SHELDON GLACIER, Antarctica (Reuters) - More rain on the Antarctic Peninsula is speeding a melt of glaciers such as the Sheldon, which has retreated 2 km (1.2 miles) in 20 years and is nudging up world sea levels, a leading expert said.

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“Rain is very corrosive to glaciers and at least in part the reason this glacier is retreating,” David Vaughan, a British Antarctic Survey glaciologist, said on an inflatable speedboat in a bay that had been blanketed by ice for thousands of years.

“The glacier has retreated since 1989 and left this open water. That’s the same pattern for 87 percent of 400 glaciers along the Antarctic Peninsula,” he told Reuters.

The ice cracks and growls as the 70-meter-high (230 feet) ice cliffs at the front of the Sheldon glacier slide downhill, some of the ice a bluish white. Icebergs sometimes split off into the sea, where penguins and seals swim.

The front edge of the Sheldon -- a small glacier by Antarctic standards -- has receded 2 km since 1989, apparently because of global warming blamed on greenhouse gases emitted by burning fossil fuels, Vaughan said.

Temperatures were above freezing with clear skies on Friday and Thursday. Rain spattered the glacier on Wednesday and has fallen several times this month.

Vaughan said rain was becoming more frequent in summertime on the peninsula, the northernmost part of Antarctica that sticks up toward South America. The peninsula is warming faster than anywhere else in the southern hemisphere.

SEA LEVEL RISE

And the thaw of Sheldon, near the British Rothera reseach station and other peninsula glaciers, is part of a wider melt adding to world sea levels.

“It doesn’t add up to much on its own but by the time we’ve added Patagonia, Alaska, all those other areas where glaciers are receding, we have 1 millimeter (0.04 inch) at least (a year) of sea level rise around the globe,” Vaughan said.

Adding in other factors including that water expands as it warms up, ocean levels are rising 3 mm a year -- or 30 cm (11.8 inches) a century. And the rate is accelerating after a gain of 17 cm in the 20th century.

Vaughan, a leading member of the U.N. Climate Panel, said there were worrying signs that vast glaciers to the south were also starting to spill more water into the sea.

“The concern is ... that the much bigger glaciers (further south) are going to start doing the same thing,” he said. The large Pine Island glacier about 500 km to the south has also accelerated.

The peninsula covers a fraction of Antarctica, a frozen continent bigger than the United States that contains most of the world’s fresh water.

More than 190 governments have agreed to work out by the end of 2009 a new U.N. treaty to fight global warming, partly fearing that rising sea levels could swamp low-lying Pacific Islands or flood coastal cities from Amsterdam to Sydney.

Average temperatures on the peninsula have risen by up to 3 Celsius (5.4 F) in the past 50 years against a world average of 0.7 Celsius in the past century.