* Pine Island glacier shrank 8,000 years ago, lasted decades
* New melt similar, will add to sea level rise-Science study
By Alister Doyle, Environment Correspondent
OSLO, Feb 20 (Reuters) - A thawing Antarctic glacier that is the biggest contributor to rising sea levels is likely to continue shrinking for decades, even without an extra spur from global warming, a study showed on Thursday.
Scientists said the Pine Island Glacier, which carries more water to the sea than the Rhine River, also thinned 8,000 years ago at rates comparable to the present, in a melt that lasted for decades, perhaps for centuries.
"Our findings reveal that Pine Island Glacier has experienced rapid thinning at least once in the past, and that, once set in motion, rapid ice sheet changes in this region can persist for centuries," they wrote in the U.S. journal Science.
A creeping rise in sea levels is a threat to low-lying coasts from Bangladesh to Florida, and to cities from London to Shanghai. Of the world's biggest glaciers, in Antarctica and Greenland, Pine Island is the largest contributor.
The trigger of the ancient thinning, of about a metre (3 ft) a year, was probably a natural climate shift that warmed the sea and melted the floating end of the glacier, removing a buttress that let ice on land slide more quickly into the sea.
"It seems to be a similar mechanism now ... it could easily continue for decades," Professor Mike Bentley of Durham University in England, a co-leader of the project that included experts in the United States and Germany, told Reuters.
Other studies indicate that a build-up of man-made greenhouse gases, rather than natural shifts, is behind the warmer waters blamed for an accelerating thinning and retreat of the glacier in the past two decades, he said.
Regardless of the cause, the glacier's history suggests that nations may have to factor several centimetres of rising sea level from Pine Island alone into their planning for coastal defences. Experts are studying the history of other glaciers for clues to their future.
TEN PINTS A DAY
"The amount of ice being lost from Pine Island glacier is equivalent to every person on our planet pouring 10 pints (5.7 litres) of water into the ocean every day," Professor Andrew Shepherd, an expert at the University of Leeds who was not involved in the study, told Reuters. "That's the last thing our flood defences need right now."
The United Nations' panel on climate change says that global warming means that sea levels are likely to rise 26 to 82 cms (10-32 inches) by the late 21st century, after a gain of almost 20 cms over the last 100 years.
Scientists uncovered the Pine Island glacier's past thinning by studying quartz rocks in which the element beryllium changes when exposed to cosmic rays that bombard the planet's surface.
"It's like a stopwatch," James Smith, an author of the study at the British Antarctic Survey, told Reuters. (Reporting By Alister Doyle; Editing by Larry King)