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.
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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