Trillion-tonne iceberg breaks off Antarctica

An iceberg the size of Delaware, one of the largest on record, was set adrift after snapping off a West Antarctic ice shelf that is now at increased risk of collapse, scientists said Wednesday.

A crack in the Larsen C ice shelf, a drifting extension of the land-based ice sheet, finally broke through after inching its way across the ice formation for years.

The calving of ice shelves occurs naturally, though global warming is believed to have accelerated the process. Warmer ocean water erodes the underbelly of the ice shelves, while rising air temperatures weaken them from above.

The calving created an iceberg about 5,800 square kilometres (2,200 square miles) big, with a volume twice that of Lake Erie, one of the North American Great Lakes. It is about 350 metres (1,100 feet) thick.

"The iceberg weighs more than a trillion tonnes, but it was already floating before it calved away so has no immediate impact on sea level," said a team of researchers from the MIDAS Antarctic research project.

It will likely be named A68.

"The calving of this iceberg leaves the Larsen C Ice Shelf reduced in area by more than twelve percent, and the landscape of the Antarctic Peninsula changed forever," the team added.

Separation occurred somewhere between Monday and Wednesday, and was recorded by a NASA satellite.

Icebergs calving from Antarctica are a regular occurrence. But given its size, this behemoth will be closely watched for any potential risk to shipping traffic.

The fate of the berg is hard to predict. It may stay in one piece, or break up.

"Some of the ice may remain in the area for decades, while parts of the iceberg may drift north into warmer waters," said lead investigator Adrian Luckman.

According to the European Space Agency, ocean currents could drag the berg, or pieces of it, as far as the Falkland Islands, posting a threat for ships in the Drake Passage.

Records show that large icebergs from the western Weddell Sea, where Larsen C is, tend to make their way into the Antarctic Circumpolar Current, which flows clockwise from west to east around the southernmost continent, or into the South Atlantic.

Will it collapse?

This may heighten the risk of the remaining shelf disintegrating.

Floating ice shelves are fed by slow-flowing glaciers from land. Without them, the glaciers would flow directly into the ocean.

With its new shape and size, Larsen C may be less stable than before, the team warned.

"There is a risk that Larsen C may eventually follow the example of its neighbour, Larsen B, which disintegrated in 2002 following a similar rift-induced calving event in 1995," they said. d Larsen A collapsed in 1995.

If the glaciers held in check by Larsen C were to spill into the Antarctic Ocean, it would lift the global water mark by about 10 centimetres (four inches), other researchers have said.

Swansea University glaciologist Martin O'Leary, another MIDAS project member, said this is the furthest back Larsen C's ice front has been in recorded history.

"We're going to be watching very carefully for signs that the rest of the shelf is becoming unstable," he said in a statement.

"In the ensuing months and years, the ice shelf could either gradually regrow, or may suffer further calving events which may eventually lead to collapse -- opinions in the scientific community are divided," added Luckman.

"Our models say it will be less stable, but any future collapse remains years or decades away."

Human actions have lifted average global air temperatures by about one degree Celsius (1.8 degrees Fahrenheit) since pre-industrial levels, according to scientists.

Antarctica is one of the world's fastest-warming regions.

O'Leary said "we're not aware of any link to human-induced climate change" for the latest calving.