EARTH & ENVIRONMENT
Warming blamed for ice shelf collapse
Meltwater pressure may have triggered Larsen B’s breakdown
B Y THOMAS SUMNER
Balmy surface temperatures, not an
unstable underbelly, probably prompted
the largest ice shelf collapse ever
recorded, researchers report.
In 2002, a Rhode Island–sized hunk
of the Larsen B ice shelf on the Antarctic
Peninsula shattered into thousands of
icebergs during the area’s hottest summer
on record (SN: 3/30/02, p. 197). Radar
maps indicated that the point where the
shelf floated off the seabed, called the
grounding line, had retreated farther
inland, triggering the overhanging ice to
break off. Now seafloor sediments reveal
that the grounding line was stable and
had been for thousands of years before
The finding, reported in the Sept. 12
Science, demonstrates that the grounding line wasn’t involved in Larsen B’s
breakup, says study coauthor Eugene
Domack, an earth scientist at the University of South Florida in St. Petersburg.
“Up until now the community accepted
that grounding line instability is needed
for ice shelves to disintegrate,” he says.
“We now show that surface warming
alone can cause ice shelves to collapse.”
He says other ice shelves could follow
Larsen B’s lead as Antarctic surface temperatures rise (SN: 7/27/13, p. 18).
Ice shelves line 45 percent of Ant-
arctica’s coast, helping stem the flow of
the continent’s ice sheets and glaciers
into the ocean. Warm seawater can melt
an ice shelf’s underside, pushing the
grounding line inward until the ice snags
on a seafloor formation such as a cliff or
hill. Because the grounding line is too far
under the ice to observe directly using
submersibles or ice drills, scientists glean
these deep ice movements from radar
data collected by satellites and airplanes.
After Larsen B’s remains floated away,
the location of the grounding line before
the breakup became open water. That
allowed Domack and colleagues to sample
seafloor sediments that had accumulated
over thousands of years beneath the ice.
Based on the radiocarbon ages of shells
found in the sediments, the team estimated how long ago each layer formed.
Because distinctive types and amounts
of sediment form where an ice shelf meets
the seafloor, the researchers could track
the location of Larsen B’s grounding line
over time. The team determined it hadn’t
budged for at least 11,000 years. Domack
suggests that scientists in 2002 simply
mistook a seafloor trough for a grounding line on the radar maps. Grounding
line instability, Domack concludes, did
not contribute to the shelf’s collapse.
Domack thinks surface melting, previously considered a secondary mechanism
in Larsen B’s collapse, was the prime trigger. During Antarctic summers, a layer of
snow usually sits on top of the shelf and
soaks up melt water from thawing ice and
glaciers, preventing it from forming large
pools. During the warm summers leading
up to the Larsen B collapse, the snow on
the ice shelf melted and water collected in
large lakes on the surface. Pressure from
these lakes probably opened cracks in the
ice, destabilizing the shelf, Domack says.
Glaciologist Eric Rignot of the University of California, Irvine remains unconvinced that surface melting on its own
could have caused Larsen B’s collapse. He
suggests other factors, such as warming
ocean temperatures thinning the shelf’s
underside, could have played a role. s
Surface melting may have caused the 2002
collapse of Antarctica’s Larsen B, seen in
the left half of a satellite photo before the
“We have no strong hints of new physics,” Savard says.
While Tait admits it would have been
exciting if the photon excess had held up,
he stresses that there is plenty of opportunity for discovery in the future. Many
theories positing new particles and forces
predict only subtle deviations in LHC
data from the standard model — effects
that are too small to be weeded out until
even more data are in hand.
And much more data are on the way.
The LHC was shut down in February
2013 for upgrades, but it will resume
particle smashing in spring 2015 (SN
Online: 6/23/14). The improved collider
will slam protons together at higher
energies, perhaps revealing heavier particles inaccessible to the first-generation
LHC. Plus, collisions will occur more
frequently, providing more data to help
physicists separate signal from noise.
“To find new particles, we want to give it
everything we’ve got,” Tait says. s
immune system proteins to fuel or calm
inflammation, or pave the way for a particular bacterium to settle into a niche.
Dorrestein predicts researchers will find
that fungi that live in the body make a
plethora of manipulative molecules, too.
Most of the gene clusters in human-dwelling bacteria — 30,000 — have no
known function. “It’s the stuff of bio-science fiction to imagine what all these
things are doing,” Mekalanos says. For
example, obese people’s microbes may
make antibiotics that kill off bacteria that
keep people lean. Microbes’ antibiotics
may do double duty: In addition to killing
rival bacteria, these molecules may also
help microbes communicate with their
human hosts, he speculates.
The newly discovered antibiotic, lactocillin, is a type of molecule called a
thiopeptide. A similar molecule is in
human tests for treating intestinal infections with Clostridium difficile. A bacterium called Lactobacillus gasseri makes
lactocillin in the vagina, and some bacteria in the mouth also make the antibiotic.
Other bacteria throughout the body produce their own thiopeptide antibiotics. s