Cracks in the ice accelerate ice shelf collapse | Polarjournal
The satellite image shows the Pine Island glacier in 2011, when a 30-kilometer-long crack formed and led to the calving of a 720-square-kilometer iceberg. The glacier itself is about 250 kilometers long and its catchment area is about 10 percent of the West Antarctic ice sheet. Photo: NASA

The Pine Island Glacier (PIG) in West Antarctica is considered the fastest retreating glacier in Antarctica, along with its neighbor, Thwaites Glacier. Overall, the ice loss per year is in the double-digit billion range. The grounding line of the glacier tongue has retreated so rapidly that the glacier is now considered too weak to hold back the ice masses of the West Antarctic ice sheet. Now, an international research team has found that one of the main reasons for the retreat is a weakness on the southern side of the glacier, so the glacier flow is no longer able to stop.

The weaknesses in the glacier’s so-called shearing zone, which lie on the side of the glacier, were caused by damages first observed at the end of the 1990s, the scientists write in their work. Since then, this damage has increased further, destabilizing the southern side of the glacier. According to the researchers, this condition would further promote the dissolution of the ice shelf (the area of the glacier floating on the water) and was partly responsible for the massive decline of the shelf over the past 20 years. As a reason for the weakening on the side, the scientists cite increased movement forces on thinning ice.

The video shows how, over the past two years, huge cracks and crevices on the southern side have accelerated the glacier’s flow, while there has been little movement on the northern side. The time-lapse images were created using satellite images. Video: Lhermitte et al (2020) PNAS

The study’s lead author, Dr. Stef Lhermitte of the TU Delft, writes on the university’s website: “We hypothesize that these damage preconditions these ice shelves for further disintegration.Firstl, because it already compromises the integrity of both ice shelve; Second, because the weakening due to damage makes the future response of both ice shelves more sensitive to varying and extreme future atmospheric, oceanic and sea ice conditions. Third, the damage initiates a feedback process, where damage enhances speedup, shearing and weakening, hence again promoting additional damage.” To test their hypothesis, the research team created models with different weakening conditions and examined satellite images from the past decades. It was shown that the damage on the side had an enormous influence on the loss of ice mass and the retreat of the grounding line. The researchers suspect that this region of the glacier will play an important role in further ice mass losses.

The video shows the development of the grounding line and, at the same time, the damage to the side (P1). The researchers were able to use models to show that these damages caused the decline and were not the result of the retreat. Video: Lhermitte et al (2020) PNAS

The Pine Island Glacier had already gained sad fame in 2014/15, when the front of the ice shelf retreated several tens of kilometres inland. It was assumed that the reasons for this massive retreat were not only by the climate-related warming of the water masses but also by a significant influence of the seabed. In 2018, a research team also discovered volcanism and another team the fast-lifting terrain under the West Antarctic ice sheet as other possible influences. Stef Lhermitte and his team believe that their results need to be incorporated into the models used for predictions for the further development of the Pine Island Glacier. At present, the glacier is considered destabilized and retreats about 10 meters per day. A collapse of the glacier would lead to a chain reaction and allow an uncontrolled draining of part of the West Antarctic ice sheet. This would make an increase in global sea level of around 1 metre inevitable.

The Pine Island Glacier and Thwaites Glacier are part of the West Antarctic ice drainage basin in the Amundsen Sea. They make up a large proportion of this ice outflow, as large parts of the coast have no such outflows.

Dr Michael Wenger, PolarJournal

Link to the study: Lhermitte et al (2020) PNAS, EPub; Damage accelerates ice shelf instability and mass loss in Amundsen Sea Embayment, https://doi.org/10.1073/pnas.1912890117

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