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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.