By Gerard Wynn
500 MILES FROM THE NORTH POLE (Reuters) - The minimum summertime volume of Arctic sea ice fell to a record low last year, researchers said in a study to be published shortly, suggesting that thinning of the ice had outweighed a recovery in area.
The study estimated that last year broke the previous, 2007 record for the minimum volume of ice, which is calculated from a combination of sea ice area and thickness.
The research adds to a picture of rapid climate change at the top of the world that could see the Arctic Ocean ice-free within decades, spurring new oil exploration opportunities but possibly also disrupted weather patterns far afield and a faster rise in sea levels.
The authors developed a model predicting thickness across the Arctic Ocean based on actual observations of winds, air and ocean temperatures.
"The real worrisome fact is downward trend over the last 32 years," said Axel Schweiger, lead author of the paper, referring to a satellite record of changes in the Arctic.
He was emailing Reuters at the Greenpeace icebreaker Arctic Sunrise, in the Arctic Ocean between the Norwegian island of Svalbard and the North Pole.
"(It fell) by a large enough margin to establish a statistically significant new record," said the authors in their paper titled "Uncertainty in modeled Arctic sea ice volume."
The researchers at the University of Washington in Seattle checked the model results against real readings of ice thickness using limited submarine and satellite data.
The approach has some detractors because it is focused is on modeling rather than direct observations of thickness, and therefore contains some uncertainty.
Sea ice area is easier to measure by satellite than ice thickness, and so has not needed a modeling approach.
Ice thickness is just as important or more so in helping understand what is happening in the far north. Some experts argue that part of the reason the ice area has dramatically fallen in recent years is because it has been thinning for decades.
The authors said their Pan-arctic Ice Ocean Modeling and Assimilation System (PIOMAS) in general agreed well with actual observations, although "modeling error" was possible.
The Arctic sea ice area fell below 4.6 million sq km last week with two weeks of the melt season still to go, compared with the record low of 4.13 million sq km in 2007.
By comparison, the minimum ice extent in the early 1970s was about 7 million square km. Ice melts every year during the summer and reaches a minimum extent in mid-September.
Most experts now agree that the Arctic Ocean will be ice-free in late summer at some point this century but disagree about exactly when.
While sea ice itself does not raise sea levels when it melts, a warmer Arctic could speed up melting of the Greenland ice sheet, which is freshwater ice trapped over land and contains enough water to raise world sea levels by 7 metres.