There’s a fair chance Thwaites Glacier has passed its tipping point. But is it a ‘doomsday’ scenario?

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A study published earlier this month has raised fresh questions about exactly how fast Antarctica’s Thwaites Glacier, the widest glacier on the planet, is melting.

One of the study authors described the glacier as now holding on “by its fingernails“.

So it’s worth unpacking just what this new study, published in Nature Geoscience, is saying, and what it tells us about how the glacier might impact sea-level rise.

The Thwaites Glacier is sometimes called the “doomsday glacier” — a dramatic nickname first used in a Rolling Stone article, though that’s not a term favoured by most scientists. More on that point shortly.

The glacier itself contains enough ice to raise sea levels by more than 60 centimetres. But it also acts as a buffer, holding back a large part of the West Antarctic Ice Sheet. If that is released, it will raise sea levels by several metres.

The glacier flows from West Antarctica over the land’s surface to the coast and into the Amundsen Sea.

Aerial view of Thwaites glacier.
The glacier was found to have retreated at double the rate observed by satellite, during a 6-month period, some time in the past 200 years.(Photo: NASA/OIB/Jeremy Harbeck)

Because of its huge mass, the glacier pushes deep offshore before it begins to lift off the seabed at what is called the grounding line or grounding zone.

We can think of the grounding line as being “like a hinge”, connecting the inland part of the glacier to a floating extension known as an ice shelf, according to Matt King, director of the Australian Centre for Excellence in Antarctic Science at the University of Tasmania.

The Thwaites Glacier’s grounding line has been steadily melting and retreating in the direction of the ice sheet interior over decades.

What did this new research find?

A diagram of a glacier forming ridges on the seafloor.
”Back-stepping sedimentary ridges” are formed where the ice lifts and settles on the seabed at the grounding line.(Supplied: Graham et al/Nature Geoscience)

For the first time, due to recent sea-ice loss — that’s the ice on the sea’s surface that forms each winter — researchers were able to send down an autonomous underwater vehicle to survey the area of the seafloor that the glacier used to cover.

On the seafloor, more than 600 metres down, they found an isolated region interspersed with long, parallel ridges, mostly around 20cm high, and ranging mostly between 4m and 8m apart.

They worked out these raised ribs, or “back-stepping sedimentary ridges”, were left as the ice at the grounding line lifted and settled on the seabed with the tide.

Because the geology and slope of the seabed changes and undulates, there isn’t a nice, continual track of these ridges all the way to the present-day grounding line.

Black and white scan of seafloor ridges.
The distance between the seafloor ridges corellated to the rate of retreat of the glacier grounding line.(Supplied: Graham et al/Nature Geoscience)

So while the researchers are certain they were laid down before we have satellite data — pre-1992 — and they’re fairly certain they formed before the 1950s, beyond that they’re not sure exactly how old the ridges are.

But by using the distance between them, the researchers were able to work out how fast the grounding line was retreating at the time.

Satellite data between 2011 and 2017 recorded the average rate of retreat of the Thwaites Glacier grounding line was between 0.6-0.8 kilometres per year, or a few metres a day.

But the distance between ridges indicated that for 5.5 months, sometime prior to the 1950s, the grounding lines retreat suddenly sped up to around 2.3km per year.

So what does this tell us?

“It tells us first of all that the glacier seems to have retreated faster than we’ve previously observed through satellites,” Professor King said.

“It tells us that the glacier can behave in more dramatic ways than we’ve seen.”

And what are the implications of that?

The bulk of the Thwaites Glacier that’s over land is always slowly flowing towards the sea.

But glaciers and their associated ice shelves have what are called pinning points — regions where the glaciers come in contact with geological features like sea-floor plateaus.

A glacier schematic.
The grounding line or zone is the point where the glacier lifts off the seabed.(Supplied: The International Thwaites Glacier Collaboration)

Friction between the part of the glacier in contact with the pinning point slows the flow of the glacier “for hundreds of kilometres upstream”, according to earlier research on the Thwaites Glacier.

Rapid melting at the grounding line can allow it to detach from these pinning points, releasing that friction, in turn allowing hundreds of kilometres of glacier to flow more rapidly.

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