Caption: Heavily crevassed terminus of Kapp Mohn outlet glacier, Austfonna, in May 2013, after 25-fold increase in flow speed (Dunse et al., The Cryosphere Discussions, 2014). Credit: Thorben Dunse, University of Oslo

For years, scientists have documented the rapid retreat of Arctic ice, from melting glaciers in Greenland to shrinking snow cover in far northern Eurasia. Now researchers have discovered one Arctic ice cap that appears to be literally sliding into the sea.

Ice is disappearing at a truly astonishing rate in Austfonna, an expanse of frozen rock far north of the Arctic Circle in Norway’s Svalbard island chain. Just since 2012, a portion of the ice cap covering the island has thinned by a whopping 160 feet, according to an analysis of satellite measurements by a team led by researchers at Britain’s University of Leeds.

Put another way, the ice cap’s vertical expanse dropped in two years by a distance equivalent to the height of a 16-story building. As another comparison, consider that scientists were recently alarmed to discover that one of Western Antarctica’s ice sheets was losing vertical height at a rate of 30 feet a year.

“It is a very large signal,” said Mal McMillan, a geophysicist and one of two researchers at Leeds’  Center for Polar Observation and Modelling who worked on the study. “The ice cap has slumped out into the ocean with a substantial loss of ice.”

McMillan and colleague Andrew Shepherd analyzed changes in Austfonna’s ice using data from satellites that measure, among other things, changes in elevation. They found that the gradual melting of the island’s 1,550-cubic-mile ice cap recently shifted into overdrive, for reasons that aren’t fully understood.  Small ice caps like the one over Austfonna are believed to be more vulnerable to climate change-related thawing because relatively more surface area is exposed to the air and sea.

The image shows the rate of ice cap elevation change between 2010 and 2014 observed by the CryoSat satellite, which is overlaid onto an image acquired by the Sentinel-1A satellite. Red regions show where the ice surface has lowered due to ice loss. Credit: CPOM/GRL

In this case, the ice cap lost one-sixth of its original thickness in two years, and the flow of ice from the summit to the sea accelerated by 25 fold, to a rate of several kilometers a year, a fast clip by glacier standards, the study found.

“What we see here is unusual because it …  appears to have started when ice began to thin and accelerate at the coast,” Shepherd said.

The research, published in the journal Geophysical Research Letters, illustrates how quickly ice caps can evolve, highlighting the challenges associated with predicting future impact of climate change, the scientists said. Arctic experts are closely watching changes in polar ice because of the potentially profound implications for sea-level rise. About a third of the increase in sea level in recent decades is attributed to melting glaciers and ice sheets, and researchers worry that more rapid melting could eventually swamp coastal cities around the world.

Still, researchers say, it’s too early to say definitively if the shrinking of the Austfonna ice cap is due to global warming. Ice caps can shift suddenly for reasons that have nothing to do with climate, McMillan said. But in this case the list of possible culprits would certainly include warmer ocean water and air temperatures, both of which have risen more rapidly in the Arctic compared to the rest of the planet, he said.

“We’ve only seen this for a couple of years,” he said of the Austfonna meltdown, “so we really need to monitor it further.”