13 Jul 2017

Trillion-tonne iceberg breaks off Antarctica

2:44 pm on 13 July 2017

One of the biggest icebergs ever recorded has broken away from Antarctica.

an image obtained from NASA showing the Antarctic Peninsula's rift in the Larsen C ice shelf from NASA's IceBridge mission Digital Mapping System. A trillion-tonne iceberg, one of the biggest on record, has snapped off the West Antarctic ice shelf.

An image obtained from NASA shows the rift in the Larsen C Ice Shelf (from NASA's IceBridge mission Digital Mapping System). Photo: AFP PHOTO / SWANSEA UNIVERSITY/ADRIAN LUCKMAN

The giant block is estimated to cover an area of roughly 6000 sq km, about a quarter the size of Wales - or more than three times the size of New Zealand's Stewart Island.

It is probably in the top 10 biggest icebergs ever recorded.

A US satellite observed the iceberg while passing over a region known as the Larsen C Ice Shelf, near the south Atlantic.

Scientists were expecting it as they have been following the development of a large crack in Larsen's ice for more than a decade and the cracking has accelerated in the past three years.

The more than 200m-thick tabular berg will not move very far, very fast in the short term. But it will need to be monitored.

Currents and winds might eventually push it north of the Antarctic where it could become a hazard to shipping.

An infrared sensor on NASA's Aqua satellite spied clear water in the rift between the shelf and the berg on Wednesday. The water is warmer relative to the surrounding ice and air - both of which are sub-zero.

An illustration depicting the iceberg's detachment from the Larsen C Ice Shelf. A trillion-tonne iceberg, one of the biggest on record, has snapped off the West Antarctic ice shelf, said scientists on July 12, 2017 who have monitored the growing crack for months.

An illustration depicting the trillion-tonne iceberg's detachment from the Larsen C Ice Shelf. Photo: AFP PHOTO / SWANSEA UNIVERSITY/ADRIAN LUCKMAN

"The rift was barely visible in these data in recent weeks, but the signature is so clear now that it must have opened considerably along its whole length," explained Prof Adrian Luckman, whose Project Midas at Swansea University has followed the iceberg's evolution most closely.

The event was confirmed by other spacecraft such as Europe's Sentinel-1 satellite-radar system.

Scientists think Larsen C is now at its smallest extent since the end of the last ice age some 11,700 years ago, and about 10 other shelves further to the north along the Peninsula have either collapsed or greatly retreated in recent decades.

The two nearby, smaller shelves, Larsen A and Larsen B, disintegrated around the turn of the century; and a warming climate very probably had a role in their demise.

But Larsen C today does not look like its siblings. Prof Helen Fricker, from the Scripps Institution of Oceanography, told BBC News: "The signs we saw at Larsen A and B - we're not seeing yet.

The thinning we saw for Larsen A and B - we're not seeing. And we're not seeing any evidence for large volumes of surface meltwater on the order of what you would need to hydro-fracture the ice shelf.

"Most glaciologists are not particularly alarmed by what's going on at Larsen C, yet. It's business as usual."

The largest iceberg observed in the satellite era was an object called B-15.

It came away from the Ross Ice Shelf in 2000 and measured some 11,000 sq km. Six years later, fragments of this super-berg still persisted and passed by New Zealand.

In 1956, it was reported that a US Navy icebreaker had encountered an object of roughly 32,000 sq km. That is bigger than Belgium. Unfortunately, there were no satellites at the time to follow up and verify the observation.

It has been known also for the Larsen C Ice Shelf itself to spawn bigger bergs. An object measuring some 9,000 sq km came away in 1986. Many of Larsen's progeny can get wound up in a gyre in the Weddell sea or can be despatched north on currents into the Southern Ocean, and even into the South Atlantic.

A good number of bergs from this sector can end up being caught on the shallow continental shelf around the British overseas territory of South Georgia where they gradually wither away.

- BBC / RNZ

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