Pope, Smith and Kohler: These three glaciers in West Antarctica have probably not been heard of until now, as they are overshadowed by their large neighbors – the Thwaites and Pine Island glaciers. These have been losing ice for decades. If they melted completely, they could raise sea levels by 1.2 meters and destabilize the rest of West Antarctica. The ice volume from Pope, Smith and Kohler looks comparatively puny at the equivalent of two inches of sea level rise. And yet you should remember their names.
Because these smaller glaciers are melting faster than expected, their ice shelves are thinning, a group of scientists from the US and Europe recently found in the journal naturee Geosciences has stated. “The glaciers retreated faster than the glaciers in the Alps,” says lead author Pietro Milillo of the University of Houston. “This is bad news for us.”
Because the melting processes are likely to be very similar for other glaciers around the Amundsen Sea. “If the glaciers melt so quickly all over West Antarctica, we have real problems,” says Paola Rizzoli from the Institute for High Frequency Technology and Radar Systems at the German Aerospace Center (DLR) in Oberpfaffenhofen, who was involved in the study.
Where others see a relatively intact ice shelf, Rizzoli is beginning to be suspicious. The remote sensing expert knows that what satellite images or aerial photos of Antarctica show only represents half the truth at best. The essential eludes our view, because it takes place on the underside of the glacier.
“A glacier behaves like chewing gum”
For a long time it was unclear what was happening under the thick layer of ice. In early 2020, researchers drilled 600 meters through the Thwaites Glacier ice shelf and lowered a submersible robot. This encountered warm deep water that undermined the ice shelf. However, such boreholes are only possible selectively and at irregular intervals. Rizzoli, on the other hand, does not have to wait years to be able to study the processes under the ice. You can analyze them on a monthly basis; and on a large scale – thanks to a new generation of radar satellites.
Since October 2010, for example, a radar satellite called Tandem X around the earth and flies slightly offset to the almost identical satellite Terrasar-X, reminiscent of an oversized gold bar. In contrast to satellites with optical sensors, those twin satellites can penetrate thick cloud layers and darkness with their radar systems and scan the earth’s surface even in the polar night.
One of the two satellites emits electromagnetic radiation, which is reflected back from the ground and received by both radar satellites. Depending on the background, the backscatter is stronger or weaker. And thanks to the distance between the two satellites, using the so-called interferometry create a three-dimensional elevation model. Numbers become a topographical map. And over the years, the map will become an accurate depiction of the melting processes in West Antarctica.
Scientists suspect that a change in the westerly winds around Antarctica has brought warm deep water into Amundsen Bay, which is now undermining the ice shelf. The Smith Glacier, for example – as demonstrated by Milillo and his colleagues – melted by five meters per year over land between 2011 and 2019, but by around 22 meters per year on the freely floating underside of the glacier.
This erosion also causes Pope, Smith, and Kohler’s touchdown line to recede, which is the limit at which the ice loses contact with the mainland and begins to protrude out to sea. In 2017 alone, Pope Glacier retreated 3.5 kilometers in less than four months. And that has consequences: Because the glacier bed slopes inland, the touchdown line moves down and the ice thickness above it increases. Due to gravity, the outlet glaciers slide even more and more ice is lost. This in turn causes the glacier to thin out even faster. “A glacier behaves like chewing gum,” says Milillo. “The faster it flows, the thinner it gets.”
For Paola Rizzoli, the new findings are worrying. “We are seeing an acceleration of these phenomena, which had not previously been taken into account in the models,” says the DLR researcher. “The danger is that the entire West Antarctic Ice Sheet will become unstable.”
Climate models can now be fed with the new knowledge of what is actually happening on and below the outlet glaciers of West Antarctica. And thus create much more accurate forecasts than before for sea level rise. “We have probably underestimated the extent so far,” says Milillo.