By Alister Doyle, Environment Correspondent

TROLL STATION, Antarctica, Jan 31 (Reuters) - A deep freeze holding 90 percent of the world's ice, Antarctica is one of the biggest puzzles in debate on global warming with risks that any thaw could raise sea levels faster than U.N. projections.

Even if a fraction melted, Antarctica could damage nations from Bangladesh to Tuvalu in the Pacific and cities from Shanghai to New York. It has enough ice to raise sea levels by 57 metres (187 ft) if it melted, over thousands of years.

A year after the U.N.'s Intergovernmental Panel on Climate Change (IPCC) projected sea level rises by 2100 of about 20 to 80 cms (8-32 inches), a Reuters poll of 10 of the world's top climatologists showed none think that range is alarmist.

Six experts stuck by the projections, saying the response of ice sheets in Antarctica and Greenland was still unclear, and four other experts, including one of the authors of the IPCC report, projected gains could be 1 or even 2 metres by 2100.

"Most people looking at it are thinking more in terms of a metre," said John Moore of the Arctic Centre at the University of Lapland. "Insurance companies don't know to a factor of 100 where to set their insurance premiums for coastal areas in Florida."

Some island nations, such as the Maldives in the Indian Ocean, are building defences costing millions of dollars and want to know how high to build.

"I think it will be...certainly at the high end of the range," said Kim Holmen, research director of the Norwegian Polar Institute, at the Troll Station 250 km (155 miles) from the coast in Antartica. Set amid jagged mountains like the mythical homes of troll giants, this part of east Antarctica is the world's deep freeze with no sign of a thaw. Temperatures were about minus 15 Celsius (5.00 F) at the height of the Antarctic summer.

"It's my view that more than a metre of sea-level rise can't be ruled out," said Stefan Rahmstorf, of the Potsdam Institute for Climate Impact Research in Germany. He said many experts "think the IPCC range is unfortunately not the full story".

MORE ICE
Even so, most experts said it is still impossible to model how the ice will react. Antarctica may accumulate more ice this century because of warming, blamed by the IPCC mainly on human use of fossil fuels, rather than slide faster into the sea.

"The crux of this problem is that we are moving into an era where we are observing changes in the climate system that have never before been seen in human history," said Gerald Meehl, of the U.S. National Center for Atmospheric Research.

"Ice sheets fall into that category. Quite simply, at this time we don't have a good upper-range estimate of 'how much sea- level rise and how fast'," he said. Meehl, a coordinating lead author of the IPCC report, said that gave the best view.

The core prediction for sea-level rise by the IPCC, which shared the 2007 Nobel Peace Prize with former U.S. Vice President Al Gore, is for a gain of 18 to 59 cms (7-23 inches) in the 21st century, after 17 cms in the 20th.

The forecast rate includes faster ice flow from Antarctica and Greenland observed from 1993-2003 but the IPCC said this could increase or decrease in future. If the flow grows in line with temperature rises, it would add a further 10 to 20 cms.

"The IPCC range only takes into account things that can be modelled," said Jonathan Gregory of the University of Reading, who was also among authors who stuck by the conclusions. "There are lots and lots of reasons why you can say there will be large changes. But you can't say it without more evidence," he said.

Among worrying scenarios is the chance Antarctica will slide faster into the sea, perhaps if a ring of sea ice melts away in warmer oceans. Or melt water might flow under the ice sheets of Antarctica and Greenland, and act a lubricant to speed a slide.

But glaciers can slow down as well as speed up.

"We don't know much about changes in the speed of outlet glaciers. Some of these in Antarctica and Greenland tend to speed up -- or slow down. Nobody knows," said Philippe Huybrechts at the Free University of Brussels.

Another factor that could dampen any rise is that warmer air can absorb more moisture -- which may paradoxically bring more snow to Antarctica that would thicken the ice sheet and contribute to lower sea levels this century.

Most of the projected sea-level rise by 2100 will be because water in the oceans expands as it warms, with little being added by the ice sheets. Beyond 2100, the IPCC said sea-level rises are likely to go on for centuries.

"In the long term we are in trouble...Greenland is close to a 'tipping point'," or an irreversible meltdown that would last hundreds of years, Huybrechts said. Greenland has enough ice to raise world sea levels by 7 metres if it all vanished.

One IPCC author said the uncertainties are stacking up towards rising seas. "I firmly believe sea-level estimates are conservative," said Andrew Weaver of the University of Victoria, Canada.
"The lower bound should probably be more like 25 cm and the upper bound closer to a metre if you take everything into consideration now," he said.

Moore at the University of Lapland said a so-far unpublished study by his centre showed seas could rise by 1-2 metres by 2100, based on observational records of sea level in the last 150 years.

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-- For Reuters latest environment blogs click on: http://blogs.reuters.com/environment/ (Editing by Sara Ledwith)