Doomsday glacier could melt 'with just a small kick'

From the ABC (Australian Broadcasting Corporation)

Scientists have used an unmanned submarine to map underwater terrain near a massive melting glacier in West Antarctica for the first time, showing it has the potential to retreat at twice the speed observed by satellites over the past decade.

The Thwaites Glacier, named after glacial geologist Fredrik Turville Thwaites, is known as the "doomsday glacier" in some circles because of its potential to significantly raise global sea levels, as well as its vulnerability to rapid melting due to warming ocean water.

A team of researchers from the United States, the United Kingdom and Sweden travelled to Antarctica on the icebreaking research vessel Nathaniel B. Palmer in early 2019 to map a former grounding zone of the glacier — the area where an ice shelf attached to solid ground gradually transitions into a floating ice shelf.

The researchers hoped that by studying Thwaites Glacier's past retreat, they could gain a new understanding of what it has the potential to do in the future, amid speculation it could be a possible candidate for climate engineering projects to prevent further collapse.

They used a bright orange autonomous underwater vehicle (AUV) named Rán, mounted with two types of geophysical sensors, to produce 3D scans of the underwater surface.

The 19-hour mission produced about 13 square kilometres of new geophysical data, gathered from between 50 and 90 metres above the sea floor.

By analysing the amplitude and spacing of ridges left by the glacier on a raised part of the sea bed — an isolated underwater outcrop they called "the bump" at the south-west corner of the glacier's tongue — they were able to determine that the markings, or "ribs", had been left by the glacier lifting and settling with the ocean tide each day.

They found the Thwaites Glacier was indeed susceptible to rapid retreat, having been shrinking at a rate of more than 2.1 kilometres per year when the markings were left.

That is twice the rate of retreat observed via satellite between 2011 and 2019, previously thought to have been the upper limit of the glacier's potential shrinking speed.

The University of Florida's Dr Alastair Graham, who led the mission to Antarctica and was lead co-author of a study published on Monday in Nature Geoscience, said the glacial retreat documented by the team would have occurred sometime over the past two centuries, possibly as recently as the mid-1900s.

While he called the opportunity to map the area "truly a once-in-a-lifetime mission", he was at pains to point out that what they had found shattered the previously held hope that Antarctic ice sheets could be sluggish and slow to respond to changes in climate.

"Just a small kick to Thwaites could lead to a big response," Dr Graham said.

Dr Robert Larter, a marine geophysicist and co-author of the study, said once the glacier retreats beyond a shallow ridge in its bed, it has the potential to shrink at an even greater rate.

"Thwaites is really holding on today by its fingernails, and we should expect to see big changes over small timescales in the future — even from one year to the next," he said.

Multibeam bathymetry images captured by Rán show "ribs" and melting channels on underwater terrain near the Thwaites Glacier.(Nature Geoscience)