Ice shelf stability has weakened over eight decades

On 26 February this year, the journal PNAS looked back at the front of the Thwaites Glacier – a 130-kilometre stretch of coastline in West Antarctica. In the paper, a team of geologists show that the glacier started retreating in at least the 1940s. This coincides with data from the equally famous neighbouring Pine Island glacier. The ice giants’ anchorages have gradually disappeared from coastal islands and underwater landforms as a result of external factors: atmospheric and oceanic changes. “This is part of a wider context of climate change,” says Rachel Clark, a geologist at the University of Houston. In this case, an extreme El Niño event could have triggered the process.

Retreat means less ice and Thwaites’ mass loss has accelerated since the 1970s. The difference between upstream snowfall and the flow of ice towards the sea represents a deficit of solid water of 50 billion tonnes, which has contributed to 4% of the current sea level rise.

Image: Alex Mazur

This process is thought to have spread to the Antarctic region. Writing in the journal Nature on 21 February, geologists from the University of Edinburgh used satellite images to go back to 1973 and show that the reduction of underwater anchoring points on ice shelves has accelerated over the last 50 years, doubling in speed. During the first period (1973-1989), ice shelves lost 15% of their attachment points, mainly in the Amundsen Sea (bottom left of the map). The ice tongues concerned lost thickness locally, but for Thwaites, “the anchor points had already been thinning for decades, before the first […] satellite observations”, explain the authors.

Beginning in 1990, this became more widespread from the Antarctic Peninsula and has continued to accelerate to the present. According to the authors: “The general acceleration in the loss of anchor points is striking and portends a bleak future for many ice shelves,” with new signs of weakness coming from the coastal glaciers between Dumont d’Urville and Casey (bottom right on the map).

Map: B. Miles and R. Bingham, 2024 / Nature

The seabed and the islands off the coast are less and less able to hold back these floating platforms, and more and more ice is spilling into the sea. On February 5 this year, a study published in AGU looked back at the rapid opening of a crack in the Pine Island ice shelf in 2012. An 11-kilometre fissure opened 400 metres of ice into the sea within five minutes. Its propagation speed was estimated at 35 meters per second. Stephanie Olinger, a geologist at the University of Waterloo (for this study), says: “To our knowledge, this is the fastest rift formation ever observed. The ice behaved like glass,” the physicists note, “but the seawater still slowed the breakthrough, which could have taken much longer.”

Image: Rob Soto