Global warming has created a new normal in the Arctic, with a thinner blanket of sea ice now more susceptible to seasonal variation, according to NASA research.

Over the past 60 years, much of the older, thicker ice has disappeared, to be replaced with a mostly younger, thinner version. That means the rate of decrease in ice thickness has slowed, but only because much of the old permanent stuff is gone.

The younger ice is more vulnerable to the elements – it can even be pushed around by the wind – but it also grows more quickly. Today, 70% of Arctic ice cover forms and melts within a single year.

To make this finding Ron Kwok from NASA’s Jet Propulsion Laboratory in California combined decades of declassified US Navy submarine measurements with more recent data from four satellites to provide very much a big picture.

His results are published in the journal Environmental Research Letters.{%recommended 1799%}

He found that since 1958 Arctic ice cover has lost about two-thirds of its thickness, measured as the average across the region at the end of summer. Older ice has shrunk in area by more than two million square kilometres.

Seasonal ice now grows to a depth of about two metres in winter, but most of it melts in summer. That pattern is likely to continue.

“The thickness and coverage in the Arctic are now dominated by the growth, melting and deformation of seasonal ice,” Kwok explains.

The increase in seasonal ice also means record-breaking changes in ice cover, such as those of recorded in the 1990s and 2000s, are likely to be less common. There has not been a new record sea ice minimum since 2012, despite years of warm weather in the Arctic.

“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 says.

As ice survives through several seasons, it becomes thicker, stronger and rougher than seasonal ice. It is also much less salty; Arctic explorers used it as drinking water.

Satellite sensors can pick out enough differences between this and seasonal ice to allow scientists to use spaceborne data to distinguish between the two.

Kwok used data from US Navy submarine sonars from 1958 to 2000; satellite altimeters on NASA’s ICESat and the European CryoSat-2, which span from 2003 to 2018; and scatterometer measurements from NASA’s QuikSCAT and the European ASCAT from 1999 to 2017.