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SCIENCE OBSERVER

Greenland or Whiteland?

David Schneider

According to most experts, global warming will lead to some worrisome changes in the not-too-distant future. One that is expected to become quite troubling, at least for coastal residents, is rising sea level. The latest assessment of the Intergovernmental Panel on Climate Change, for example, suggests that the world may see anywhere from 11 to 77 centimeters' rise in sea level before the century runs out. One of the several sources for all this water, the panel's scientists concluded, will be the partial loss of the Greenland ice sheet. That view seems quite consistent with recent news reports of Greenland's slushiness, 2002 having been a record year for summer melting there. But a new study suggests that the Greenland ice sheet may, in fact, grow as the climate warms over the next several decades, which would modestly diminish the rate at which the oceans are expected to rise.

Will 21st-century climate warmingClick to Enlarge Image

This surprising result was published earlier this year in the Journal of Geophysical Research. Martin Wild, Pierluigi Calanca, Simon C. Scherrer and Atsumu Ohmura of the Swiss Federal Institute of Technology in Zurich evaluated the changes to the Greenland ice sheet that would ensue in a world where the carbon dioxide concentration had doubled. (Unless current trends abate, we'll reach this level in about 70 years.) As with several other modeling efforts of this sort, they found that the accumulation of snow and ice will increase over Antarctica. The reason is simply that the warming of the nearby ocean puts more moisture in the atmosphere, which then falls as snow over land. Melting over that frigid continent should remain negligible, even with doubled CO2.

The net growth expected of the Antarctic ice sheet thus acts to slow the rise of sea level, which is expected to come about primarily because ocean water expands as it warms. Greenland, being not nearly so chilly as Antarctica, experiences both accumulation and melting. So the issue there is which process dominates. Wild and his colleagues found that doubled CO2 will lead to an increase in accumulation that more than compensates for the enhanced melting (by a factor of almost four). Greenland's net contribution to sea level works out to a lowering of 0.32 millimeter per year, or 3.2 centimeters per century. That is, the magnitude of the beneficial effect is between 4 and 29 percent of the change in sea level that was expected to take place over the next century.

"It definitely is an exciting new result," says Konrad Steffan, an investigator at the University of Colorado at Boulder who maintains a network of automated weather stations in Greenland. Steffan says that although he had expected accumulation to increase somewhat over the interior of the Greenland ice sheet over coming decades, he was surprised to see the considerable increase that Wild and his coworkers found with their model. And he was similarly surprised to see that the melting in this doubled-CO2 simulation was so much smaller than estimated previously. Wild reports that field workers have generally had such startled reactions to his new result. "It's a bit counter-intuitive for them," he says.

Why the big difference from past assessments? The short answer is resolution. Running global climate models is a computer-intensive exercise, which limits how fine a grid one can use to divvy up the Earth for calculation. A coarse mesh fails to capture the key feature of the topography of Greenland, where elevation rises abruptly near the coast. Even the best models thus end up representing Greenland as a gently rounded mound rather than as a steep-walled mesa. And because melting takes place only at lower elevations, the area prone to melting gets exaggerated in the models.

Wild and his colleagues managed to overcome this problem by combining their latest model results for the temperature and precipitation change over Greenland with a high-resolution topographic map. This maneuver provided a realistic picture of where enhanced melting would actually take place—and that zone covered a far smaller swath than had been determined previously without taking the detailed topography into account.

One weakness of their procedure, which the Swiss investigators candidly admit, is that they assume that the rate at which glaciers near the margins carry ice to the sea will not change. They were essentially forced to make that simplification because the global climate model they used could not simulate complex ice dynamics. "This is one big weakness," says Calanca, who points out that the meltwater on some spots in western Greenland is known to percolate down all the way to bedrock, where it lubricates the base of the glacier, allowing it to flow more easily.

This phenomenon was demonstrated for the first time last year, when H. Jay Zwally of NASA's Goddard Space Flight Center and his coworkers described the motion of a 4-meter pole stuck in the ice of a Greenland glacier since 1996. Using the Global Positioning System, they determined that the pole (and presumably the glacier it marks) moves fastest each year in late summer, when meltwater is available to, in effect, grease its skids.

But Zwally doesn't make a big deal of Wild's inability to model the lubricating effect of meltwater: "Just how significant that can be—we really don't know yet." One aid to finding out may be circling the Earth now, a probe called ICESat, which contains a laser altimeter designed to map changes in the Greenland and Antarctic ice sheets. Unfortunately the slated three-to-five-year mission of ICESat has gotten off to a rocky start. Zwally, the lead scientist for ICESat, explains dryly that "it operated for 36 days before the first laser failed." So right now ICESat is out of commission. But the satellite carries two other laser transmitters, which might suffice for the remainder of the planned mission, assuming that controllers can figure out what damaged the first unit and then correct their procedures. Zwally is guardedly optimistic that ICESat will yet provide critical ground truth for climatologists and offers that "we're hoping we can rise to the occasion."

Wild too is guardedly optimistic about the value of ICESat—and about what his study means for the Earth's rising seas, noting that "if things stay below two times CO2, it may not be all that bad." That is, we'll only have to cope with the thermal expansion of the oceans: Taken together, Greenland, Antarctica and the Earth's many small mountain glaciers in total will probably contribute little if anything to rising sea level. But Wild cautions that this conclusion applies only to a situation in which CO2 has doubled in concentration. Should atmospheric CO2 increase further, he believes that that the margin of Antarctica will begin to experience summer melting. So we should be thankful that both the Greenland and Antarctic ice sheets are helping to slow the rise of sea level, for the moment at least, and not push our luck—or rather, our grandchildren's luck—too hard.—David Schneider

 

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