Scientists discover a new cause of Antarctic ice shelf melting

An international team of researchers has discovered that nearby ice shelves play a role in causing instability on other downstream ice shelves. The University of East Anglia in the United Kingdom conducted research that found that the amount of glacial meltwater flowing under the Thwaites Ice Shelf could be due to a small ocean eddy next to it. The weaker circulation causes more warm water to enter the area under the ice shelf, causing it to melt.

The Thwaites Ice Shelf is one of the largest ice shelves in West Antarctica and supports the eastern portion of the Thwaites Glacier, which has retreated rapidly over the past 20 years and is the largest source of global sea level rise among Antarctic glaciers.

Using a unique dataset collected by sensors placed under the Thwaites Ice Shelf, which has thinned and weakened significantly in recent years, the researchers observed that the shallow layers of the ocean beneath warmed significantly between January 2020 and March 2021. Much of this warming was caused by large volumes of glacial meltwater originating from the Pine Island Ice Shelf further east and flowing into the area below the Thwaites Ice Shelf.

As the ocean melts the bottom of the ice shelf, glacial meltwater mixes with salt and can form a layer of floating water that is warmer than the surrounding waters. This lighter, relatively fresher and warmer water brings the heat that melts the bottom of the Thwaites Ice Shelf.

Lead author Dr Thiago Dotto, from the UEA’s Center for Ocean and Atmospheric Sciences, said: “We have identified another process that can affect ice shelf stability, demonstrating the importance of local ocean and sea ice circulation. Circumpolar Deep Water, a warm variant of Antarctic waters, is the bottom of the ice shelf floor. However, in this study we have shown that a large amount of heat in shallow layers below an ice shelf can be supplied by water from other nearby melting ice shelves, so what happens to one ice shelf can affect the neighboring 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 is adjacent to another, and heat output from one ice shelf can reach the other via ocean circulation.”

Dr Dotto added: “These atmosphere-sea-ice-ocean interactions are important because they can extend warming times under the ice shelves by allowing warm and melt-enriched water to enter nearby ice shelf spaces. The eddies that potentially exist in other regions around Antarctica, may cause the ice shelf to undergo intense basal melting associated with prolonged warm conditions and, as a result, further contribute to global sea level rise.”

In January 2020, colleagues from the US drilled holes in the ice and placed sensors under the Thwaites Ice Shelf that monitor temperature, salinity, and ocean currents.

For more than a year, these sensors have sent back data via satellite, which is used to determine ocean fluctuations, such as how temperature and meltwater content change. From these observations, the researchers suggested that the extreme heat could not have occurred locally on the Thwaites Ice Shelf, as they did not see much melting in the areas where the sensors were placed.

Combining the information with computer modeling to identify the source of this heat, they found that water leaving the Pine Island Ice Shelf may be reaching areas below the Thwaites Ice Shelf.

The mechanism describing how these waters reached the Thwaites Ice Shelf was identified using data collected using model simulations and labels affixed to seals. Both showed that the vortex near the Thwaites Ice Shelf weakens in winter, allowing more heat to reach the shallower areas below the ice shelf.

Satellite images also showed that the 2020/2021 Southern Hemisphere summer season was unusual as areas near the Thwaites Ice Shelf had a high sea ice concentration.

Based on modeling and previous research, the team hypothesized that the circulation was even weaker, so excess meltwater from nearby ice shelves could not be carried out of the area by currents and instead ended up at the Thwaites Ice Shelf. This further reduced the strength of the circulation and allowed water with a higher concentration of glacial meltwater to flow under the ice shelf.