Hugh Jenkyns, of the University of Oxford in England, and scientists from the Royal Netherlands Institute for Sea Research analysed organic material in mud in the core and calculated that the mean sea surface temperature of the Arctic Ocean was 15 degrees Centigrade (59 Fahrenheit) about 70 million years ago.

"Further back in time if would have been even warmer, so it would have been a good place to swim," said Jenkyns, who reported the findings in the journal Nature on Wednesday.

Twenty million years earlier it could have reached 20 degrees Centigrade (68F).

"It was certainly very much a greenhouse world," he told Reuters.

Although the scientists don't know why it was so warm, they suspect it may have been due to high concentrations of atmospheric carbon dioxide, the greenhouse gas blamed for contributing to current global warming.

In a commentary in the journal, Christopher Poulsen, of the University of Michigan in the United States, said the levels of carbon dioxide were probably three to six times what they are today which contributed to a "super-greenhouse climate".