The rapid retreat of Antarctic glaciers triggers a scientific “whodunit”

When an Antarctic glacier caused rapid retreat three years ago, scientists wondered what could have caused it.

The Hektoria Glacier retreated more than 8 km (5 miles) in just two months at the end of 2022 – and now a new study claims to have the answer.

The authors believe Hektoria may be the first modern example of a process by which the front of a glacier resting on the seafloor rapidly destabilizes.

This could lead to much faster sea level rise if it happened elsewhere in Antarctica, they say.

But other scientists say this part of the glacier was actually floating in the ocean. So while the changes are impressive, they’re not that unusual.

Floating tongues of glaciers extending into the sea – called ice shelves – are much more likely to break off than glacier fronts resting on the seafloor.

This is because they can be more easily eaten away by the hot water underneath.

The fact that Hektoria has undergone enormous changes is not in dispute. Its front retreated about 25 km (16 miles) between January 2022 and March 2023, according to satellite data.

But elucidating the causes is akin to a “whodunit” mystery, according to the study’s lead author, Naomi Ochwat, a research affiliate at the University of Colorado at Boulder and a postdoctoral researcher at the University of Innsbruck.

The case began in 2002 with the extraordinary collapse of an ice shelf called Larsen B in the eastern Antarctic Peninsula. Around 3,250 square kilometers (1,250 square miles) of sea ice was lost, roughly the size of Cambridgeshire or Gloucestershire.

Larsen B had effectively held back the Hektoria glacier. Without it, Hektoria’s movement accelerated and the glacier thinned.

But the bay freed by the ice floe was eventually filled with sea ice “attached” to the seafloor, helping to partially stabilize Hektoria.

That was until early 2022, when the ice floe broke up.

What followed was further loss of floating ice on the front of Hektoria, as large flat-topped icebergs broke off or “calved”, and the ice behind accelerated and thinned.

This is not unusual. Iceberg calving is a natural part of ice sheet behavior, although human-driven climate change makes the loss of ice shelves much more likely.

What was unprecedented, the authors say, was what happened in late 2022, when they suggested the glacier front was “stranded” – resting on the seafloor – rather than floating.

In just two months, Hektoria has moved back 8.2 km. That would be nearly ten times faster than any previously recorded failed glacier, according to the study published in Nature Geoscience.

This extraordinary change, the authors say, could be due to an ice plain – a relatively flat area of ​​bedrock on which the glacier rests lightly.

They say the upward forces of ocean water could “lift” the thinning ice all at once, causing the icebergs to break up and the glacier to rapidly retreat.

“Glaciers don’t usually retreat this fast,” said co-author Adrian Luckman, professor of geography at Swansea University.

“The circumstances may be a bit unusual, but this rapid retreat shows us what can happen elsewhere in Antarctica, where glaciers are lightly anchored and sea ice is losing its grip,” he added.

What makes this idea even more appealing is that this process has never been observed in the modern world, the authors say. But marks on the seafloor suggest this may have triggered a rapid loss of ice in the ocean in Earth’s past.

“What we see in Hektoria is a small glacier, but if something like this were to happen in other parts of Antarctica, it could play a much bigger role in the rate of sea level rise,” Dr Ochwat said.

This could include Thwaites – the so-called “doomsday” glacier because it contains enough ice to raise sea levels by 65cm (26 inches) if it melted entirely.

“It’s really important to understand whether or not there are other areas of ice plains that would be susceptible to this type of shrinkage and calving,” Dr. Ochwat added.

But other researchers have disputed the study’s findings.

The controversy centers on the position of the “grounding line” or “grounding zone” – where the glacier loses contact with the seafloor and begins to float in the ocean.

“This new study offers a tantalizing glimpse of what could be the fastest rate of retreat ever observed in modern Antarctica,” said Dr. Frazer Christie, glaciologist and senior Earth observation specialist at Airbus Defense and Space.

“But there is significant disagreement within the glaciological community over the precise location of the grounding line of the Hektoria Glacier, because it is very difficult to obtain accurate recordings from radar satellites in this fast-flowing region,” he added.

The location of the ground line may seem trivial, but it is crucial to determine whether the change was truly unprecedented.

“If this part of the ice sheet actually floated [rather than resting on the seabed]”The conclusion would rather be that the icebergs calved from an ice shelf, which is much less unusual behavior,” said Dr Christine Batchelor, senior lecturer in physical geography at Newcastle University.

“I think the proposed mechanism and rate of retreat is plausible in the Antarctic ice plains, but due to the uncertainty over the location of the stranding zone at Hektoria, I am not entirely convinced that it was observed here,” she added.

But where there is little debate is that the fragile white continent – ​​once thought to be largely immune to the impacts of global warming – is changing before our eyes.

“Even though we disagree about the process behind this change in Hektoria, we absolutely agree that changes in the polar regions are frighteningly rapid, faster than we expected ten years ago,” said Anna Hogg, professor of Earth observation at the University of Leeds.

“We need to collect more satellite data, to better monitor and understand why these changes are happening and what their implications are. [for sea-level rise]”.

Additional reporting from the visual journalism team