The break-off last month of a 500 billion tonne chunk of the Larsen Ice Shelf - 650 feet (200 metres) thick and with a surface area of 1,250 sq miles (3,240 sq km) - is the second big break since a giant iceberg broke away in 1995 and is well beyond normal activity, scientists say.

The production of vast amounts of icebergs is a threat to the world's climate and the way the ocean's function, they say. And the process, once started, cannot be reversed.

The fear is that a snowball effect will lead to disintegration of the vast West Antarctic ice shelf, kilometres thick in parts.

"The (first) break-off said 'this is not theory, it's real - a rapid and dramatic collapse of an ice shelf can happen'," says Neal Young, glaciologist with the Antarctic Cooperative Research Centre (CRC) in Hobart.

"This is saying 'that wasn't a one-off thing.'"

Significant warming in parts of the pristine Antarctic wilderness is expected to continue to send huge icebergs into the Southern Ocean, and lead to the disintegration of other sections of ice shelves that fringe Antarctica's continental ice cover.

A longer-term effect would be if the disintegration led to a meltdown of the grounded West Antarctic ice sheet, which would cause the world's oceans to rise by up to five metres (17 feet).

As they delve deeper into the mysteries of the southern continent, scientists are finding a jigsaw on a gigantic scale.

The Antarctic Peninsula, which juts out into the Southern Ocean, has warmed by 2.5 degrees Celsius over the past 50 years, while some other areas have cooled. Some parts of West Antarctica have been losing ice, while, like shifting grains of sand on a beach, ice has built up elsewhere.

LONG-TERM FEARS

But the main message from the world's biggest concentration of Antarctic scientists in Hobart, in Australia's southernmost city, is of retreating West Antarctic ice and massive break-offs.

Scientists are not too worried for the moment about rising sea levels. This is because floating ice shelves displace large amounts of sea water, and sea levels would effectively remain unchanged if the ice shelfs disappeared.

The real problems arise if the ice built up over millions of years on parts of Antarctica's land mass melts.

"We aren't too worried about the first 100 years or so when the ice shelves go, because there's no real effect on sea level and feedback on global climate is really rather small," said Bill Budd, Professor of Meteorology at the CRC.

The CRC is a co-operative body between Australia's Antarctic Division, the Commonwealth Scientific and Industrial Research Organisation (CSIRO), the University of Tasmania and other bodies.

But scientists believe that the expected loss of half the Antarctic's sea ice by the end of the century will have important consequences for Earth's entire natural system.

They are finding that the world's deep ocean circulation system will slow as the Antarctic produces smaller amounts of dense oxygen-rich seawater, possibly within 30 years, threatening marine life.

"We can't reverse it. Because the greenhouse gas levels are already up, we can't bring them down, they just get higher, and the (ocean) cutoff will be stronger at higher levels," Budd said.

The Antarctic is normally the source for a large part of the "bottom water" which feeds oxygen to global ocean depths. And computer modelling results indicate production of this dense, rich water has fallen by 20 percent from pre-industrial times.

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