LONDON - Global warming will leave the Arctic Ocean ice-free during the summer within 20 years, raising sea levels and harming wildlife such as seals and polar bears, a leading British polar scientist said on Thursday.

Peter Wadhams, professor of ocean physics at the University of Cambridge, said much of the melting will take place within a decade, although the winter ice will stay for hundreds of years.

The changes will mean the top of the Earth will appear blue rather than white when photographed from space and ships will have a new sea route north of Russia.

Scientists say evidence of melting Arctic ice is one of the clearest signs of global warming and it should send a warning to world leaders meeting in Copenhagen in December for U.N. talks on a new climate treaty.

The data supports the new consensus view -- based on seasonal variation of ice extent and thickness, changes in temperatures, winds and especially ice composition -- that the Arctic will be ice-free in summer within about 20 years, Wadhams said in a statement. Much of the decrease will be happening within 10 years.

Wadhams, one of the world's leading experts on sea ice cover in the North Pole region, compared ice thickness measurements taken by a Royal Navy submarine in 2007 with evidence gathered by the British explorer Pen Hadow earlier this year.

Hadow and his team on the Catlin Arctic Survey drilled 1,500 holes to gather evidence during a 280-mile walk across the Arctic. They found the average thickness of ice-floes was 1.8 meters, a depth considered too thin to survive the summer's ice melt.

Sometimes referred to as the Earth's air-conditioner, the Arctic Sea plays a vital role in the world's climate. As Arctic ice melts in summer, it exposes the darker-colored ocean water, which absorbs sunlight instead of reflecting it, accelerating the effect of global warming.

Dr Martin Sommerkorn, from the environmental charity WWF's Arctic program, which worked on the survey, said the predicted loss of ice could have wide-reaching affects around the world.

The Arctic Sea ice holds a central position in our Earth's climate system. Take it out of the equation and we are left with a dramatically warmer world, he said.

This could lead to flooding affecting one-quarter of the world's population, substantial increases in greenhouse gas emissions .... and extreme global weather changes.

Britain's Energy and Climate Change Secretary Ed Miliband said the research sets out the stark realities of climate change.

This further strengthens the case for an ambitious global deal in Copenhagen, he added.

(Editing by Jon Boyle)

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