Sea Ice Loss And Ocean Swells Triggering Collapse Of Antarctic Ice Shelves

The contribution of the Antarctic Ice Sheet is the 'greatest source of uncertainty' in global sea rise projections.

Ice shelves in Antarctica slowly breaking off and collapsing into the ocean are nothing new. 

How this “catastrophic disintegration” affects global sea levels stirs great uncertainty among scientists -- as does the question why it occurs.

Now a previously under-appreciated link between ice shelf collapse and the loss of sea ice has been identified by a team of international researchers, led by Australians.

Ocean swells, following sea ice loss hve been found to trigger ice shelf collapse. Image: Nick Roden

Ice shelves on the Antarctica Peninsula have been increasingly exposed to ocean swells due to reduced sea ice coverage since the late 1980s, according to new research published in Nature journal on Thursday.

These swells have caused them to flex and break.

What are ice shelves? And sea ice?

An ice shelf is a thick plate of ice that juts out into sea.

Up to several hundred metres thick, they are floating extensions of grounded ice sheets, which are formed through the build up of snowfall.

“Over thousands of years the layers of snow build up and compact, forming a sheet of ice up to thousands of metres thick and thousands of kilometres wide,” according to the Antarctic Climate and Ecosystems Cooperative Research Centre.

Scar inlet Ice Shelf. Image: Ted Scambos

Sea ice is different. It originates from the freezing of seawater.

It typically forms a “thin and highly-dynamic veneer” up to a few metres thick.

The sea ice acts as a “protective buffer” to ice shelves “by dampening destructive ocean swells before they reach the ice shelf edge,"said Lead study author Dr Rob Massam, from the Australian Antarctic Division.

“But where there is loss of sea ice, storm-generated ocean swells can easily reach the exposed ice shelf, causing the first few kilometres of its outer margin to flex,” he said. 

“(This) triggers the runaway collapse of large areas of ice shelves weakened by pre-existing fracturing and decades of surface flooding.”

Sea ice. Image: Jessica Fitzpatrick
When does this happen?

Back in 1995, 1600 square kilometres of ice disintegrated from the Larsen A ice shelf.  Seven years later, 3320 square kilometres were “abruptly and rapidly” lost from the Larsen B Ice Shelf.

That collapse in 2002 happened in a matter of days, removing an are of ice shelf that had been in place for the previous 11,500 years.

These were among the five major ice shelf disintegrations the team of international researchers analysed by combining modelling with satellite images and surface and ocean data.

They found each event occurred during periods when sea ice was significantly reduced or absent -- or when ocean swells were large.

Ice calves off an ice shelf. Image: Ian Phillips
Why does this matter?

Ice shelves fringe about three quarters of the Antarctic coast, according to study co-author Dr Luke Bennetts, from the University of Adelaide’s School of Mathematical Sciences.

Their collapse does not directly raise sea levels because they are breaking off floating ice. 

But as climate changes, Bennetts said the findings will help to predict the contribution of Antarctica’s Ice Sheet -- an area of about 14 million square kilometres -- to sea levels.  

“They play a crucially important role in moderating sea level rise by buttressing and slowing the movement of glacial ice from the interior of the continent to the ocean,” he said.

“While ice shelf disintegration doesn’t directly raise sea levels … the resulting acceleration of the tributary glaciers behind the ice shelf, into the Southern Ocean, does.”

He said the research highlights the need for ice sheet modelling to include sea ice and ocean waves.

In turn, this could help scientists to more accurately forecast the fate of what ice shelves remain.