Multiyear Arctic sea ice – ice that has survived at least one summer – is an important indicator of long-term changes in polar and global climate. But, with such ice now covering less than one-third of the Arctic Ocean, scientists may soon have to rely on other measures.

“The [ice] thickness and coverage in the Arctic are now dominated by the growth, melting and deformation of seasonal ice,” says Ronald Kwok of NASA’s Jet Propulsion Laboratory and ICESat-2 science team. “We’ve lost so much of the thick ice that changes in thickness are going to be slower due to the different behaviour of this ice type.”

Kwok’s analysis of satellite data and submarine measurements covering six key areas of the Arctic shows that between 1958–1976 and 2011–2018, the average ice thickness near the end of the melt season decreased by 2 m, some 66%,.

If these losses continue, scientists expect trends in ice thickness and volume to be more closely controlled by shorter-term seasonal changes. Younger ice is more susceptible to variations in wind and weather.

“The transition from a thicker older ice cover to a largely seasonal ice cover has implications on the types of thickness changes we expect to see,” says Kwok. “Even though seasonal ice grows faster than multiyear sea ice, there is a limit to how thick seasonal ice can grow during a given winter; multiyear ice has more than a year’s worth of growth and deformation to add to its thickness.”

An update to work carried out a decade ago, the study includes CryoSat-2 observations as well as the record of multiyear sea ice coverage from NASA’s QuikSCAT satellite and EumetSat’s ASCAT.

Scatterometer data gathered from 1999 to 2017 indicate that the Arctic has lost more than 2 million sq. km of multiyear ice – a decrease of over 50%. According to the report, multilayer ice now covers less than one-third of the Arctic Ocean. It can be found to the north of the coast of Greenland and around the Canadian Arctic Archipelago.

The next steps are to examine regional effects and the impact of ice deformation on the behaviour of the ice cover.

“Ice extent — satellite data cells containing ice above a threshold — gives information about coverage while ice thickness provides additional insights into the impact of thermodynamics and dynamics on the behaviour of the ice cover,” explains Kwok.

The team published the findings in Environmental Research Letters (ERL).