"When we had similar weather patterns in the past, they didn't appear to have as strong an effect on sea ice," said Jennifer Kay, an atmospheric scientist who led the U.S. research team.
"Now because the ice is thinner you can have a chain reaction of runaway melting with a reduction in cloud cover," she said.
Research has linked the thinning of Arctic ice to warmer average temperatures caused by rising levels of greenhouse gases from human activities. Readings from U.S. submarines indicate a widespread reduction in sea ice thickness of 40 per cent since 1960.
The melting is also increased because the darker surface of open water absorbs the sun's rays as heat rather than reflecting them back into space like ice and snow.
The discovery of this additional vulnerability significantly ratchets up the prospects of international shipping within a decade through the fragile Canadian Arctic archipelago for months every year.
Using modern satellite imagery, the scientists from the U.S. National Center for Atmospheric Research linked last summer's record loss of sea ice to unusual cloudless weather in June and July that allowed the sun to relentlessly beat down on first-year ice formed over the previous winter.
Yet such clear skies were not unprecedented, according to historical records going back to 1946. Five times since then the Arctic skies had been even more cloud-free than last summer, yet without the massive shrinking of sea ice cover.
First-year ice makes up the bulk of the floating Arctic sea ice.
But last summer only 13 per cent of this first-year ice survived the summer melt, instead of the customary 30 per cent. The 4.1 million kilometres of old ice remaining by September was the lowest since accurate satellite measurements began in 1979.
Kay and her colleagues calculated that in three months the increase in sunshine sped up the surface melting of the ice by an extra one-third of a metre as well as accelerating the thinning from below by raising water temperatures by more than 2C.
David Barber, a top Canadian expert, applauded the findings for helping improve the computer models that predict the future pattern of Arctic sea ice. "Increased shortwave flux (sunshine) will play a very important role in the melt of sea ice," Barber said in an email from the research icebreaker Amundsen in the western Arctic.
The landmark study was made possible by new cloud-profiling radar and laser-based detectors that began sending back readings from NASA satellites in June 2006.
© 2008 Toronto Star