Arctic sea ice is on track to recede to a record low this year, suggesting that northern waters free of summer ice are coming faster than anyone thought.
The latest satellite information shows ice coverage is equal to what it was in 2007, the lowest year on record, and is declining faster than it did that year.
"Could we break another record this year? I think it's quite possible," said Mark Serreze of the National Snow and Ice Data Center in Boulder, Colo.
"We are going to lose the summer sea-ice cover. We can't go back."
In April, the centre published data showing that sea ice had almost recovered to the 20-year average. That ignited a flurry of interest on climate change skeptic blogs.
But much of that ice was thin and new. The warmest April on record in the Arctic made short work of it.
Ice cover has already fallen back to where it was in 2007 at this time of year and is disappearing at a faster pace than it did then. Dr. Serreze said winds, cloud cover or other weather conditions could slow the melt, but he points out that the decline is likely to speed up even more in June and July.
"Will [thawing]this year be particularly fast?" he asked. "We don't know. We really don't know."
One of Canada's top sea-ice experts suggests things might even be worse than Dr. Serreze thinks. His data could be underestimating the collapse of summer ice cover, said David Barber of the University of Manitoba. Researchers can't learn anything from satellite data about the state or thickness of the ice.
"What we think is thick multiyear ice late in the summer is in fact not," he said. "It's heavily decayed first-year ice. When that stuff starts to reform in the fall, we think it's multiyear ice, but it's not."
Arctic explorers and scientific expeditions are finding more open water and untrustworthy ice ever, Prof. Barber said.
He pointed out the Arctic continued to lose multiyear ice even in 2008 and 2009, when total ice coverage rebounded somewhat.
True multiyear ice - the thick, hard stuff that stops ships - now comprises about 18 per cent of the Arctic ice pack. In 1981, when Prof. Barber first went north, that figure was 90 per cent.
"This is all just part of a trajectory moving toward a seasonally ice-free Arctic," he said. "That's happening more quickly than we thought it would happen."
Once northern waters are clear in the summer, there will be little difference between navigating the Northwest Passage and the Gulf of St. Lawrence, he suggested.
He recounts sailing through degraded ice in an icebreaker. The ship's top speed in open water was 13.7 knots. Its speed through the decayed ice was 13 knots.
"It was almost like it didn't exist."