Gore told the first conference devoted to melting ice, held in the Norwegian town of Tromsoe ahead of the UN meeting in Copenhagen in December, that melting was worse than the worst-case scenarios presented by experts a few years ago.

"This conference is a global wake-up call," Gore said. "The scientific evidence for action in Copenhagen in December is continuing to build up week by week."

He explained why the melting ice posed such a threat to the planet.

"Ice is important through the ecological system of the Earth for many reasons, but one of them has to do with its reflexivity," he said.

Ice reflects 90 percent of the sun's radiation back into the atmosphere. If the ice were to melt, the dark water would not reflect the heat but instead absorb it, thereby accentuating the effect of global warming.

"As it disappears we have to keep in mind that it can come back only if we act fairly quickly," said Gore, who shared the 2007 Nobel Peace Prize with the UN's Intergovernmental Panel on Climate Change.

He explained that "if we keep turning the temperature of the Earth up, then the heat will go to lower depths of the Arctic Ocean and it will be impossible for the ice to come back."

The Arctic ice cap measured 4.13 million square kilometres (1.59 million square miles) in September 2007, its smallest size ever. It is also thinner than ever, making it more susceptible to rapid melting.

Ice melting in the Antarctic and Greenland as well as on the world's glaciers will also have dramatic consequences, Gore said, warning that each one-meter (3.3-foot) rise of water levels will cause 100 million people to become climate refugees.

Melting snow in the Himalayan mountain range, dubbed "the third pole," will meanwhile lead to flooding, then droughts, for 40 percent of the planet's population which depends on that water for survival.

Scientists attending the Tromsoe conference said the future was ominous, with the scope and rapidity of global warming exceeding the UN climate panel's worst scenarios.

"We are in trouble," said Robert Correll, a US scientist and chair of the Arctic Climate Impact Assessment, a report published in 2004 which shed light on the effects of climate change on the region.

Temperatures in the Arctic are climbing twice as fast as in the rest of the world.

Action envisaged around the world to counter global warming will only reduce carbon dioxide (CO2) emissions by a third of what is necessary, Correll said.

And if no other action is taken, the planet's temperature will rise by 4.5 degrees by the end of the century, he said. Experts have warned that a 2.0 percent increase is the most the planet can tolerate.

If his forecast were to come true, sea levels would rise by one metre (yard), he said.

Following the conference, a working group will write a report to raise awareness among decision-makers about the issue of melting ice ahead of the Copenhagen summit.

Meanwhile, the top US climate negotiator said Tuesday he was more optimistic about reaching a new global warming treaty this year after two days of talks in Washington among 18 major economies.

Climate envoy Todd Stern told reporters that the talks were not a "head-butting exercise."

Secretary of State Hillary Clinton on Monday told delegates from major European countries, China, India, Indonesia and other powers that President Barack Obama "and his entire administration are committed to addressing this issue and we will act."

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