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.

Dr Martin Sommerkorn, from the environmental charity WWF's Arctic programme, 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."