Arctic sea ice this month slipped to its second-lowest minimum extent since satellite observations began, matching almost precisely the mark set in 2007 but not as low as in the record-setting summer of 2012, scientists reported.
Extent, the area where ice covers at least 15 percent of the sea surface, fell to the year's low point of 4.14 million square kilometers (1.598 million square miles) on Saturday, the National Snow and Ice Data Center said. The 2007 minimum extent was 4.15 million square kilometers (1.602 million square miles), the Boulder, Colorado-based center said.
The result is considered a statistical tie because the difference lies within the margin of error for the satellite data, said NSIDC Director Mark Serreze.
This year's minimum extent was about a third lower than the 1981-to-2010 average, according to the NSIDC.
The big ice loss was remarkable because Arctic weather conditions were cloudy and cool earlier in the summer, Serreze said. Such conditions would normally favor ice preservation, not the big meltdown that occurred, he said.
It signals an important shift — possibly because thinner ice overall is more susceptible to melt from warmer waters beneath it, he said.
"I think that this is telling us that the old rules are changing," he said.
Rapid ice loss very late in the melt season — a time when loss has slowed in past years — supports the idea that a warmed-up ocean is becoming an important factor in the fate of sea ice, Serreze said.
"When you're getting into late August, early September, there is not a lot of surface melt going on," he said.
By that time of the year, the air is getting cooler, but this year the melt happened at a rapid clip during those recent days.
"So where does it come from? It has to come from underneath," he said.
Though this year's minimum was virtually the same as that in 2007, weather conditions were vastly different. Then, clear skies allowed solar heat to beat down on the ice from the top, and wind patterns aided in the meltdown. Serreze described 2007 as "the perfect storm." Had those conditions been replicated this year, melt would have been much bigger, he said.
Freeze-up has just started. Freezing usually goes from about mid-September to about mid-March, when melt resumes.
This year's melt season started out with the lowest winter maximum ever measured since satellite records began in 1979. Several monthly records were hit earlier this year.
Atmospheric circulation patterns last winter were similar to those of the winter before the 2007 minimum, said Xiangdong Zhang, a University of Alaska Fairbanks scientist who predicted in March that this year's minimum would wind up around the 2007 level — or lower.
The disappearance of summer and fall sea ice is a long-term trend with profound self-reinforcing effects. Loss of sea ice stimulates more melt and yet more warming, since dark ocean surfaces absorb the solar heat that white ice would reflect away. Vaster areas of open water in the Arctic have other implications — for weather patterns, for ice-dependent wildlife and for commerce.
"The warmed Arctic atmosphere and ocean suppress winter sea ice growth, leading to an enhanced vulnerability of sea ice to melt in the following season," Zhang said in an email.
Among the most famous victims are polar bears, classified in 2008 as a threatened species because of Arctic climate warming.
All populations of polar bears are susceptible to ice loss, a new University of Washington study said. Sea ice extent has declined over the long term in all of 19 Arctic regions that are home to polar bear populations, says the study, published in Wednesday's issue of The Cryosphere.
The season when ice cover is available has been shortened by seven to 19 days per decade in the different regions, and summer-fall ice concentration has also been steadily declining in all of the regions, according to the study.
But this year, Pacific walruses, which also use sea ice as resting platforms in between food-seeking dives into the water, have caught a lucky break.
The Chukchi Sea right now holds fragmented ice that, though in scattered pieces, is floating in the places where walruses need them, said Jim MacCracken, a supervisory wildlife biologist with the U.S. Fish and Wildlife Service.
Unlike the case in most of the recent years, when lack of ice forced walruses away from prime offshore foraging grounds, there have been no reports of the animals crowding in large numbers on Arctic Alaska beaches, MacCracken said. He expects them to avoid such beach crowds through the fall.
"It's unlikely that they're going to come to shore," he said.
This year's ice retreat opened watery pathways in the legendary Northwest Passage across Arctic Canada, which a luxury cruise ship used for an unprecedented month-long journey from Seward to New York. The company, Crystal Cruises, plans a repeat Northwest Passage trip next year.
The Northern Sea Route over the coast of Arctic Russia is also open and clear of ice, current reports show.