Arctic sea ice, the vast sheet of frozen seawater floating on the Arctic Ocean, has been hit with a double whammy over the past decades: its extent has shrunk and the oldest and thickest ice has thinned or melted away. That leaves a sea ice cap more vulnerable to the warming ocean and atmosphere.

The phenomenon of melting Arctic ice has implications for international trade and shipping. As more ice melts, the Arctic could emerge as a new shipping lane from Asia to Europe and the Americas. Such a northern passage would slash transit times and costs, as well as fuel usage.

“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.”

In September 1984, there were 1.86 million square kilometers of old ice (five years or older) left throughout the Arctic sea ice cap during its yearly minimum extent; in September 2016, there were only 110,000 square kilometers of older sea ice left.

“Ice age is a good analog for ice thickness because basically, as ice gets older it gets thicker,” Meier said. “This is due to the ice generally growing more in the winter than it melts in the summer.”

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 is 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.

Arctic sea ice has not only been shrinking in surface area in recent years, it’s becoming younger and thinner as well.

“What’s happening now more is that the old ice is melting within the Arctic Ocean during the summertime,” Meier said. “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. We’ve lost most of the older ice. In the 1980s, multiyear ice made up 20 percent of the sea ice cover. Now it’s only about three percent. 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.”