An iceberg floats in Baffin Bay near Pituffik in Greenland, as captured from the ground during a NASA mission to measure melting Arctic Sea ice. Observations from ICESat-2 show a remarkable level of Arctic Sea ice thinning in a mere three years. Over the past two decades, the Arctic has lost about a third of its winter sea ice volume, largely due to a reduction in ice that persists over several years called multi-year ice, according to a new study. The study also found that sea ice is likely thinner than previous estimates.