Longer Arctic summers and thinning sea ice are threatening the habitats of polar bears and the livelihood of native people, scientists said Wednesday.

Arctic ice has lost as much as 40 percent of its thickness in the past 50 years.

But researchers, who measured it from space for the first time, said it varies more widely than previously thought and is mainly because of summer melting.

"We found a direct link between lengthening summers and thinning ice," said Seymour Laxon of University College London.

A female polar bear and her cubs are shown in the Arctic National Wildlife Refuge in Alaska in this undated photo. A new NASA study found a rise in Arctic temperatures in the last years.  [AP/File]

The thinning sea ice will not cause sea levels to rise but it could have a knock-on effect for the climate, Arctic ecosystems and wildlife.

"There is a lot of concern about the effect on the people in the high Arctic. They rely on the ice to do their hunting and fishing and each year the ice season is getting shorter and shorter," Laxon said.

"They are very concerned about how that is going to change their way of life," he added in an interview.

The thinning ice could also increase the impact of global warming in the northern hemisphere and may influence the operation of the Gulf Stream, according to Laxon.

Instead of using computer models, scientists developed a technique to measure the thickness of the ice covering the circumference of the Arctic Ocean with radar data from a European Space Agency satellite and images from an American satellite.

Their research is reported in the science journal Nature.

The satellite bounces radar off the ice surface and by measuring the height of the ice protruding above the water surface, the scientists can measure how thick the ice is.

"When we compared the data from the two satellites we were astonished by the similarity between the changes in the ice thickness and the length of the summer melt season," Laxon said.

"This result suggests that if this continues, further melting will occur, leading to the eventual disappearance of the ice during summer," he added.

The satellite technique gives scientists an idea of what has been happening to the ice over the last decade and allows them to cover greater areas than other methods.

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