Cores retrieved from up to 430 metres (1,397 feet) below the seafloor in waters 1,300 metres (4,550 feet) deep show that, for a brief period which occurred around 55 million years ago, the Arctic Ocean was around 20 C (68 F), compared with today's typical average temperature of minus 1.5 C (29.3 F), they said on Tuesday.

"It occurred during a period called the Palaeocene-Eocene thermal maximum, which was already known as a warm period," Andy Kingdon of the British Geological Survey (BGS) told AFP.

"What no-one expected was how much warmer it really was. That is a huge surprise."

The BGS joined other scientific agencies in a 12.5-million-dollar

million-euro) initiative called the European Consortium for Ocean Research Drilling (ECORD).

Under it, a Swedish-flagged, Norwegian-operated drillship ventured to within 238 kilometers (148 miles) close to the North Pole, protected by a Russian and a Swedish icebreaker, which broke up sea ice around the vessel as it drilled into the sediment.

The evidence of big climate change comes in the form of fossilised marine plants and animals which died out en masse within a relatively short time because they could not cope with the surge in temperatures.

The cores also revealed a band of fossilised extinct algae that only lived in sub-tropical conditions.

"The early history of the Arctic basin will be re-evaluated based on the scientific results collected on this expedition," one of the mission's chief scientists, Jan Backman, a professor at Stockholm University, said.

The expedition returns to Tromso, Norway on September 14 and its members will meet in Bremen, Germany, in November to assess the results.

Although the data is only preliminary, the scientists "are absolutely confident" in their conclusion about the massive temperature swing, said Kingdon.

"This is convincing proof that global warming has a planetary-wide effect, having a major impact on locations even as distant as the North Pole."

As for what have caused the big warming during this period, two theories are being aired, both based on the greenhouse principle in which gases were released into the atmosphere, trapping the Sun's heat.

One is that gigantic volanic eruptions disgorged billions of tonnes of carbon dioxide (CO2) into the air; the other is that frozen methane, held under layers of seabed sediment, was released and melted, also causing a runaway greenhouse effect.

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