OSLO (Reuters) - Global warming will melt most of the Arctic icecap in summertime by the end of the century, a report showed Wednesday.

The three-year international study indicated that ice around the North Pole had shrunk by 7.4 percent in the past 25 years with a record small summer coverage in September 2002.

"The summer ice cover in the Arctic may be reduced by 80 percent at the end of the 21st century," said Norwegian Professor Ola Johannessen, the main author of the report funded by the European Commission.

The Arctic Barents Sea north of Russia and Norway could be free of ice even in winter by the end of the century, said Johannesssen, who works at the Nansen Environmental and Remote Sensing Center in Norway.

"This will make it easier to explore for oil, it could open the Northern Sea Route (between the Atlantic and Pacific oceans)," he said of the report, dubbed the Arctic Ice Cover Simulation Experiment.

Moscow and Norway reckon the Barents Sea could be a promising new area for oil and gas. The Northern Sea passage could save shippers about 10 days on a trip from Japan to Europe compared to traveling through the Suez Canal.

Johannessen said that the report, published on the Internet ahead of peer review, also indicated that a recent thinning of the polar icecap was linked to human emissions of gases like carbon dioxide blamed for blanketing the planet.

But the study showed a thinning of the icecap from 1920-1940 was caused by natural climate fluctuations, such as ocean currents and winds, rather than by a build-up of greenhouse gases.

Johannessen said the new survey added to evidence of a gradual thinning of the icecap and gave firmer signs that human emissions, such as exhausts from cars and factories, were mainly to blame.

Climate experts say that polar areas are heating up more than other regions.