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The Arctic Ocean's blanket of sea ice has changed since 1958 from predominantly older, thicker ice to mostly younger, thinner ice, according to a NASA study published Monday.
With so little thick, old ice left, the rate of decrease in ice thickness has slowed, the US space agency said in a statement.
According to the study by Ron Kwok from NASA's Jet Propulsion Laboratory (JPL), new ice grows faster but is more vulnerable to weather and wind, so ice thickness is now more variable, rather than dominated by the effect of global warming.
The study, published in the journal Environmental Research Letters, combined decades of declassified US Navy submarine measurements with more recent data from four satellites to create the 60-year record of changes in Arctic sea ice thickness.
It found that since 1958, Arctic ice cover has lost about two-thirds of its thickness, as averaged across the Arctic at the end of summer.
Older ice has shrunk in area by more than two million square kilometers, researchers said.
Today, 70 per cent of the ice cover consists of ice that forms and melts within a single year, which scientists call seasonal ice.
Sea ice of any age is frozen ocean water. However, as sea ice survives through several melt seasons, its characteristics change.
Multiyear ice is thicker, stronger and rougher than seasonal ice. It is much less salty than seasonal ice; Arctic explorers used it as drinking water, researchers said.
Satellite sensors observe enough of these differences that scientists can use space-borne data to distinguish between the two types of ice.
Thinner, weaker seasonal ice is innately more vulnerable to weather than thick, multiyear ice.
It can be pushed around more easily by wind, as happened in the summer of 2013, researchers said.
During that time, prevailing winds piled up the ice cover against coastlines, which made the ice cover thicker for months, they said.
The ice's vulnerability may also be demonstrated by the increased variation in Arctic sea ice thickness and extent from year to year over the last decade.
In the past, sea ice rarely melted in the Arctic Ocean. Each year, some multiyear ice flowed out of the ocean into the East Greenland Sea and melted there, and some ice grew thick enough to survive the melt season and become multiyear ice.
As air temperatures in the polar regions have warmed in recent decades, however, large amounts of multiyear ice now melt within the Arctic Ocean itself. Far less seasonal ice now thickens enough over the winter to survive the summer.
As a result, not only is there less ice overall, but the proportions of multi-year ice to seasonal ice have also changed in favour of the young ice.
Seasonal ice now grows to a depth of about six feet (two metres) in winter, and most of it melts in summer. That basic pattern is likely to continue, Kwok said.
"The thickness and coverage in the Arctic are now dominated by the growth, melting and deformation of seasonal ice," he said.
