Witness: Arctic ice breaks up as polar bears stalk ship

500 MILES FROM THE NORTH POLE (Reuters) - Stepping onto an Arctic ice floe on Monday, an unusually mild, easterly breeze blew at the end of the annual summer melt. The footprints of two polar bears from the night before were disintegrating in a dusting of snow.

At nearly 81 degrees latitude, the air temperature was 2.5 degrees Celsius -- normal for a winter's day in Europe but rather mild for the high Arctic, even in late summer. The previous day had been 4 degrees colder.

The monochrome scene was calm after the rolling swell of the Fram Strait between the Norwegian island of Svalbard and Greenland. We were embedded in an ice pack stretching half the area of Brazil, across the North Pole.

The Greenpeace icebreaker Arctic Sunrise nearby shuddered occasionally, nudged by white slabs of ice the size of a small car park, which jostled among threads of open water.

The water temperature was below zero, the ship's log read, and the air was filled by the hum of its generators. The ship's mooring ropes were driven by two giant stakes into ice up to 10 meters thick.

This entire Arctic landscape is forecast to disappear within decades and replaced by open sea each summer, perhaps for the first time in 7,000 years or more. The dramatic retreat signals the scale of humankind's impact on the climate, experts say.

On Wednesday, the shrinking sea ice was closing in on the 2007 record low area of 4.1 million square km, according to the U.S.-based National Snow and Ice Data Center. The annual minimum was 7 million square km in the early 1970s.

Environmental group Greenpeace wanted to draw attention to changes in the high Arctic, and ferried Cambridge University researchers from Svalbard to measure the thickness of the ice. Experts say it has been thinning for decades, possibly hastening an entirely ice-free summer as soon as 2020.

The sea ice area is easily read from satellites overhead. Measuring thickness is more difficult, and the most direct approach is to drill a hole and poke a tape measure down.

While doing just that the researchers on Monday were confronted directly with the annual seasonal melt which ends around mid-September each year.

They raced to evacuate the floe when a 3-meter wide crack appeared suddenly, in under a minute. A combination of melting, the swell of the sea and wind broke the floe apart.

The night before, a polar bear and her cub blundered through research equipment left on the ice, adding to the novel dangers of Arctic field experiments.

The polar bear is called "Nanuk", which means the Great Wanderer in the local Arctic Inuit language. But it was clear that there was nothing random about the bears which stalked our ship from floe to floe.

Experts say Arctic wildlife, including bears, depend on hunting from the ice. They are likely to suffer from an ice-free summer, while warmer air and water could shift birds and fish.

Wider, "wild card" consequences could see disrupted global weather patterns, when a warmer, open sea, without its insulating layer of ice, releases more heat into the polar autumn air. The difference in temperature between the poles and equator is the basic engine of world weather.

Meanwhile, some experts say a warmer Arctic could speed up the melt of the Greenland ice sheet, which contains enough water to raise sea levels by 7 meters.

On this research trip, the most dramatic recording was simply our arrival time back in Svalbard, hastened many hours by a sea ice retreat of 8 miles in just three days.

Researchers who go to the Arctic rarely find themselves in the same place twice and it is a privilege to measure such a change so precisely.

"We were basically at the same point where we entered and left the ice, and you could see there was a difference," said Arne Sorensen, Arctic Sunrise ice.

(Editing by Karolina Tagaris)

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