Greenland’s ice is thinner at the margins

Two new NASA studies show that the central portion of Greenland’s ice sheet isn’t, on the whole, getting any thinner. However, one of the investigations finds substantial thinning along most margins of the ice sheet—changes that are contributing to rising sea levels.

The thick bars bordering Greenland’s 1-million-square-kilometer central ice sheet show the relative magnitude and direction of ice flow. Science

Scientists at NASA’s Wallops Flight Facility in Wallops Island, Va., led both studies, which they report in the July 21 Science.

One group used the Global Positioning System to measure the downhill flow of ice at elevations of about 2,000 meters above sea level from 1993 to 1997. Survey points were typically about 30 kilometers apart. Together, these roughly 150 points bordered the approximately 1 million square kilometers of Greenland’s central ice sheet.

On average, the high-altitude portion of the ice sheet is neither gaining nor losing thickness, glaciologist Robert H. Thomas and his colleagues report. Ice flowing down past the 2,000-m contour is almost balanced by the accumulation of snow above that level. Particular areas, however, show significant net gains and losses.

For example, the extreme southwestern portion of the central ice sheet thickens by some 210 millimeters each year. Although increased snowfall accounts for some of this growth, the scientists suggest that this area also accumulates ice flowing from higher elevations, a result of warming temperatures.

During the same period, the far southeastern portion of the ice sheet has been thinning by almost 300 mm annually, primarily due to increased melting. Northern portions of the ice sheet experience smaller changes due to lower precipitation and colder temperatures.

The second research team studied Greenland’s ice between 1993 and 1999, using airplanemounted laser altimeters to calculate its thickness. William B. Krabill and his colleagues confirmed that the central ice sheet remained stable. However, their research also shows that about 70 percent of the ice sheet’s margins are thinning substantially—in some places, Krabill notes, up to 1 meter per year. Moreover, they find, thickening of the margins elsewhere is not compensating for these losses.

Overall, more than 50 cubic kilometers of Greenland’s ice melts away annually, Krabill says. That’s enough to raise Earth’s sea level by about 0.13 mm each year, he adds, or about 7 percent of the rise observed during the study. Melting Greenland’s entire ice sheet, which is more than 4,000 m thick in places and covers almost all the island, would raise sea level about 7 m.

Currently, researchers can’t separate the effects of recent warming from those due to long-term trends in climate. The key to predicting future sea level changes is distinguishing the effects of increased average temperatures during the 1990s from those due to long-term trends, says Dorthe Dahl-Jensen of the University of Copenhagen, in a commentary accompanying the articles. “[T]he records need to be extended to several decades before the long-term trend can be estimated,” she says.

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