For the past 40 years, thanks to satellite measurements, scientists have known that sea ice coverage in the Arctic is shrinking. Global warming has reduced the extent of ice in the region in summer, when it is at its lowest, by nearly 13 percent a decade. That has led some researchers to predict that the Arctic could be ice-free in summers by the middle of the century.
But ice extent is only part of the story. Scientists want to know thickness, too, because together with extent that tells them the total volume of ice in the Arctic.
Average thickness has also declined sharply, as melting of multiyear ice leaves a greater proportion of thinner, first-year ice. Winds and currents can also move more older ice out of the Arctic.
Measuring the thickness of sea ice is trickier than measuring its extent. There’s a European satellite, Cryosat-2, that can do the job using radar to determine ice elevation and therefore thickness. But Cryosat-2 works best in winter; in summer, when the ice is melting, it has difficulty distinguishing between ice and open water. ICESat-2, a NASA satellite that was launched this month, will provide even more precise thickness data but will have similar summer limitations.
To fill the data gap, some governments and other groups have conducted summer measurement campaigns from aircraft. The latest was undertaken in late July and August by researchers from the Alfred Wegener Institute, which is based in Bremerhaven, Germany.
Operating from Station Nord, a small Danish military and scientific outpost in Greenland, about 575 miles from the geographic North Pole, the researchers measured ice thickness in the Arctic Ocean and in the Fram Strait, which separates Greenland from the Norwegian archipelago of Svalbard.
The Wegener institute program, which is led by Thomas Krumpen, a sea-ice physicist, employs an electromagnetic device similar to a metal detector. It uses the difference in electrical conductivity between ice and seawater, coupled with precision elevation data measured by a laser scanner, to determine thickness.
The institute has been using such detectors since 2002, first on a sledge that traveled across the ice and then suspended from a helicopter. But in those cases the area that could be covered was limited.
Beginning in 2011, the torpedo-shaped device, called the EM-Bird, has been used from a plane. Suspended by a cable just 70 feet above the surface while the plane, a rebuilt and extensively modified DC-3, flies a few hundred feet higher, it can take measurements over a much wider area.
But the low-altitude flights require a lot of planning, and good visibility is a must. So the Wegener team — which this summer included two pilots, an engineer, a mechanic and another scientist in addition to Dr. Krumpen — spends a lot of time discussing the weather.
“For this kind of flying, the key is good forecast data,” he said. “It’s all about finding the best spot to go to make the operation as safe as possible.”