FROM: NASA
WASHINGTON -- Warm ocean currents attacking the underside of ice
shelves are the dominant cause of recent ice loss from Antarctica, a
new study using measurements from NASA's Ice, Cloud, and land
Elevation Satellite (ICESat) revealed.
An international team of scientists used a combination of satellite
measurements and models to differentiate between the two known causes
of melting ice shelves: warm ocean currents thawing the underbelly of
the floating extensions of ice sheets and warm air melting them from
above. The finding, published today in the journal Nature, brings
scientists a step closer to providing reliable projections of future
sea level rise.
The researchers concluded 20 of the 54 ice shelves studied are being
melted by warm ocean currents. Most of these are in West Antarctica,
where inland glaciers flowing down to the coast and feeding into
these thinning ice shelves have accelerated, draining more ice into
the sea and contributing to sea-level rise. This ocean-driven
thinning is responsible for the most widespread and rapid ice losses
in West Antarctica and the majority of Antarctic ice sheet loss
during the period studied.
"We can lose an awful lot of ice to the sea without ever having
summers warm enough to make the snow on top of the glaciers melt,"
said the study's lead author Hamish Pritchard of the British
Antarctic Survey in Cambridge, United Kingdom. "The oceans can do all
the work from below."
To map the changing thickness of almost all the floating ice shelves
around Antarctica, the team used a time series of 4.5 million surface
height measurements taken by a laser instrument mounted on ICESat
from October 2003 to October 2008. They measured how the ice shelf
height changed over time and ran computer models to discard changes
in ice thickness because of natural snow accumulation and compaction.
The researchers also used a tide model that eliminated height changes
caused by tides raising and lowering the ice shelves.
"This study demonstrates the power of space-based, laser altimetry for
understanding Earth processes," said Tom Wagner, cryosphere program
scientist at NASA Headquarters in Washington." Coupled with NASA's
portfolio of other ice sheet research using data from our GRACE
mission, satellite radars and aircraft, we get a comprehensive view
of ice sheet change that improves estimates of sea level rise."
Previous studies used satellite radar data to measure the evolution of
ice shelves and glaciers, but laser measurements are more precise in
detecting changes in ice shelf thickness through time. This is
especially true in coastal areas. Steeper slopes at the grounding
line, where floating ice shelves connect with the landmass, cause
problems for lower-resolution radar altimeters.
ICESat was the first satellite specifically designed to use laser
altimetry to study the Earth's polar regions. It operated from 2003
to 2009. Its successor, ICESat-2, is scheduled for launch in 2016.
"This study demonstrates the urgent need for ICESat-2 to get into
space," said Jay Zwally, ICESat project scientist at NASA's Goddard
Space Flight Center in Greenbelt, Md. "We have limited information on
the changes in polar regions caused by climate change. Nothing can
look at these changes like satellite measurements do."
The new research also links the observed increase in melting that
occurs on the underside of a glacier or ice shelf, called basal melt,
and glacier acceleration with changes in wind patterns.
"Studies have shown Antarctic winds have changed because of changes in
climate," Pritchard said. "This has affected the strength and
direction of ocean currents. As a result warm water is funnelled
beneath the floating ice. These studies and our new results suggest
Antarctica's glaciers are responding rapidly to a changing climate."
A different picture is seen on the Antarctic Peninsula, the long
stretch of land pointing towards South America. The study found
thinning of the largest ice shelf on the peninsula can be explained
by warm summer winds directly melting the snow on the ice shelf
surfaces. The patterns of widespread ocean-driven melting and summer
melting on the Antarctic Peninsula can be attributed to changing wind
patterns.
The study was carried out by an international team from the British
Antarctic Survey, Utrecht University in Utrecht, Netherlands, the
University of California in San Diego and the non-profit research
institute Earth and Space Research in Corvallis, Ore.
