HOBART, AUSTRALIA - The Antarctic Peninsula ice shelves are cracking up and, on the face of things, it is the most serious thaw since the end of the last ice age 12,000 years ago.

The break-up of the ice shelves in itself is a natural process of renewal, but the size and rate of production of icebergs -- some the size of major cities -- is alarming scientists, who blame global warming.

The break-off last month of a 500 billion ton chunk of the Larsen Ice Shelf -- 650 feet thick and with a surface area of 1,250 sq. miles -- 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, kilometers 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 Center (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 meters (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 Organization (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 modeling results indicate production of this dense, rich water has fallen by 20 percent from pre-industrial times.

ROBOTIC FLOATS CHECK ANTARCTIC

Two technology-crammed research ships, the 1,594 ton former Arctic trawler "The Southern Surveyor" and its bigger cousin, the bright orange "Auora Australis," ride at anchor next to CSIRO Marine Research headquarters at Hobart harbor .

Both vessels are allowing scientists to probe the southern seas as never before, as they deploy thousands of robotic floats and tons of sensitive equipment in parts of the Antarctic.

Senior physical oceanographer Nathan Bindoff is conducting the first study of ocean circulation under East Antarctica's Amery Ice Shelf.

"(Results show) the ice shelves are vulnerable to climate change," Bindoff said. "An increase in temperature over the continental shelf (leads to) slightly warmer water at the back of the ice shelves...the melt rate goes up."

A small increase in ocean temperature from climate warming could produce a doubling of the melt, which would cause the ice shelf to shrink dramatically, recede and break off, he said.

Two years of physical research is proving model results, that the entire coastal shape of the 550 km long, 200 km wide Amery Ice Shelf could soon change as it melts back, he said.

A 1999 expedition to the Antarctic south of Tasmania, near Commonwealth Bay, yielded even more alarming results.

An open coastal area near Dumont d'Urville in French territory has been found to produce the most important source in East Antarctica of bottom water -- "the lungs of the ocean."

In the depths of winter, strong freezing winds cascade down the Arctic continent to race across the ocean surface, pushing ice floes away, forming new sea in open water near the coastline.

The oxygen-rich highly-saline seawater which remains sinks to the ocean floor to form 20-25 percent of Antarctica's total bottom water production, which then circulates the globe, promoting ocean circulation and life.

Bottom water is also sensitive to climate change, with no production near Dumont d'Urville in some years, Bindoff said.

"These patterns are beyond natural variability," he said.

CHANGING SEAS

One question occupying Tom Trull, leader of Biogeochemical Cycles Program at the CRC, is whether disappearance of half the Antarctic's sea ice by the end of the century would also halve the Southern Ocean's krill, the tiny planktonic crustaceans which are most abundant animal organism on earth.

Krill, the keystone of the Antarctic ecosystem and bread and butter for seals, penguins and whales, need ice for sanctuary and for food from algae.

Trull says CRC scientists predict a 15 percent drop in total global marine phytoplankton production by the end of the century because of slowing ocean circulation.

By then, melting of the grounded Antarctic ice sheet could be adding to predicted sea level rises of 30-50 centimeters this century. And fears remain about the long-term stability of the West Antarctic ice sheet because of rises in ocean temperature.

"It is unlikely to collapse over the next 100 years, but projections on a longer term are uncertain," said John Church, Polar Waters Program leader in the CRC.

Copyright © 2002 Reuters Limited

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