• Sun. May 14th, 2023

Greenland’s Petermann Glacier melting faster than expected, heat clawing at core

ByGurinderbir Singh

May 14, 2023

A crucial glacier in Greenland is melting far faster than expected. Now scientists have found unnatural heat clawing away at its core – and that means all previous calculations for the rate of sea level rise may be wildly wrong.

Petermann Glacier, in the country’s northwest, acts as a giant plug holding back an enormous expanse of land-based ice. But that 70km long, 15km wide wall is weakening.

“It’s bad news,” University of California Irvine glaciologist Eric Rignot says.

Satellite measurements have observed the glacier rising and falling with the daily tides. Those same measurements have found an enormous 204m cavity at the grounding line – where the solid wall of ice digs into the sea floor, holding the glacier back.

Put simply, the thinning glacier isn’t pushing into the sea floor as heavily as it used to. And that means a rising tide lifts its “lip”, allowing warm water to spill deeper inland under the ice flow than ever expected. This is eating away at the glacier like plaque eats away at a tooth. And, like a loose tooth, the tidal wobbling is weakening the ice’s structure.

“These dynamics are not included in models,” says Prof Rignot, one of the authors of a new study published in The Proceedings of the National Academy of Sciences (PNAS). “If we were to include them, it would increase projections of sea level rise by up to 200 per cent – not just for Petermann but for all glaciers ending in the ocean, which is most of northern Greenland and all of Antarctica.”

Grounding line

The Arctic ice cap is melting at about 13 per cent each year. At the current rate, no ice will be left after 2040.

But this melting ice is not contributing to sea level rise.

That’s caused by water from ice that sat on the land.

In recent decades, scientists have been nervously watching several key glaciers across the globe. This includes Antarctica’s “doomsday” Thwaites Glacier, which is three times the size of Tasmania. It holds back enough ice to cause a global 3m rise. (It’s called the doomsday glacier because of this potential catastrophic rise in sea levels.)

Petermann Glacier performs a similar role for a chunk of Greenland’s ice sheet. It will add 0.5m to global sea levels if it melts.

Exactly how fast this happens largely depends on the “grounding line”, where the glacier’s leading edge ploughs into the bedrock. Beyond this point, the ice turns into a floating sheet – the edges of which regularly “calve” icebergs.

Until now, glaciologists did not believe daily tides had any real impact on the health of a grounding line. But warmer water has made these tides “a very powerful mechanism,” Prof Rignot says.

Petermann Glacier is now bouncing back and forth over a distance of 2km to 6km each time the tides rise and fall. And as it lifts, large amounts of ocean heat pulse under the ice front at high speeds.

“The seawater actually goes much farther beneath the grounded ice, kilometres, not hundreds of meters,” Prof Rignot says. “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.”

If it holds, a glacier’s slide is slowed. Less ice gets exposed to the warmer water.

If it lets go, there’s little to stop the weight of ice behind it from plunging into the sea.

Recalculating predictions

This is the second study this year to reveal the world’s most important glaciers are in trouble.

“Thwaites is really holding on today by its fingernails,” British Antarctic Survey marine geophysicist Robert Larter warned in February. “We should expect to see big changes over small timescales in the future.”

His team found warm sea currents were gouging vast underwater caverns into that glacier’s grounding line.

Now the UCI researchers have found similar erosion under Petermann Glacier.

It helps explain why Greenland’s ice sheet is melting faster than expected. It has already lost billions of tons of ice that have added 1.4cm to global sea levels since 1972. But its acceleration can now be attributed to the addition of warm water to the effects of warm winds.

At Petermann, the 204m-tall abscess at the glacier’s base allows warm currents to consistently flow past the grounding line and under the ice.

The glacier’s grounding line is now melting 50 per cent faster than it was when last observed between 2016 and 2019. And that’s causing it to retreat at about 1km each year.

“These ice-ocean interactions make the glaciers more sensitive to ocean warming,” Prof Rignot says.

And that means sea level rise from melting glaciers is happening faster than scientists previously thought.

It’s also a deadly feedback loop.

Warming oceans are melting glaciers faster. That causes sea levels to rise. That increases the amount of glacial ice exposed to warmer water. It also helps loosen glacial grounding zones. And both cause more ice to melt.

“We have reasons to believe that the current [sea level] projections are too low, not too high. They could be as much two times too low,” Prof Rignot says.

Jamie Seidel is a freelance writer | @JamieSeidel

Read related topics:Time Is Now

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