Tuesday , June 15 2021

Surface lakes cause Antarctic ice shelves to "bend"



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IMAGE: Lakes of Antarctic Mineral Water.
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Yours sincerely: Allison Banwell

It has been found that filling and draining lakes from the melt leads to a floating Antarctic ice shelf that could endanger its stability.

A team of British and American researchers led by Cambridge University has measured how much McMurdo's ice shelf in Antarctica bends in response to the filling and draining of melting lakes on its surface. This type of bending was hypothetical before and simulated by computer models, but this is the first time the phenomenon is measured in the field. The results are reported in the magazine Nature Communications,

The results show a link between surface melting and the weakening of Antarctic ice shelves and support the idea that separating the ice shelf around the Antarctic peninsula may have been caused, at least in part, by large amounts of surface melting water produced in response to atmospheric warming. ,

As the climate continues to warm up, more and more ice shelves can become prone to bending, breaking and collapsing in the next century.

The vast majority of the Antarctic continent is covered by the Antarctic ice sheet, which is up to four kilometers thick and contains enough ice to raise the world's sea levels by about 58 meters. In most of the continent and throughout most of the year, air temperatures are well below zero and the ice surface remains frozen. But about 75% of the ice cover is surrounded by drifting ice shelves, which are up to one kilometer thick, mostly below sea level, but with tens of meters of their total height protruding above the water. In the summer months, when the air temperatures rise above the freezing temperature, the surfaces of these shelves are susceptible to melting.

"Surface water on shelves has been known for a long time," said co-author Ian Willis of the Scott Polar Research Institute. "More than 100 years ago, Shackleton's Nimrod team and Scott's Northern Antarctic expedition team mapped and recorded water on the Nansen Ice Shelf, about 300 kilometers from where we surveyed the ice shelf of McMurdo. For decades it has been possible to see the widespread surface melting water formed on many ice shelves every summer by satellite imagery. "

What is not fully known is the extent to which surface water can destabilize the ice shelf, especially in warmer summers when more melted water is produced. If the slope of the ice shelf is steep enough, water can flow from the ice shelf to the ocean into large surface rivers, mitigating potential instability.

The danger comes if the water pours into surface depressions on the ice shelf to form large lakes. The extra weight of the water will drop on floating ice, which will lead to sinking into the sea. Around the edge of the lake the ice will bend up to compensate. "If the lake drains away, the ice shelf will now fold back, rising where the lake was, sinking down the edge," says lead author Dr Alison Banuel, also from SPRI. "It is this filling and draining of the lakes causing the ice shelf to fold, and if the stresses are large enough, fractures can also develop."

Banuel and co-author Doug McCaley of the University of Chicago earlier suggested that filling and draining hundreds of lakes could have led to the catastrophic breakdown of the Larsen B 2002 ills, when 3250 square kilometers of ice was lost in just a few days.

"We have succeeded in modeling the rapid disintegration of this ice shelf through our meltdown destruction mechanism," Banuel said. The problem, however, is that no one actually measured the folding and breaking of the ice shelf in the field, so we could not fully limit the parameters in our model. This is partly the reason to try to measure the McMurdo process. ice shelf. "

Using helicopters, snow machines and their own legs, the researchers created a series of pressure sensors to monitor the rise and fall of water levels in pond-filled depressions and GPS receivers to measure small vertical movements of the ice shelf.

"It was a lot of work to get the data, but they reveal an overwhelming story," McCain said. "The majority of the GPS signal is due to ocean tides that move the floating ice shelf up and down a couple of meters twice a day, but when we removed this tide, we found some GPS receivers moving down, then around one meter, for several weeks, while others, just a few hundred meters, barely moved, and those that moved downward were located where the lakes were filled and drained, and the lakes had relatively little movement. this differential vertical movement, co it shows that the ice shelf bends. We expected this result, but it was very nice when we found him.

The team hopes that their work will inspire others to look for evidence of bending and breaking other ice shelves around Antarctica. Their work will also help to develop large-scale ice plates that could be used to predict the stability of ice shelves in the future and to understand the size of the ice shelf as they act as buffers against the rapidly moving ice from the continent. As ice shelves shrink, the glaciers and ice streams behind them run faster towards the ocean, contributing to the global rise in sea level.

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The work is funded by the US National Science Foundation, Leverhulme Trust, NASA and CIRES, University of Colorado, Boulder.

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