Washington, Oct 29 (IANS): As the extent of Arctic Sea ice, the vast sheath of frozen seawater floating on the Arctic Ocean and its neighboring seas, shrinks, its oldest and thickest ice has either thinned or melted away, leaving the sea ice cap more vulnerable to the warming ocean and atmosphere, a NASA study says.

"What we've seen over the years is that the older ice is disappearing," said Walt Meier, a sea ice researcher at NASA's Goddard Space Flight Center in Greenbelt, Maryland.

"This older, thicker ice is like the bulwark of sea ice: a warm summer will melt all the young, thin ice away but it can't completely get rid of the older ice. But this older ice is becoming weaker because there's less of it and the remaining old ice is more broken up and thinner, so that bulwark is not as good as it used to be," Meier noted in a NASA statement.

Direct measurements of sea ice thickness are sporadic and incomplete across the Arctic, so scientists have developed estimates of sea ice age and tracked their evolution from 1984 to the present.

Now, a new NASA visualisation of the age of Arctic sea ice shows how sea ice has been growing and shrinking, spinning, melting in place and drifting out of the Arctic for the past three decades.

"Unlike in the 1980s, it's not so much as ice being flushed out -though that's still going on too," Meier said.

"What's happening now more is that the old ice is melting within the Arctic Ocean during the summertime. One of the reasons is that the multiyear ice used to be a pretty consolidated ice pack and now we're seeing relatively smaller chunks of old ice interspersed with younger ice. These isolated floes of thicker ice are much easier to melt," he added.

"We've lost most of the older ice: In the 1980s, multiyear ice made up 20 per cent of the sea ice cover. Now it's only about three percent," Meier said.

"The older ice was like the insurance policy of the Arctic sea ice pack: as we lose it, the likelihood for a largely ice-free summer in the Arctic increases," Meier noted.