Scientists are warning that apparently stable glaciers in the Antarctic can "switch very rapidly" and lose large quantities of ice as a result of warmer seas.
Their finding comes after a research team involving scientists at Lancaster University, and led by the University of Leeds, used satellites to track the Cadman Glacier, which drains into Beascochea Bay, on the west Antarctic peninsula.
Between November 2018 and May 2021, the glacier retreated eight kilometres as the ice shelf at the end of the glacier - where ice extends out into the sea and is anchored onto the sea floor at what is known as the grounding zone - collapsed.
The ice shelf would have acted as a buttress, slowing the movement of the glacier towards the sea.
Surrounded by warmer ocean waters, the scientists believe the ice shelf thinned and became ungrounded, and the ice shelf was no longer able to hold back the glacier.
As a result, the speed at which the glacier was flowing rapidly accelerated - doubling its speed - increasing the amount of ice it discharges into the sea as icebergs, through a process known as iceberg calving.
Benjamin Wallis, study lead author from the University of Leeds, said: "We were surprised to see the speed at which Cadman went from being an apparently stable glacier to one where we see sudden deterioration and significant ice loss.
"What was also curious was that the neighbouring glaciers on this part of the west Antarctic Peninsula did not react in the same way, which may hold important lessons for the way we can better project how climate change will continue to affect this important and sensitive polar region.
"Our study brought together data from three decades, nine different satellite missions, and in-situ oceanographic measurements to understand the changes happening in Antarctica. This demonstrates how important it is to have long term monitoring of the Earth's polar regions with a range of sensors which all tell us a different piece of the story."
According to the scientists, the Cadman Glacier is now in a state of "substantial dynamic imbalance". The ice on the glacier has continued to thin, with elevation being lost at a rate of around 20 metres a year.
And around 2.16 billion tonnes of ice are draining from the Cadman Glacier into the ocean each year.
Lancaster University scientists contributed to the study by mapping the rate and spatial pattern of Cadman Glacier thinning. This was done using extremely high-resolution satellite imagery to create digital elevation models of the ice surface; before and after the collapse event.
These data allowed the research team to determine the magnitude of glacier thinning, and also how far inland the thinning had spread.
Co-author of the study Mal McMillan, Professor in Earth Observation at Lancaster Environment Centre and the Centre of Excellence in Environmental Data Science (CEEDS), said: "This study demonstrates just how rapidly remote Antarctic glaciers can respond to changes in their surrounding environment, entering phases of rapid retreat and increasing their discharge of ice to the ocean. Given that the Antarctic Peninsula has many hundreds of these small and highly dynamic glaciers, which are notoriously hard to monitor, this new analysis reinforces the importance of developing new, high resolution satellite observations of ice sheet change, and of investing in long-term satellite programmes to monitor future ice loss."
The researchers have published their analysis - Ocean warming drives rapid dynamic activation of marine-terminating glacier on the west Antarctic Peninsula in the scientific journal Nature Communications.
Unusually high ocean water temperatures in early 2018/19 around the west Antarctic peninsula are believed to have triggered the rapid dynamic change on the Cadman Glacier system.
By analysing historic satellite data, the scientists believe warmer ocean waters gradually thinned the glacier's ice shelf from the early 2000s and possibly since the 1970s.
The warmer water was not carried on the surface of the ocean but deep in the water column. This warmer water may have reached the ice shelf where it is grounded on the sea floor. The result is the ice shelf begins to melt from the bottom up.
In 2018/19, the ice shelf was so thin that it broke free from the grounding zone and started to float, in effect slipping anchor and enabling the Cadman Glacier to drain more ice into the seas.
But the scientific team still faced one big question. Why had the Cadman Glacier collapsed when the neighbouring Funk and Lever Glaciers remained relatively stable?
By analysing subsea oceanographic data, they believe a series of subsea rock structures called ridges or sills, at a depth of 200 metres and 230 metres, acts as a defensive barrier, deflecting channels of warmer water from reaching the glaciers. Although they warn that a rise in ocean warming could compromise the ability of the ridges to protect some glaciers.
Professor Michael Meredith, from the British Antarctic Survey and one of the authors of the paper, said: "We have known for some time that the ocean around Antarctica is heating up rapidly, and that this poses a significant threat to glaciers and the ice sheet, with consequences for sea level rise globally.
"What this new research shows is that apparently stable glaciers can switch very rapidly, becoming unstable almost without warning, and then thinning and retreating very strongly. This emphasises the need for a comprehensive ocean observing network around Antarctica, especially in regions close to glaciers that are especially hard to make measurements."
Writing in the paper, the researchers say what has happened to the Cadman Glacier can be seen as an example of a "glaciological tipping point", where a system in a steady state can take one or two paths based on a change in an environmental parameter.
A tipping point was reached in 2018 caused by the arrival of unusually warm ocean water, which caused the ice shelf to unground. Reaching this tipping point caused the Cadman Glacier to increase its ice discharge by 28% in 13 months.
The researchers say other glaciers on the Antarctic Peninsula may be vulnerable to similar sudden changes because of subsea geology.
The paper is availabole at https://www.nature.com/articles/s41467-023-42970-4
Lancaster researchers involved in the study are Professor Mal McMillan, Romilly Close and Dominic Hardy.
