A new NASA study says that an increase in
Antarctic snow accumulation that began 10,000 years ago is currently
adding enough ice to the continent to outweigh the increased losses from
its thinning glaciers.
The research challenges the conclusions of other studies, including
the Intergovernmental Panel on Climate Change’s (IPCC) 2013 report,
which says that Antarctica is overall losing land ice.
According to the new analysis of satellite data, the Antarctic ice
sheet showed a net gain of 112 billion tons of ice a year from 1992 to
2001. That net gain slowed to 82 billion tons of ice per year between
2003 and 2008.
“We’re essentially in agreement with other studies that
show an increase in ice discharge in the Antarctic Peninsula and the
Thwaites and Pine Island region of West Antarctica,” said Jay Zwally, a
glaciologist with NASA Goddard Space Flight Center in Greenbelt,
Maryland, and lead author of the study, which was published on Oct. 30
in the
Journal of Glaciology. “Our main disagreement is for
East Antarctica and the interior of West Antarctica – there, we see an
ice gain that exceeds the losses in the other areas.” Zwally added that
his team “measured small height changes over large areas, as well as the
large changes observed over smaller areas.”
Scientists calculate how much the ice sheet is growing or shrinking
from the changes in surface height that are measured by the satellite
altimeters. In locations where the amount of new snowfall accumulating
on an ice sheet is not equal to the ice flow downward and outward to the
ocean, the surface height changes and the ice-sheet mass grows or
shrinks.
But it might only take a few decades for Antarctica’s growth to reverse, according to Zwally.
“If the losses of the Antarctic Peninsula and parts of
West Antarctica continue to increase at the same rate they’ve been
increasing for the last two decades, the losses will catch up with the
long-term gain in East Antarctica in 20 or 30 years — I don’t think
there will be enough snowfall increase to offset these losses.”
The study analyzed changes in the surface height of the Antarctic ice
sheet measured by radar altimeters on two European Space Agency
European Remote Sensing (ERS) satellites, spanning from 1992 to 2001,
and by the laser altimeter on NASA’s Ice, Cloud, and land Elevation
Satellite (ICESat) from 2003 to 2008.
Zwally said that while other scientists have assumed that the gains
in elevation seen in East Antarctica are due to recent increases in snow
accumulation, his team used meteorological data beginning in 1979 to
show that the snowfall in East Antarctica actually decreased by 11
billion tons per year during both the ERS and ICESat periods. They also
used information on snow accumulation for tens of thousands of years,
derived by other scientists from ice cores, to conclude that East
Antarctica has been thickening for a very long time.
“At the end of the last Ice Age, the air became warmer
and carried more moisture across the continent, doubling the amount of
snow dropped on the ice sheet,” Zwally said.
The extra snowfall that began 10,000 years ago has been slowly
accumulating on the ice sheet and compacting into solid ice over
millennia, thickening the ice in East Antarctica and the interior of
West Antarctica by an average of 0.7 inches (1.7 centimeters) per year.
This small thickening, sustained over thousands of years and spread over
the vast expanse of these sectors of Antarctica, corresponds to a very
large gain of ice – enough to outweigh the losses from fast-flowing
glaciers in other parts of the continent and reduce global sea level
rise.
Zwally’s team calculated that the mass gain from the thickening of
East Antarctica remained steady from 1992 to 2008 at 200 billion tons
per year, while the ice losses from the coastal regions of West
Antarctica and the Antarctic Peninsula increased by 65 billion tons per
year.
“The good news is that Antarctica is not currently
contributing to sea level rise, but is taking 0.23 millimeters per year
away,” Zwally said. “But this is also bad news. If the 0.27 millimeters
per year of sea level rise attributed to Antarctica in the IPCC report
is not really coming from Antarctica, there must be some other
contribution to sea level rise that is not accounted for.”
“The new study highlights the difficulties of measuring
the small changes in ice height happening in East Antarctica,” said Ben
Smith, a glaciologist with the University of Washington in Seattle who
was not involved in Zwally’s study.
“Doing altimetry accurately for very large areas is extraordinarily
difficult, and there are measurements of snow accumulation that need to
be done independently to understand what’s happening in these places,”
Smith said.
To help accurately measure changes in Antarctica, NASA is developing
the successor to the ICESat mission, ICESat-2, which is scheduled to
launch in 2018. “ICESat-2 will measure changes in the ice sheet within
the thickness of a No. 2 pencil,” said Tom Neumann, a glaciologist at
Goddard and deputy project scientist for ICESat-2. “It will contribute
to solving the problem of Antarctica’s mass balance by providing a
long-term record of elevation changes.”
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