The Arctic on thin ice

The Arctic Ocean’s floating sea-ice cover naturally waxes and wanes with the seasons. But the decline in area covered by perennial ice—that which survives the summer melt—has been accelerating in recent years. The nonlinear trend, no doubt, reflects the Arctic’s response to a warming climate: Thanks to the ice albedo–ocean feedback, a drop in ice cover increases the absorption of solar radiation, which warms the ocean, prolongs the melting, and reduces ice cover yet further. For quantitative details on the ice pack’s changing mass and heat capacity, though, researchers need more than areal measurements. Ronald Kwok and colleagues at NASA and the University of Washington have now published what may be the most comprehensive thickness maps of the entire Arctic basin. The lidar system they use aboard NASA’s ICESat can precisely distinguish height differences between the sea surface and ice floes. After measuring that “freeboard,” Kwok and company use Archimedes’ principle to calculate the portion that’s underwater. Because much of the brine drains from sea ice as it ages, making it more reflective to microwaves, the researchers are also able to distinguish between young, seasonal ice and older, perennial ice. The surveys taken by ICESat over five years reveal that the Arctic has thinned by about 0.7 m and lost 1.5 × 10⁶ km² of perennial ice—more than twice the area of Texas. Moreover, the volume of perennial ice shrank by 57%—so much that seasonal ice has become dominant for the first time on record. (R. Kwok et al., J. Geophys. Res. 114, C07005, 2009, doi:10.1029/2009JC005312.)—R. Mark Wilson