One of the world’s most dangerous glaciers could soon be retreating at an unexpectedly rapid rate – and no one knows by how far. Studies of the seafloor in front of the 100km-wide Thwaites Glacier in Antarctica indicate that not long ago the ice flow was falling at almost three times its current rate, reports a team led by Alastair G.C. Graham from the University of South Florida.
This is shown by a series of submarine ribs that document the surprisingly rapid retreat of the glacier to the day, as the working group writes in a publication in “Nature Geoscience”.
In particular, the traces probably come from a situation very similar to what is now found at the front edge of the glacier. The traces are probably no more than 200 years old and come from an undersea hill.
This stabilized – just like today – the glacier front. Until it stopped and the glacier began to retreat faster and faster. Should Thwaites Glacier lose contact with its supporting rise today, it is likely to experience a similar boost, experts conclude. Nobody knows when and how such a boost would end.
Thwaites Glacier is considered to be one of the key sites of climate change. Not only is it big enough to raise the sea level by 65 centimeters with the water it contains. The glacier is also believed to hold the key to the fate of the West Antarctic Ice Sheet.
If it disintegrates, the ice masses could follow the entire region as if someone had pulled a plug – and possibly quite quickly. The problem is that Thwaites Glacier, like the entire West Antarctic Ice Sheet, lies in a mile-deep bowl that deepens inland from what is now the ice margin. This idiosyncratic landform beneath the ice feeds apocalyptic scenarios.
A possible cause is a mechanism called “marine ice-cliff instability”. The basic idea is that as the glacier retreats further into the bowl, the front of the glacier gets higher and higher. Eventually the ice cliff will be so high that it will collapse under its own weight. However, this creates an even higher ice cliff because the ground slopes inland. This cliff crumbles even faster, and so on, until large chunks of ice float in the now water-filled bowl – raising sea levels by several meters in a short period of time.
However, it is quite debatable whether such scenarios will actually occur and, more importantly, how quickly this could happen. But there are a number of other troubling signs that Thwaites Glacier is now “hanging on with its fingernails,” as co-author Robert Larter of the British Antarctic Survey put it to CNN. The ice shelf, the floating mass of ice that is in front of the actual glacier and dams the current with its sheer weight, is riddled with cracks.
Some forecasts see it largely disintegrating within the next few years, with the likely result that Thwaites would lose ice even more rapidly. And the glacier is already shrinking faster than most in Antarctica.
Studies have identified several specific factors affecting glaciers and shelves. Relatively warm water thaws the floating ice from below. And according to studies, this probably also happens with the glacier itself, under which warm, salty seawater also penetrates due to the tides. No one knows what that really means: whether there really is a risk of rapid collapse or whether the glacier and the ice cap behind it are so massive that they will continue to exist for centuries even in the face of climate change.
However, the new study raises concerns that the glacier is much more dynamic than one would think – and the pessimistic scenarios, including devastating sea level rise, could be closer to the truth.
Graham’s working group used an underwater drone to map the seabed in front of the glacier and found remarkable, previously unknown structures on an approximately 200 meter high hill called “The Bump”. Hundreds of parallel ribs, mostly less than 20 centimeters high, cover the seabed at intervals of around seven meters on average.
They once formed along the line where the glacier front lay on the sea floor – the touchdown line. But the edge of the glacier does not lie still. With the ebb and flow of the tide, the ice rises from the sea floor and sets again. And that’s where the ribs came from, experts say.
Several features speak in favor of this – among other things, their heights vary cyclically about every 14 ribs, which fits in with the tidal cycle typical of the region of one high and one low tide per day and particularly high and low tides every 14 days, as the working group writes. The team attributed the fact that the ribs formed at such regular intervals to the fact that the base of the glacier melted away with every high tide due to the ingress of warmer seawater. As a result, the touchdown line shifted landward by around two to ten meters per day.
Using a particularly elongated series of 164 daily ripples, the working group found that not only was the glacier retreating significantly faster than contemporary glaciers in the region over those five and a half months, but that the process also accelerated over time.
At the start of the series, the glacier was receding at 1.5 miles per year, and it ended at 1.4 miles per year. That is about three times faster than the grounding line of Thwaites Glacier is currently receding and about 15 percent faster than the current fastest retreating glaciers in the region. Last but not least, studies show that it can be even faster. For example, in 2017, Pope Glacier retreated four kilometers over a period of four months.
Exactly when the ribs formed is unclear, but they are young. Depending on the rate of glacier retreat one assumes, they were formed sometime between 1800 and 1960. Thus, the conditions at the time of their formation were similar to those in today’s ocean – and the hump on which the ribs are imprinted is not very different Hill where the glacier touchdown line is anchored today.
According to the working group, the marks on the “bump” leave little doubt that Thwaites Glacier can retreat much faster in a short period of time if it loses contact with the highest points below the touchdown line. How fast actually is unknown. And nobody knows whether the process will simply stop again at the next hump around 35 kilometers further – or whether it can no longer be stopped this time, possibly driven by the effects of climate change in the Southern Ocean.
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The original for this article “Riddles about the ice apocalypse” comes from Spektrum.de.
