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Why the "Doomsday glacier" is melting from below

The Thwaites Ice Shelf in the Amundsen Sea is one of the biggest ice sheets in West Antarctica but it is highly unstable and disintegrating rapidly. It buttresses and braces against the mighty Thwaites glacier and slows down its rate of melting, at least on its eastern edge. This is important because the Thwaites glacier, also nicknamed the “Doomsday glacier,” has been retreating rapidly over the past 20 years and has become the largest net contributor to sea-level rise among the Antarctic glaciers. 

For this reason, an international team of scientists, led by researchers from the University of East Anglia in the UK, investigated the factors influencing the melt rate of the Thwaites Ice Shelf. Using two automated sensor systems, moored about 4 km from one another, the researchers collected data from different depths under the Thwaites Ice Shelf. The sensors recorded water temperature, salinity, pressure and horizontal current speed at an upper depth of a few hundred meters, and at a lower depth of over 700 meters. 

In their research study, published in Nature Communications, the researchers found that the shallow layers of the ocean underneath the Thwaites Ice Shelf warmed considerably during the period between January 2020 and March 2021. It did not appear that the excess heat originated locally because there was little evidence of melting at the base of the ice sheet where the sensors were installed. In addition, the content of glacial-meltwater under the ice increased in the upper layers at the same time. 

Combining the measurements with computer simulations, the researchers determined that most of this meltwater likely originated from the Pine Island Ice Shelf, further to the east, and flowed into the area beneath the Thwaites Ice Shelf under the influence of a gyre (circulating ocean current) centered in Pine Island Bay.  This proposed mechanism explaining how warm meltwaters are found under the Thwaites Ice Shelf was also supported by data collected using tags that were attached to seals. 

Both the model simulations and the data from seal tags showed that a gyre near the Thwaites Ice Shelf weakens in winter, which allows more heat to reach shallow areas beneath the ice shelf. Satellite images showed that the Southern Hemisphere summer season of 2020/2021 was unusual because it had a high concentration of sea ice in regions near the Thwaites Ice Shelf, and this is known to weaken the gyre in Pine Island Bay, meaning that excess meltwater from adjacent ice sheets is not removed by the gyre but instead enters the cavity under the Thwaites Ice Shelf.

When the base of the Pine Island Ice Shelf comes into contact with slightly warmer ocean water, it melts and the fresh water mixes with the saltwater. This makes the ocean water less saline and, therefore, less dense. It forms a buoyant layer of water that is warmer than the surrounding water, and when this flows under the Thwaites Ice Shelf, it melts the base of the ice there. This means that the melting of one ice shelf can influence the melting of another ice shelf nearby. This is a factor that has not been considered before.

“We have identified another process that could impact the stability of ice shelves, revealing the importance of local ocean circulation and sea-ice,” said study lead author Dr. Tiago Dotto of the Centre for Ocean and Atmospheric Sciences at UEA.

“Circumpolar Deep Water, a warm variety of Antarctic waters, is a key player in melting the base of ice shelves,” he said. “However, in this study, we show that a great amount of heat at shallow layers beneath one ice shelf can be provided by waters originating from other melting ice shelves nearby. Therefore, what happens to one ice shelf, can impact the adjacent ice shelf, and so on.”

“This process is important for regions of high ice shelf melting such as the Amundsen Sea because one ice shelf sits next to the other, and the export of heat from one ice shelf can reach the next one through the ocean circulation.”

“These atmosphere-sea-ice-ocean interactions are important because they can prolong warm periods beneath ice shelves by allowing warm and meltwater-enriched water to enter adjacent ice-shelf cavities,” said Dr. Dotto.

“Gyres potentially existing in other regions around Antarctica may also cause a greater number of ice shelves to be prone to intense basal melting associated with prolonged warm conditions, and as a result further contribute to global sea-level rise.”

The authors conclude that ice shelves are connected through the coastal circulation and that what happens under one ice shelf greatly influences what happens under the ice shelves further downstream in the coastal current. They emphasize that future models should consider the meltwater pathways from adjacent ice shelves if they are to simulating the fate of the Antarctic ice shelves accurately. 

Image Credit: Karen Alley

By Alison Bosman, Staff Writer

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