By Environment Correspondent Alister Doyle
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.
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)