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Recent satellite observations and
analysis have shown that Greenland
will be a far greater contributor to
sea level rise than previously expected.
Image 1. depicts the changes in elevation
across Greenland's vast ice sheet, which
covers roughly 80 per cent of the total
landmass. The dark blue colouring
indicates a decrease in elevation of 0.5
meters or less per year, while red indicates
a decrease of over 1.5 meters per year.
The ice sheet velocity was calculated
by using data from the European Space
Agency's new Sentinel-1A satellite, which
captured 12-day repeat acquisitions
throughout 2015. About 1200 radar
scenes from the satellite's wide-swath
mode were used to produce the map. It
clearly shows dynamic glacier outlets
around the Greenland coast such as the
vulnerable Zachariæ Isstrøm glacier in the
northeast, which in 2015 was found to be
destabilising from the mainland and had
begun crumbling into the Atlantic Ocean.
Clearly identifiable as the large
red area in the top right of Image 1.,
Zachariæ Isstrøm drains ice from an
area of 91,780 square kilometres - about
five per cent of the Greenland ice sheet.
A 2015 study led by the University of
California, Irvine, found that Zachariæ
Isstrøm holds enough water to raise
global sea level by more than 46
centimetres if it were to melt completely,
and new satellite data shows that it's
currently leaking 5 billion tonnes of mass
into the ocean every year.
These developments come after much
of Greenland's expansive ice sheet was
found to be far more vulnerable to warm
ocean waters from climate change than
had been thought, according to studies
of Greenland's underlying topography by
UC Irvine and NASA glaciologists. It was
previously impossible to get an estimate
of Greenland's ground topography,
however, ice thickness data was calculated
by a 'mass conservation algorithm' that
combined ice thickness measurements
derived from radar with information on
the velocity and direction of its movement
and estimates of snowfall and surface melt.
The topography was then calculated
by subtracting the ice thickness from
the surface elevation data, revealing
estimates of the full subterranean
landscape as shown in Image 2. The
findings exposed what were thought to
be shallow glaciers along the coast were
actually long, deep fissures stretching
more than 100 kilometres inland and
descending far below sea level.
The deformation of Greenland's ice
sheet is also accelerating: between 2012
and 2015, the Zachariæ Isstrøm accelerated
its descent into the Atlantic by 125 metres a
year, every year, three times faster than the
speed between 2000 and 2012.
"Zachariæ Isstrøm is being hit from
above and below," said Eric Rignot,
professor of Earth system science at UC,
Irvine and another of the authors. "The
top of the glacier is melting away as a
result of decades of steadily increasing
air temperatures, while its underside
is compromised by currents carrying
warmer ocean water."
The researchers predict that the
Zachariæ glacier will go on retreating for
another 20 to 30 years, while continuing
to increase its discharge of ice and
contribution to sea level rise. ■
Far left: Image1. Ice sheet
in motion. Colour scale in
metres per day. Contains
Sentinel data. Source:
Left: Image 2. Bed
topography in Greenland.
Source: M. Morlighem,
E. Rignot, J. Mouginot,
H. Seroussi and E.
Larour, "Deeply incised
valleys beneath the
Greenland Ice Sheet",
7, 418-422, 2014.
4 position December/January 2016
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