The Cryosphere, 16, 2545–2564, 2022
https://doi.org/10.5194/tc-16-2545-2022© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Rapid fragmentation of Thwaites Eastern Ice Shelf
Douglas I. Benn1 , Adrian Luckman2 , Jan A. Åström3 , Anna J. Crawford1 , Stephen L. Cornford2 , Suzanne L. Bevan2 ,Thomas Zwinger3 , Rupert Gladstone4 , Karen Alley5 , Erin Pettit6 , and Jeremy Bassis7
1 School of Geography and Sustainable Development, University of St Andrews, St Andrews, KY16 9AL, UK
2 Department of Geography, Swansea University, Swansea, SA2 8PP, UK
3 CSC-IT Center for Science, 02101 Espoo, Finland
4 The Arctic Centre, University of Lapland, 96101 Rovaniemi, Finland
5 Department of Environment and Geography, University of Manitoba, Winnipeg, MB R3T 2M6, Canada
6 College of Earth, Ocean and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331-5503, USA
7 Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109-1005, USA

Correspondence: Douglas I. Benn (dib2@st-andrews.ac.uk)
Received: 8 September 2021 – Discussion started: 20 September 2021
Revised: 6 March 2022 – Accepted: 25 April 2022 – Published: 27 June 2022

Abstract. Ice shelves play a key role in the dynamics ofmarine ice sheets by buttressing grounded ice and limiting
rates of ice flux to the oceans. In response to recent climaticand oceanic change, ice shelves fringing the West Antarctic
Ice Sheet (WAIS) have begun to fragment and retreat, withmajor implications for ice-sheet stability. Here, we focus on
the Thwaites Eastern Ice Shelf (TEIS), the remaining pinned
floating extension of Thwaites Glacier. We show that TEIS
has undergone a process of fragmentation in the last 5 years,
including brittle failure along a major shear zone, formationof tensile cracks on the main body of the shelf, and a releaseof tabular bergs on both the eastern and western flanks. Simulations with the Helsinki Discrete Element Model (HiDEM)
show that this pattern of failure is associated with high backstress from a submarine pinning point at the distal edge of the
shelf. We show that a significant zone of shear, upstream ofthe main pinning point, developed in response to the rapid
acceleration of the shelf between 2002 and 2006, seedingdamage on the shelf. Subsequently, basal melting and positive feedback between damage and strain rates weakened
TEIS, allowing damage to accumulate. Thus, although backstress on TEIS has likely diminished over time as the pinning point shrunk, accumulation of damage has ensured thatthe ice in the shear zone remained the weakest link in thesystem. Experiments with the BISICLES ice-sheet model indicate that additional damage to or unpinning of TEIS is unlikely to trigger significantly increased ice loss from WAIS,
but the calving response to the loss of TEIS remains highly

uncertain. It is widely recognised that ice-shelf fragmentation and collapse can be triggered by hydrofracturing and/or
unpinning from ice-shelf margins or grounding points. Our
results indicate a third mechanism, backstress triggered failure, that can occur if and when an ice shelf is no longer able
to withstand stress imposed by pinning points. In most circumstances, pinning points are essential for ice-shelf stability, but as ice shelves thin and weaken, the concentration ofbackstress in damaged ice upstream of a pinning point may
provide the seeds of their demise.

1

Introduction

Ice shelves play a key role in the dynamics of marine ice
sheets. By transmitting resistive stresses from lateral or basal
pinning points to the grounding line, ice shelves buttressgrounded portions of the ice sheet and constrain ice flow
(e.g. Doake et al., 1998; Dupont and Alley, 2005). If buttressing is reduced or lost following the retreat or disintegration of ice shelves, tributary glaciers can accelerate andincrease ice flux to the ocean (Scambos et al., 2004). Accelerated ice discharge may be temporary and reversible, butmodelling studies indicate that in some circumstances (e.g.
where the bed is steeply retrograde) the ice-shelf breakup
may initiate sustained loss of grounded ice and irreversible
retreat through the marine ice-sheet and marine ice-cliff instabilities (Schoof, 2012; Sun et al., 2020; DeConto et al.,Published by Copernicus Publications on behalf of the European Geosciences Union.

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D. I. Benn et al.: Rapid fragmentation of Thwaites Eastern Ice Shelf

Figure 1. The floating extensions of Thwaites Glacier, showing thelocation of the Thwaites Eastern Ice Shelf (TEIS), the Thwaites
Western Ice Tongue (TWIT), and the position of grounding lines
and the location of pinning points in 2011 from Rignot et al. (2014).

we use a high-frequency time series of Sentinel-1 imagery
to show that within the last 5 years it has transitioned froman intact ice shelf to a highly fragmented state. The ongoing acceleration of parts of the ice shelf suggests the crossing of a threshold from stable to unstable. Our simulations
with the Helsinki Discrete Element Model (HiDEM) show a
distinct fracture pattern that supports this threshold-crossing
behaviour occurring due to the failure of weakened ice in response to stresses associated with the pinning point, and not
due to progressive unpinning. Pinning points, therefore, are
not necessarily always a stabilising factor for ice shelves, butinstead may have a destabilising effect when ice around themis sufficiently weakened. Finally, we use the ice-sheet model,
BISICLES, to explore the possible near-term consequencesof damage evolution, shelf thinning and decoupling of TEIS
from the pinning point. The recent behaviour and imminent
breakup of TEIS have important implications for both iceshelf stability and the effectiveness of buttressing from intact
and fragmented ice shelves.

22021; Bassis et al., 2021). Large areas of the West Antarctic
Ice Sheet (WAIS) are vulnerable to this process, particularly
Thwaites Glacier and Pine Island Glacier in the Amundsen
Sea sector (Scambos et al., 2017).
In response to recent climatic and oceanic change, the geographical extent of ice-shelf retreat and disintegration has
spread south and westward from the Antarctic Peninsula into
West Antarctica (Cook and Vaughan, 2010; Liu et al., 2015).
If the response of WAIS to a variety of climate change scenarios is to be predicted with confidence, understanding theprocesses affecting ice-shelf stability is a matter of urgency(Fox-Kemper et al., 2021). The prominent role of melt ponddrainage and hydrofracturing in the demise of Larsen B in
2002 (Scambos et al., 2003) has focused attention on surface melting as a trigger for ice-shelf disintegration (e.g. Robel and Banwell, 2019; Lai et al., 2020). Recent observations, however, indicate that ice-shelf retreat and disintegration can occur in the absence of surface melt, if basal melting causes ice to lose contact with lateral or seafloor pinningpoints. Recent examples include major rifting and calvingfrom the Pine Island Glacier following weakening of lateralpinning points (Arndt et al., 2018; Lhermitte et al., 2020) andfragmentation of the Thwaites Western Ice Tongue (TWIT;
Fig. 1) following progressive loss of a seafloor pinning point(Tinto and Bell, 2011; Miles et al., 2020).
In this paper, we focus on the Thwaites Eastern Ice Shelf(TEIS), the remaining pinned floating extension of Thwaites
Glacier. TEIS is currently pinned at its distal end by aseafloor ridge (Fig. 1), but rates of ice thinning over this pinning point suggest that complete unpinning could occur inless than 1 decade (Alley et al., 2021; Wild et al. 2021). HereThe Cryosphere, 16, 2545–2564, 2022

2.1

Methods
Observations

We use satellite data to monitor surface features which indicate the evolution of surface and basal fractures, and to derive
surface velocity fields that monitor change in ice flow rate
and patterns of surface strain. The main source of satellite
data is Sentinel-1 (image resolution ∼ 10 m, velocity product resolution ∼ 100 m), but we also use velocity products
from MODIS (velocity product resolution ∼ 500 m; Alley etal., 2021), MEaSUREs (velocity product resolution ∼ 1 km;
Mouginot et al., 2017) and ITS_LIVE (velocity product resolution ∼ 240 m, Gardner et al., 2019) to provide historicalcontext.
Surface velocity fields are derived from the Sentinel-1 Interferometric Wide Swath (IW) mode using standard feature/speckle tracking procedures (e.g. Luckman et al., 2015).
We employ the whole satellite data archive since 2014 andcombine velocity maps by averaging following a spatial filter based on signal-to-noise ratio and local variability. We
present results of individual velocity maps on key dates andmean velocity products on an annual, quarterly or monthly
basis to assess the development of speed and strain. We use
a combination of 6 and 12 d Sentinel-1 image pairs fromthe available archive and our feature tracking window sizeis 416 × 128 pixels, which equates to ∼ 1 km in range andazimuth. We sample the velocity field at 50 × 10 pixels before geocoding to the Antarctic Polar Stereographic projection (EPSG:3031) at 100 m resolution using the REMA mosaic DEM (Howat et al., 2019) gap-filled by Bedmap2 surface topography data (Fretwell et al., 2013). Strain rates arederived from selected 6 d pair velocity maps with high coherhttps://doi.org/10.5194/tc-16-2545-2022

D. I. Benn et al.: Rapid fragmentation of Thwaites Eastern Ice Shelfence and low noise, and are calculated in a 3 × 3 neighbourhood for optimum resolution.
2.2

Modelling experiments

Modelling experiments were conducted with HiDEM and the
BISICLES ice-sheet model to investigate fracture processes
underway at TEIS and ice-sheet dynamic responses to the ice
shelf’s evolution, respectively.
HiDEM is a brittle–elastic fracture model that can be used
to simulate ice fracture and calving processes by representing
ice as arrays of particles linked by breakable elastic beams(Åström et al., 2013). Particles are stacked together in ahexagonal close-packed lattice to form a 3-D domain representing observed ice geometries. The version used in thisstudy (HiDEM2.0) was developed by Jan A. Åström andFredrik Robertsen at the CSC-IT Center for Science, Finland, with data structures and a parallelisation scheme optimised for effective computation. On a modern HPC system
HiDEM2.0 can compute 106 time steps for 108 particles inabout 24 h, with a time step length of 0.001 s. This is 1–2
orders of magnitude faster than older versions of HiDEM,
allowing simulations of much larger domains with smaller
particle sizes.
Five parameters determine the bulk tensile and shear
strength of the ice: particle size, beam width to particle diameter ratio, beam tensile breaking strain, maximum beam endpoint bending angle, and density of randomly distributed prebroken beams (damage). In the simulations reported here, we
use a particle size of 40 m, a beam width to particle diameter
ratio of 0.6, tensile breaking strain of 0.0005, and a maximum bending angle of 0.03 radians. These values were calibrated against observed fracture and calving patterns on theGreenlandic glacier Sermeq Kujalleq (a.k.a. Jakobshavn Isbræ). The calibration was focused at Sermeq Kujalleq given
the glacier’s large ice thickness and the availability of remotely sensed observations of calving.
The HiDEM domain incorporates the entire area of TEIS
and extends 20 to 30 km upstream of the grounding line. The
domain is based on BedMachine v2 (Morlighem et al., 2020),
which incorporates the REMA mosaic ice-surface elevation
DEM (Howat et al., 2019), hydrostatic ice thickness for the
fully floating regions, and recent updates to the Thwaites
Glacier bed and adjacent seafloor (Jordan et al., 2020). The
glacier bed DEM does not incorporate recent data on the
TEIS pinning point presented by Wild et al. (2021). The implications of this omission are discussed below.
The model domain was adjusted in a short (2.5 d) surface
relaxation in Elmer/Ice, to ensure that the ice-shelf base near
the grounding line was at equilibrium with stresses in the ice,
as well as buoyant forces. The extent of the pinned ice, which
consists of two separate pinning regions, was unchanged over
the relaxation period. The surface height changed minimallyand the height above flotation over the pinned regions (approximately 20 to 30 m) is comparable to values shown by
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Wild et al. (2021). Any differences are negligible considering the 40 m particle size used in the HiDEM simulations. Ice
viscosity and basal resistance were then estimated following
the serial inversion workflow described in Wang et al. (2022)
using MEaSUREs v.2 velocity data (Rignot et al., 2011a, b;
Mouginot et al., 2012, 2017). The region of the REMA mosaic covering TEIS is based on data acquired in 2013–2014,
and the velocity data are from 2007 and 2009. The model domain should thus be regarded as representing an approximation of conditions in recent years rather than a specific snapshot in time. Surface velocities following the inversion werein agreement with observed velocities over the grounded region (approximately 150 m a−1 or less). Subglacial friction
coefficients for grounded portions of the domain were determined assuming a linear bed friction law, and friction coefficients were converted to SI units and rescaled (×10−5 ) for
use in HiDEM. Rescaling bed friction coefficients is necessary to produce useful ice-displacement magnitudes, because
HiDEM simulates glacier sliding and fracture taking placeover timescales of seconds, whereas in reality these processes
take place over timescales of hours to years (van Dongen etal., 2020).
Experiments with HiDEM were designed to test two hypotheses on the causes of the observed fragmentation of
TEIS: (1) fragmentation is related to a reduction of backstress on the shelf, consequent upon progressive unpinning;and (2) fragmentation is related to progressive weakening of
the ice and continuing backstress from the pinning point. To
test these hypotheses, we conducted a matrix of runs with different combinations of ice damage density and pinning point
friction. The latter is used to represent the degree to which the
TEIS was pinned. Simulations that utilised the rescaled bed
friction coefficient noted above, hereafter referred to as baseline friction, represented a less-pinned TEIS. A fully pinned
TEIS was represented by placing “no-slip” constraints onparticles that were in contact with the pinned region. It is appropriate to apply this no-slip boundary condition given that
HiDEM is used to simulate the elastic component of the TEIS
deformation. Other, slower processes of ice motion would
not result in substantial movement over the short-duration
timescales of motion considered in HiDEM simulations.
Ice damage density is represented by the proportion of initially broken bonds in a HiDEM simulation. In the present
study, ice strength was varied by adjusting the initial ice
damage density d from 0 (no damage) to 0.3 (moderate
damage) to 0.6 (highly damaged). This damage index represents a reduction in the load-bearing area and thus has asimilar physical meaning to damage as commonly defined(e.g. Lemaitre, 2012; Borstad et al., 2012). However, the dependence of modelled ice properties on particle and bondparameters means that values of d are not directly comparable to damage variables used in other studies, including the
BISICLES model discussed below. Ice damage typically increases during model runs as beams are broken in response
to inter-particle stresses. Unlike the initial prescribed damage
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D. I. Benn et al.: Rapid fragmentation of Thwaites Eastern Ice Shelfd, which is macroscopically uniform and isotropic, emergent
damage is localised and anisotropic, typically taking the formof fractures and shear zones and therefore a more realistic
simulation of damaged ice (Åström and Benn, 2019).
The HiDEM settings and experiments are summarised inTable 1. In the form used in this study, HiDEM is a purely
brittle–elastic model and does not include viscous deformation. For simulations of short-lived calving processes it issufficient to allow the domain to evolve under the gravitational and buoyant forces arising from ice geometry and water depth. Crevasses and rifts on ice shelves, however, typically propagate on long timescales during which ice can undergo large displacements, and for such cases some external model forcing is often desirable (e.g. Åström and Benn,
2019; Åström et al., 2021). In the runs reported here, we applied a force at the upstream boundary of the domain that
produced a close approximation of the observed velocity
structure near the grounding line. The simulated TEIS domain contained 109 particles and was run over 36 h using
2048 cores on the Mahti supercomputer at the CSC-IT Center for Science.
BISICLES is a continuum ice-flow model based on a vertically integrated stress balance equation (Cornford et al.,2020). Its treatment of fracture processes is limited to thecalculation of a scalar damage D(x, y, t), which modifies
the effective viscosity. BISICLES includes a model of damage evolution that attempts to evolve damage according to asimple relationship with local stress and damage advectedfrom upstream (Sun et al., 2017). That model, however,proves too simplistic to model TEIS and TWIT accurately.
Instead, we estimate D(x, y, t), together with basal friction
τ b (x, y), through regularised optimisations conducted formonthly intervals from 2016 to 2020, using our velocity data
derived from Sentinel-1 imagery. The model domain covers
the Amundsen Sea embayment drainage basins including theentire Thwaites Glacier, and is based upon the BedMachine
v.2 ice thickness and bedrock elevation (Morlighem et al.,2020). As in the HiDEM simulations, the pinning point at
the northern tip of TEIS comprises two separate pinning regions. In contrast to HiDEM, the vertically integrated stressmodel within BISICLES does not depend on thickness above
flotation in the pinning point, but only on its extent.
To assess the possible dynamic response of Thwaites
Glacier to further changes on TEIS, we conducted a set ofsimulations to 2116 with varying damage, ice-shelf thickness h(x, y, t) and pinning point friction. All simulations
start with the basal friction τ b calibrated to March 2016,
which then evolves according to a regularised Coulomb law(Joughin et al., 2019; Zoet and Iverson, 2020),
1|τ b | =β 2 |u| 3
1

,

(1)

(|u| + u0 ) 3
where β 2 (x, y) is a basal friction coefficient, u is the horizontal ice velocity, and u0 is a regularisation parameter.
The Cryosphere, 16, 2545–2564, 2022

A simple set of calving criteria are also in play: ice is removed wherever (1 − D)h < 5 m or where ice speed exceeds
104 m a−1 . The damage–calving criterion is similar to thefull-depth crevasse calving law of Nick et al. (2010) and determines the position of the ice front rather than a calving
rate. Fourteen experiments are reported here and summarisedin Table 2, designed to assess the impact of different physicalprocesses. Each experiment is named according to the evolution of the ice-shelf damage D(x, y) and the choice of ice
shelf melt rate and the choice of the basal friction regularisation parameter in Eq. (1), with a three-letter code (e.g. EMC,
meaning “Extrapolation of damage, ice shelf Melting, andCoulomb-like friction”)
Damage evolution follows one of three possibilities:– Control experiments have D(x, y) set to the optimal values determined for March 2016.

– Extrapolation experiments have D(x, y) increases linearly in time within the ice shelf from 2016 in orderto reach the optimal values for March 2020, and thencontinue to increase linearly to 2026 at which point
D(x, y) ∼ 1 across the shear zone between the pinningpoint and the remainder of TEIS, and across the shear
margin between TEIS and TWIT.

– Unpinning experiments set the ice thickness to zero in anarrow region surrounding the pinning point from 2016
onward, creating a new northern calving front.
Ice shelf melt rates also follow one of three different
schemes. Control experiments fix the ice-shelf thickness intime. Retreat experiments allow the ice shelf to evolve dynamically and fix the melt rate to zero but prevent presentday floating ice regions from grounding, so that the grounding line may retreat but not advance. Melting experiments
apply one of the melt rate formulae described in Yu etal. (2018), leading to melt rates:

M=











0
160(zd − 150)
850
160


zd < 150 
−1
150 ≤ zd < 1000  m a ,1000 < zd(2)where zd is the depth of the lower surface of the ice shelf,
measured in metres.
The basal friction regularisation parameter was set to either u0 = 50 m a−1 , resulting in plastic bed (Coulomblike) friction
 across much of Thwaites Glacier, or to u0 =300 m a−1 , where hard bed (Weertman-like) sliding dominates.
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Table 1. Key parameters in simulations using the Helsinki Discrete Element Model (HiDEM).
ModelParameter

Value

HiDEM

Beam width: particle diameter
Tensile breaking strain
Maximum bending angle (radians)
Young’s modulus (Pa)
Damage (‘d’; proportion of initially broken bonds)
Friction scale

0.6
0.00050.03
1.0 × 10−90.1; 0.3; 0.6
1.0 × 10−5 (baseline friction); 1 ×
10−4 + no-slip constraints over the pinning point (“no-slip”)
40
0.001
1200

Particle size (m)Time step (s)Duration of simulation (s)Table 2. BISICLES experiments.

3
3.1

Experimentu0 (m a−1 )Damage evolution

Melt rate evolution

00C
00W
0MW
0MV

50
300
50
300

Control:D(x, y) fixed at the values determined
for March 2016

Control: ice-shelf thickness remains
constant
Melting: melt rate up to 160 (m a−1 )

EOW
EOC
ERW
ERC
EMW
EMC

50
300
50
300
50
300

Extrapolation: D(x, y) increases linearly within
the ice shelf from 2016 to reach the value determined for March 2020, and then continues toincrease at the same rate until 2026.

Control

URW
URC
UMW
UMC

50
300
50
300

Unpinning: ice thickness in the shear margin
between TEIS and TWIT set to zero in 2016.

Observations
Velocity structure 2015–2021

The large-scale velocity structure of TEIS underwent littlechange over the period 2015–2020 (Fig. 2). On all velocity
maps, ice-flow vectors onto the shelf are aligned approximately normal to the general trend of the grounding line andover most of the shelf progressively veer toward the northeast as ice approaches the pinning point. The only exception
to this pattern is in a relatively small region west and southwest of the pinning point, where ice flow is predominantly towards the northwest. Over most of the shelf, flow speeds are
typically in the range 1–3 m d−1 and an order of magnitude
lower at and around the pinning point. Sustained shrinkage ofthe area bounded by the 0.2 m d−1 velocity contour indicatesprogressive loss of traction over the pinning point, consistent
with previous work that demonstrated ongoing ice thinningin this region (Fig. 2; Alley et al., 2021; Wild et al., 2021).
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Retreating: melt rate calculated to prevent ice-shelf grounding.
Melting
Retreating
Melting

3.2

Fracture and strain 2015–2021

Although the large-scale velocity structure of TEIS has remained relatively constant from 2015 to 2020, considerable
changes in fracture patterns and strain rates occurred during
this period.
In Sentinel-1 imagery, evidence for fracturing takes two
forms: (1) broad surface troughs indicating depressions
above basal crevasses, and (2) sharp-edged linear features indicating surface crevasses or full-depth rifts (e.g. Luckman
et al., 2012). In the earliest Sentinel-1 images (2014), setsof subparallel basal crevasses occur downflow of the grounding line and upstream of the pinning point (Fig. 3). The latter set extends diagonally across the shelf from southwest tonortheast, coincident with the transition between rapidly andslowly moving ice shown in Fig. 2. This region is interpreted
as a narrow shear zone (S; Fig. 3) between slow-moving ice
over the pinning point (P; Fig. 3) and more rapidly flowing
ice on the main part of the shelf, hereafter termed the TEIS
shear zone. In late 2016 and early 2017, a set of secondary
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D. I. Benn et al.: Rapid fragmentation of Thwaites Eastern Ice Shelf

Figure 2. Time series of mean annual velocity from TEIS derived from Sentinel-1 speckle tracking between 2015 and 2020. Arrows show
flow direction and indicate no significant change in flow orientation during this period. White lines delineate the 0.2 m d−1 velocity contour
which is centred on the main established pinning point. Sustained shrinkage over time of the area bounded by this contour confirms that
TEIS is losing contact with the pinning point as the shelf thins (Alley et al., 2021; Wild et al., 2021).

basal crevasses started to develop at a high angle to the TEIS
shear zone, on the upstream (southern) side (T; Fig. 3). These
subsequently propagated southward across the shelf and increased in number. During 2017, the easternmost of thesesecondary crevasses developed into a full-depth rift, visible
as a sharp-edged feature on the Sentinel-1 imagery and leading to the detachment of a tabular berg ∼ 18 km long and
up to 2 km wide (C1; Fig. 3). Other areas of active calving are evident on both the eastern (C2; Fig. 3) and western

The Cryosphere, 16, 2545–2564, 2022

(C3; Fig. 3) flanks of TEIS, the latter releasing tabular bergs
into the area formerly occupied by the shear margin between
TEIS and TWIT. Evidence for full-depth rifting within the
TEIS shear zone was initially confined to short wing cracks
across blocks bounded by basal crevasses, but became increasingly widespread over time.
Rapid evolution of the shear zone since 2018 is further illustrated by velocity gradients and patterns of shear strain(Figs. 4 and 5). In 2018, a narrow band of high strain existed

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Figure 3. Fracture patterns on TEIS 2014–2021. Labels indicate key features described in the text: S–S: TEIS shear zone; T: tensile cracks;
C1–C3: calving along rifts; P: slow-moving, relatively intact ice above pinning point.

in the northeastern portion of the shear zone, while strain in
the remainder of the shear zone was lower in magnitude anddistributed over a much wider area. In late 2020, a second
narrow shear band appeared at the southwest end of the shear
zone, then both bands connected and extended across the full
extent of the shelf by early 2021, indicating a transition to
full-thickness fractures. Velocity gradients across the southwestern half of the shear zone (Fig. 5) show distributed strain
in the first quarter of 2020, suggesting predominantly viscous
deformation or strain distributed across numerous fractures.
During 2020 all velocity profiles showed increasing localisation of strain as the shear zone transitioned to full-depth
fractures. Increasing shear localisation is accompanied by anincrease in ice velocity on the upstream side of the shear zone
and a decrease on the downflow (pinning point) side. On
the upstream side, ice acceleration is substantial and ongoing (Fig. 6), with > 40 % increase in ice speed in the central
part of TEIS between 2018 and 2021.

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3.3

Long-term perspective: velocity and strain from
MODIS data 2002–2021

The shear zone on TEIS was already a well-established feature in 2014, when the Sentinel-1 data began. Evolution of
TEIS prior to 2014, and the possible origin of the shear zone,
can be inferred using MODIS and other historical data (Alley et al., 2021). These data are at a lower spatial resolution(500 m) than the Sentinel-1 imagery, and strain rates were
calculated over a longer length scale (2500 m). This precludes detailed analysis, but the MODIS data provide a valuable record of large-scale patterns of velocity and strain andtheir changes over time.
Figure 6 shows a time series of velocity in the central partof TEIS, ca. 15 km upstream of the pinning point (magenta
circle in Fig. 4). The ice accelerated between 2002 and 2006,
then rapidly decreased to a minimum in 2009 before gradually increasing until 2020 when the current acceleration
began. The 2002–2006 acceleration is attributed to strong
coupling between TEIS and TWIT (Alley et al., 2021). The
TWIT increased in speed after 2002 (Miles et al., 2020), possibly in response to progressive unpinning (Tinto and Bell,2011), and the strong shear margin between the two portions
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D. I. Benn et al.: Rapid fragmentation of Thwaites Eastern Ice Shelf

Figure 4. Evolution of velocity (first and third columns) and shear strain rate (second and fourth columns) on TEIS derived from Sentinel-1
speckle tracking between 2016 and 2021. Specific image pairs are chosen for excellent coherence and minimal noise, and to focus on recent
months. Cyan lines are MEaSUREs InSAR-derived grounding lines from 2011 showing the location of historic known pinning points. Whitelines show positions of numbered profiles used to extract velocities presented in Fig. 5. The magenta circle indicates the location of thevelocity time series shown in Fig. 6.

of the shelf allowed TEIS to be dragged forward (Alley etal., 2021). After 2006, the shear margin between TEIS andTWIT fragmented and weakened, reducing coupling and allowing TEIS to decelerate. Meanwhile, TWIT fragmented
and transitioned into a “mélange ice shelf” and continued toaccelerate (Miles et al., 2020; Alley et al., 2021).
Patterns of strain on TEIS for three key time periods areshown in Fig. 7. During the TEIS acceleration event (2005–
2006), a band of positive (dextral; blue) shear strain outlines
the shear margin between TEIS and TWIT, while a zone of
negative (sinistral; red) shear strain is evident around the locus of the modern TEIS shear zone. Very high compressive
longitudinal strain rates upstream of the TEIS pinning point
indicate significant backstress during the acceleration event.
After the acceleration event, the shear margin between TEIS
and TWIT weakened, reducing the coupling between them,and allowing TEIS to slow down again. This appears in the
middle column of Fig. 7, where high shear strain rates delimit the weakened shear margin between TEIS and TWIT.
In contrast, both shear strain and longitudinal strain rates arevery low upstream of the TEIS pinning point. However, thefirst large fractures within the TEIS shear zone are visible in
the Landsat imagery (marked with an arrow). Data for 2019–
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2020 show a renewed increase of shear strain in the vicinityof the shear zone and the development of extensive rifting.
This phase of shear zone evolution has been discussed in detail above (Figs. 4 and 5).
In summary, TEIS has undergone a process of fragmentation in the last 5 years, including brittle failure along a major
shear zone up-glacier of the pinning point, formation of longitudinal tensile cracks on the main body of the shelf, andrelease of tabular bergs on both eastern and western flanks of
TEIS. This pattern of failure is consistent with longitudinal
compression and transverse extension of the shelf. The origins of the shear zone can at least be traced to the 2002–2006
acceleration event on TEIS, when zones of high longitudinaland shear strain developed upstream of the pinning point, indicating high backstress at that time. Fragmentation of the
shelf has increased since 2014, with increasing shear localisation and extensive full-thickness rifting since late 2020.
Weakening of the shear zone has been accompanied by rapid
and ongoing acceleration of the ice on the upstream side.

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and calving of tabular icebergs, and tensile fractures are developed in the central part of the shelf (Fig. 8d).
The fracture pattern in the no-slip simulation exhibits
many similarities with the pattern observed in February 2021
(Fig. 8c and d). Key common features are (1) a triangularzone of slowly moving or stagnant ice extending upstream
from the pinning point; (2) deflection of ice flow around thestagnant zone; (3) development of a shear zone (TEIS shearzone) upstream of the pinning point; (4) and rifting and calving along the eastern and western flanks of TEIS. Tensile
cracks appear on the mid-shelf in the HiDEM simulation,
but unlike the separate linear fractures observed on TEIS,
they are part of stepped fractures with both shear and tensile components. This difference probably reflects the lack ofviscous processes in the model. The otherwise close similarity between the observed pattern of fracture and the no-slip
HiDEM simulation supports the conclusion that the recent
fragmentation of TEIS occurred in response to uniaxial compression (i.e. opposing driving stress and backstress from thepinning point) and the absence of transverse confining pressure.
4.2
Figure 5. Profiles of mean quarterly surface velocity during 2020
and 2021 from Sentinel-1 speckle/feature tracking along the whitelines shown in Fig. 4.

4
4.1

Modelling results
Fracture modelling with HiDEM

The results of our HiDEM simulations show that widespread
fracturing of the shelf does not occur in runs with low damage density (d < 0.3), but does occur for runs with high damage density (d = 0.6). Results from two runs with d = 0.6
and different pinning point friction are presented here.
Results for d = 0.6 and baseline friction are shown in
Fig. 8a. The pinning point does not exert any significant influence on the pattern of ice displacement, and ice is able
to slide over the proximal side of the submarine ridge thencalves at its crest. Extensive rifting and calving occur on theeastern flank of TEIS (highlighted by discontinuities in displacement magnitudes), but only limited fracturing occurs on
the western flank.
Figure 8b shows results for d = 0.6 and a no-slip boundary condition over the pinning point. In this case, a teardropshaped area of stagnant, largely intact ice extends upstream
from the pinning point. Patterns of ice displacement on TEIS
show progressive veering towards the northeast as it approaches the stagnant zone. Mobile and stagnant ice are separated by a shear zone, indicated by the sharp discontinuity
in displacement magnitudes. The eastern and western flanks
of TEIS exhibit widespread propagation of subparallel rifts
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BISICLES model results: damage evolution andfuture dynamics

The observed increase in speed in TEIS between 2016 and2020 can be reproduced in the optimised BISICLES modelby reduced friction close to the 2016 TWIT grounding line
and by increased damage in both the shear zone region southeast of the TEIS pinning point as well as the shear margin between TEIS and TWIT. The optimised damage andbasal friction fields computed for March 2016 to 2020 are
shown in Fig. 9 together with the associated observed andmodel speeds. Model speeds match observed speeds acrossthe majority of Thwaites Glacier: the notable exception is
TWIT, where brittle fracture and the formation of a mélange
shelf are poorly described by the continuum model. There
the mismatch is as much as 500 m a−1 , or 10 % of the observed speed. Figure 10 shows the difference in basal traction, and the difference in damage, between March 2016 andMarch 2020, together with the difference in both observed
and model velocities. The greatest relative changes in thebasal friction are concentrated close to the TWIT grounding line and on the TEIS pinning point, perhaps due to thegrounding line retreat, or at least the reduction in effective
pressure. Damage around the TEIS pinning point increases
from around 0.4 to around 0.8, while TEIS and TWIT are
essentially separated from one another.
We used the optimised basal traction and damage values
as the starting point for forward model simulations (Figs. 11,
12, Appendix). In all cases, changes that are quite large occur in the velocity pattern on TEIS. The control experiments(00W, 00C) exhibit a reduction in speed of ∼ 200 m a−1
across both ice-shelf regions and immediately upstream by2032, caused by thinning of the ice upstream and the reThe Cryosphere, 16, 2545–2564, 2022

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D. I. Benn et al.: Rapid fragmentation of Thwaites Eastern Ice Shelf

Figure 6. Velocity evolution at a point in the centre of TEIS from MODIS (blue dots; Alley et al., 2021), ITS_LIVE (pink), MEaSUREs(green) and Sentinel-1 (small red) showing the 2002–2006 acceleration, the 2006–2009 slowdown, the 2009–2020 modest acceleration andrecent rapid acceleration as the shelf crossed the fracture transition. Location of velocity data is shown by the magenta circle in Fig. 4.

Figure 7. Mean longitudinal strain rates (a–c) and shear strain rates (d–f), from MODIS feature tracking in 2005–2006 during the TEIS
acceleration event (a, d), 2009–2010 (b, e), and 2019–2020 (c, f). The TEIS–TWIT shear margin is near the bottom of each image andthe TEIS shear zone is to the right of the twin “islands” of the pinning point, outlined in black, from the MEaSUREs 2011 grounding
line (Rignot et al., 2016). Background images are from Landsat-7 (a, d: 13 January 2005; b, e: 13 December 2010) and Landsat-8 (c, f:
12 November 2019). The first major crevasses within the location of the current shear zone became visible in Landsat imagery around 2010,
and examples are indicated by the arrows in the middle column.

sulting reduced gravitational driving stress. Damage extrapolation and unpinning experiments without sustained future
melting (E0W, E0C, ERW, ERC, URW, URC) see an increasein speed on TEIS upstream from the pinning point, attributed
to reduced ability of the shelf to support backstress. In contrast, ice around the grounding line of TWIT and the easterngrounding line of TEIS decreases in speed for similar reasons to that in the control experiments. Experiments ERW
and ERC (extrapolated damage, dynamically evolving iceshelf thickness) result in faster flow over most of TEIS thaneither E0W or E0C (extrapolated damage, constant ice-shelf
thickness), with the ice-shelf acceleration leading to thinning
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and a further loss of buttressing. The greatest speed increase
is seen in experiments URW and URC, because a new calving front is created; hence additional tensile stress is imposedon the now unconfined shelf. In all cases, there is at most amodest increase in the ice flux across the grounding line onto
TEIS, even when large acceleration occurs on the shelf. On
the other hand, experiments with sustained future melt forcing (0MW, 0MC, EMW, EMC, UMW, UMC) see ice flowing faster across the whole region, more so when the TEIS
damage is greater (EMW, EMC, UMW, UMC). These experiments also see a grounding line retreat ∼ 1 km a−1 in boththe TEIS and TWIT areas. The much greater retreat in thesehttps://doi.org/10.5194/tc-16-2545-2022

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Figure 8. Modelled and observed fractures on TEIS. Panels (a) and (b) show ice displacement patterns simulated in HiDEM. Panel (c) shows
the satellite observations and labels key features as in Fig. 3 (S–S: TEIS shear zone; T: tensile cracks; C1–C3: calving along rifts; P: pinningpoint). Panel (a) illustrates that the observed shear zone does not develop in the simulation with the baseline friction boundary condition
over the pinning point (i.e. values derived from the Elmer/Ice inversion rescaled for HiDEM) and damage density = 0.6. The zone of large
displacements around TEIS (yellow) consists of mélange and calved icebergs. Panel (b) shows the sharp displacement gradient (shear zone)
between the main body of TEIS and the triangular area of stationary ice extending from the pinning point in the HiDEM simulation witha no-slip boundary condition over the pinning point and damage density = 0.6. Panel (d) illustrates fracture patterns resulting from the
HiDEM simulation, depicting surface elevation in greyscale shading and bonds broken during the simulation in blue. Features with close
correspondence to the observations are also indicated.

cases indicates a substantial loss of buttressing when the ice
shelf thins close to the grounding line.
The future contribution of Thwaites Glacier to sea-level
rise computed by the BISICLES model depends only weakly
on changes around the TEIS pinning point. Figure 12 shows
the discharge of ice above flotation, that isZ
Ff (t) = ∇ H · (u(h − hf )) dG ,(3)
Gand the change in volume above flotation (VAF),Z
1Vf (t) = (hf (t) − hf (t = 2016))dG ,

(4)

G

where u is the horizontal ice velocity, h is ice thickness, hf
is the flotation thickness, ∇ H is the horizontal gradient operator, and G is the grounded part of the Thwaites drainage
basin. The most important cause of variation between simulations is the sustained melt rate. Simulations that apply
the melt rate of Eq. (2) exhibit discharge of ice above flotation in excess of 180 Gt a−1 for the entire century-long simulations, resulting in around 25 mm eustatic sea-level risehttps://doi.org/10.5194/tc-16-2545-2022

by 2100, consistent with the comparable simulations of Yu
et al. (2018). Simulations without sustained ice shelf meltshow a long-term trend with discharge declining from 180 to
100 Gt a−1 and less than 10 mm eustatic sea-level rise. The
second most important cause of variations is the choice offriction law. Coulomb-like (plastic) friction models produce
more discharge than Weertman-like simulations given sustained melt rates, where the grounded ice is generally accelerating with less discharge otherwise. The effects of damage extrapolation and TEIS unpinning are visible as minor
additional discharge early in each simulation, most prominently in the difference between experiments UMC and 0MC
to 2040. TEIS unpinning and the consequent ice-shelf acceleration (experiments ERW, ERC, URW, URC) result insome additional grounding line retreat compared to the control experiments (00W, 00C) by the end of the 21st century(Figs. A1–A3).

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D. I. Benn et al.: Rapid fragmentation of Thwaites Eastern Ice Shelf

Figure 9. BISICLES inverse problem results: March 2016–2020. The first and second columns show the optimised model velocities andobserved velocities; the third column shows the misfit between modelled and observed velocities; and the fourth and fifth columns show theresulting optimised basal traction and damage. Each row shows data for March of 1 year.

5
5.1

Discussion
Observed and modelled fracturing

Sentinel-1 data, available since 2014, show that TEIS hastransitioned from a largely intact ice shelf into an extensively fractured state, with full-depth fracturing along the
TEIS shear zone, rifting and tabular calving along both theeastern and western flanks, tensile fracturing in the central
part of the shelf, and acceleration of ice flow. Concurrently,
ice has been progressively decoupling from the submarine
pinning point at its distal end.
The HiDEM modelling highlights the key role of backstress from the pinning point in this pattern of fracturing. The
TEIS shear zone does not develop in runs where friction overthe pinning point has the baseline values derived by rescaling
output from the Elmer/Ice inversion. In contrast, a shear zone
and other features similar to the observed fracture pattern are
The Cryosphere, 16, 2545–2564, 2022

seen to develop in runs where a no-slip boundary condition
is imposed over the pinning point. It may appear paradoxicalthat the shear zone fails to develop under model boundary
conditions derived from observed ice geometry and dynamics. Two reasons may be suggested for this.
First, the inversion-derived friction coefficients over thepinning point may be unrepresentative because of uncertainties associated with the seafloor topography, the inversion
process in Elmer/Ice, and the subsequent rescaling for HiDEM. Radar data show that the bedrock topography is up to
200 m shallower than indicated by the gravity inversion usedas the basis for the model bed (Wild et al., 2021). Therefore, some small local differences between our bed and theseafloor topography shown by Wild et al. (2021) may exist. However, the height above flotation is reasonable when
considering the full pinning point region in our HiDEM domain. In the Elmer/Ice inversion, the relatively small area ofthe pinning point may mean that the inverted basal resistancehttps://doi.org/10.5194/tc-16-2545-2022

D. I. Benn et al.: Rapid fragmentation of Thwaites Eastern Ice Shelf

2557

Figure 10. BISICLES model inverse problem results: difference between March 2016 and March 2020. Observed speed differences (a).
Model speed difference (b). Normalised difference in basal friction magnitude |τ b (x, y, t) | (c). Difference in damage D(xyt) (d).

Figure 11. Output of BISICLES simulations of TEIS, showing differences in velocity between 2016 and 2032, and the 2032 grounding line
(see Appendix for equivalent panels with simulation end years of 2024, 2064 and 2112). Control experiments (00W, 00C) see TEIS and the
upstream ice slow down as the geometry relaxes. Damage extrapolation and unpinning experiments without sustained future melting (E0W,
E0C, ERW, ERC, URW, URC) see parts of TEIS speed up and a lower rate of upstream slow down. Experiments with sustained future melt
forcing (0MW, 0MC, EMW, EMC, UMW, UMC) see ice flowing faster across the whole region, more so when the TEIS damage is greater(EMW, EMC, UMW, UMC).

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D. I. Benn et al.: Rapid fragmentation of Thwaites Eastern Ice Shelf

Figure 12. Discharge of ice above flotation Ff and change in volume above flotation (VAF), 1Vf associated with modelled changes to TEIS.
Colours and line styles are grouped according to melt forcing and friction parameterisation. Dark (blue/grey/black) lines show the results ofsimulations with sustained future melting, bright lines (red, orange, purple) show the results of simulations without sustained future melting.
Solid lines represent simulations with Coulomb-like friction and dashed lines show Weertman-like friction.

is too low as a result of spatial smoothing in the regularisation process. Rescaling of friction to account for the different
functional timescales of HiDEM and Elmer/Ice introduces
further uncertainties, particularly because HiDEM does notincorporate viscous processes. Taken together, these factors
could mean that the bed is “too slippy” over the pinning pointin the HiDEM simulations using the baseline friction values.
Second, the necessity for high pinning point friction to create a shear zone in HiDEM may mean that the observed TEIS
shear zone was initiated at a time when pinning point friction
was greater than it was when the model input data were acquired (2007 and 2009 for MEaSUREs and 2013–2014 for
REMA). It is therefore possible that the TEIS shear zone developed when the pinning point provided greater backstress,
and that progressive weakening through shear localisationand damage evolution has allowed the shear zone to persist
even though pinning point friction was diminishing. Whatever the case, the HiDEM results are unequivocal: the observed fracture pattern requires sufficient backstress from thepinning point to initiate and sustain shear failure in the shelf.
That is, backstress must consistently exceed the evolving effective strength of TEIS.
In the HiDEM runs, a high initial damage density (d =0.6) is required to produce a shear zone. This does not imply
that a similar degree of pervasive damage was required toinitiate the TEIS shear zone. As noted above, the propertiesof ice in HiDEM reflect a range of parameters and we chosedamage density as a transparent and easily tunable controlof ice strength. It must also be emphasised that the versionof HiDEM used here is purely brittle–elastic, and does notincorporate viscous deformation. This means that the modelcannot include processes likely to have been important on
TEIS, particularly viscous shear localisation in the early de-

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velopment of the shear zone. In addition, evolution of TEIS
likely involved a number of factors not included in the simulations, such as basal roughness elements, including advectedcrevasses and basal channels.
5.2

Causes of fragmentation of TEIS

The transformation of TEIS from a largely intact ice shelfinto its current fragmented state suggests the pinning point
has shifted from being a stabilising to a destabilising influence. Formerly, backstress from the pinning point was sufficient to constrain ice flow, but insufficient to initiate fracturing. At some point the balance shifted, such that backstressfrom the pinning point exceeded the effective strength of theice, and damage was able to accumulate. This shift may haveoccurred for three fundamental reasons:
1. changes in resistive stresses at the grounded margins of
the shelf
2. changes in stress patterns due to interactions between
TEIS and TWIT
3. changes to the effective strength of the shelf.
The grounded margins of TEIS have undergone overall retreat in recent decades. The area of ice-bed contact at the
TEIS pinning point has been reduced (Wild et al., 2021;
Fig. 2), and the grounding line at the upstream boundary of
TEIS has retreated (e.g. Rignot et al., 2014; Milillo et al.,2019). In addition, interferograms of TEIS for the 1990s indicate the presence of a possible additional pinning point in
the middle of TEIS (Rignot, 2001), which is not evident inmore recent times. While reduction in the area of the TEIS
pinning point may be expected to have reduced backstress on
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D. I. Benn et al.: Rapid fragmentation of Thwaites Eastern Ice Shelf
the shelf, the other changes may have increased longitudinal
compressive stresses on the floating ice.
Concomitant with the changes to grounded margins, there
have been large changes in the relationship between TEIS
and TWIT. First, acceleration of TWIT after 2002 (itself
likely a response to weakening of a sub-shelf pinning point)
was transmitted across shear margin to TEIS, causing the observed speed-up between 2002 and 2006 (Fig. 6). This speedup was associated with increased longitudinal and shear
strain on TEIS, indicating increased loading on the shelf.
Subsequently, fragmentation and opening of the shear margin exposed the western flank of TEIS, reducing lateral confining stress on that side.
Several factors may have contributed to the weakening
TEIS in recent decades. Basal melting in response to incursions of warm, deep water has reduced ice-shelf thickness, with basal melt rates typically on the order of 5 m yr−1
(Seroussi et al., 2017; Wåhlin et al., 2021; Alley et al., 2021).
Focused basal melting in sub-shelf channels can increase
basal roughness, create local stress concentrations and interact with fracture processes. Weakening may also occur
through the advection of damage from upstream, and basal
crevasses or other fractures could seed additional damage
when they reach different stress regimes. Finally, damage canaccumulate over time via positive feedback between damage
and strain (Åström and Benn, 2019; Lhermitte et al., 2020).
The recent evolution of TEIS may have involved a combination of all the above factors. On the balance of available
evidence, we propose the following sequence of events as
the most likely cause of the recent fragmentation. The TEIS
shear zone was initiated, or was significantly modified, in the
mid-2000s, when TEIS accelerated in response to stresses
transferred across the shear margin with TWIT which wasstrong at that time (Alley et al., 2021). During the acceleration event, high longitudinal and shear strain rates developed upstream on TEIS in response to elevated, longitudinal compression supported by backstress from the pinningpoint. Large fractures within the TEIS shear zone first appeared in satellite imagery after the acceleration event, suggesting that damage was initiated or increased in response toelevated stresses (Fig. 7). Fragmentation, opening and significant weakening of the shear margin between TEIS andTWIT removed confining pressure from the western flank of
TEIS, encouraging transverse extension in response to ongoing longitudinal compression. Concurrently, damage on the
shelf continued to accumulate due to positive feedback between damage and strain. Sentinel-1 data show increasing
development of tensile fractures since 2017 in response totransverse extension (Fig. 3), and increasing concentrationof strain and extensive rifting within the TEIS shear zone
since 2020 (Fig. 5). Thus, although backstress from the pinning point has likely diminished over time (Wild et al., 2021),
accumulation of damage on TEIS has ensured that the ice inthe shear zone remained the weakest link in the system. Fragmentation of TEIS was not a consequence of unpinning; on
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2559

the contrary, fragmentation reflects stresses originating at thepinning point acting on progressively weakening ice.
5.3

Future implications of TEIS loss

Results of the BISICLES model indicate that loss of strength
in the shear zone or unpinning of TEIS will likely have little impact on basin-wide discharge from Thwaites Glacier.
There is currently no evidence that the imminent loss of
TEIS will hasten marine ice sheet instability or the demiseof Thwaites Glacier. However, future evolution of Thwaites
may be significantly influenced by calving processes along
unbuttressed grounding lines, and it is possible that calving
and iceberg overturn may increase, perhaps substantially, if
the ice shelves are lost.
Water depths along the grounding line of TEIS are currently in the range of 400–700 m, which is below the likely
threshold for ice-cliff instability (cf. Bassis and Walker,
2012; Crawford et al., 2021). The glacier bed deepens substantially 40 km upstream of the current grounding line, but
ice retreat to that point will involve calving by processes
such as melt undercutting, longitudinal extension and buoyant calving (e.g. Benn et al., 2007; Benn and Åström, 2018).
Detailed observations and process modelling will be required
to understand how ice-shelf removal will affect calving processes and ice-retreat rates in this region (e.g. Winberry etal., 2020; Crawford et al., 2021).

6

Conclusion

Backstress from the pinning point, once a crucial source of
support for TEIS, is now hastening its destruction. Complete
fragmentation of TEIS appears to be imminent and disintegration could occur sooner than the ∼ 10-year timescale
implied by rates of thinning and unpinning (cf. Wild et al.,2021). Complete loss of the shelf may follow, although theformer pinning point and fast ice may retard the evacuationof icebergs in much the same way as currently observed on
TWIT.
The main implications of this work extend farther than theimmediate future of Thwaites Glacier. Past observations of
Antarctic ice-shelf disintegration implicate two main mechanisms for ice shelf collapse: hydrofracture and unpinning.
It is already clear that surface melting is neither a necessary
nor sufficient condition for ice-shelf disintegration, and ourmodelling results show that unpinning does not produce theobserved fracture patterns on TEIS.
Our remote-sensing observations and modelling results
suggest that a third mechanism, the concentration of backstress in damaged ice upstream of a pinning point, will ultimately be responsible for the destruction of TEIS, and may
operate on other Antarctic ice shelves in contact with pinning points. The mechanism for damage and development of
the shear zone on the TEIS was a dynamic interaction with
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D. I. Benn et al.: Rapid fragmentation of Thwaites Eastern Ice Shelf

the neighbouring TWIT. The TWIT, which accelerated anddisintegrated in response to unpinning, caused TEIS to accelerate and develop the prominent shear zone where strain
is continuing to concentrate. This shear zone may have beenfurther weakened by direct basal melt (Wåhlin et al., 2021),
but the continued strain concentration and rift formation implicate dynamics as the ultimate destabilising mechanism.
For ice shelves in steady state, pinning points play a crucial role in their stability and in buttressing the upstream flowof ice off the Antarctic continent. If basal melt and/or dynamic interactions upset this balance, stresses are likely toconcentrate upstream of pinning points, causing irreversible
damage. In most circumstances, pinning points are essential
for ice-shelf stability, but as ice shelves thin and weaken, pinning points can provide the seeds of their demise.
Appendix A

Figure A1. As Fig. 11, except showing differences in velocity between 2016 and 2024.

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Figure A2. As Fig. 11, except showing differences in velocity between 2016 and 2064.

Figure A3. As Fig. 11, except showing differences in velocity between 2016 and 2112.

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D. I. Benn et al.: Rapid fragmentation of Thwaites Eastern Ice Shelf

Code availability. BISICLES is an open-source model available
at: https://commons.lbl.gov/display/bisicles/BISICLES (last access: 28 February 2022; Cornford et al., 2013, 2020). HiDEM
is an open-source model available at: https://github.com/joeatodd/
HiDEM (last access: 25 February 2022).

Data availability. Sentinel-1 data are freely available via theCopernicus Open Data Hubs (https://scihub.copernicus.eu/, last access: 10 December 2021). MODIS data are freely available fromNASA (http://nsidc.org/daac/modis/index.html, last access: 22 August 2021).

Video supplement. Video animation of Sentinel-1 imagery is available via the TIB AV-Portal (https://doi.org/10.5446/57379; Luckman, 2022).

Author contributions. DIB and AL designed and oversaw the study
in discussion with JB and EP; remote sensing analyses were conducted by AL (Sentinel-1), SB (DEMs) and KA (MODIS); computer modelling was done by JAÅ and AJC (HiDEM), RG, AJC andTZ (Elmer/Ice), and SLC (BISICLES). DIB wrote the manuscriptwith input from all authors. Post-review manuscript amendments
were handled by AJC and AL.

Competing interests. The contact author has declared that neither
they nor their co-authors have any competing interests.

Disclaimer. Publisher’s note: Copernicus Publications remains
neutral with regard to jurisdictional claims in published maps andinstitutional affiliations.

Acknowledgements. This work is from the DOMINOS andTARSAN projects, components of the International Thwaites
Glacier Collaboration (ITGC). Logistics provided by NSF-U.S.
Antarctic Program and NERC-British Antarctic Survey. Sentinel1 data were provided by the Copernicus Program of the European
Commission. The ITGC contribution number is ITGC-055.

Financial support. Funding was provided by the National Science
Foundation (NSF: grant nos. 1738896 and 1929991) and Natural
Environment Research Council (NERC: grant no. NE/S006605/1).
Rupert Gladstone is supported by Academy of Finland (grantno. 322430), Thomas Zwinger by grant no. 322978.

Review statement. This paper was edited by Josefin Ahlkrona andreviewed by Stef Lhermitte and Holly Still.

The Cryosphere, 16, 2545–2564, 2022

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