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.

 

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.

 

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.