GLOBAL warming 49 million years ago was of such intensity that it left the Arctic warm enough to become overgrown with vegetation, research shows.

Studies of sediment cores from the sea bottom reveal that long before the North Pole froze it was covered with floating ferns. Six million years earlier the water on the surface of the Arctic Ocean was a balmy 23C (73F) or higher, equivalent to today’s subtropical seas.

Scientists have had to reassess their understanding of the region because previously it was thought to have frozen only 15 million years ago. Sea ice, however, started forming 45 million years ago.

The findings were made during analysis of 430 metres of sedimentary core drilled from the seabed in 2004. The core, the subject of three studies published in the journal Nature, provides scientists with a geological record of the region dating back 57 million years.

The Arctic Ocean was, 50 million years ago, a basin largely surrounded by land, which meant that much of the surface consisted of fresh or slightly brackish water.

Such conditions, combined with “extreme global warming”, allowed the azolla fern to grow in floating mats over a period lasting 800,000 years.

Today most azollas grow in freshwater ponds, canals and rice fields in tropical and subtropical regions and their presence near the North Pole “suggests a substantial rise in Arctic sea surface temperature to subtropical or tropical levels”.

During the warmest period the surface of the Arctic Ocean was up to 33C (59F) warmer than it is today. At this time, 55 million years ago, “temperatures peaked near 24C, which is notably higher than previous estimates”, the international team of scientists reported.

Such high temperatures conflict with previous models, which estimated that the surface would have been 10-15C while assuming that carbon dioxide levels were at 2,000 parts per million — today’s level is 381ppm.

“This suggests that higher than-modern greenhouse gas concentrations must have operated in conjunction with other feedback mechanisms,” the scientists said.

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