Science - USA (2020-03-20)

25-year altimetry period is pronounced in the

Amundsen and Bellingshausen Sea sectors

of West Antarctica and Wilkes Land in East

Antarctica ( 3 ). Pine Island, Thwaites, and Smith-

Pope-Kohler Glaciers experienced the greatest

elevation drop over this period, with changes

of up to 9 m/year (Fig. 4). Supporting the

observed GRACE mass gain, some margins

of East Antarctica are increasing in elevation

because of increased snowfall (Fig. 4) ( 29 ).

Along the Siple Coast, the interior of Kamb Ice

Stream has thickened at a rate of ~0.65 m/year

over the past 20 years because of its stagnation

(Fig. 4) ( 3 ). Velocity measurements based on

interferometric synthetic aperture radar (SAR)

measurements and speckle tracking ( 4 ) allow

the flow of large regions to be observed with

accuracies of several tens of meters per year.

The changes in ice sheet velocity are striking in

the Peninsula, where a substantial accelera-

tion of glaciers feeding the Larsen ice shelves

was observed after their collapse, as well as in

the Amundsen Sea sector. In this region, Pine

Island Glacier’s velocity doubled from the 1990s

to the 2010s (Fig. 4), while its grounding line

position, accurately estimated by differential

interferometric SAR, retreated by more than

30 km ( 7 ). The velocity observations are used

to calculate the flux of ice discharge into the

ocean and, combined with modeled surface

accumulation, to estimate the ice mass gain

or loss for the different catchment basins.

Through an international collaboration, the

scientific community has demonstrated the ro-

bust agreement between these three different

methods and highlighted the ongoing changes

of the Antarctic Ice Sheet ( 5 ).

Between 1950 and 2000, the average air tem-

perature in the Peninsula increased by 4°C ( 30 ).

During this warming period, the Larsen A

and B ice shelves collapsed in 1995 and 2002,

respectively (Fig. 4). The glaciers feeding the

Larsen B Ice Shelf sped up after the loss of the

backward stress or buttressing ( 6 , 31 ). Before

crystalline bedrock, and active volcanic ter-

rains ( 12 , 13 ). The low-viscosity warm litho-
sphere beneath the region is reflected in the
ongoing vertical movement of up to 4 cm/year

measured with GPS ( 25 ) and caused by ice sheet
thinning. The basal hydrology beneath West
Antarctica is complex, with hundreds of pockets
of water that fill and drain on a decadal time
scale producing meter-scale surface elevation

changes ( 1 ). Two sides of the West Antarctic
Ice Sheet are buttressed by the large Ross and
Ronne-Filchner ice shelves, whereas the Amund-
sen Sea sector drains into small ice shelves

(Fig. 4) ( 26 ). The ice sampled at the base of
West Antarctica is 68,000 years (Fig. 2) ( 27 ).
The Antarctic Peninsula is the northernmost
and warmest region of Antarctica. More than
500 glaciers drain ice from the central plateau.
Along the eastern side, facing the Weddell
Sea, the Peninsula glaciers feed the Larsen ice
shelves (Fig. 4).

Evidence for change

The evidence for recent changes of Antarctic
ice is quantified by three independent meas-
urements primarily derived from satellite and
airborne systems: decreasing mass from gravity,
dropping surface elevation, and increased sur-
face velocities (Fig. 4).
Changes in ice mass are measured from
space with the pair of GRACE (Gravity Recov-
ery and Climate Experiment) satellites, which
capture changes in the gravity field experienced
by each spacecraft as they orbit Earth together

( 28 ). The original pair of satellites resolved
monthly changes in Antarctic ice mass from
2002 to 2017 (Fig. 4), and a new pair of satellites,
GRACE Follow-On, was launched in 2018 to
continue the record of the Earth and Antarctic
mass changes. The observed changes must be
corrected for modeled changes in motion of the
solid Earth to include the crust and the mantle
rebound owing to past and ongoing ice mass
changes. Determining both the Earth structure
and the history of past ice sheet changes are the
greatest challenges in separating the observed
mass changes into the solid Earth component
and the changes in ice mass. The GRACE data

( 28 ) show mass loss in West Antarctica, focused
in the Amundsen and Bellingshausen Sea sec-
tors, and mass gain in some regions of East
Antarctica and along Kamb Ice Stream (Fig. 4).
Lowering of the surface elevation has been
measured with altimeters from space and air-
craft in the same regions where mass loss is
observed. Both radar (Cryosat and European
Remote Sensing satellites) and laser altimeters
[Ice,Cloud,andlandElevationSatellite1(ICESat1)
and ICESat2 satellites, Operation IceBridge air-
borne] are used to measure ice surface elevation.
Laser observations are impeded by cloud cover,
whereas radar measurements penetrate into the
upper portions of the snowpack, introducing
some ambiguity. Dropping elevation over the

SCIENCE 20 MARCH 2020•VOL 367 ISSUE 6484^1323

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A

B

B'

A'

C

Filchner-Ronne Ice Shelf

Pine Island

Glacier

Larsen C

Ice Shelf

Ross Ice Shelf

Thwaites Glacier

D

Gamburtsev

Antarctic Peninsula Mountains

Admundsen Sea

Bellinghausem

Sea

Transantarctic Mountains Wilkes Land

Lake

Vostok

Elevation (m)

Elevation (m)

Dome Fuji Dome A Vostok Dome C

Lake Vostok

Gamburtsev Mountains

-2000

-1000

0

1000

2000

3000

4000

WAIS Divide

Marine Ice Sheet

Ross Ice Shelf Sea level

Byrd Subglacial Basin

0 500 1000 1500 2000 2500 3050

Distance (km)

-2000

-1000

0

1000

2000

3000

4000

A

A A'

B B'

B

Amery Ice

Shelf

Ice velocity (m/yr) Bed elevation (m)

MEaSUREs Ice Velocity Bed Elevation (BEDMAP v2)

East Antarctica

West Antarctica

Marine Ice Sheet

Fig. 2. Structure of the Antarctic Ice Sheet.(A) Eastand (B) West Antarctic cross section profiles. (Cand

D) Location of profiles are shown on (C) surface velocity and (D) subglacial bed topography ( 17 ). West

Antarctica maximum elevation of 2200 m is nested in the deep Byrd Subglacial Basin, with depths 2500 m

below sea level. The East Antarctic Ice Sheet nucleated on the high Gamburtsev Mountains, with the

maximum elevation at Dome A reaching 4200 m, covers deep subglacial lakes such as Lake Vostok and has

portions that are marine. The portions of the ice sheets with bedrock elevation below sea level are shaded blue.