Science 2011-rignot-science.1208336

Ice Flow of the Antarctic Ice Sheet
E. Rignot,1,2* J. Mouginot,1 B. Scheuchl1
1
Dept. Earth System Science, University of California Irvine, Irvine, CA, 92697 USA. 2Jet Propulsion Laboratory, California
Institute of Technology, Pasadena, CA 91109, USA.

*To whom correspondence should be addressed. E-mail: erignot@uci.edu

We present a reference, comprehensive, high-resolution, from topographic divides to a set of narrow, fast-moving
digital mosaic of ice motion in Antarctica assembled from glaciers that control most of the ice sheet discharge into the
multiple satellite interferometric synthetic-aperture radar ocean. This lack of broad-scale and detailed observation of
data acquired during the International Polar Year 2007- ice motion has placed a fundamental limit on the capability
2009. The data reveal widespread, patterned, enhanced and reliability of numerical models of ice sheet evolution (6).

Downloaded from www.sciencemag.org on August 25, 2011
flow with tributary glaciers reaching hundreds to On the eve of the international polar year, international
thousands of kilometers inland, over the entire continent. space agencies worked together to enable a complete InSAR
This view of ice sheet motion emphasizes the importance survey of Antarctica. Here, we employ spring 2009 data from
of basal-slip dominated tributary flow over deformation the Canadian Space Agency (CSA)'s and MacDonald,
dominated ice sheet flow, redefines our understanding of Dettwiler and Associates Ltd. (MDA)'s RADARSAT-2,
ice sheet dynamics, and has far-reaching implications for spring 2007-2008-2009 data from European Space Agency
the reconstruction and prediction of ice sheet evolution. (ESA)'s Envisat ASAR, and fall 2007-2008 data from the
Japan Aerospace Exploration Agency (JAXA)'s Advanced
Ice velocity is a fundamental characteristic of glaciers and ice
Land Observing Satellite (ALOS) PALSAR, complemented
sheets that measures the rate at which ice is transported from
by patches of CSA's RADARSAT-1 data from fall 2000 (7)
the interior regions toward the ocean, the location of preferred
and ESA's ERS-1/2 data from spring 1996 (2). Each radar
channels of ice transport, and how ice mass evolves with
instrument contributes its unique coverage and performance
time. Traditional measurements from ground-based stations
level (see Fig. S1 in supporting online material (SOM)).
are limited relative to the size of the continent, leading to an
The final mosaic assembles 900 satellite tracks and more
incomplete picture of Antarctica. Satellite radar
than 3,000 orbits of radar data (Fig. 1). The data are geo-
interferometry or InSAR has been successfully employed to
referenced with a precision better than one pixel, here 300 m,
map glacier flow independent of cloud cover, solar
to an Earth-fixed grid using a digital elevation model (DEM)
illumination or the presence of surface features (1). Until
(8). Absolute calibration of the surface velocity data relies on
now, however, the vast extent of East Antarctica, which
control points of zero motion distributed along the coast
comprises 77% of the continent, has been devoid of quality
(stagnant areas near ice domes or emergent mountains) and
data; only a few floating ice shelves have been mapped, and
along major ice divides (areas of zero surface slope in the
comprehensive velocity mapping has been limited to the
DEM) in a set of coast-to-coast ASAR tracks (fig. S1). The
lower reaches of key outlet glaciers (2).
mapping precision varies with instrument, location, technique
Balance velocity calculated from ice thickness, surface
of analysis, repeat cycle, time period and data stacking.
slope and snow accumulation data provides insights about the
Nominal errors range from 1 m/yr along major ice divides
potential flow pattern of the ice sheet (3), but the technique
with high data stacking to about 17 m/yr in areas affected by
assumes an ice sheet in mass equilibrium, which is not correct
ionospheric perturbations (fig. S2). In terms of strain rate, or
everywhere, and that ice flows perpendicular to surface
changes in velocity per unit length, data noise is at the 3 x
contours, which is a simplification of the gravitational driving
10−4 /yr level, which is sufficient to reveal effective strain
stress equation of motion (4). The precision of balance
rates along tributary shear margins over the vast majority of
velocity is affected by uncertainties in snow accumulation,
the continent (Fig. 2A).
major gaps in ice thickness data, and poor topographic
Ice velocity ranges from a few cm/yr near divides to a few
coverage near the South Pole. Nor does the technique apply
km/yr on fast moving glaciers and floating ice shelves, or 5
to floating ice shelves or replicate the correct width and
orders of magnitude. The histogram in surface velocity has a
extent of ice streams and tributaries (5). As a result, there is
bi-modal distribution with a main peak at 4-5 m/yr
no clear picture of ice sheet motion at the continental scale,

/ www.sciencexpress.org / 18 August 2011 / Page 1 / 10.1126/science.1208336

corresponding to slow motion in East Antarctica, and a suggests uncertainties in DEM-derived ice divide or recent
second peak at 250 m/yr driven by the fast flow of glaciers changes in ice dynamics and enhanced flow to the West. The
and ice shelves. The fastest glaciers, Pine Island and western tributary of Foundation Ice Stream offers a natural
Thwaites, are several times faster than any other glacier, and division between east and west Antarctica flow that was not
unique in terms of how far inland fast flow prevails. This is known before. Academy Glacier, a major eastern tributary of
indeed the sector of most rapid change at present, over the Foundation, extends to the West over the South Pole and to
widest area, and with the greatest impact on total ice sheet the East in parallel to Support-Force Ice Stream for more than
mass balance (2). Other fastest-moving glaciers include the 400 km, which is counter to the traditional view of well-
Larsen B glaciers in the Peninsula, which accelerated in defined drainage basins.
response to ice shelf collapse (9); the Ferrigno and Land Our map reveals to its full extent the tributaries of
glaciers in the West; and the Ninnis, Frost, Totten, Denman Recovery, Slessor and Bailey ice streams, which are
and Shirase glaciers in the East. grounded below sea level and are likely underlaid by thick
There is reasonably good agreement between observed marine sediments that favor rapid basal motion (16–17). The
surface velocity and balance velocity (Fig. 2B) near ice tributaries share common sources that extend 1,000 km inland
divides, however, large differences exist within each basin of the grounding line through a meander of slow moving
and especially near the coast, demonstrating that the direct areas. Recovery Ice Stream's two main tributaries broaden
measurement of ice velocity is crucial to capture continental- inland and reach beyond the four subglacial lakes thought to

wide ice motion accurately. Figure 1 reveals a wealth of new be a possible source of fast motion (18). The presence of 2
information. For instance, the exact pathway of ice along the subglacial lakes may accelerate the flow of the tributaries
coastline is not without surprise. In Queen Maud Land, the coming from the East, but much less clearly for the tributaries
main trunk of Jutulstraumen is not to the South through Penck coming from the West. The impact of the abrupt reduction in
trough but to the east of Neumayer Cliffs (10). The Sor basal friction associated with subglacial lakes is not apparent
Rondane Mountains were known to deflect ice flow to the in the broad pattern of ice motion. Similarly, at Lake Vostok,
East and to the West through Hansenbreen and Brydbreen the largest subglacial lake in Antarctica, flow disturbances
glaciers, but the main ice sheet discharge is from two large remain at the sub-meter scale per year (19) and are not
(80-km), un-named ice streams that flow at 100-200 m/yr expressed beyond the lake.
around the Belgica Mountains for more than 500 km inland Bedrock topography has a clear impact on the flow pattern
(11). Farther east, the fast-flowing core of Shirase Glacier of ice (Fig. 2C). Restricted ice motion is observed over major
does not extend far inland, but little-studied Rayner Glacier to subglacial (Gamburtsev) and around emergent mountains
the east flows above 100 m/yr for more than 200 km inland, (Newfoundland, Sor Rondane, Victoria Land, TransAntarctic,
presumably along a deep subglacial trough (12). In the Executive Committee Range, Ellsworth, Graham Land),
Antarctic Peninsula, the tributaries of Wilkins Ice Shelf and which deflect flow around them or limit flow through narrow
of the northern sector of George VI Ice Shelf abruptly glaciers. The tributaries of Lambert Glacier extend inland
transition to zero velocity when they mix with the floating ice among slow-moving areas of higher ground, hence higher
shelves. We attribute this spectacular termination of the basal friction, with some tributaries emerging as narrow, fast
glaciers to massive rates of basal ablation of the ice shelves streams from the flanks of the Gamburtsev Mountains, which
by the underlying warm ocean (13). is surprising this far inland from the coast. The ice flowing
A most interesting aspect is the spatial pattern of tributary down Byrd Glacier, the most extensive basin drained by a
flow. Each major glacier is the merger of several tributaries single glacier, originates from 4 tributaries that spread inland
that extend hundreds of km inland. While this was observed over more than 1,000 km. The tributaries are wider and more
in the partial mapping of Siple Coast (14) and Pine Island diffuse than in the balance velocity map, which reveals a
(15), this is now observed over the entire ice sheet. The map form of ice motion that is neither ice stream flow nor ice
reveals to its full extent the dendritic nature of drainage sheet flow but an intermediate regime which we denote
systems, the anastomosing distribution of tributaries, the patterned enhanced flow.
narrowing (acceleration) and widening (deceleration) of Conversely, major flow pathways do not follow the
tributaries inland, and their extension well beyond grounding deepest subglacial basins, hence the divide between Pine
lines toward topographic divides. On Pine Island, Thwaites Island, Rutford and Evans runs across Byrd and Bentley
and Siple Coast, the tributaries extend to ice divides trenches, and the divide between Cook Ice Shelf and David
everywhere we have data. Pine Island and Rutford share a Glacier runs across the Wilkes subglacial basin. Totten
common source in the South that connects two different sides Glacier extends 1,000 km inland through two major
of West Antarctica. Furthermore, the velocity divide is offset tributaries that reach the Aurora subglacial basin, alongside
to the East of the topographic divide by 10-15 km, which Denman, another marine-based fast-flowing glacier. In


Wilkes Land, Mertz Glacier splits around a high-ground This organization of ice sheet flow into a complex set of
region to reach Astrolabe trench to the West and Wilkes meandering, size-varying, speed-varying, anastomosing
subglacial basin to the East. But Ninnis Glacier extends tributaries most certainly dominated by basal-slip motion
straight, deep south into Wilkes subglacial basin. These two challenges the view of ice sheet flow constrained by internal
glaciers exhibit an acceleration in flow at a bedrock step (20). deformation, disconnected from the coastal regions, that was
A similar initiation is visible on one western tributary of adopted as the background model for continental scale ice
Recovery Ice Stream and the western tributary of Totten, but sheet modeling (6, 21). Actual observations of continental-
over the vast majority of Antarctica, we find no area where scale ice motion reveal a new flow regime that initiates near
fast flow initiates at a bedrock step; instead, we observe topographic divides and involves a significant amount of
tributary flow extending slowly, diffusively and gradually basal-slip motion. Much remains to be understood about the
inland. mechanisms of basal motion and patterned enhanced flow,
Tributary shear margins, detected as zones of effective but our observations already imply a tighter connection
strain rate orders of magnitude higher than the background between coastal sectors and interior regions than in the
(Fig. 2A), appear at velocities of about 30 m/yr, i.e., at hypothetical case of a uniform ice sheet flow because the
substantially smaller values than the 100 m/yr quoted from concentration of ice fluxes along preferred channels enhances
the partial mapping of Siple Coast (14). This extensive the diffusivity of perturbations. It is likely that this patterned
network of shear margins reaches a few hundred km inland enhanced flow is not unique to Antarctica but a common

and expresses a strong coupling between coastal and inland feature of ice sheets. The mapping of Antarctic ice motion
flow of ice. Yet patterned enhanced flow initiates at even therefore redefines our view of ice sheet flow dynamics and
lower speed, in both west and east Antarctica. Patterned the way ice sheets have been modeled in the past, with
enhanced flow here means flow that is not uniform, implications for improving reconstructions of past and
dominated by deformational velocity, but includes a major ongoing changes, and especially for modeling the evolution
basal slip component which varies spatially. Basal slip may of ice sheet dynamics in a warming climate.
be attributed the presence of sub-glacial valleys that
channelize thicker ice that generates more friction, more heat References and Notes
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21. K. Cuffey and W. S. B. Paterson, The physics of glaciers Figs. S1 to S5
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Ice Data Center. Digital media. Published online 18 August 2011; 10.1126/science.1208336
23. E. Rignot, J. Mouginot, B. Scheuchl, Antarctic grounding
line mapping from differential satellite radar Fig. 1. Antarctic ice velocity derived from ALOS PALSAR,
interferometry. Geophys. Res. Lett. 38, L10504 (2011). Envisat ASAR, RADARSAT-2 and ERS-1/2 satellite radar
24. M. B. Lythe, D.G. Vaughan and the BEDMAP interferometry color coded on a logarithmic scale and
Consortium, BEDMAP: a new ice thickness and subglacial overlaid on a MODIS mosaic of Antarctica (22) with
topographic model of Antarctica. J. Geophys. Res. 106, geographic names discussed in the text. Pixel spacing is 300
11335 (2001). m. Projection is polar stereographic at 71oS secant plane.
Thick black lines delineate major ice divides (2). Thin black


lines outline subglacial lakes discussed in the text. Thick
black lines along the coast are interferometrically-derived ice
sheet grounding lines (23).
Fig. 2. (A) Effective strain rate, · , color coded from 3 x 10−4
yr−1 to greater than 15 x 10−3 yr−1; (B) balance velocity (3)
color coded on a logarithmic scale as in Fig. 1; (C) bed
elevation (24) with geographic names discussed in the text;
(D) surface velocity calculated from internal deformation
with a creep parameter, A = 9 x 10−25 s−1 Pa−3. Thick black
lines in (B-D) and white lines in (A) delineate major ice
divides (2) and grounding lines (23).



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