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WASHINGTON (Reuters) - The Arctic ice cap is melting much faster than expected and is now about 30 years ahead of predictions made by the Intergovernmental Panel on Climate Change, a U.S. ice expert said on Tuesday.

This means the ocean at the top of the world could be free or nearly free of summer ice by 2020, three decades sooner than the global panel's gloomiest forecast of 2050.

No ice on the Arctic Ocean during summer would be a major spur to global warming, said Ted Scambos, a glaciologist at the National Snow and Ice Center in Colorado.

"Right now ... the Arctic helps keep the Earth cool," Scambos said in a telephone interview. "Without that Arctic ice, or with much less of it, the Earth will warm much faster."

That is because the ice reflects light and heat; when it is gone, the much darker land or sea will absorb more light and heat, making it more difficult for the planet to cool down, even in winter, he said.

Scambos and co-authors of the study, published in the journal Geophysical Research Letters, used satellite data and visual confirmation of Arctic ice to reach their conclusions, a far different picture than that obtained from computer models used by the scientists of the intergovernmental panel.

"The IPCC report was very careful, very thorough and cautious, so they erred on the side of what would certainly occur as opposed to what might occur," Scambos said in a telephone interview.

ICE-FREE SUMMER

The wide possibility of what might occur included a much later melt up north, or a much earlier one, Scambos said.

"It appears we're on pace about 30 years earlier than expected to reach a state where we don't have sea ice or at least not very much in late summer in the Arctic Ocean," he said.

He discounted the notion that the sharp warming trend in the Arctic might be due to natural climate cycles. "There aren't many periods in history that are this dramatic in terms of natural variability," Scambos said.

He said he had no doubt that this was caused in large part by greenhouse gases in the atmosphere, which he said was the only thing capable of changing Earth on such a large scale over so many latitudes.

Asked what could fix the problem -- the topic of a new report by the intergovernmental panel to be released on Friday in Bangkok -- Scambos said a large volcanic eruption might hold Arctic ice melting at bay for a few years.

But he saw a continued warm-up as inevitable in the coming decades.

"Long-term and for the next 50 years, I think even the new report will agree that we're in for quite a bit of warming," Scambos said.

"We just barely now, I think, have enough time and enough collective will to be able to get through this century in good shape, but it means we have to start acting now and in a big way."