year after year without melting away in
the summer — meltwater ponds are only
about 1 centimeter deep, meaning they
are lighter than the ponds on younger
ice. “Is there something we’re missing
about the permeability structure that
allows water to pool in first-year ice?”
asks Eicken.
For now, the answer seems to be
maybe. One way to make progress on
such questions may be to once again pull
back on perspective, leaving the world of
ice microphysics and zooming out to a
full view of the ice cap.
Here, too, researchers have much to do.
In particular, scientists lament the lack
of regular measurements of ice thickness
across the Arctic ice cap. Such observations can help distinguish between thinner first-year ice and thicker multiyear
ice, which behave very differently from
each other. But there are few options for
obtaining this data reliably.
NASA’s ICESat mission gauged ice
thickness by bouncing a laser beam off
the surface of the snow and measuring the
time it took to return. But ICESat ceased
operations last fall after six years, and
its replacement is not due to fly until at
least 2015. The European Space Agency’s
newly launched CryoSat- 2 mission
is taking up some of the slack with its
own radar altimeter instrument. NASA
is also flying research airplanes in the
IceBridge mission over some regions of
the Arctic.
Pulling all these views together will
give polar scientists a much fuller picture of the sea ice situation. And all will
be looking to see what this year’s summer melt season brings, to forecast how
much time they may have left to study
the ice at all.
Explore more
s k.m. golden. “climate change and
the mathematics of transport in sea
ice.” Notices of the american Math-
ematical society. may 2009.
s national snow and ice data center’s
page on arctic sea ice: nsidc.org/
arcticseaicenews/
s sea ice outlook: www.arcus.org/
search/seaiceoutlook/
www.sciencenews.org
Sea ice’s yearly retreat
each september, polar scientists get the number they’ve been betting on all
summer: how small the arctic ice cap will get that year.
at the end of the summer melt season, sea ice covers only a fraction of the
arctic ocean compared with the ice’s winter reach. in september 2007 ice
reached a record low, covering about 4. 2 million square kilometers —about
23 percent less than the previous record minimum, in 2005. few researchers
had seen the 2007 record low coming.
so in 2008, scientists began putting together predictions for what the upcom-
ing summer might bring. international teams of researchers submit to a central
organizer their “outlooks,” which include a number —how small they expect the
arctic ice cap to get that year —and a rationale for that number. reports are due
by June and updated monthly throughout the summer to incorporate information
about changing ice conditions. this June will mark the third annual effort.
so far, the project has primarily shown how hard it is to predict sea ice cover.
many factors determine how much ice exists from year to year, from oceanic
heating patterns to wind and water conditions that can either pile up ice into
sturdy blocks or break it apart and flush it out of the arctic ocean basin.
for instance, all of the project teams underestimated how much ice would
remain in september 2009. the median estimate was 4. 6 million square kilo-
meters, but the final number was 5.36 million square kilometers, in large part
because of weather patterns in august and september that kept things chilly
across large parts of the basin.
this spring, arctic sea ice reached its maximum on march 31 (above), the lat-
est date recorded for a maximum since satellite measurements began in 1979.
the late maximum probably won’t affect the september minimum much, since
most of that ice is thin first-year ice on the southern fringes that will melt away
quickly once temperatures rise, says arctic expert walt meier of the national
snow and ice data center in boulder, colo. “what really is key is how much of
the thick ice we have,” he says. “we have more thick ice than we did the last
couple of years at this point.” as of late may his team’s predictions for the
2010 minimum had not been released publicly, but meier says that this year’s
ice might look similar to that in 2009— not the dramatic low seen in 2007.
marika holland, a climate modeler at the national center for atmospheric
research in boulder, likens the forecasting exercise to the early days of predict-
ing the el niño climate pattern. unreliable decades ago, el niño predictions
have improved to the point that today planners use them regularly.
the arctic outlook project hopes one day to have similarly useful forecasts for
sea ice. in the meantime, the project has started issuing a much smaller-scale
forecast for regional use. the new “walrus outlook” predicts coastal sea ice
patterns for the upcoming week, for use by native hunters. —alexandra Witze
