Arctic ice is thinning at a "frightening" pace and has lost one-third of its volume in the past 18 years, a satellite survey has found.
Researchers have warned that sea ice is thinner than previous estimates – and that ice-free summers in the Arctic could be a reality by mid-century.
Seasonal sea ice, which melts completely each summer rather than accumulating over years, is replacing thicker, multi-year ice.
This is accelerating the disappearance of the ice, according to the research.
Scientists make satellite estimates of sea-ice thickness using snow depth and the height of the floating ice above the sea surface.
Snow can weigh ice down, changing how ice floats in the ocean.
The researchers estimated sea ice snow depth using a combination of data from the ICESat-2 satellite.
Using these estimates of snow depth and the height of sea ice exposed above water, the study found multi-year Arctic sea ice has lost 16% of its winter volume, or approximately 1.5 feet of thickness, in the three years since the launch of ICESat-2.
The study was published in the AGU journal Geophysical Research Letters.
Lead study author Sahra Kacimi, a polar scientist at the California Institute of Technology’s Jet Propulsion Laboratory, said: "We weren't really expecting to see this decline, for the ice to be this much thinner in just three short years."
Ron Kwok, a polar scientist at the University of Washington’s Applied Physics Laboratory, who co-authored the study, said: "Arctic snow depth, sea ice thickness and volume are three very challenging measurements to obtain.
"The key takeaway for me is the remarkable loss of Arctic winter sea ice volume – one-third of the winter ice volume lost over just 18 years – that accompanied a widely reported loss of old, thick Arctic sea ice and decline in end-of-summer ice extent."
"This is the first time anyone has several years’ worth of data from the difference between lidar and radar data for snow depth," said Robbie Mallett, a polar ice researcher at University College London who was not involved in the study.
"This is really old ice we’re losing at quite a frightening rate."
The study used an 18-year record of sea-ice observations from ICESat and the newer ICESat-2 and CryoSat-2 satellites to capture monthly changes in Arctic sea ice thickness and volume.
It showed a loss of about 6,000 cubic kilometres of winter ice volume, largely driven by the switch from predominantly multi-year ice to thinner, seasonal sea ice.
Older, multi-year ice tends to be thicker and therefore more resistant to melting.
As that "reservoir" of old Arctic sea ice is depleted and seasonal ice becomes the norm, the overall thickness and volume of Arctic sea ice is expected to decline.
Kacimi said: "Current models predict that by the mid-century we can expect ice-free summers in the Arctic, when the older ice, thick enough to survive the melt season is gone."
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