underwater, there are huge variations," Dixon said.

"It's really the underwater part that is pinning the glacier back and preventing it from calving. By precisely measuring the surface elevations, we were able to get a handle on the much bigger sub-surface variations, which define melange thickness," said. Earlier this year, NASA scientists reported Jakobshavn Glacier, which has been Greenland's fastest -thinning glacier for the last 20 years, was slowing in its movement towards the ocean in what appears to be a cyclical pattern of warming and cooling.

However, because Jakobshavn is still giving up more ice than it accumulates each year, its sheer size makes it an important factor in sea level rise, the NASA scientists said. "Our study helps understand the calving process. We are the first to discover that melange isn't just some random pile of icebergs in front of the glacier. A melange wedge can occasionally 'hold the door' and keep the glacier from calving," Dixon said.

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