The area covered by sea ice in the Arctic is relatively easy to measure with satellite imagery. NASA reports sea ice is decreasing about 13 percent per decade. But the thickness of that ice—which affects wildlife, hunting, fishing and shipping—is more difficult to estimate because the ice is partially submerged and weighed down by snow.
Research published on June 4 in the journal Cryosphere uses new estimates of snow cover to update models of sea ice thickness, Damian Carrington reports for the Guardian. Previous snow cover estimates mostly come from data collected between 1954 and 1991, so the new study combines radar measurements with models of temperature, snowfall and the movements of ice sheets to find more accurate calculations of ice thickness. The evidence suggests that in some areas, ice is thinning at about twice as fast as previously estimated.
“Sea ice thickness remains highly uncertain compared to the area that the sea ice covers. However, this paper is a significant advance in characterizing the trends that we’re seeing in the thickness, and those are trends that reflect an Arctic warming at three times the global rate,” says Robbie Mallett, an expert in the physics of sea ice at University College London and first author on the new study, to Adam Vaughan at New Scientist.
To measure the height of ice sticking out of the ocean, satellites send radar toward the ground and map the ground based on the reflection of the radar. A thick layer of snow will push the ice so that it sits lower in the water, so scientists seeking to calculate ice thickness need to take snow into account.
But past calculations of snow didn’t take the effects of climate change into account.
"When it was developed, the Arctic was mostly covered with multi-year ice," says Nathan Kurtz, a sea ice expert at NASA, to Chelsea Harvey at E&E News. Multi-year ice is thicker than first-year ice, but much of the Arctic’s multi-year ice has disappeared in the last couple of decades.
When Mallett and the research team combined modern radar data with modern computer models of snow on first-year ice, they found that between 2002 and 2018, Arctic sea ice was becoming thinner at least 60 percent faster than previous estimates. In the coastal areas of the Chukchi seas, the rate of ice decline increased by 110 percent compared to past calculations.
Variable, thinning sea ice can make hunting and ice fishing more difficult for coastal and Indigenous communities in the Arctic. The thinner ice also allows storms to more easily reach and erode the coast, Mallett tells the Guardian.
However, thinner sea ice also opens up new routes for shipping. In February, a cargo ship made the first round trip in winter between Sabetta in northern Russia and Jiangsu in China, using the Northeast passage, per the Guardian. The shorter passage could save on fuel, and make drilling for oil easier—simultaneously increasing the chances of Arctic oil spills.
Speaking to New Scientist, University of Cambridge ocean physicist Peter Wadhams criticizes the study because it omits measurements of sea ice thickness taken by submarines up to 2007. Although Mallett tells New Scientist that the submarine measurements were taken at single “points,” while the new model uses larger swaths of ice.
“We are still learning about the changes to the Arctic environment, and one of the big unknowns – or less well-knowns – is snow cover,” says U.S. National Snow and Ice Data Center research scientist Walt Meier, who was not involved in the study, to the Guardian. “The approach in the study is a significant improvement over older methods, and the results fit with other changes we’re seeing with Arctic sea ice, including earlier melt onset, lower summer ice cover, and later freeze-up.”