Climate change: Arctic sea ice thins in two big jumps, and now more vulnerable

Climate change attacked crucial Arctic sea ice thickness in two sudden big gobbles instead of steady nibbling, a new study says.

A little more than 15 years ago, sea ice quickly lost more than half its thickness, becoming weaker, more prone to melting and less likely to recover, according to the study that emphasises the importance of two big “regime shifts” that changed the complexion of the Arctic.

Those big bites came in 2005 and 2007. Before then, Arctic sea ice was older and misshapen in a way that made it difficult to move out of the region. That helped the polar area act as the globe's air conditioner even in warmer summers. But now the ice is thinner, younger and easier to push out of the Arctic, putting that crucial cooling system at more risk, the study's lead author said.

Before 2007, 19 per cent of the sea ice in the Arctic was at least 4 metres (13 feet) thick — taller than most elephants — but now only about 9.3 per cent of ice is at least that thick. And the age of the ice has dropped by more than a third, from an average of 4.3 years to 2.7 years, according to the study in the journal Nature on Wednesday, which cited “the long-lasting impact of climate change on the Arctic sea ice”.

“Ice is much more vulnerable than before because it's thinner; it can easily melt,” said study lead author Hiroshi Sumata, a sea ice scientist at the Norwegian Polar Institute. Thicker sea ice is crucial to all sorts of life in the Arctic, he said.

The study shows “how the Arctic sea ice environment has undergone a fundamental shift”, said Walt Meier, a scientist at the National Snow and Ice Data Centre, who wasn't part of the research. “This paper helps explain why the sea ice has not recovered from those big drops.”

Past studies concentrated more on the extent of Arctic sea ice, or how widespread it is, because that's easily measured by satellites, which don't observe volume well. But 90 per cent of the sea ice eventually is pushed out of the Arctic through the Fram Strait by Greenland, so Sumata overcame the challenges of measuring from space by focusing his observations on that ground-based choke point.

Ice getting younger

He found that first ice was getting younger, which made it thinner and more uniform, and easier to push out through the Fram Strait. Thicker ice has all sorts of edges and weird shapes that make it harder to force out of the Arctic because of aerodynamics, but that's not the case for sleeker, younger ice, Sumata said.

Scientists had known before that sea ice was shrinking in extent and getting thinner, but this “flushing” is key, said Mark Serreze, director of the National Snow and Ice Data Centre, who wasn't part of the study.

“Such flushing episodes have reduced the residence time of ice in the Arctic Ocean by more than a year so there's less time for it to thicken and it's the thick ice that's resistant to melting out,” Serreze said in an e-mail. “But since the Arctic is quickly warming up, we're probably past the point of hoping the Arctic Ocean can recover.”

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