the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Nov 2025
Brief communication: Updated grounding line mapping in the Amundsen Sea Embayment, Antarctica, from 1-day repeat Sentinel-1 SAR data
Abstract. Knowledge of Antarctic glacier grounding lines, which mark the transition between grounded and floating ice, is a vital parameter in determining the stability of major ice shelves and hence the ice sheet. Rapid grounding line retreat and associated mass loss has been documented at numerous Antarctic glaciers, particularly in the Amundsen Sea Embayment. However, few comprehensive grounding line mappings exist, particularly from recent years. Here, we utilize a unique record of Sentinel-1 Synthetic Aperture Radar 1-day repeat-pass imagery to generate a comprehensive retrieval of grounding line location in the Amundsen Sea Embayment in 2025 and evaluate recent changes.
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Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
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Supplement
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- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2025-4471', Anonymous Referee #1, 02 Dec 2025
- AC1: 'Reply on RC1', Jonas Kvist Andersen, 05 Feb 2026
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RC2: 'Comment on egusphere-2025-4471', Bryony Freer, 19 Dec 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-4471/egusphere-2025-4471-RC2-supplement.pdf
- AC2: 'Reply on RC2', Jonas Kvist Andersen, 05 Feb 2026
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2025-4471', Anonymous Referee #1, 02 Dec 2025
In this brief communication, Andersen et al. present an updated mapping of glacier grounding lines in the Amundsen Sea Embayment of West Antarctica, using Sentinel-1 Synthetic Aperture Radar (SAR) images acquired with a 1-day repeat-pass during the commissioning phase of Sentinel-1C. These data offer an excellent opportunity to comprehensively update the grounding line record for Pine Island, Thwaites and other glaciers in the Amundsen-sea, which are crucial in the mass balance and stability of West Antarctica.
The authors find grounding line retreat of between 2 and 7 km on Pine Island Glacier since the last available grounding line data from 2011, while Thwaites Glacier grounding lines are within previous mapping efforts of 2023. In the Crosson-Dotson region they find that grounding lines have largely stabilised on their 2020 position.
The manuscript is well written and easy to follow, with excellent quality data. The new up-to-date GL product which the authors make available is valuable to the Antarctic glaciology community. I commend the authors for making this available promptly after the Sentine-1 acquisitions earlier this year. The authors also do a good job of highlighting the value in short-repeat pass interferometry for ice sheet monitoring, I thank them for this effort.
Overall, the manuscript is timely, significant and will be useful to all glaciologists with an interest in West Antarctica. I have one main general comment which I think it is important to address, plus minor comments on a line-by-line and figure-by-figure basis. I hope that these will be useful in improving and revising the paper.
General comment:
My main concern with this manuscript is in how uncertainty in the grounding line position and grounding zone width is reported. I understand that there is a limited amount of data for historic periods and that any grounding line delineation from a single (3 or 4 constituent acquisition) differential InSAR result is only a snapshot of grounding line position in a zone that we know displays substantial tidal migration. However, even with this caveat, I think that the authors could and should provide a more detailed discussion of the impact of this uncertainty on the measured GL retreat. In my opinion, the sentence at line 105: 'Consequently, the reported changes in GL location should be viewed as approximate, as they stem from a comparison of discrete acquisitions' does a disservice to the volume and quality of data available.
The authors quote from Mohajerani et al. 2021 that ‘estimates of grounding zone width in the ASE are mostly around 1km but may locally exceed 5km’. This range of 1 to 5 km is comparable to the retreat rates that the authors quote throughout the article and consequently deserves a more thorough quantitative treatment. I do not want to be too prescriptive, given the Brief Communication format, but suggestions for how to do this could include quoting the grounding zone width from the Mohajerani data where possible, or expanding the supplementary figures to show the estimated grounding zone width where data is available, as it has been done for Cosgrove Ice Shelf and Getz Ice Shelf.
Improving the quantification of these impacts in the manuscript would make the grounding line retreat results substantially more useful to the community by qualifying the results with an uncertainty.
Line-by-line comments:
8: In my opinion, it would be useful to quantify the increase in mass loss or ice discharge in this opening sentence.
11: perhaps clarify with ‘As ocean temperatures rise…’
14: The source for the quoted 1.2m potential SLR is not clear to me, as the references that follow this are all about GL retreat. This is not trivial, as there are multiple bed products that could support this statement, each with different methodologies and associated errors.
22: Here I would also recommend citing Milillo et al. (2022) and Freer et al. (2023) for remote-sensing observations of the tidal GL migration process.
54: I think the text in brackets is unnecessary here
58: Here the authors might also comment on the impact of deformation and strain on InSAR coherence. Decorrelation is observable in the shear margins of PIG and Smith glacier in Figure 2, so it would be useful to include in this paragraph.
86: Can the authors give an indication of the impact of not having the POE products for S-1C?
121: If manuscript length allows it would be good to give an explicit example of one of these local exceptions.
123: I find the references to East/West in this paragraph to describe the geometry of PIG to be quite confusing. I think the authors are referring to East/West in terms of the grid directions of the polar stereographic (EPSG:3031) grid. Typically, many authors talk about the geometry of the shelf with respect to true north, eg the northern and southern shear margins, and to me this is far more intuitive.
132: If article length allows it would be interesting to mention the evolution of the pinning point/ephemeral grounding point in the central PIG shelf which appears in your interferograms.
Figures:
Figure 1 – I understand that this colourmap has been used in the past to represent MEaSUREs ice speed data, so there are some historical reasons to use it, but in my opinion this is not well-aligned to Copernicus Publications’ guidelines for figure composition (https://publications.copernicus.org/for_authors/manuscript_preparation.html#manuscriptcomposition) . I encourage the authors to choose a colourmap that is perceptually uniform.
The underlying optical image mosaic could be clipped to the boundary used for the ice speed data; this would make the figure much neater.
The figure caption should specify which underlying image mosaic is being used.
Figure 2 – Including a boundary for the current ice shelf calving front, perhaps in white, would be useful for identifying where the InSAR is incoherent due to factors like the shear margin and where it is just ocean.
Figure 3 – the use of green for MEaSUREs, blue for CCI and blue for bathymetry make this figure difficult to interpret. It’s quite hard for me to make out the GL in some places, for example on PIG for the 2014 MEaSUREs GL and on Smith West. For a colour deficient reader I’m concerned that it could be prohibitively difficult. A solution may be to simplify the plot so that MEaSUREs, CCI and Milillo are on a single colour scheme based on the year.
Milillo is spelt wrong in the legend for Figure 3.
Citation: https://doi.org/10.5194/egusphere-2025-4471-RC1 - AC1: 'Reply on RC1', Jonas Kvist Andersen, 05 Feb 2026
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RC2: 'Comment on egusphere-2025-4471', Bryony Freer, 19 Dec 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-4471/egusphere-2025-4471-RC2-supplement.pdf
- AC2: 'Reply on RC2', Jonas Kvist Andersen, 05 Feb 2026
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Jonas Kvist Andersen
Romain Millan
Eric Rignot
Bernd Scheuchl
Jean Baptiste Barré
Anders Anker Bjørk
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(83201 KB) - Metadata XML
-
Supplement
(100504 KB) - BibTeX
- EndNote
- Final revised paper
In this brief communication, Andersen et al. present an updated mapping of glacier grounding lines in the Amundsen Sea Embayment of West Antarctica, using Sentinel-1 Synthetic Aperture Radar (SAR) images acquired with a 1-day repeat-pass during the commissioning phase of Sentinel-1C. These data offer an excellent opportunity to comprehensively update the grounding line record for Pine Island, Thwaites and other glaciers in the Amundsen-sea, which are crucial in the mass balance and stability of West Antarctica.
The authors find grounding line retreat of between 2 and 7 km on Pine Island Glacier since the last available grounding line data from 2011, while Thwaites Glacier grounding lines are within previous mapping efforts of 2023. In the Crosson-Dotson region they find that grounding lines have largely stabilised on their 2020 position.
The manuscript is well written and easy to follow, with excellent quality data. The new up-to-date GL product which the authors make available is valuable to the Antarctic glaciology community. I commend the authors for making this available promptly after the Sentine-1 acquisitions earlier this year. The authors also do a good job of highlighting the value in short-repeat pass interferometry for ice sheet monitoring, I thank them for this effort.
Overall, the manuscript is timely, significant and will be useful to all glaciologists with an interest in West Antarctica. I have one main general comment which I think it is important to address, plus minor comments on a line-by-line and figure-by-figure basis. I hope that these will be useful in improving and revising the paper.
General comment:
My main concern with this manuscript is in how uncertainty in the grounding line position and grounding zone width is reported. I understand that there is a limited amount of data for historic periods and that any grounding line delineation from a single (3 or 4 constituent acquisition) differential InSAR result is only a snapshot of grounding line position in a zone that we know displays substantial tidal migration. However, even with this caveat, I think that the authors could and should provide a more detailed discussion of the impact of this uncertainty on the measured GL retreat. In my opinion, the sentence at line 105: 'Consequently, the reported changes in GL location should be viewed as approximate, as they stem from a comparison of discrete acquisitions' does a disservice to the volume and quality of data available.
The authors quote from Mohajerani et al. 2021 that ‘estimates of grounding zone width in the ASE are mostly around 1km but may locally exceed 5km’. This range of 1 to 5 km is comparable to the retreat rates that the authors quote throughout the article and consequently deserves a more thorough quantitative treatment. I do not want to be too prescriptive, given the Brief Communication format, but suggestions for how to do this could include quoting the grounding zone width from the Mohajerani data where possible, or expanding the supplementary figures to show the estimated grounding zone width where data is available, as it has been done for Cosgrove Ice Shelf and Getz Ice Shelf.
Improving the quantification of these impacts in the manuscript would make the grounding line retreat results substantially more useful to the community by qualifying the results with an uncertainty.
Line-by-line comments:
8: In my opinion, it would be useful to quantify the increase in mass loss or ice discharge in this opening sentence.
11: perhaps clarify with ‘As ocean temperatures rise…’
14: The source for the quoted 1.2m potential SLR is not clear to me, as the references that follow this are all about GL retreat. This is not trivial, as there are multiple bed products that could support this statement, each with different methodologies and associated errors.
22: Here I would also recommend citing Milillo et al. (2022) and Freer et al. (2023) for remote-sensing observations of the tidal GL migration process.
54: I think the text in brackets is unnecessary here
58: Here the authors might also comment on the impact of deformation and strain on InSAR coherence. Decorrelation is observable in the shear margins of PIG and Smith glacier in Figure 2, so it would be useful to include in this paragraph.
86: Can the authors give an indication of the impact of not having the POE products for S-1C?
121: If manuscript length allows it would be good to give an explicit example of one of these local exceptions.
123: I find the references to East/West in this paragraph to describe the geometry of PIG to be quite confusing. I think the authors are referring to East/West in terms of the grid directions of the polar stereographic (EPSG:3031) grid. Typically, many authors talk about the geometry of the shelf with respect to true north, eg the northern and southern shear margins, and to me this is far more intuitive.
132: If article length allows it would be interesting to mention the evolution of the pinning point/ephemeral grounding point in the central PIG shelf which appears in your interferograms.
Figures:
Figure 1 – I understand that this colourmap has been used in the past to represent MEaSUREs ice speed data, so there are some historical reasons to use it, but in my opinion this is not well-aligned to Copernicus Publications’ guidelines for figure composition (https://publications.copernicus.org/for_authors/manuscript_preparation.html#manuscriptcomposition) . I encourage the authors to choose a colourmap that is perceptually uniform.
The underlying optical image mosaic could be clipped to the boundary used for the ice speed data; this would make the figure much neater.
The figure caption should specify which underlying image mosaic is being used.
Figure 2 – Including a boundary for the current ice shelf calving front, perhaps in white, would be useful for identifying where the InSAR is incoherent due to factors like the shear margin and where it is just ocean.
Figure 3 – the use of green for MEaSUREs, blue for CCI and blue for bathymetry make this figure difficult to interpret. It’s quite hard for me to make out the GL in some places, for example on PIG for the 2014 MEaSUREs GL and on Smith West. For a colour deficient reader I’m concerned that it could be prohibitively difficult. A solution may be to simplify the plot so that MEaSUREs, CCI and Milillo are on a single colour scheme based on the year.
Milillo is spelt wrong in the legend for Figure 3.