Arctic sea ice losing its bulwark against warming summers

The oldest and thickest ice has either thinned or melted away, leaving the sea ice cap more vulnerable to the warming ocean and atmosphere.

Washington

: The older, thicker layer of Arctic ice, which is most resistant to melting in summer, has shrunk alarmingly over the past decades, leaving the sea ice cap more vulnerable to global warming, according to NASA.

Arctic sea ice, the vast sheath of frozen seawater floating on the Arctic Ocean and its neighbouring seas, has been hit with a double whammy over the past decades. As its extent shrunk, the oldest and thickest ice has either thinned or melted away, leaving the sea ice cap more vulnerable to the warming ocean and atmosphere, NASA said.

"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 Centre in the US. "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," said Meier. "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," he said.

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. In the early 2000s, scientists at the University of Colorado developed a way to monitor Arctic sea ice movement and the evolution of its age, primarily by using data satellite passive microwave instruments.

"We're keeping track of sea ice as it moves around, up until it melts in place or leaves the Arctic," said Meier, from University of Colorado. Every year, sea ice forms in the winter and melts in the summer. The sea ice that survives the melt season thickens with each passing year: newly formed ice grows to about three to seven feet of thickness during its first year, while multi-year ice (sea ice that has survived several melt seasons) is about 10 to 13 feet thick.

The older and thicker ice is more resistant to melt and less likely to get pushed around by winds or broken up by waves or storms. The motion of sea ice is not limited to its seasonal expansion and shrinkage. Except for coastal regions where sea ice is attached to the shore, the sea ice cap is in almost constant movement.

"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 per cent per cent," 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," he said.