Here's what scientists think is really happening with the calving of a huge iceberg in Antarctica

Land emerging from the ice. Image: Australian Antarctic Division

There have been mixed reactions from scientists on the underlying forces at work to make a 1-trillion-tonne iceberg, more than four times the size of the ACT, break away from Antarctica.

Some say this is a normal calving events at Larsen C and that there is no evidence of surface melt that led to the rapid and very dramatic collapse of Larsen B in 2002.

Others think global warming is a major factor and that these types of events will contribute to rises in sea levels.

The iceberg is about 30 metres above sea level, with another 200 metres hidden beneath the ocean. It covers an estimated 6,000 square km.

Marine physicist Natalie Robinson, at the National Institute of Water and Atmospheric Research in New Zealand, says this is a normal but relatively large calving event.

“This large iceberg is the result of the relentless seawards slide of the land-based glaciers feeding the ice shelf,” she says.

“In this case, there were a couple of ‘pinning points’ (islands) that meant pressure built up within the ice shelf — and very likely contributed to determining the exact location of the fracture that ultimately created the iceberg.

“The significance of this particular event will come in the next weeks to months as scientists (not me, or anyone in NZ) monitor its response to the calving. Will it flow faster? Will it find new locations to pin or ground? Will there be a response from sea ice in the local/regional vicinity?”

At the University of Melbourne, Professor Ian Simmonds from the School of Earth Sciences, says the calving is deeply troubling.

“This follows the collapse of part of the Larsen B ice shelf (to the north) in February 2002,” he says.

“The causes of these breakups are similar. Temperatures have risen dramatically in the region over recent decades. This has meant that summer temperatures now frequently get above freezing, and the associated surface melting significantly weakens the ice shelves.

“In addition the major warming of the sub-surface Antarctic Ocean is strongly impacting on the thick ice shelves from beneath.”

Professor Nathan Bindoff, Professor of Physical Oceanography at the University of Tasmania, says the big iceberg is a critical component of the story around the fate of the Antarctic Ice Sheet.

“The ice shelves buttress the Antarctic Ice Sheet and slow the rate of ice loss from Antarctica,” he says.

“So a major ‘berg like this one means we will see an acceleration of the grounded glaciers behind the Larsen C shelf.

“Amazingly, this glacier acceleration will contribute to further sea-level rise in next few years. We saw precisely this behaviour for sea-level when the Larsen B ice shelf broke up.”

Bindoff says the big picture concern is that ice shelves are thinning around Antarctica, implying more events like this one, and more acceleration of the glaciers that feed the ice shelves.

“To my mind, this is the million dollar question around Antarctica: how fast is it going to melt going into the future?” he says.

Pat Langhorne, an Antarctic sea ice researcher at the University of Otago, says iceberg calving events are essential to keep the Antarctic ice sheet in mass balance and don’t necessarily mean climate change alarm bells should go off.

“Snow accumulates on the Antarctic continent, turns to ice due to the pressure of the snow above it, and then the ice flows to the edge of the continent to float out on the ocean as a massive ice shelf,” he says.

“These ice shelves lose ice mass by melting at the base and by calving icebergs.

“This process has gone on over the millennia and is the planet’s mechanism for keeping Antarctica’s ice in approximate balance.”

He says it will be very interesting to see what iceberg A68 does next.

Currently, it is mid-winter in the Southern Ocean and the new iceberg is surrounded by sea ice that extends about 1000 km from the Antarctic coastline in some places.

“This sea ice will likely be less than a metre thick, while the iceberg is about 200m thick. But the sea ice will act like a glue that may help to ‘pin’ the iceberg in place until the sea ice starts decreasing in November.”

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