The papers report a mission in which European scientists aboard a drillship braved flowing walls of ice to delve deep into the Lomonosov ridge on the floor of the Arctic Ocean.

The precious cores of sediment, retrieved from up to 430 metres (1,397 feet) below the sea bed, give an idea of the planet's climate going back 55 million years thanks to the fossilised creatures, plants and stones buried in them.

This surveyed period kicks off with an astonishingly warm period called the Palaeocene-Eocene thermal maximum. At one point during this era, the Arctic Ocean was 23 C (73 F) -- the same temperature as a tepid bath.

Then, around 49 million years ago, large volumes of cool freshwater for some reason were dumped into the Arctic, chilling the sea to around 10 C (50 F) and diluting its saltiness so much that in summer months, a species of green freshwater fern covered much of its surface.

At 45 million years ago, the first ice started to form, as evidenced by pebbles dropped by icebergs, and the relative cooling has continued to the modern era.

The studies, published in the weekly British journal Nature, were carried out in an initiative called Arctic Coring Expedition (ACEX).

A Swedish-flagged, Norwegian-operated drillship, the Vidar Viking, manoeuvred in water 1,000 metres (3,250 feet) deep just 238 kilometers (148 miles) from the North Pole, protected by a Russian and a Swedish icebreaker.

"At times, the drill site was covered with ice two to three metres (seven to 10 feet) thick," said one of the lead authors, Jan Blackman, a professor at Stockholm University.

"We encountered an iceflow of multi-year ice, harder and denser than ice from just one Arctic winter. It was like driving into a brick wall."

In a history spanning some 4.5 billion years, Earth has gone through natural shifts in climate change.

The drivers for this include changes in solar radiation, surges in volcanic activity, releases of methane stored underground, shifts in vegetation and the light that is reflected back into space by polar icecaps.

The study delves into the distant past and does not cover recent history, especially the Industrial Revolution, whose fossil-fuel emissions are blamed for global warming.

Evidence has accumulated in recent years that Arctic ice cover is thinning and shrinking in response to this warming, which in turn may have a big impact on polar bears and other species.

The icecap at the North Pole floats on the Arctic Ocean, which means its melting does not affect global sea levels.

In Antarctica, it lies mainly on rock, which means that even a partial melting would threaten coastal cities and deltas around the world.

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