Arctic sea ice in 1984. Image via NASA

Arctic sea ice in 2016. Image via NASA

The extent of Arctic sea ice, the vast sheath of frozen seawater that floats on the Arctic Ocean, has been shrinking steadily. But, NASA scientists report, there is an additional problem: As the ice extent has 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.

In recent years, that old ice – known as perennial sea ice – has declined rapidly. Perennial sea ice, also known as multi-year ice, is the portion of the sea ice that survives the summer melt season. Perennial ice, which might have a life-span of 9 years or more, is the thickest component of the sea ice. Perennial ice can grow up to 4 meters (13.12 feet) thick, in contrast to ice that grows during a single winter, which is about 2 meters (6.6 feet) thick at most.

Walt Meier is a sea ice researcher at NASA’s Goddard Space Flight Center. Meier said in a statement: