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Race against time

THE WASHINGTON POST – Scientists studying one of Greenland’s largest glaciers said 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.

The scientists fear the phenomenon observed at Petermann Glacier could be happening to other glaciers in both Greenland and Antarctica, possibly leading to faster, more dramatic levels of sea level rise worldwide – “potentially double” what is currently expected from glaciers, according to a study published on Monday.

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 as 250 feet a year in some places.

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

Scientists Andreas Muenchow and Keith Nicholls work on Petermann glacier in Greenland. PHOTO: THE WASHINGTON POST

“We think that could change sea level projections quite a bit,” he said. The study was published on 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 ten 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 one 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 two 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. 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 tonnes 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 centre of the grounding line. It is nearly eight 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, means 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 suggested.

“Probably a lot of other glaciers are in that situation, with tidal flushing,” said Rignot. 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 United States institutions – the University of California, 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.

Several scientists not affiliated with the study reached by The Post were impressed by the new measurements, but not entirely convinced to make the same conclusions. “The melt rates reported are very large, much larger than anything we suspected in this region,” said Hélène Seroussi, a glaciologist at Dartmouth College who uses models to study glaciers and sea level rise.

However, Seroussi said, the models that researchers use to project sea level rise – complex suites of equations that are used to predict how glaciers all over the world will respond to warmer oceans and air – would not immediately change based on the results of the current study.

“We are many years away from implementing these processes correctly in numerical models,” Seroussi said. “It is important to understand that there are always long delays between the discovery of a new process and its inclusion in numerical models as these processes need to be perfectly understood from a physical point of view”, requiring more research.

In particular, Seroussi said, the process in question is generally not included because the scale over which it operates is not fully understood. Until that happens, some models could show too much ice loss because of it, simply because they represent the process as playing out over too large of a region.

Andreas Muenchow, a scientist at the University of Delaware who studies Petermann Glacier, also had some cautionary notes.

“I very much like the idea of a ‘tidal heart beat’ of the glacier’s grounding zone, the glacier flapping up with warm water intruding during the incoming tide and flapping down with colder water exiting during the outgoing tide,” Muenchow said.

However, he noted that “the very high melt rates are real, but they are estimated over very small areas”.

“My main takeaway is that models need to be improved,” Muenchow concluded. “The study provides a sharper focus for what processes we need to study near floating glaciers in Greenland or Antarctica, if we want to project rising sea level into the future using models.”

Overall the new study again underscores that we don’t really know how quickly one of the largest consequences of climate change – sea level rise from the melting ice sheets of Greenland and Antarctica – will occur. We’re still finding out new details – and new reasons to think that it could be faster than expected.

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