The North Pole, synonymous with all things very cold, once had a subtropical climate, according to scientists now returning from the Arctic.

The international scientific team, taking part in the Integrated Ocean Drilling Program's Arctic Coring Expedition (ACEX), recently has discovered that the Arctic Ocean once was ice-free because of prehistoric global warming.

The scientific team from eight nations has recovered sediment cores from nearly 1,312 feet below the seafloor, in waters 4,265 feet deep. “The early history of the Arctic Basin will be re-evaluated based on the scientific results collected on this expedition,” says Professor Jan Backman, Stockholm University, one of the co-chief scientists.

The cores show evidence of subtropical, shallow seas in the form of tiny fossils - extinct marine plant and animals. These date back to a period known as the “Palaeocene-Eocene thermal maximum”, a brief period that occurred around 55 million years ago. Scientists identified the interval through specific algae, which lived only in subtropical conditions. The algae fossils reveal that the Arctic Ocean once was much warmer around 68 degrees F, similar to the waters around New York in August - compared with today's freezing temperatures that average around 29 degrees F.

Prof Backman adds: “We also were surprised to find fresh water conditions and periods of extreme warmth. This indicates environmental conditions were more variable than anticipated. We have now sediment records going back to 56 million years, which are resting on 80-million-year-old bedrock.”