West Antarctica warming fast, may quicken sea level rise-study

OSLO, Dec 23 (Reuters) - West Antarctica is warming almost

twice as fast as previously believed, adding to worries of a

thaw that would add to sea level rise from San Francisco to

Shanghai, a study showed on Sunday.

Annual average temperatures at the Byrd research station in

West Antarctica had risen 2.4 degrees Celsius (4.3F) since the

1950s, one of the fastest gains on the planet and three times

the global average in a changing climate, it said.

The unexpectedly big increase adds to fears the ice sheet is

vulnerable to thawing. West Antarctica holds enough ice to raise

world sea levels by at least 3.3 metres (11 feet) if it ever all

melted, a process that would take centuries.

"The western part of the ice sheet is experiencing nearly

twice as much warming as previously thought," Ohio State

University said in a statement of the study led by its geography

professor David Bromwich.

The warming "raises further concerns about the future

contribution of Antarctica to sea level rise," it said. Higher

summer temperatures raised risks of a surface melt of ice and

snow even though most of Antarctica is in a year-round deep

freeze.

Low-lying nations from Bangladesh to Tuvalu are especially

vulnerable to sea level rise, as are coastal cities from London

to Buenos Aires. Sea levels have risen by about 20 cms (8

inches) in the past century.

The United Nations panel of climate experts projects that

sea levels will rise by between 18 and 59 cms (7-24 inches) this

century, and by more if a thaw of Greenland and Antarctica

accelerates, due to global warming caused by human activities.

GLACIERS

The rise in temperatures in the remote region was comparable

to that on the Antarctic Peninsula to the north, which snakes up

towards South America, according to the U.S.-based experts

writing in the journal Nature Geoscience.

Parts of the northern hemisphere have also warmed at

similarly fast rates.

Several ice shelves - thick ice floating on the ocean and

linked to land - have collapsed around the Antarctic Peninsula

in recent years. Once ice shelves break up, glaciers pent up

behind them can slide faster into the sea, raising water levels.

"The stakes would be much higher if a similar event occurred

to an ice shelf restraining one of the enormous West Antarctic

ice sheet glaciers," said Andrew Monaghan, a co-author at the

U.S. National Center for Atmospheric Research.

The Pine Island glacier off West Antarctica, for instance,

brings as much water to the ocean as the Rhine river in Europe.

The scientists said there had been one instance of a

widespread surface melt of West Antarctica, in 2005. "A

continued rise in summer temperatures could lead to more

frequent and extensive episodes of surface melting," they wrote.

West Antarctica now contributes about 0.3 mm a year to sea

level rise, less than Greenland's 0.7 mm, Ohio State University

said. The bigger East Antarctic ice sheet is less vulnerable to

a thaw.

Helped by computer simulations, the scientists reconstructed

a record of temperatures stretching back to 1958 at Byrd, where

about a third of the measurements were missing, sometimes

because of power failures in the long Antarctic winters.

(Reporting By Alister Doyle; Editing by Janet Lawrence)

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