A Greenland glacier’s rapid melting may signal faster sea level rise

Scientists studying one of Greenland’s largest glaciers say it is melting far faster than expected in its most vulnerable region, a worrying sign that glaciers perched in the ocean could contribute to sea level rise more quickly than currently forecast.

Using satellite measurements of its surface, researchers found that Petermann has been bouncing up and down, dramatically shifting its seafloor moorings in response to the tides. All this movement has carved a large cavern at the base of the glacier and allowed warm water to regularly stretch beneath it. As the glacier lifts and migrates, the water can rush in for over a mile, thinning the ice by as much a 250 feet a year in some places.

“You have this constant flushing of seawater going many kilometers below the glacier and melting the ice,” said Eric Rignot, one of the study’s authors and a glaciologist at the University of California at Irvine and the Jet Propulsion Laboratory at the California Institute of Technology.

“We think that could change sea level projections quite a bit,” he said. The study was published Monday in the Proceedings of the National Academy of Sciences.

Petermann Glacier is, in the context of climate change, the next big thing that our greenhouse gas emissions may break. The vast glacier, some 10 miles wide, is one of several major outlets for ice to escape from Greenland’s interior into the ocean. In total, the massive region of ice queued up behind Petermann could, if it all melted, raise global sea levels by over 1 foot.

Petermann has not changed as much as some other Greenland glaciers, likely in part because it is so far north. But it has seen important shifts.

Petermann lost two massive chunks of ice from its floating ice shelf in 2010 and 2012, causing the shelf to lose roughly a third of its area. It has not since recovered.

The glacier has also started to move backward, as the central region of its grounding line — where it sits on the floor of the deep fjord — retreated more than 2 miles inland toward Greenland’s interior. This has occurred in response to a warming of the water in the fjord in front of the glacier. The warming only amounts to a fraction of a degree, according to Rignot, but the water is now slightly above zero degrees Celsius (32 Fahrenheit). But it is more than warm enough to melt ice, especially at the depths and pressures seen at the grounding line.

At the same time, the ice has begun to flow outward more rapidly, meaning that Petermann has swung from a more or less stable state to losing a few billion tons of ice to the ocean each year. It’s not that much compared with a few other major glaciers in Antarctica or Greenland, but it could be only the beginning.

All of this likely reflects changes at the grounding line, which is extremely difficult to observe. But satellites can detect both changes in the surface height of the glacier, which can be used to infer to what is going on beneath, and how glaciers respond to cycles in the tides.

This is what the new research captures at Petermann — showing that the tidal cycles have very large implications for the glacier’s melting. The satellites showed that there is no real grounding “line” — rather, there is a vast zone, over a mile in length, over which the glacier moves back and forward along the seafloor. This movement accelerates melting as it allows seawater to mix in close to and even beneath the glacier.

The research also found that a large cavity — 650 feet in height — has now been hollowed out in the center of the grounding line. It is nearly 8 square miles in area, and in this region, the ocean can enter and cause melting even without help from the tides that move and lift up the glacier.

All of this, according to the researchers, has a very large implication — we may need to adjust our current models to take into account rapid melting at the bobbing grounding lines of large glaciers. And this, in turn, could cause sea level rise projections from these behemoths to “potentially double,” the study suggests.

“Probably a lot of other glaciers are in that situation, with tidal flushing,” Rignot said. He believes that Petermann is, overall, a good analogue for what may also be happening in Antarctica, where there is far more ice at stake than in Greenland.

The research was conducted by scientists at the three U.S. institutions — the University of California at Irvine, the Jet Propulsion Laboratory at the California Institute of Technology, and the University of Houston — in collaboration with international colleagues at institutions in China, Finland, Germany and Italy.