The multiyear ice covering the Arctic Ocean has effectively vanished, a startling development that will make it easier to open up polar shipping routes, an Arctic expert said on Thursday.

Vast sheets of impenetrable multiyear ice, which can reach up to 80 meters (260 feet) thick, have for centuries blocked the path of ships seeking a quick short cut through the fabled Northwest Passage from the Atlantic to the Pacific. They also ruled out the idea of sailing across the top of the world.

But David Barber, Canada’s Research Chair in Arctic System Science at the University of Manitoba, said the ice was melting at an extraordinarily fast rate.

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