Arctic was the Mediterranean of its day (55 million years ago)

The ice-strewn strewn waters of the Arctic Ocean were once as warm as the Mediterranean Sea, an international scientific study has found.

The ice-strewn strewn waters of the Arctic Ocean were once as warm as the Mediterranean Sea, an international scientific study has found.

The Arctic Coring Expedition (Acex) made the surprising discovery after extracting sediment from 400m below the seabed. Samples of fossilised algae showed that, 55 million years ago, the Arctic was a sub-tropical, shallow sea with an average temperature of about 20C.

Scientists from eight countries recovered the samples from waters 1300m deep on the Lomonosov Ridge which runs under the polar ice cap between Siberia and Greenland.

The tiny algal fossils, containing traces of marine plants and animals, dated back to the Palaeocene Eocene period, the warmest period in history in the history of the earth when huge amounts of carbon were released into the sea and atmosphere causing a supercharged greenhouse effect after which the planet gradually cooled.

Dr Michael Kaminski, a palaeontologist from University College London, said very few living things would have survived the conditions. He said: "We're seeing a mass extinction of sea-bottom-living organisms caused by these conditions. Moving forward in time, we see many species disappear. Only a few hardy survivors endure the thermal maximum."

Geologists already knew something about the last 250,000 years of Arctic history thanks to cores taken from the Greenland ice cap. But the six-week Acex expedition - which also set out to find how long the now-melting Arctic ice caps had been in existence - is the first to find evidence of conditions at the North Pole 55 million years ago.

Professor Jan Backman of Stockholm University, one of the two chief scientists of Acex, said: "We now have sediment records going back to 56 million years, which are resting on 80-million-year-old bedrock. The early history of the Arctic Basin will be re-evaluated based on the scientific results collected on this expedition."

The waters of the Arctic are today so cold - on average -1.5C - that the expedition employed three ice breakers - two to clear a path for the drill