"Today's models under-predict how warm the poles were back then, which tells you something disturbing -- that the models, if anything, aren't sensitive enough to greenhouse gases," said Matthew Huber of Purdue University in the United States.

The co-author of two of three reports in the journal Nature that analysed the 400-metre (yard) long sediment core said it provides climate modellers with a once in a lifetime experience.

In addition to the soaring temperatures which scientists think were triggered by a massive release of carbon from volcanoes or underwater stores of methane, the sediment revealed evidence that the Arctic Ocean was covered in fresh water ferns in the summer months around 50 million years ago.

"Imagine that the Arctic Ocean was a giant lake, with this vegetation growing on it," said Henk Brinkhuis, a researcher from Utrecht University in the Netherlands.

"What these findings say is that the Arctic Ocean must have been isolated, or nearly cut off, from the Atlantic Ocean by land masses that later shifted into the present continents," he added.

Pea-sized pebbles trapped inside the core also show that glaciers formed in the Arctic Ocean about 14 million years earlier than scientists had thought.