"The climate is changing fast," said Geir Johnsen, a professor at the Norwegian University for Science and Technology who was among experts who found the bivalves. Molluscs were a "very good indicator that the climate is warming," he said.

"It seems like the mussels we found are two to three years old," he told Reuters. Such shellfish have not been recorded off the islands since Viking times 1,000 years ago during another warm period.

U.N. scientists say the Arctic is now warming faster than any other region because of human emissions of carbon dioxide and other greenhouse gases released from burning fossil fuels in cars, factories and power plants.

As the white ice and snow melts, it exposes darker ground or water that soaks up heat and so accelerates warming compared to regions further south. By comparison, ice in Antarctica is thicker and acts as a deep freeze resisting global warming.

Inuit peoples in Canada, for instance, are seeing robins for the first time and hunters are falling through previously solid sea ice. In Scandinavia, birch trees are moving northwards into previously icy areas used for reindeer herding.

The scientists monitoring Svalbard also said they had found seas free of ice further north than for 250 years at one point this summer.

"The climate has been warming," said Bjorn Gulliksen, a professor at the University of Svalbard. "The ice limit...has not been as far north since 1751."