Half a century of dynamic instability following the ocean-driven break-up of Wordie Ice Shelf

Domain: earth_science
Published: 2025-04-29
Editorial outcome: minor_revision

Abstract
Ice shelves restrain grounded ice discharge into the ocean, and their break-up contributes significantly to Antarctica’s sea level rise. Using aerial imagery from the 1960s and modern satellite data, we construct a long-term record of Wordie Ice Shelf’s disintegration and its effects on tributary glaciers. Early changes in pinning points and ocean warming in Marguerite Bay since the 1960s strongly suggest increasing basal melt as the primary driver of the ice shelf disintegration. Some glaciers responded immediately to the ice shelf break-up, with surface velocities tripling, thinning up to 160 m, and grounding line retreat of 7.5 km, while others reacted decades later due to buttressing from remnant parts of the ice shelf. Our findings emphasize the importance of long-term observations to understand ice shelf disintegration and its impacts, offering crucial insights for assessments of future ice loss from the Antarctic Ice Sheet.

Introduction
Ice shelves, covering nearly two-thirds of Antarctica’s coastline, are essential to the stability of the Antarctic Ice Sheet. Acting as buttressing forces, they regulate the amount of ice discharged into the ocean by glaciers flowing from the interior of the ice sheet1,2. The potential weakening and break-up of these ice shelves can have significant consequences for the grounded tributary glaciers, including rapid thinning3, velocity acceleration4, and grounding line retreat5, as observed during the collapse of Larsen B Ice Shelves in the Antarctic Peninsula, where the discharge of certain outlet glaciers more than tripled from 1996 to 20036.

Ice shelf retreat styles vary widely, from rapid collapses to a protracted retreat7, and are driven by a wide range of factors8. These include atmospheric warming7,9, meltwater ponding and hydrofracturing10, brine infiltration11, increased basal melting from intrusion of warm circumpolar deep waters12,13, ice mélange removal14, and sea ice loss accompanied by ocean swell15. Moreover, ice shelf changes have been linked to damage and structural weakening that precondition ice shelves for disintegration16–18, including the shifting role of pinning points, which can transition from stabilizing to destabilizing features during this process19–21. This emphasizes the complexity of ice shelf dynamics and stresses the critical need for long-term observations, as the initiation of ice shelf break-up may occur decades before their eventual disintegration21.

Despite the limited availability of such observations, they are essential for improving our understanding of ice shelf behavior and establishing accurate baseline data for future modeling efforts21,22. Additionally, there is a notable lack of observations on the long-term impacts of ice shelf disintegration on tributary glaciers, which contributes to large uncertainties in projecting ice sheet contributions to future sea-level rise22,23. This is largely due to the fact that there are few observations of ice shelves that have disintegrated, as many of these events occurred prior to the availability of extensive satellite observations (e.g., Larsen A in 1995, Larsen B in 2002, Wordie in ~1980–1990s, Ostenfeld in 2004 in Greenland).

Since the early 20th century, the pursuit for polar exploration and mapping led to the acquisition of a large quantity of historical aerial images of Antarctica24–27. The search ives present enormous potential for reconstructing the long-term evolution of Antarctic glaciers and ice shelves3,28, but remain underutilized29. In this study, we utilize more than 450 historical aerial images of the Wordie Ice Shelf, captured in November and December 1966 and January 1969, in combination with various satellite and climate datasets to conduct a comprehensive analysis of its break-up along with the implications for the tributary glaciers.

The Wordie Ice Shelf is located in the Antarctic Peninsula, in Marguerite Bay (Fig. 1). Originally, multiple glaciers (Hariot, Fleming, Prospect, Carlson, and three unnamed glaciers, here called Foster Ice Piedmont (FIP) 1 and 2, and Unnamed) were flowing into the bay, forming a single unified ice shelf (Fig. 1), with Fleming being the largest and fastest among them. Around 1986 the ice shelf split into two parts19 and during the early 2000s it almost completely disintegrated, leaving behind a set of small individual ice shelves30. Doake and Vaughan19 were the first to analyze fracture dynamics in this area, and attributed the initial disintegration to warming air temperatures and increased melt ponding19. However, their study was limited to only five images covering the ice shelf from 1974 to 1989. While numerous recent studies have investigated the region, their primary focus has been on the dynamic changes at Fleming Glacier during the more recent decades30–33, rather than on the processes underlying the ice shelf disintegration. Consequently, a detailed long-term investigation is essential to unravel both the climatic and glaciological factors contributing to the ice shelf disintegration, as well as the impacts on the tributary glaciers feeding into Wordie. Such information is crucial for determining the timing and timescales over which these ice shelf and glacier processes occur, offering critical insights into the mechanisms underlying ice shelf disintegration and enhancing our ability to project potential future changes in Antarctica.

Results
After reaching its maximum observed extent of approx. 2200 km2 in 1966, the Wordie Ice Shelf entered a period of nearly constant retreat and loss of pinning points (Fig. 1), which triggered substantial changes of the tributary glaciers. In the following section, we first describe the observed changes of the Wordie Ice Shelf, including variations in ice shelf size and elevation, pinning points, structural damages, and melt ponds. Subsequently, we examine the ice dynamic response of the tributary glaciers, focusing on changes in grounding line positions, ice flow velocities, and thinning rates.

Ice shelf retreat and structural changes
In 1966 and 1974, ice shelf damage was limited, consisting mainly of transverse rifts found near the large pinning points, at the intersection between Fleming Glacier and FIP1-2 and Prospect Glacier, at the outflow of Carlson Glacier, and close to the 1966 Fleming Glacier grounding line (Fig. 1B, Supplementary Figs. S1, S2). From the historical 1966 DEM we estimate the median freeboard elevation of the ice shelf in front of Fleming, Hariot, and Unnamed Glacier to be 33 m ± 11 m (Supplementary Fig. S3) relative to sea level, which translates to an ice shelf thickness of 307 m ± 105 m (Methods). By 1979, the extent of ice shelf damage had increased, marked by larger and wider rifts and crevasses, and the emergence of longitudinal rifts, many originating upstream of the largest pinning points (Supplementary Fig. S2). This eventually led to the unpinning of many of these pinning points due to calving (Fig. 1B). Moreover, we observe unpinning occurring far from the ice shelf margin, driven by a thinning of the ice shelf (Fig. 1B.) Figure 1D illustrates the evolution of a pinning point at the outlet of Prospect Glacier, showing its gradual change between 1966 and 1979, culminating in its complete disappearance by the next available image in 1986. Unfortunately, precise thinning rates cannot be quantified from the available historical data.

In 1979, Carlson Glacier started disconnecting from the ice shelf, and by 1986 it had lost both its pinning points at the ice front where the glacier flow becomes unconfined (Fig. 1B). By 1986, Wordie Ice Shelf split into two separate parts: i) a Western part, consisting mainly of Prospect Glacier, FIP1 and FIP2, and ii) an Eastern part consisting of Fleming Glacier, Hariot, and Unnamed Glacier (Fig. 1B). By 1990, the main branch of the Fleming ice shelf was completely disconnected from Hariot and Unnamed, removing all lateral drag, and subsequently retreated ~19 km (Supplementary Fig. S4). Simultaneously, the connected ice shelf of Prospect Glacier, FIP1, and FIP2 retreated to a confined area (Supplementary Figs. S2, S4). The increase in ice shelf size during the 1990s is mostly related to a re-advance of the main branch of Fleming and (to a lesser extent) Carlson glaciers. After the re-advance, the ice shelf area decreased markedly, and at Hariot and Unnamed glaciers pinning points were lost at the ice shelf front both due to retreat and thinning (Fig. 1B, C). Between 1996 and 1998 large crevasses propagated far inland at Flemming Glacier, preceding its near-complete disappearance around 2000 (Supplementary Figs. S2, S5). Similarly, Carlson Ice Shelf had nearly vanished by 2002, while Hariot and Unnamed Ice Shelves disappeared by 2009 and 2023, respectively (Supplementary Figs. S2, S4). Additionally, the combined ice shelf of Prospect Glacier, FIP1, and FIP2 disintegrated between 2012 and 2014 (Supplementary Fig. S4).

Surface melt ponds are predominantly observed on the eastern sector of the Wordie Ice Shelf near Hariot and its neighboring glaciers (Fig. 1), with initial observations of meltwater ponding dating as far back as December 1966 (Supplementary Fig. S6). Moreover, thanks to the high resolution of the orthomosaics, small ponds (<0.025 km2) were detected in front of Fleming Glacier in 1969 (Supplementary Fig. S7). On the ice shelf in front of Hariot Glacier, large melt ponds persistently appeared until 1991 (Supplementary Fig. S6). By 2001, the melt ponds had mostly disappeared, with only small ponds remaining, primarily at the smaller glaciers adjacent to Hariot. As cloud-free optical imagery was unavailable from 1992 to 2001, the melt ponds could have extended into the late 90s. Moreover, there are notable annual variations in the area and location of melt ponds (Supplementary Fig. S6), with the largest single melt pond (~6 km2) observed on January 12, 1969. These variations may partly reflect seasonal differences in acquisition times, as images captured close in time show different pond distributions. Due to sparse image availability, we cannot rule out that melt ponds have been present at other parts of the ice shelf during the early stages of the disintegration. However, we consider this unlikely, as no melt ponds were observed in any other areas of the ice shelf during years when image coverage was more consistent.

Acceleration, grounding line retreat and thinning of tributary glaciers
Figures 2 and 3 presents the evolution of ice velocities, grounding line positions, and ice elevation for the Wordie Bay glaciers between 1966 and 2023, illustrating a variable dynamical response between the glaciers during the disintegration of Wordie Ice Shelf.

At Carlson Glacier, velocities nearly doubled from ~500 m/yr in 1966 to ~950 m/yr in 1986 at the 1966 grounding line position, followed by a further 40% increase to ~1350 m/yr in 1989 (Fig. 2). Similarly, Fleming Glacier experienced a 50% velocity increase from ~1100 m/yr in 1966 to ~1700 m/yr in 1986 at the 1966 grounding line, with a subsequent rapid acceleration between 1989 and 1991, rising from ~1900 m/yr to ~3200 m/yr (Fig. 2). Since the early 1990s, velocities were relatively stable, except for a ~300 m/yr decrease at Carlson Glacier between 2000 and 2007 and an acceleration of both glaciers from 2007 to 2013. Moreover, during the initial observation period from 1966 to 1996/2001, the grounding line of Fleming Glacier retreated by 7.5 km (Fig. 2), while Carlson Glacier experienced a retreat of 6.6 km. By 2014, the grounding line at Fleming Glacier had retreated an additional ~5.5 km (Fig. 2). The early changes in grounding line position and velocity acceleration were accompanied by 160 ± 14.6 m of thinning at the grounded section of Fleming Glacier between 1966 and 2007 (Method). This corresponds to a thinning rate of 4.0 ± 0.4 m/yr (Fig. 3A), exceeding the 2007–2022 rate of 3.2 ± 0.7 m/yr (Fig. 3B).

In contrast to the early dynamic response of Carlson and Fleming glaciers, FIP1, FIP2, Prospect, Hariot, and Unnamed glaciers show minimal changes in grounding line and ice velocity during the first three decades, with the most substantial changes occurring over the past two decades (Fig. 2). On the eastern part of the ice shelf, grounding line positions remained stable from 1966 to 1996, with no detectable retreat at Unnamed Glacier and only 1.1 km at Hariot Glacier. No grounding line estimates are available after 1996 for these glaciers (Methods). At Hariot Glacier the most notable velocity acceleration occurred between 1994 and 2000, with an increase of around 40% (~900 m/yr to ~1250 m/yr) at the grounding line. At Unnamed Glacier, we observe velocity fluctuations ranging from ~220 to 330 m/yr from 1966 to 2007, with the lowest velocities recorded in 1966. The largest flow acceleration occurred between 2013 and 2021, with an increase from ~400 m/yr to ~550 m/yr (Fig. 2). At Hariot Glacier, thinning rates remained similar from 1966–2007 to 2007–2022, whereas at Unnamed Glacier, rates more than doubled from 0.7 ± 0.4 m/yr in 1966–2006 to 1.5 ± 0.7 m/yr in 2007–2022. On the western part of the ice shelf, the grounding line of Prospect Glacier was stable from 1966 to 1996 but retreated 2.5 km from 1996 to 2023. Similarly, FIP1 exhibited a retreat of less than 1 km between 1966 and 1996, followed by a more substantial retreat of 5 km from 1996 to 2023. At FIP2 there was a total retreat of 7 km, with 4 km occurring after 1996. Historical velocities could only be extracted from the floating part of the ice shelf at FIP1, where the velocity doubled between its 1966 minimum and the 1994 maximum (Fig. 2). In contrast, near the 1966 and 1996 grounding line, there were limited velocity changes between 1986 and 2000, ranging from ~600 m/yr to 850 m/yr. Between 2000 and 2013, the velocity more than doubled, increasing from ~850 m/yr to ~1900 m/yr, with a slight subsequent decrease in 2021 (Fig. 2). At FIP2, velocities at the 1996 grounding line remained relatively stable from 1989 to 2013, after which they accelerated rapidly from ~700 m/yr in 2013 to ~1300 m/yr in 2021. The velocity at Prospect Glacier accelerated gradually from 1986 to 2007 at the 1966/1996 grounding line, after which we observed a large acceleration from ~1100 m/yr in 2007 to ~1850 m/yr in 2013 (Fig. 2). Unfortunately, no historical elevation data is available for Prospect and FIP1-2 glaciers. However, between 2007 and 2022, all experienced significant thinning, with rates ranging from 3.9 ± 0.7 m/yr to 5.9 ± 0.7 m/yr, the highest at FIP1 (Fig. 3).

Methods
American Aerial photographs
We utilize 468 black and white aerial photographs of the Wordie Ice Shelf collected mainly by the US NAVY from 1966 to 1969 (Table 1). The images were captured using a trimetrogon camera setup, wherein left-oblique, on-nadir (vertical), and right-oblique images are captured simultaneously27. Coverage of the Wordie Bay region varies across years, with the November 28, 1966 images providing nearly complete coverage.

We process the images using a standard structure from motion workflow48 in Agisoft Metashape v. 2.0.2, with photographs from different years processed separately. Over the eastern part of the ice shelf, including Fleming, Unnamed, and Hariot glaciers, sufficient visible bedrock enabled the generation of a DEM with accurate absolute elevations from the November 28, 1966 images (Supplementary Fig. S3). For the remaining study area and images from other dates, limited visible bedrock allowed for the production of only relative DEMs and orthomosaics.

First, the images are scaled and aligned by detecting the image fiducials using the Detect Fiducial function. All fiducials are manually inspected and misplaced ones are corrected. Subsequently, the images are aligned by extracting up to 50,000 key points per image. In certain instances, additional tie points are manually placed to ensure precise image alignment. To produce the absolute DEM from November 28, 1966, 42 ground control points (GCPs) were manually placed in the aerial images. The GCPs are distinct stable bedrock features that can be identified in both the historical imagery and the 2 m resolution Reference Elevation Model of Antarctica (REMA) Mosaic49, which serves as the reference model. For the relative DEMs and orthomosaics, this step is unnecessary, as georeferencing occurs later in the workflow. Next, we generate a dense point cloud at maximum image resolution, and obvious outliers are manually filtered. The dense point cloud is then gridded into a DEM and used to create a mesh, from which the orthomosaics are generated.

The relative orthomosaics and DEMs are georeferenced to 15 m resolution panchromatic Landsat 8 satellite imagery of the Wordie Bay region. The orthomosaics and DEMs are generated at similar pixel resolution, which allows us to transfer the GCPs from the orthogeorectification to the DEM. We georeference the orthomosaic using distinct surface features such as nunataks and islands and distribute the control points evenly across the image. To constrain the georeferencing over the ice shelf area, we use ice rises detectable in the 1974 and 1979 Landsat images. However, we acknowledge that these points may exhibit larger uncertainties compared to the control points over stable bedrock. The georeferencing is performed using an affine transformation, with the number of control points placed varying from 15 to 25 depending on the coverage of the orthomosaic. The georeferenced orthomosaics have a resolution of 4–6 m and a Root Mean Square (RMS) error from the placed control points ranging from 40.1 to 43.2 m. Among the three generated orthomosaics, the one from November 28, 1966 is based on the largest selection of images (Table 1) and provides the most extensive and complete coverage of the entire ice shelf area, along with the glaciers flowing into it (Supplementary Fig. S1). For the georeferencing of this orthomosaic a total of 25 control points were placed, achieving an RMS error of 41.8 m. Following the georeferencing of the orthomosaic, we transfer the GCPs to the corresponding DEM to ensure consistent georeferencing between the two products. Despite the relative elevations of the precisely georeferenced DEMs, they are still useful for accurately detecting changes in slope.

DEM post-processing and glacier elevation change
First, the historical absolute DEM from November 28, 1966 is clipped to the minimum boundary area defined by the placed GCPs28. Next, the clipped DEM is co-registered50 to the REMA reference DEM, and the uncertainty is estimated to 11.2 m based on the Normalized Median Absolute Deviation (NMAD) of the elevation differences over stable bedrock51.

To analyze elevation changes over time, we incorporate two additional DEMs: one from 2007 and another from 2022. The 2007 DEM, was generated from SPOT5-HRS stereopair images captured on February 21, 200752 using the Ames Stereo Pipeline software53 with an initial co-registration to the Copernicus (GLO-30) DEM. After a filtering step, the SPOT DEM was vertically adjusted with synchroneous ICESat altimetry data (closest campaign in time). An additional co-registration to the REMA reference DEM Mosaic was performed giving an estimated uncertainty of 9.3 m. The 2022 DEM is created by mosaicking multiple REMA strips acquired over a short timespan from November 22, 2021 to April 6, 2022 to minimize errors due to variable ice cover. Each strip is individually co-registered before mosaicking, with the final DEM assigned a date of January 28, 2022 and an estimated uncertainty of 5.4 m. For each glacier we estimate changes by calculating the mean elevation differences within an area located behind the 2022 frontal position and the 1996 grounding line (Supplementary Fig. S10) during the two periods 1966–2007 and 2007–2022.

From the 1966 DEM we estimate the ice shelf freeboard elevation relative to sea level by calculating the median elevation of the ice shelf area defined by the 1966 grounding line position. From this freeboard elevation we determine the 1966 ice shelf thickness by setting the density of the shelf ice and seawater to 917 kg/m³ and 1027 kg/m³, respectively.

Grounding line location
Historical grounding line positions from 1966 are determined using the break of slope method from Mouginot et al.54, with an uncertainty of 500 m. The grounding line mapping is performed using a slope and a hillshade raster (Supplementary Fig. S11), generated from the November 28, 1966 georeferenced historical DEM at 100×100 m resolution to even out local errors. In addition to the historical grounding lines, we incorporate MEaSUREs 1996 data derived from Differential Synthetic Aperture Radar (DInSAR) data55. However, as no DInSAR grounding line is available for Carlson Glacier, we utilize the data from Antarctic Surface Accumulation and Ice Discharge (ASAID)56 project derived from the break of slope using Landsat 7 imagery and ICESat altimetry data with a temporal coverage from 1999 to 200357.

In an effort to retrieve recent grounding line locations, we used DInSAR measurements from the EU/ESA Sentinel-1 satellites and the conventional method of double-difference interferometry58. We processed 200 image pairs from 2016 to 2023 with a temporal baseline of 6 days (when available, otherwise 12 days). However, in all cases, interferometric coherence was consistently lost over the full region of interest, meaning that no useful vertical displacement measurements could be extracted. Applying the method of differential range offset tracking59 to the Sentinel-1 measurements equally did not yield meaningful measurements. This limitation may be due to the difficulty of tracking the GL with DInSAR for a relatively small floating section (e.g limited resolution and loss of coherence due to precipitation, surface melting, or fast ice flow and high deformation).

Modern grounding line estimates from 2014 are available for Fleming Glacier31,33, with notable disparities in the findings. Friedl et al.33 suggest the grounding line to be situated 5–8 km further upstream than Walker and Gardner31. We have opted to adopt the Friedl et al.33 estimate, as they calculate the hydrostatic equilibrium of the glaciers center, whereas Walker and Gardner31 refer to the 1996 grounding line. Moreover, utilizing optical satellite imagery60 we determine approximate 2023 grounding lines for FIP1, FIP2, and Prospect Glacier. This methodology is employed only when the surface shading in the optical satellite imagery provides a clear distinction between the grounded and floating sections of the glacier (Supplementary Fig. S12). As a result, we are unable to determine optical-derived grounding lines for Hariot, Unnamed, Carlson, and Fleming Glacier. By comparing the 1996 DInSAR grounding line to the break of slope from the surface shading in the 1990 optical images (Supplementary Fig. S12), we observe a close alignment with the largest variation of 1000 m observed at the FIP2. The average variation is ~300 m between the grounding line estimates.

The grounding zone marks the region where the glacier transitions from being completely grounded to floating. As a result the different mapping techniques map features within the grounding zone that act as proxies for the true grounding line60. Consequently, some of the changes observed in the grounding line may be attributed to variations in the mapping method, and to the tide magnitude at the time of measurement. However, due to the close alignment between 1996 DInSAR and the 1990 break of slope grounding line (Supplementary Fig. S12), we anticipate this factor to have minimal influence.

Ice velocity
We compile a time series of surface ice velocities spanning over 50 years by integrating data from multiple satellites and aerial orthomosaics. Velocities from 1966 to 1969 are obtained from orthomosaics, which are precisely georeferenced prior to feature tracking using stable bedrock areas. For three glaciers (Unnamed, Fleming, and FIP1), we manually track the movement of distinct surface features on the glacier and ice shelf surface between 28-11-1966 and 12-01-1969. The uncertainties of the manual tracking are determined as the combined uncertainty of the georeferencing error and the manual error attributed to feature positioning which is set to 1 pixel. This approach gives a combined uncertainty of 60 m/yr. For the Carlson glacier, the relatively short time between repeat acquisitions, approximately 1 month (28-11-1966 and 27-12-1966), enables us to employ automatic feature tracking. We first perform a sobel filter in the x and y direction to enhance surface features on images. Finally, we feed the resulting images to the feature tracking algorithm using image correlation windows of 64×64 pixels, search windows of 20×20 pixels with spacing of 10 pixels. From 1986 to 2021, we use time series of surface flow velocities originating from feature and speckle tracking using satellite observations from ESA’s ERS-1&2, Sentinel-1a/b and Sentinel-2, RADARSAT-1&2, JAXA’s ALOS/PALSAR, NASA’s LANDSAT-1-8. The methodology follows the approaches described by Mouginot et al.61, Mouginot et al.62, and Millan et al.63, with velocities exhibiting a mean uncertainty of 38 m/yr, ranging from 29 to 55 m/yr. To account for missing years of interest, we augment the time series with data from the ITS_LIVE annual mosaics64. Velocities are extracted along an approximate center flowline of each glacier. The flowlines are positioned as close to the center as possible, with slight adjustments made based on areas with the most complete historical velocity estimates. For each year, we exclude all measurements seaward of the frontal position to avoid comparing velocities over sea ice or mélange.

Ice shelf area and pinning points
Frontal positions of the Wordie Ice Shelf and its glaciers spanning 1966–2022 are mapped using the generated orthomosaics and the optical Landsat images (1974–1991 and 2001–2022), with a preference for images captured during the austral summer. The ice shelf area is determined with respect to the MEaSUREs 1996 grounding line. We also supplemented our analysis with data from ESA’s ERS-1 and ERS-2 satellites in the period 1991–2004. Single Look Complex (SLC) images were processed into amplitude and geocoded using the 200 meter resolution DEM from the Radarsat Antarctic Mapping Project (RAMP). Given the lower resolution of ERS backscatter images and distortions due to the different satellite geometries, the frontal positions from 1992 to 2000 are more difficult to accurately delineate than for the Landsat data.

We map all ice shelf pinning points found in both the 1966 orthomosaic and 1974 Landsat images to exclude ephemeral features, as these may gain contact or unpin due to tidal processes and thus provide limited buttressing65. Subsequently, we map when the ice shelf unpins from the pinning point either due to thinning or due to retreat (Figs. 1 and 2). However, during the period from 1993 to 1999, the low resolution and high distortion levels of the ERS radar images prevent us from determining whether pinning points disappeared due to thinning.

Climate, SMB, ocean, and sea ice concentration of Wordie region
We extract RACMO2.3p2 mean annual surface mass balance (SMB), snowfall, and rainfall from 1979 to 202266 for the Wordie region (Supplementary Fig. S9). Additionally, we calculate the mean austral summer temperature (December, January, February) from 1941 to 2022 using 0.25° ERA5 data67 and since 1976 using observational records from Rothera Station located approx. 150 km from Wordie Ice Shelf extracted from the Scientific Committee on Antarctic Research (SCAR) Met-READER project (https://legacy.bas.ac.uk/met/READER/). Monthly sea ice concentrations in the Wordie Bay area are extracted from the Bootstrap Sea Ice Concentrations dataset68 derived from Nimbus-7 and Defense Meteorological Satellite Program (DMSP) satellites. Subsequently, we calculate the annual mean November to March sea ice concentration in the region.

To track the progression of ocean temperatures in the Marguerite Bay, we used Conductivity Temperature and Depth (CTD) measurements obtained from the Hadley Centre (bodc.ac.uk) spanning the years 1960 to 2019. Using the temperature profiles, we calculated the average temperature within the 150–500 m depth range (Supplementary Fig. S13). We then used a locally weighted scatterplot smoothing69 to plot the temporal evolution of the warm water layer through time.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "General Comments\nThe manuscript by Dømgaard et al. presents an in-depth analysis of the demise of Wordie\nIce Shelf located in the Antarctic Peninsula region. They set out to identify the main drivers\nbehind the step-wise retreat and ultimate disappearance of Wordie Ice Shelf. To this end,\nthey analyse a wide range remote sensing data dating back to 1966. These images are supplemented with data from\nregional climate model output, weather station data, and ocean\ntemperature profiles. First of all, I would like to commend the authors for extending the\nobservational record of Wordie Ice Shelf. However, the scientific interpretation and the conclusions drawn in the paper are in\nmy view not always well supported. I am left wondering\nwhat the manuscript’s scientific contribution is and throughout the manuscript I am missing\na coherent story. Therefore, I cannot support publication in its current form. In the following, I will outline my main concerns.\nSpecific comments:\n1. One of my biggest issues concerns the story of the manuscript and its framing. In\nits present form, the manuscript fails to clearly state what its main contribution is. I\nunderstand that the authors manage to extend the observational record, but what is\nthe new scientific insight that we gain from this. From the abstract, I suspected that\nthey show that atmospheric warming did not contribute to the initial retreat of Wordie\nIce Shelf. But while this is mentioned in the main text (e.g. l60f, l263f), it is never\nhighlighted in the introduction or in the conclusion section as the main finding. Moreover, there\nis only the indirect evidence of meltwater ponding during the advance phase for the\n”minimal role a rise in air temperature plays in the role of the collapse”. However, given the relative large time gaps in the\nimages presented in Fig. 2, I am not entirely convinced\nthat such a strong statement can be made. In addition, the individual subsections read\ndisjointed, are not well connected, and the authors often fail to embed their findings\ninto the present body of literature. For example, in line 103-104, the authors write\n”FIP1 and FIP2 retreated to a confined area, maintaining some lateral buttressing”.\nThis would be the ideal point to link to their velocity observations to check that this is\nindeed the case (in this case I would expect no or very little speed-up).\n2. This leads me to my second point. Scattered throughout the manuscript, the authors\nare guilty of imprecise language. Often their wording suggests a certainty that, based\non the presented data, is not warranted. For example in line 74 the authors write ”we have compiled a continuous time-\nseries of shelf area, ice shelf damage...”. Even though I am a little unclear from which years data are available, I do not think,\nthe authors can claim to have a ”continuous time-series” when you have time gaps of 3-years and beyond. Moreover, the\nauthors claim that ”pinning\npoints must provide important buttressing (l91)”. While it is certainly true that in most cases pinning points provide\nbuttressing, there are also counter-examples, where the opposite is true because pinning points structurally weaken the ice\nshelf through enhanced fracturing [Doake and Vaughan, 1991]. To their credit, the authors mention this process (l90), but it is\nunclear how they decide when pinning points provide more or less buttressing. A more nuanced and circumspect use of\nlanguage would certainly be appreciated.\n3. There are two terms that repeatedly confused me when reading the manuscript. It appears that the authors use the term\n”ice shelf, shelf, floating tongue, floating extension” interchangeably. My suggestion would be to NOT use ”shelf” as this is\nalso commonly used to refer to the ”continental shelf in the ocean bathymetry”. I would recommend using ”ice shelf”\nthroughout unless you refer to really a marine-terminating glacier that has a small floating ice tongue like most marine-\nterminating glaciers in Greenland. My aim of this is not that the authors adopt my convention, but it would be good to have a\nconsistent naming convention throughout the manuscript. The second confusion revolved around the use of word ”collapse”.\nWhen I use this with reference to ice shelves\nI presume when an ice shelf has collapsed it is not there anymore. Otherwise I would refer to it as a ”retreat” or maybe ”a\npartial collapse”. This would make it much easier to follow for readers not from the glaciology community.\nTechnical corrections:\nL33: the sea-level rise potential of 65 cm needs a citation\nL33: What is a ”turning point”? Are you referring to a tipping point? If so, I do not\nthink the Rignot citation is best suited for that.\nL35-39: I feel you can expand this paragraph a little and, in brief, make the reader aware\nof what processes have previously been hypothesised to have caused ice-shelf collapse.\nL37: I do not understand how the intrusion of warm water can trigger ice damage. Please explain!\nL57, 58: As mentioned above, please use consistent naming, preferably ”ice shelf”.\nL62: introduce GL at first mention. I also do not think that you need to abbreviate\ngrounding line with GL in the main text\nL70: What about the study, maps of Ferrigno et al. [2008]. They also have aerial photos\ndating back to 1946.\nL71: Here would be a good point to spell out when exactly you have remote sensing\ndata e.g. 1966, 1969, 1974 etc. Also the way it is written it sounds like you have 450\nimages spanning the time from 1966 – 1969, where I believe you only have images from\nNov. and Dec. 1966 and Jan. 1969.\nL75-76: The last part of this sentence sounds awkward. Please rephrase.\nL101: It is very rare that you get unrestricted flow. If there is still some basal drag\nfrom pinning points this would be incorrect. Maybe better to just say: ”removing all\nlateral drag” or similar\nL103: Does the velocity data back this up that there is still buttressing?\nL107: ”readvance” should be ”re-advance”\nL112-113: ”...with their fronts retreating to grounding ice” sounds odd. Please rephrase.\nL115: Figure S6 starts at year 1974, so I cannot verify this 300% claim. Also please\nprovide additionally absolute numbers.\nL155: ”behind” change to ”upstream”\nL198: I would not call it a collapse. At best maybe a partial collapse.\nL199: Why is this now 1966/1968? Is it not 1966/1969. And why are they bunched\ntogether? Is there only limited spatial coverage?\nL227: Citation format\nL227-231: I do not think these processes are mutually exclusive, but could both be\nresponsible for the observed changes. Please adjust.\nL232: I do not think a 20% increase is minuscule.\nL249-255: Here it would be good to add some background information on what the\nbias is for ERA5 and how much we can trust it. These reanalysis products tend to\nbe particularly bad for the southern hemisphere and even more so for the polar south\nbecause of the paucity of observations.\nL285: Citation or refer to your figure for ocean temp. increase\nL287: ”halted the collapse”, maybe better ”stabilised the retreat”?\nL298-300: This warrants a bit of explanation. Why does it not buttress, but can\nstabilise the ice front? If it previously stabilised the ice front should it not also provide\nsignificant buttressing?\nL301-307: I find this not a great ending and it circles back to my main point. I strongly\ndoubt that the change in calving style is the main contribution of this manuscript.\nWhat are the main findings and their wider implications?\nL314: Why is there a time gap of three years between the aerial images?\nL344: ”The grounding line ... ”\nL435: 150 km?\nFigures:\nThe Figures are overall of good quality but could use some larger axis fonts.\nFig. 2: What are the thick black stripes in the second panel of the first row?\nFig. 3: Grounding line in second column is very difficult to see. Third panel is very\ndark, but I guess the purpose of this column is to highlight the slope break. Still might\nbe worth trying a different color scale\nFig. 4: Velocity map is really dark. Consider changing the colormap. Also I would\nrecommend moving the legends such that it is clear that the velocity legend belongs to\nthe remaining subplots. For all velocity transects, add a line in the caption that x and\ny-axes do not match.\nFig. S2: Please write ice shelf in figure caption.\nFig. S4: Here or somewhere in the main text, please specify how retreat distance is\ncalculated (along a flowline, maximum retreat for the whole drainage basin front)? Use\nproper exponentiation for km2\nFig. S7: How many pixels are you using from ERA5? Please increase fontsize of axis\nlabels and axis titles.\nReferences:\nC. S. M. Doake and D. G. Vaughan. Rapid disintegration of the wordie ice shelf in response\nto atmospheric warming. Nature, 350(6316):328–330, Mar. 1991. ISSN 1476-4687. doi:\n10.1038/350328a0. URL http://dx.doi.org/10.1038/350328a0.\nJ. Ferrigno, A. Cook, A. Mathie, R. Williams Jr, C. Swithinbank, K. Foley, A. Fox, J. Thom-\nson, and J. Sievers. Coastal-change and glaciological map of the larsen ice shelf area,\nantarctica: 1940–2005: Us geological survey geologic investigations series map i–2600–b, 1\nsheet, scale 1: 1,000,000, with 28-p. pamphlet. Also available online at http://pubs. usgs.\ngov/imap/2600/B/, 2008.", "reviewer_2": "General comments:\nThis manuscript describes the long-term evolution of the Wordie Ice Shelf on the western Antarctic Peninsula, which\ncollapsed in the 1970s and 1980s and transitioned to a series of floating ice tongues by the 2000s. The authors investigate\nthe roles of unpinning and damage, meltwater ponding, grounding line retreat and the dynamical responses of tributary\nglaciers in response to the ice-shelf collapse.\nPrevious ice-shelf collapses on the Antarctic Peninsula (most notably Larsen A and B), have been linked to ice flow\nacceleration and increased discharge of grounded ice tributary glaciers. Atmospheric warming and surface meltwater\naccumulation has been proposed as mechanisms that triggered disintegration. As the Wordie Ice Shelf collapse does not\nappear to have been meltwater-driven, an investigation of possible causal mechanisms in the context of its long-term\nevolution is important.\nThe authors did a large amount of work in processing the historical American aerial photographs, which provide a valuable\naddition to their time series and enable them to connect ice-shelf dynamics to changes in pinning points associated with ice-\nshelf thinning. The manuscript is generally written clearly but lacks some details, as well as context of the findings within the\nexisting literature, requiring some clarifications before I can recommend it for publication.\nThe introduction is lacking citations to the literature on previous ice-shelf collapses which provide usual context. For\nexample, I would suggest citing Leeson et al. (2020), who discuss the role of supraglacial lakes in the collapse of Larsen B\nin detail and is relevant for this study. There are also several relevant studies on the Larsen A collapse that could be cited\n(e.g. Doake et al., 1998 and Rott et al., 2002).\nI think also the introduction could be strengthened by discussing damage and how this affects ice-shelf stability. (i.e. that\ncrevasses and open fractures constitute structural weakening that precondition ice shelves for disintegration; e.g. Benn et al.\n2022). I also hate to be *that* reviewer, but Arthur et al. (2021) provides relevant discussion on how structural weakening\npreconditions for ice-shelf disintegration, and could be added later in the Discussion about the stabilising effect of ice\nmélange. Similarly, the discussion could better reference the existing literature to support the authors’ conclusions. For\nexample, in the discussion of the minimal buttressing role played by mélange, the authors could reference Sun et al. (2023).\nAs it stands, the manuscript does not fully communicate how it advances the state-of-knowledge related to this ice-shelf\ncollapse event and Antarctic ice-shelf stability more broadly. More work could be done to go one step further by discussing\nthe results in terms of what this means for identifying precursors to future destabilization of other ice shelves around\nAntarctica.\nI am unsure on the relevance or usefulness of using tabular iceberg production estimates as a proxy for the condition and\nlocal thickness changes of ice mélange. For example, this metric doesn’t distinguish between ice blocks that have already\ncalved and subsequently remain ‘locked’ into the mélange.\nTo me, the manuscript ends quite abruptly with a paragraph discussing iceberg production and calving regimes. It lacks a\nmore synthesising conclusion that pulls together the key findings and sets them in context of the wider implications of ice-\nshelf dynamics around the rest of Antarctica. This should include the potential for future reductions in buttressing, collapse of\nother ice shelves, acceleration in both ice discharge and mass loss, and of course sea-level rise contributions.\nOverall, it is my view that this study is of broad interest to the cryospheric community as it builds upon previous work focused\non ice-shelf dynamics and collapse. Once the authors address my comments, I think this will strengthen the manuscript and\nmake it an important contribution.\nSpecific comments:\nL20: Some vague wording in the Abstract could be more specific, e.g. ‘changes at tributary glaciers occurred at different\nintervals’. Also the last sentence, ‘consequences for tributary glaciers’ – in terms of what – velocity? Grounding line retreat?\nL30: I would suggest specifying over what timescale this increase in discharge occurred following the ice-shelf collapses.\nL77: Figure 1: Could you adjust your colour ramp slightly, as it is very difficult to visually distinguish between the 1993 and\n1998 ice fronts.\nL163: As this section is quite descriptive, could you add 1 or 2 sentences at the end of this section to summarise the key ice\ndynamical changes?\nL192: What velocity dataset is this Figure based on? This needs specifying in the caption.\nL198: Refer specifically to the amount of % acceleration again here to help the reader.\nL232: I would suggest using a different comparative here instead of miniscule, as 20% is not insignificant.\nL245: I think a few more sentences could be added here to discuss in greater depth the significance of this interesting finding\n(that there is a variable response between different tributary glaciers in response to the ice-shelf collapse).\nL259: ‘Peak size’ is vague, unclear wording.\nL269: I find ‘pond formations’ unclear, and suggest changing this sentence to: ‘Given the absence of melt ponds, meltwater\nis likely to have drained through these extended crevasses.’\nL276: Confusing end to the paragraph, concluding that meltwater has not played a major role and then concluding that\nmeltwater hydrofracturing may have destabilized the ice shelf.\nL286: You highlighted a sustained period of high sea-ice concentration in the bay during the 1990s, yet there were several\nother years with similarly large or larger anomalies, for example in the early 2000s when the ice-shelf area had already\nbegan to decline.\nL296: Do you have mélange thickness measurements, for example from laser altimetry? Otherwise, I’m not sure you can\nspecify that the mélange was thick.\nL323: From which satellite were images used to georeferenced the orthomosaics and DEMs?\nL391: Can you provide details of what automatic feature tracking algorithm you used?\nL412: Can you explain how you distinguish between stable pinning points and features that ground ephemerally over tidal\ncycles (e.g. without comparing imagery with tidal or altimetry data?)\nL415: Clarify why you are not able to determine whether pinning points disappear due to thinning.\nL583: Nature Communications requires an author contributions statement, so please add this.\nL618: Figure S7 caption: make clear what the pale blue and pale gray lines are in the Precipitation and SMB sub-plots, and\nlabel panels.\nL623: Figure S8: What is the red polygon in Panel A? Add to the Figure caption the source of the grounding line dataset.\nAlso add temperature units to Panel (c) y axis.\nL636: Dashed front line positions don’t seem to match the glacier front lines that are visible in the underlying ERS radar\nimages?\nTechnical/minor corrections:\nL23: Lowercase ‘bay’.\nL29: Write as ‘Larsen A and B ice shelves’, here and throughout.\nL33: Hyphenate sea-level rise.\nL36: I suggest ‘events such as’ is unnecessary here.\nL43: observations ‘of’, rather than ‘on’? Otherwise, specify observations of what.\nL56: Add comma after ‘three unnamed glaciers, (…)’\nL58: Be more specific than ‘around 1989’?\nL59: Add comma for readability: ‘almost completely disintegrated, leaving behind (…)’\nL61: Would suggest adding how high these basal melt rates were.\nL63: Specify what recent period.\nL64: Use ‘ice shelf’ rather than just ‘shelf’ for clarity, and change this throughout.\nL65: Comma not needed after ‘the other tributary glaciers feeding Wordie’.\nL67: Suggest changing to ‘the main causes of the initial ice shelf collapse remain unclear.’\nL151: Lower case ‘western’ (western part of the ice shelf).\nL173: Use a tilde ~ here for consistency with the rest of this section.\nL249: ‘On the Antarctic Peninsula,’.\nL257: Hyphenate ‘atmospherically-driven’.\nL282: Hyphenate ‘ocean-driven’.\nL302: Suggest ‘collapse dynamics’.\nL333: Remove comma after ‘November 28, 1966’.\nL345: ‘The grounding line’\nL355: ‘(when available, otherwise 12 days). However, (…)’.\nL375: ‘that act as’.\nL403: ‘(…) the generated orthomosaics and the optical Landsat images’.\nL627: Typo in Figure S9 legend.\nReferences\nArthur, J.F., Stokes, C.R., Jamieson, S.S., Miles, B.W., Carr, J.R. and Leeson, A.A., 2021. The triggers of the disaggregation\nof Voyeykov Ice Shelf (2007), Wilkes Land, East Antarctica, and its subsequent evolution. Journal of Glaciology, 67(265),\npp.933-951. doi:10.1017/jog.2021.45.\nBenn, D.I., Luckman, A., Åström, J.A., Crawford, A.J., Cornford, S.L., Bevan, S.L., Zwinger, T., Gladstone, R., Alley, K., Pettit,\nE. and Bassis, J., 2022. Rapid fragmentation of Thwaites Eastern Ice Shelf. The Cryosphere, 16(6), pp.2545-2564.\ndoi.org/10.5194/tc-16-2545-2022.\nDoake, C., Corr, H., Rott, H. et al. Breakup and conditions for stability of the northern Larsen Ice Shelf, Antarctica. Nature\n391, 778–780 (1998). https://doi.org/10.1038/35832\nLeeson, A.A., Forster, E., Rice, A., Gourmelen, N. and Van Wessem, J.M., 2020. Evolution of supraglacial lakes on the\nLarsen B ice shelf in the decades before it collapsed. Geophysical Research Letters, 47(4), p.e2019GL085591.\ndoi.org/10.1029/2019GL085591.\nRott H, Rack W, Skvarca P, Angelis HD. Northern Larsen Ice Shelf, Antarctica: further retreat after collapse. Annals of\nGlaciology. 2002;34:277-282. doi:10.3189/172756402781817716\nSun, Y., Riel, B. and Minchew, B., 2023. Disintegration and buttressing effect of the landfast sea ice in the Larsen B\nembayment, Antarctic Peninsula. Geophysical Research Letters, 50(16), doi.org/10.1029/2023GL104066.", "reviewer_3": "The paper by Domgaard et al. provides a careful review and new details of the retreat and loss of the Wordie Ice Shelf, using\nimage data spanning 1966 to ~2010, and reviewing / compiling existing ocean data and ice velocity data as well as adding\nsome new data to the mix there. The main tenet of the paper appears to be (from the title and abstract) that the evidence\nsupports basal melting, rather than hydrofracture, as the main cause of the break-up and loss of the ice shelf.\nBut the paper is all over the place. The first figure starts off in the right direction, showing the timing of loss of contact with\npinning points in the Wordie Bay, although its unclear if these pinning points are let go by calving or by basal melting -- a\nfigure in the Supplemental Information (S2) shows what I would expect to see next in the main paper -- more evidence of a\ngradual fade in the appearance of ice rumples or ice rises as the shelf thins due to ocean-driven basal melt. (using image\nseries, rather than elevation change, because the remaining Wordie is too small for accurate direct elevation measurements\nfrom radar altimetry later in the 1990s - is that correct?)\nInstead the next section is a review of the extent of surface ponding on the ice shelf (with Figure 2), which is fine -- but\nbecause the images are just single-day snapshots, acquired at various points in the summer season, its hard to make a case\nfor any kind of increase or decrease in surface ponding, or whether there is a fracturing relationship. Moreover, while I agree\nthat ocean-driven basal melting is the likely cause for weakening, increased rifting and break-up of the Wordie, it is highly\npossible that the fine-scale calving seen in the melange areas in the 1990 image were in part generated by hydrofracture.\nFigure 2 and Figures S2 and S3 bring up a point about image processing in papers that rely on morphology-based\ninterpretation. For the reader to get an accurate sense of comparison across images, they must be processed so that the\ncontrast and grayscale range match as closely as possible, within reason. Histogram matching is a standard tool in most\nimage processing toolkits; so is large-scale high-pass filtering, which the 1966 orthophoto mosaic could use -- it is relied on\nin several places but has a serious backscatter-related brightness variation across it.\nThe next section focusses on grounding line retreat based on image analysis (determination of the slope break from a 1966\nimage-based DEM). Does this DEM not provide an accurate freeboard measurement of the ice shelf? I see in the Methods\nthat there are high uncertainties in the absolute elevations. Figure 3 takes up a lot of space in the paper, when the point is\nestablishing the oldest grounding line for comparison with the more recent data, which is sort of germane to the title of the\npaper, but does not really factor into the abstract (no statement on grounding line retreat rates or timing along the various\ntributaries.)\nThe paper then moves to glacier acceleration along the various tributaries. Fleming at least has been discussed in earlier\npapers; but this account is more detailed. I don't see a discussion of errors on the velocity measurements, or errors indicated\nin Figure 4.\nOverall, the paper is a mile wide and an inch deep, and somewhat in search of a reason to exist.. Ok that's a bit harsh but the\npaper is very different from what I expected reading the title and abstract. It seems that the main points are really (1) the\ncreation of a mosaic and approximate DEM from 1966 images, which undoubtedly was a lot of work; (2) using that image\nmosiac with existing data to track in somewhat more detail the ice front retreat and fracturing of the Wordie; (3) using the\n1966 grounding line to set some timing on grounding line retreat pace for the past few decades in the major troughs; and (4)\na more detailed and extensive look at the acceleration of the feeder glaciers of the Wordie.\nThroughout, there is a need to sharpen and steer the story -- is it about grounding line retreat? or ocean melting as the cause\nof break-up? or ice flow acceleration? -- and if the latter, would it be useful to attempt a mass balance assessment or at least\na mass flux time-series? I'm sorry but it really doesn't add much to the hydrofracture story, I'm not sure what the take-away is\nfrom the early images of melt ponds except that there are more there in the north end during the 1980s- 1990s than I was\naware of before.\nReprocess the images so that they are more comparable and the morphology is semi-quantitatitive. If this is about\nhydrofracture versus ocean melting, then Figure S2 and S8 need to come into the main text, and some kind of thickness\nchange data -perhaps by using the grounding line series somehow, is needed -- and we need a closer look at the melange\nareas. If it is a more general story about the evolution of the Wordie tributary system post-break-up, it doesn't seem to have\nmuch of a punchline, but it is more detailed than I had seen previously. As it stands it would seem to be more of a paper for\nJournal of Glaciology or JGR Earth Surface; but I'm sure the authors will revise the text and extract a more consistent story.\nOr do two papers -- the data is valuable and the separate studies of morphology, grounding line retreat, and acceleration are\nuseful. .\nFor the editors, I don't see many errors; the paper is clearly written in a sentence-by-sentence basis; but the storyline is\nconfusing, at least to this reader.\n(the word 'collapse' is used, but I think its not appropriate, especially if the results are somehow given to the public. Polar\nscience needs to be careful with this term.)"}, "author_response": "Overview\nWe would like to thank reviewers #1, #2 and #3 for their detailed and constructive feedback. We are\nappreciative of the time and effort invested and believe that the comments have helped improve the\nmanuscript significantly. It is encouraging to learn that the reviewers find the manuscript well-written\nand without any obvious flaws.\nIn response to the comments, we have implemented several significant revisions:\n-Rewritten large parts of the manuscript, including the introduction, results, and discussion, to\nimprove the storyline, the framing and more clearly state the paper’s main contribution. Additionally,\nthe discussion now includes a broader discussion on the implications of our findings.\n-We have spent a lot of hours improving our processing of the 1966 historical aerial images and as a\nresult succeeded in producing reliable absolute elevations over a larger part of the ice shelf and\ngrounded glaciers. From this DEM we have estimated the 1966 ice shelf freeboard elevation as well\nas thinning of the tributary glacier over two periods from 1966-2007 and 2007-2022. This is now\nincluded in the result, discussion and method section. During this process we decided to add Maud\nBernat to the author list.\n- We have rewritten the result section and split it into two main sections looking at 1) Ice shelf changes,\nincluding shelf size and elevation, pinning points, structural damages, and melt ponds, and 2) the\ntributary glaciers focusing on changes in grounding line positions, ice flow velocities, and ice\nelevations.\n-The manuscript now includes two newly introduced figures: (i) Figure 4 in the Results section, which\nillustrates the elevation change rates, and (ii) Figure 5 in the Discussion, which presents ERA5 austral\nsummer temperatures, Rothera Station summer temperatures, and mid-ocean temperatures. These\ndatasets, previously part of the supplementary material, are now integrated into the main text with\nannotations highlighting key events during the ice shelf disintegration. Additionally, we have\nrearranged some figures between the main text and the supplementary material to improve the\nmanuscript's clarity and better present the evidence pointing to the ocean warming as the main trigger\nfor the initial disintegration of the ice shelf.\nFurthermore, numerous minor adjustments have been made, and we have addressed every referee’s\ncomment. A detailed breakdown of these modifications can be found below, with referee comments\nhighlighted in blue and our responses provided in red."}, {"round": 2, "reviewer_comments": {"reviewer_2": "General comments:\nI think that the authors have sufficiently implemented revisions to the manuscript that address the points I raised in my\noriginal review.\nI am now happy that the authors have made the Abstract and Introduction clearer and situated it better within the existing\nliterature on ice shelf disintegration, including mention of damage and structural weakening.\nI agree with the authors’ decision to remove discussion of tabular iceberg production estimates from the manuscript.\nI also think the Discussion is now much improved, with more precise wording and addresses the roles of pinning points and\nthe buttressing effect of mélange.\nI think the concluding paragraph has a much clearer main message, highlighting that ocean warming is identified as the\nmain trigger of the initial ice shelf disintegration.\nOn reading the revised manuscript, I would suggest a few additional changes:\n• Make the sub-headings in the Results more specific, for example ‘Ice shelf retreat and structural changes’ and ‘Tributary\nglacier acceleration and grounding line retreat’, or similar.\n• The title needs correcting: please remove the comma after ‘extensive’ and hyphenate ‘ocean driven’.", "reviewer_3": "The paper applies historical aerial photographs and reviews available satellite images to examine in detail the evolution and\ndemise of the Wordie Ice Shelf. Based on the new evidence of significant thinning (by differencing aerial-image-derived\nDEMS and satellite DEM information), as well as direct image evidence of ice shelf thinning through the disappearance of\nice rises, they infer that the main cause of the loss of the Wordie Ice Shelf was due to basal melting resulting from the\nintrusion or thickening of the mCDW layer in the region. Further, they document the ensuing velocity increases in the\ntributary glaciers as the ice shelf is thinned and then lost, further supporting their contention that basal melting, rather than\nsurface melt driven hydrofracture, was the main cause of the Wordie retreat.\nThe paper is very very much improved over the first version, and can be published more or less as it is (in this reviewer's\nopinion. The text is quite readable and clear, and the graphic fairly easy to follow. The study flows fairly nicely from the case\nmade for thinning by basal melting, to the effect of the thinning, i.e. glacier flow acceleration.\nThe methods section is quite long, but I guess that's fine. I had a suggestion to re-design Figure 2 a bit so that tracing the\nspeed-ups to the glaciers they pertain to is easier; Figure 3 is an absolute rip-snorter (that means good) nicely illustrating the\nlong-term thinning response long before the complete loss of the ice (strengthening the basal melt case as the main cause).\nSide note, just for fun --- Both the Byrd 3 expedition (The Antarctic Service Expedition) of 1939-1940 and the Ronne\nAntarctic Expedition of 1947-48 acquired aerial photographs of the region -- these might push the record back to a time prior\nto the sharp increase in basal melting in the region (your Figure 4 would seem to imply that). Please see an overview of the\nflight paths and the data acquired in \"Americans in Antarctica\" by Kenneth Bertrand (these expeditions are among the last\nchapters in the book)."}, "author_response": "Overview\nWe are happy that reviewers #2 and #3 find that the manuscript has significantly improved since its\ninitial submission and is now worthy of publication.\nBelow we address the reviewer’s comments highlighted in blue and our responses provided in red.\nREVIEWER COMMENTS\nReviewer #2 (Remarks to the Author):\nGeneral comments:\nI think that the authors have sufficiently implemented revisions to the manuscript that address the\npoints I raised in my original review.\nI am now happy that the authors have made the Abstract and Introducfion clearer and situated it\nbefter within the exisfing literature on ice shelf disintegrafion, including menfion of damage and\nstructural weakening.\nI agree with the authors’ decision to remove discussion of tabular iceberg producfion esfimates from\nthe manuscript.\nI also think the Discussion is now much improved, with more precise wording and addresses the\nroles of pinning points and the buftressing effect of mélange.\nI think the concluding paragraph has a much clearer main message, highlighfing that ocean warming\nis idenfified as the main trigger of the inifial ice shelf disintegrafion.\nThank you for your posifive comments. We are happy that you agree with the changes in the\nmanuscript.\nOn reading the revised manuscript, I would suggest a few addifional changes:\n• Make the sub-headings in the Results more specific, for example ‘Ice shelf retreat and structural\nchanges’ and ‘Tributary glacier accelerafion and grounding line retreat’, or similar.\nWe agree and have now changed the sub-headings to: “Ice shelf retreat and structural changes” and\n“Accelerafion, grounding line retreat and thinning of tributary glaciers”\n• The fitle needs correcfing: please remove the comma after ‘extensive’ and hyphenate ‘ocean\ndriven’.\nTo agree with editorial guidelines, we have changed the fitle of the manuscript. It now reads:\n“Half a century of dynamic instability following the ocean-driven break-up of Wordie Ice Shelf”\nReviewer #3 (Remarks to the Author):\nThe paper applies historical aerial photographs and reviews available satellite images to examine in\ndetail the evolufion and demise of the Wordie Ice Shelf. Based on the new evidence of significant\nthinning (by differencing aerial-image-derived DEMS and satellite DEM informafion), as well as direct\nimage evidence of ice shelf thinning through the disappearance of ice rises, they infer that the main\ncause of the loss of the Wordie Ice Shelf was due to basal melfing resulfing from the intrusion or\nthickening of the mCDW layer in the region. Further, they document the ensuing velocity increases in\nthe tributary glaciers as the ice shelf is thinned and then lost, further supporfing their contenfion\nthat basal melfing, rather than surface melt driven hydrofracture, was the main cause of the Wordie\nretreat.\nThe paper is very very much improved over the first version, and can be published more or less as it\nis (in this reviewer's opinion. The text is quite readable and clear, and the graphic fairly easy to\nfollow. The study flows fairly nicely from the case made for thinning by basal melfing, to the effect of\nthe thinning, i.e. glacier flow accelerafion.\nThanks a lot, we are happy that you think the new version has improved greatly.\nThe methods secfion is quite long, but I guess that's fine. I had a suggesfion to re-design Figure 2 a\nbit so that tracing the speed-ups to the glaciers they pertain to is easier; Figure 3 is an absolute rip-\nsnorter (that means good) nicely illustrafing the long-term thinning response long before the\ncomplete loss of the ice (strengthening the basal melt case as the main cause).\nWe have re-designed Fig. 2 according to your suggesfions.\nSide note, just for fun --- Both the Byrd 3 expedifion (The Antarcfic Service Expedifion) of 1939-1940\nand the Ronne Antarcfic Expedifion of 1947-48 acquired aerial photographs of the region -- these\nmight push the record back to a fime prior to the sharp increase in basal melfing in the region (your\nFigure 4 would seem to imply that). Please see an overview of the flight paths and the data acquired\nin \"Americans in Antarcfica\" by Kenneth Bertrand (these expedifions are among the last chapters in\nthe book).\nWe are aware of the 1947-48 mission and also include the 1947 ice shelf front posifion in our paper.\nUnfortunately, we have not been able to get our hands on the images. It would indeed be fantasfic if\nthese early images could extend our current historical record of structural changes further back in\nfime."}]

Decision letter