Tidal Effects on Ice Sheets Could Mean Faster Sea Level Rise

May 10, 2023

Peterman Glacier Photo: Andreas Muenchow, University of Delaware

Associated Press:

Daily tides stoked with increasingly warmer water ate a hole taller than the Washington Monument at the bottom of one of Greenland’s major glaciers in the last couple years, accelerating the retreat of a crucial part of the glacier, a new study found.

And scientists worry that the phenomenon isn’t limited to this one glacier, raising questions about previous projections of melting rates on the world’s vulnerable ice sheets.

The rapid melt seen in this study was in the far northwest of Greenland on Petermann Glacier. If it is happening in the rest of Greenland and the even bigger Antarctic ice sheet, then global ice loss and the sea level rise could jump as much as twice as fast as previously thought, according to the study in Monday’s Proceedings of the National Academy of Sciences.

“It’s bad news,” said study author Eric Rignot, a glaciologist at University of California Irvine. “We know the current projections are too conservative. We know that they have a really hard time matching the current record″ of melt.

He said this newly found consequence of tidal activity “could potentially double the projections” of global melt.

The study looks at the all-important grounding line area of glaciers on ice sheets. That’s the point where glaciers go from being on land to floating on water. Previous studies show it’s also a key spot for rapid ice loss.

At remote Petermann, where few people have been and there are no base camps, that grounding line zone is more than six-tenths of a mile (1 kilometer) wide and could be as much as 3.7 miles (6 kilometers) wide, the study said.

Scientists used to think the daily tides weren’t a big deal on melt. The snow added on top of the glacier compensated for the tides moving further in, said Rignot, the day before he left for an expedition to Petermann. 

But with an ocean that’s warmer because of climate change the tides became “a very powerful mechanism,” Rignot said. 

“The sea water actually goes much farther beneath the grounded ice (than previously thought), kilometers, not hundreds of meters,” Rignot said. “And that water is full of heat and able to melt the glaciers vigorously. And it’s kind of the most sensitive part of the glacier.”

Using satellite altitude measurements, Rignot’s team found a 669-foot tall (204 meters) cavity at the grounding line where the melt rate is 50% higher in the last three years than it was from 2016 to 2019. Previous models forecast zero melt there.

The melting in Petermann has accelerated in the last few years, later than the rest of Greenland, probably because it is so far north that the water melting it from underneath is from the North Atlantic and it takes longer for the warmer water to reach there, Rignot theorized.

Rignot this month is exploring Petermann to get more ground-based measurements using ultrasound. He hasn’t been there since 2006, a decade before the changes were seen via satellite. Visiting Petermann, even before the glacier’s retreat accelerated, Rignot said he noticed movements that make it seem like a living thing.

“When you are standing on that shelf or sleeping on the shelf you hear noise all the time, loud noises from deep inside cracks forming,” Rignot said. “That’s where the concept of a glacier being alive starts getting to you.”

Greenland ice researcher Jason Box of the Geological Survey of Denmark and Greenland, who wasn’t part of the research, called Rignot’s technique clever and said the study makes sense, showing “that ocean heat delivery to tidewater glacier grounding lines represents a potent destabilizing effect.”

Box, who uses a different technique to calculate how much ice is no longer being fed by glaciers and is doomed to melt, something called “zombie ice,” figures 434 billion metric tons of ice on Petermann is already committed to melting.

The study provides strong evidence that models need to include these tidal effects deep inland and if they don’t, then they are underestimating future sea level rise, said Pennsylvania State University glaciologist Richard Alley, who wasn’t part of the Rignot study.

Washington Post:

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 ofits 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.


Leave a Reply

Please log in using one of these methods to post your comment:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: