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

Science 9 October 2009:
Vol. 326. no. 5950, p. 217
DOI: 10.1126/science.326_217a

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Climate Change:

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

Richard A. Kerr

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. http://www.sciencemag.org/cgi/content/full/sci;320/5874/301">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. http://www.sciencemag.org/cgi/content/full/sci;325/5942/798">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."

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Bending down. The trend line of Greenland ice mass (green) curves downward with time, suggesting that losses have been accelerating.

CREDIT: ADAPTED FROM ISABELLA VELICOGNA, GEOPHYSICAL RESEARCH LETTERS