Follow
this link to skip to the main content

NASA - National Aeronautics and Space Administration

NASA Home

>    News & Features

>    News Topics

>    Looking at Earth

>    Features

Send

Print

Follow
this link to Share This PageShare

Feature

Text Size

Grow Text SizeShrink Text Size

Is Antarctica Melting?

01.12.10

The continent of Antarctica has been losing more than 100
cubic kilometers (24 cubic miles) of ice per year since 2002.
Larger Image

There has been lots of talk lately about Antarctica and whether or not the
continent's giant ice sheet is melting. One new paper 1, which states
there’s less surface melting recently than in past years, has been cited as
"proof" that there’s no global warming. Other evidence that the amount of sea
ice around Antarctica seems to be increasing slightly 2-4 is being
used in the same way. But both of these data points are misleading. Gravity data
collected from space using NASA's Grace satellite show that Antarctica has been
losing more than a hundred cubic kilometers (24 cubic miles) of ice each year
since 2002. The latest data reveal that Antarctica is losing ice at an
accelerating rate, too. How is it possible for surface melting to decrease, but
for the continent to lose mass anyway? The answer boils down to the fact that
ice can flow without melting.

The Antarctic ice sheet. East Antarctica is much higher in
elevation than West Antarctica.
Larger Image

Two-thirds of Antarctica is a high, cold desert. Known as East Antarctica,
this section has an average altitude of about 2 kilometer (1.2 miles), higher
than the American Colorado Plateau. There is a continent about the size of
Australia underneath all this ice; the ice sheet sitting on top averages at a
little over 2 kilometer (1.2 miles) thick. If all of this ice melted, it would
raise global sea level by about 60 meter (197 feet). But little, if any, surface
warming is occurring over East Antarctica. Radar and laser-based satellite data
show a little mass loss at the edges of East Antarctica, which is being partly
offset by accumulation of snow in the interior, although a very recent result
from the NASA/German Aerospace Center's Gravity Recovery and Climate Experiment
(Grace) suggests that since 2006 there has been more ice loss from East
Antarctica than previously thought 5. Overall, not much is going on
in East Antarctica -- yet.

A Frozen Hawaii

West Antarctica is very different. Instead of a single continent, it is a
series of islands covered by ice -- think of it as a frozen Hawaii, with
penguins. Because it's a group of islands, much of the West Antarctic Ice Sheet
(WAIS, in the jargon) is actually sitting on the floor of the Southern Ocean,
not on dry land. Parts of it are more than 1.7 kilometer (1 mile) below sea
level. Pine Island is the largest of these islands and the largest ice stream in
West Antarctica is called Pine Island Glacier. The WAIS, if it melted
completely, would raise sea level by 5 to 7 meter (16 to 23 feet). And the Pine
Island Glacier would contribute about 10 percent of that.

West Antarctica is a series of islands covered by ice.
Think of it as a frozen Hawaii, with penguins. (Image credit: National Science
Foundation)
Larger Image

Since the early 1990s, European and Canadian satellites have been collecting
radar data from West Antarctica. These radar data can reveal ice motion and, by
the late 1990s, there was enough data for scientists to measure the annual
motion of the Pine Island Glacier. Using radar information collected between
1992 and 1996, oceanographer Eric Rignot, based at NASA’s Jet Propulsion
Laboratory (JPL), found that the Pine Island Glacier's "grounding line" -- the
line between the glacier's floating section and the part of the glacier that
rests on the sea floor -- had retreated rapidly towards the land. That meant
that the glacier was losing mass. He attributed the retreat to the warming
waters around West Antarctica 6. But with only a few years of data, he couldn't
say whether the retreat was a temporary, natural anomaly or a longer-term trend
from global warming.

Rignot's paper surprised many people. JPL scientist Ron Kwok saw it as
demonstrating that "the old idea that glaciers move really slowly isn't true any
more." One result was that a lot more people started to use the radar data to
examine much more of Antarctica. A major review published in 2009 found that
Rignot's Pine Island Glacier finding hadn't been a fluke 7: a large
majority of the marine glaciers of the Antarctic Peninsula were retreating, and
their retreat was speeding up. This summer, a British group revisited the Pine
Island Glacier finding and found that its rate of retreat had quadrupled between
1995 and 2006 8.

How the Ice Shelf Crumbles

The retreat of West Antarctica's glaciers is being accelerated by ice shelf
collapse. Ice shelves are the part of a glacier that extends past the grounding
line towards the ocean they are the most vulnerable to warming seas. A
longstanding theory in glaciology is that these ice shelves tend to buttress
(support the end wall of) glaciers, with their mass slowing the ice movement
towards the sea, and this was confirmed by the spectacular collapse of the Rhode
Island-sized Larsen B shelf along the Eastern edge of the Antarctic Peninsula in
2002. The disintegration, which was caught on camera by NASA's Moderate
Resolution Imaging Spectroradiometer (MODIS) imaging instruments on board its
Terra and Aqua satellites, was dramatic: it took just three weeks to crumble a
12,000-year old ice shelf. Over the next few years, satellite radar data showed
that some of the ice streams flowing behind Larsen B had accelerated
significantly, while others, still supported by smaller ice shelves, had not
9. This dynamic process of ice flowing downhill to the sea is what
enables Antarctica to continue losing mass even as surface melting declines.

Michael Schodlok, a JPL scientist who models the way ice shelves and the
ocean interact, says melting of the underside of the shelf is a pre-requisite to
these collapses. Thinning of the ice shelf reduces its buttressing effect on the
glacier behind it, allowing glacier flow to speed up. The thinner shelf is also
more likely to crack. In the summer, meltwater ponds on the surface can drain
into the cracks. Since liquid water is denser than solid ice, enough meltwater
on the surface can open the cracks up deeper down into the ice, leading to
disintegration of the shelf. The oceans surrounding Antarctica have been warming
10, so Schodlok doesn't doubt that the ice shelves are being
undermined by warmer water being brought up from the depths. But he admits that
it hasn't been proven rigorously, because satellites can’t measure underneath
the ice.

Glaciologist Robert Bindschadler of NASA's Goddard Space Flight Center
intends to show just that. He's leading an expedition scheduled to start in 2011
to drill through the Pine Island Glacier and place an automated buoy into the
water below it. According to Bindschadler, Pine Island Glacier "is the place to
go because that is where the changes are the largest. If we want to understand
how the ocean is impacting the ice sheet, go to where it's hitting the ice sheet
with a sledgehammer, not with a little tack hammer."

Meanwhile, measurements from the Grace satellites confirm that Antarctica is
losing mass 11. Isabella Velicogna of JPL and the University of
California, Irvine, uses Grace data to weigh the Antarctic ice sheet from space.
Her work shows that the ice sheet is not only losing mass, but it is losing mass
at an accelerating rate. "The important message is that it is not a linear
trend. A linear trend means you have the same mass loss every year. The fact
that it’s above linear, this is the important idea, that ice loss is increasing
with time," she says. And she points out that it isn’t just the Grace data that
show accelerating loss; the radar data do, too. "It isn't just one type of
measurement. It's a series of independent measurements that are giving the same
results, which makes it more robust."

For more information about this topic, visit NASA's Global Climate Change
website.

References:

1 Marco Tedesco and Andrew J. Monaghan, "An updated Antarctic melt
record through 2009 and its linkages to high-latitude and tropical climate
variability," Geophys. Res. Lett., 36, L18502 (2009).

2
http://arctic.atmos.uiuc.edu/cryosphere/IMAGES/current.anom.south.jpg

3 http://www.sciencedaily.com/releases/2009/04/090421101629.htm

4 http://nsidc.org/seaice/characteristics/difference.html

5 J. L. Chen et al., "Accelerated Antarctic ice loss from
satellite gravity measurements," Nat. Geosci. 2, 859-862 (2009).

6 E.J. Rignot, "Fast Recession of a West Antarctic Glacier",
Science 281, 549-551 (1998)

7 P.A. Mayewski, et.al., "State of the Antarctic and Southern
Ocean Climate System," Rev. Geophys., 47, 1-38 (2009).

8 D. J. Wingham et.al., "Spatial and Temporal Evolution of Pine
Island Glacier thinning, 1995-2006," Geophys. Res.Lett. 36, L17501 (2009).

9 E. Rignot et.al., "Accelerated ice discharge from the Antarctic
Peninsula following the collapse of Larsen B ice shelf," Geophys. Res. Lett. 31,
L18401 (2004)

10 R. M. Robertson et al., "Long term temperature trends in the
deep waters of the Weddell Sea", Deep Sea Research 49, 21, 4791-4806 (2002);
http://condor.pems.adfa.edu.au/FD-Course/webpage/longterm.pdf.

11Isabella Velicogna, "Increasing rates of ice mass loss from the
Greenland and Antarctic ice sheets revealed by GRACE," Geophys. Res. Lett. 36,
L19503 (2009).

12 J. H. Mercer, "West Antarctic ice Sheet and CO2 Greenhouse
Effect-Threat of Disaster," Nature, 271 (5643), 321-325 (1978).

13 R. Kwok & D.A. Rothrock, "Decline in Arctic sea ice
thickness from submarine and ICESat records: 1958 - 2008," Geophys. Res. Lett.,
36, L15501 (2009),

Erik Conway
NASA/Jet
Propulsion Laboratory

› Back
To Top