The rapid retreat of an Antarctic glacier three years ago initially perplexed scientists, leaving them searching for the underlying cause.
Hektoria Glacier experienced a significant retreat, receding by over 8 kilometers (5 miles) in just two months during late 2022. Now, a new study purports to have identified the reason behind this phenomenon.
The study’s authors suggest that Hektoria could represent the first contemporary instance of a process wherein the front of a glacier resting on the seabed undergoes rapid destabilization.
According to the researchers, this could potentially lead to accelerated sea-level rise if similar events were to occur elsewhere in Antarctica.
However, other scientists contend that this particular section of the glacier was, in fact, floating in the ocean, suggesting that while the changes are notable, they are not entirely unprecedented.
Floating extensions of glaciers into the sea, known as ice shelves, are generally more susceptible to disintegration compared to glacier fronts that rest directly on the seabed.
This is largely due to the increased vulnerability of ice shelves to erosion from warm water circulating beneath them.
The fact that Hektoria has undergone substantial change is not in dispute. Satellite data reveals that its front retreated by approximately 25 kilometers (16 miles) between January 2022 and March 2023.
However, unraveling the specific causes is akin to solving a complex “whodunnit” mystery, according to Naomi Ochwat, lead author of the study, a research affiliate at the University of Colorado Boulder, and a post-doctoral researcher at the University of Innsbruck.
The case traces back to 2002, with the dramatic collapse of the Larsen B ice shelf in the eastern Antarctic Peninsula. This event resulted in the loss of approximately 3250 square kilometers (1250 square miles) of ice shelf, an area roughly equivalent in size to Cambridgeshire or Gloucestershire.
The Larsen B ice shelf had effectively acted as a barrier, holding back Hektoria Glacier. Its disappearance led to an acceleration in Hektoria’s movement and a thinning of the glacier.
Subsequently, the bay left vacant by the ice shelf gradually filled with sea-ice “fastened” to the seabed, contributing to a partial stabilization of Hektoria.
This stability persisted until early 2022, when the sea-ice broke apart.
The ensuing period witnessed further loss of floating ice from Hektoria’s front, as large, flat-topped icebergs detached or “calved,” and the remaining ice behind them accelerated and thinned.
While iceberg calving is a natural component of ice sheet behavior, human-caused climate change is increasing the likelihood of ice shelf loss.
The authors emphasize that what occurred in late 2022 was unprecedented, as they posit that the glacier’s front was “grounded” – resting on the seabed – rather than floating.
In a mere two months, Hektoria retreated by 8.2 kilometers. According to the study, published in Nature Geoscience, this rate of retreat would be nearly ten times faster than any previously recorded grounded glacier.
The authors attribute this remarkable change to the presence of an ice plain, a relatively flat area of bedrock upon which the glacier rests lightly.
They suggest that upward forces from the ocean water could have simultaneously “lifted” the thinning ice, leading to the detachment of icebergs and a rapid retreat of the glacier.
“Glaciers don’t usually retreat this fast,” stated co-author Adrian Luckman, professor of geography at Swansea University.
“While the specific circumstances may be somewhat unique, this rapid retreat demonstrates what could occur elsewhere in Antarctica where glaciers are lightly grounded, and sea-ice loses its grip,” he added.
The authors emphasize that this process has never been observed in the modern era, adding to the intrigue. However, markings on the seafloor suggest that it may have triggered rapid ice loss into the ocean in Earth’s past.
“What we observe at Hektoria is a small glacier, but if similar events were to occur in other regions of Antarctica, it could significantly impact the rate of sea-level rise,” explained Dr. Ochwat.
This could include Thwaites, the so-called “doomsday” glacier, which contains enough ice to raise global sea levels by 65cm (26in) if it were to melt entirely.
“It is crucial to understand whether other ice plain areas exist that would be susceptible to this type of retreat and calving,” Dr. Ochwat added.
However, other researchers have challenged the study’s conclusions.
The disagreement centers on the precise location of the “grounding line” or “grounding zone,” which marks the point where the glacier loses contact with the seabed and begins to float in the ocean.
“This new study offers a tantalizing glimpse into what could be the fastest rate of retreat ever observed in modern-day Antarctica,” said Dr. Frazer Christie, glaciologist and senior Earth observation specialist at Airbus Defence and Space.
“However, there is significant disagreement within the glaciological community regarding the precise location of Hektoria Glacier’s grounding line, as obtaining accurate records from radar satellites in this fast-flowing region is particularly challenging,” he added.
The location of the grounding line may seem inconsequential, but it is essential for determining whether the change was truly unprecedented.
“If this section of the ice sheet was, in fact, floating [rather than resting on the seabed], the conclusion would be that icebergs calved from an ice shelf, which is much less unusual behavior,” stated Dr. Christine Batchelor, senior lecturer in physical geography at Newcastle University.
“I believe that the proposed mechanism and rate of retreat are plausible in Antarctic ice plain settings. However, due to uncertainty about the location of the grounding zone at Hektoria, I am not fully convinced that this has been observed here,” she added.
Despite the ongoing debate, there is widespread agreement that the fragile white continent, once believed to be largely immune to the impacts of global warming, is now undergoing rapid transformations.
“While we may disagree about the specific processes driving this change at Hektoria, we are in complete agreement that the changes occurring in the polar regions are alarmingly rapid, exceeding our expectations from even a decade ago,” said Anna Hogg, professor of Earth observation at the University of Leeds.
“We must gather more data from satellites to improve our monitoring and understanding of why these changes are occurring and what their implications are for sea-level rise.”
Additional reporting by the Visual Journalism team
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