Q&A: Antarctica's new trillion-tonne iceberg

A trillion-tonne iceberg - three times the size of Stewart Island - has calved off Antarctica's Larsen C ice shelf into the sea. There's now disagreement among some scientists over whether human-generated climate change can be ruled out completely as a cause. Science reporter Jamie Morton spoke to Associate Professor Nancy Bertler, of Victoria University's Antarctic Research Centre, who has been observing the vast chunk's decade-long break-off.

What do we know about this event and what caused it?

A 5,800 km2 iceberg broke off the Larsen C ice shelf.

The crack causing the this breakup started a few years ago but accelerated over the past couple of years and again over the past few months.

The occurrence of icebergs is natural.

If the ice sheet, which is ice resting on the continent, is in balance, its ice shelf, which is the tongue floating on the ocean, will lose an equal amount of ice through iceberg calving and ice shelf melting - mainly from ocean melting underneath - as it receives from its parent ice sheet.

It is the sixth largest iceberg recorded and it represents about 13 per cent of the area of the Larsen C ice shelf, which is very large for average icebergs.

Why are there opposing views as to whether this is related to anthropogenic climate change?

It is impossible to determine for any single event whether it is caused by climate change or whether it represents "just" an extraordinary event - climate and environmental processes can be messy.

However, both groups have a point.

The scientists suggesting this is entirely a natural process are correct.

Ice shelves do produce icebergs.

However, scientists concerned about this iceberg point to the fact that the Antarctic Peninsula is the fastest-warming region on the planet and we have observed extensive occurrences of catastrophic collapses of ice shelves there, most notably the rapid disintegration of Larsen B ice shelf in 2002, but many more since then.

This pattern closely follows the southward migration of warming temperatures that reached the northern tip of the Antarctic Peninsula first and moves towards the Antarctic interior.

Larsen C lies south of Larsen B.

It is entirely consistent with this migration of warm temperatures to expect Larsen C to become unstable next.

The scientific community will closely watch the behaviour of the Larsen C ice shelf over the coming months.

This is a little bit like taking the pulse of a sick person, to see if the ice shelf is in trouble already - a fatal disease, if you will - or whether this was just a one-off event - or a flu - for now.

Why do you consider it's too difficult to tell?

This has been a one-off event at Larsen C so far.

Also, we don't have good enough measurements of the thinning rate of Larsen C to see if it has thinned sufficiently to become unstable.

But time will tell whether this event has destablised the shelf.

Under climate change, can we expect more of these calving events?

Yes. Ice shelves are highly sensitive to the warming of the ocean, the change of ocean currents to bring warmer waters to the underbelly of ice shelves and warming of the atmosphere.

As temperatures increase around Antarctica, in particular at the Antarctic Peninsula but more importantly around West Antarctica, and as changing winds bring warm waters to the West Antarctic coastline, ice shelves continue to thin, shrink and disintegrate.

Some ice shelves in West Antarctica lost 20 per cent of the thickness in the past 20 years alone but all ice shelves in West Antarctica reduced.

How does this event compare with the largest break-offs in history?

The largest iceberg to ever be recorded was Iceberg B-15 - we are not great in naming them - which broke off the Ross Ice Shelf.

That berg was almost twice as big as the current iceberg with 11,000 km2.

However, it represented only a very small fraction of the Ross Ice Shelf, which is the size of France and the biggest shelf on the planet.

This iceberg is to my knowledge the sixth biggest iceberg recorded, coming from a much smaller ice shelf, which is why it sits on the fence between being completely natural or indicative of a significant change.

Is this likely to have any flow-on effect to the stability of the both the Larsen C shelf and the continent itself?

The breakup of the iceberg has no direct influence on the ice sitting on the continent behind the shelf.

However, it is still unclear, whether the iceberg destabilised the Larsen C ice shelf.

If it initiated the Larsen C breakup - then that breakup would almost certainly lead to the acceleration of ice flow of the glaciers behind the ice shelf into the ocean.

The glaciers that fed the Larsen B ice shelf which disintegrated in 2002, are still galloping towards the ocean today with a speed six to eight times their pre-breakup speed, adding to global sea-level rise.

While their actual contribution is minimal, in terms of millimetres to centimetres of equivalent sea-level rise, we take careful note of this, because of the large ice shelves - including the Ross and Ronne-Filchner Shelves and the shelves of West Antarctica in the Thwaites Glacier and Pine Island region - have the potential to allow West Antarctic ice to gallop into the Southern Ocean, with very significant implications to global sea-level rise.

The Thwaites Glacier and Pine Island region alone holds about 3.3m of equivalent global sea level.

Is it likely to pose any risk to humans?

It might become a nuisance since its travel path is determined by ocean currents.

As it travels northwards it will be incorporated into the Weddell Sea gyre and either might cause a bit of havoc there for research vessels and tourist ships or could spin into the circumpolar current travelling around Antarctica.

If it runs aground somewhere it could cause a buildup of sea ice, as happened with B-15, that is troublesome for ships, or it could cause damage to other shelves.

This happened with a smaller part of B-15, which damaged first the Drygalski Ice Tongue, and more importantly later, the Mertz Glacier Tongue.

There, the removal of the Mertz Glacier Tongue changed radically the formation of Antarctic Bottom Water - 30 per cent was produced there - with important implications on global heat transport of the ocean and uptake of atmospheric CO2.