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

Vast sheets of impenetrable multiyear ice, which can reach up to 80 metres 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.

"We are almost out of multiyear sea ice in the northern hemisphere," he said in a presentation in Parliament. The little that remains is jammed up against Canada's Arctic archipaelago, far from potential shipping routes.

Scientists link higher Arctic temperatures and melting sea ice to the greenhouse gas emissions blamed for global warming.

Mr Barber spoke shortly after returning from an expedition that sought - and largely failed to find - a huge multiyear ice pack that should have been in the Beaufort Sea off the Canadian coastal town of Tuktoyaktuk.

Instead, his ice breaker found hundreds of miles of what he called "rotten ice" - 50 centimetre thin layers of fresh ice covering small chunks of older ice.

"I've never seen anything like this in my 30 years of working in the high Arctic... it was very dramatic," he said.

"From a practical perspective, if you want to ship across the pole, you're concerned about multiyear sea ice. You're not concerned about this rotten stuff we were doing 13 knots through. It's easy to navigate through."