HOBART (Reuters) - 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 (news - web sites).
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.
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.
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.
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.