Michael Lucy
The thinning of floating ice shelves around Antarctica can affect the flow of glaciers as far as 900 kilometres away, according to new research that shows the fragility of the barriers keeping the southern polar ice from sliding into the ocean. The fate of the Antarctic ice cap, which holds enough frozen water to raise sea levels by almost 60 metres, will have repercussions around the world.
“Thinning in a small, localised ice-shelf area can influence glacial movement over much larger distances than we previously thought,” says Ronja Reese of Germany’s Potsdam Institute for Climate Impact Research, lead author of the paper published in Nature Climate Change. Many of the most critical regions, she adds, “are located in areas that could be easily accessed by warm ocean waters”.
The ice shelves that surround Antarctica are floating extensions of the continental ice mass that scientists believe will in large part determine how much and how quickly sea levels rise as the world heats up.
Floating ice won’t directly increase the level of the sea when it melts, in the same way that a melting ice cube doesn’t raise the level of a drink in a glass.
“But it is known that melting of ice shelves can cause mass losses and sea-level rise indirectly,” says Reese. “Ice shelves buttress the ice flow from the continent into the ocean.”
A perfect experiment to show the effect of ice shelves occurred in 2002, when the Larsen B ice shelf collapsed entirely. In short order, the glaciers it had been holding back began to move almost 10 times as quickly as before.
Most of the changes to ice shelves are less dramatic than total collapse: the warm waters lapping at their undersides cause gradual thinning which weakens the shelf.
Reese and her colleagues set out to study what effect this thinning has on the flow of glaciers by building a computer model of the stresses and flows within the Antarctic ice shelves.
Using their model, they developed maps that show where thinning will have the greatest effect on the rate of ice flow. The greatest effects occurred where the floating ice is connected to the onshore ice, while ice loss from certain areas will not have much effect elsewhere.
One of the most interesting discoveries was a long-range effect the researchers call “tele-buttressing”, which is a result of the force and stress patterns within the ice sheets.
“This tele-buttressing can reach as far as 900 kilometres,” says Reese. Thinning near Ross Island, for instance, can accelerate the Bindschandler ice stream on the opposite side of the Ross ice shelf.
The modelling also confirms that the spectacular calving of a trillion-tonne iceberg from the Larsen C ice shelf in July 2017 occurred in a relatively “passive” part of the shelf, meaning it will have little effect on the outflow of nearby glaciers.
“We have created a risk map of ice-shelf regions,” says Reese, which shows areas that deserve careful monitoring for signs of thinning. Future research may focus on more detailed modelling of the knock-on effects of current thinning on ice flows, and how those increased flows may in turn influence sea level rise.
