Both of the World's Ice Sheets May Be Shrinking Faster and Faster

The two great ice sheets—Greenland's and Antarctica's—have had plenty of press lately, what with galloping glaciers and whole lakes of meltwater plunging into ice holes in minutes (Science, 18 April 2008, p. 301). Surveys of ice-sheet volume made from planes and satellites have quantified these losses, but those assessments have been spotty in time, space, or both. Shrinkage accelerated from the 1990s into the 2000s, but researchers couldn't be sure what would come next.

Now the latest analysis of the most comprehensive, essentially continuous monitoring of the ice sheets shows that the losses have not eased in the past few years. More ominously, losses from both Greenland and Antarctica appear to have accelerated during the past 7 years. If the acceleration out of the 1990s "was a hiccup, it was a big one, and it's getting bigger," says glaciologist Richard Alley of Pennsylvania State University, University Park, who was not involved in the work.

The results, in press at Geophysical Research Letters, are based on measurements by the Gravity Recovery and Climate Experiment (GRACE) satellite mission. Rather than measuring the volume of ice sheets every few years as most earlier surveys did, GRACE "weighs" them from month to month with a pair of spacecraft launched in March 2002 as a joint NASA and German Aerospace Center mission (Science, 14 August, p. 798). Flying in tandem 220 kilometers apart, the satellites can measure subtle variations in the pull of gravity as they pass over a large mass on the surface. By beaming microwaves from one to the other, they precisely gauge the changing distance between them as the added mass tugs first on the leading satellite and then on the trailing one. Changes in gravity from pass to pass reflect changes in the icy mass below.

The mass changes of Greenland and Antarctica during the past 7 years have all been negative, geophysicist Isabella Velicogna of NASA's Jet Propulsion Laboratory in Pasadena, California, concludes in the study. On Greenland, she calculates, the rate of ice mass loss doubled over the 7-year period, producing an acceleration of –30 cubic kilometers of water lost per year. On Antarctica, the loss rate more than doubled to produce a similar acceleration. Together, that would make for a 5% acceleration each year in the rise of sea level. Compounded year after year, "that is a big thing," says Velicogna. "We should be more concerned."

The GRACE observations counter encouraging news from southeastern Greenland that the surging glaciers there had slowed (Science, 23 January, p. 458). The slowing was probably real, says Alley, but apparently the increasing losses from melting and accelerating glaciers elsewhere around Greenland at least made up for the slowing in the southeast. Whatever is driving ice loss—warmer oceans, warmer air, or both—is persisting, he says.

Glaciologist Waleed Abdalati of the University of Colorado, Boulder, says Velicogna's analysis "suggests—that's the key word—that there's been an acceleration in the period examined. We have to be careful to not overinterpret and speculate about the future." The record is too short to be extrapolated into the future, Abdalati says. And at least in Greenland, it was affected by the extreme warmth and resulting melting in 2007, a loss surge that might not be repeated in the next 7 years.

If bursts of ice loss do occur soon, GRACE may not be around to record it. Its two satellites will fall from orbit around 2013, dragged down after a decade of orbiting through Earth's outermost atmosphere. No replacement gravity mission is yet planned.

(By Richard A. Kerr, Science Magazine, 09/10/2009)
Science 9 October 2009:
Vol. 326. no. 5950, p. 217
DOI: 10.1126/science.326_217a