Reply on RC1

The manuscript by Dr. S. Thompson and colleagues presents a two-pronged study aimed at (1) reconstructing the 60-year surface feature record and 2014-2021 surface ice velocity field of the Shackleton Ice Shelf System via analysis of satellite imagery, and (2) modeling the future response of the upstream grounded glaciers feeding the Shackleton Ice Shelf System to near-instantaneous disintegration of this ice shelf via a 400-year projection made using the BISICLES ice sheet model. The authors conclude that the Shackleton Ice Shelf System has not changed significantly over the 60-year observational period (aside from a localized acceleration in surface ice velocity near the ice front of Scott Glacier) and that the future upstream glacier response to collapse of the Shackleton Ice Shelf is minimal relative to changes projected across other East Antarctic basins.

I find the extension of the surface-observational record both spatially (to the neighboring Scott Glacier, Roscoe Glacier, and greater Shackleton Ice Shelf region) and temporally (from 2017-2021) to be the primary strength of this manuscript, as this information is very useful to the ice sheet modeling community. However, I have significant concerns regarding the scope of the paper and the applicability of the numerical modeling work, which make the manuscript difficult to follow. First, I find significant overlap in the analysis of surface features of the Denman Ice Tongue between this manuscript and that of Miles et al. (2021) (e.g. ice front positions, patterns of rifts, and location of pinning points on the Denman Ice tongue, as well as the calving of the large tabular icebergs in the 1940's and in 1984). While the manuscript does properly cite Miles et al. (2021), the repetition of the analyses and findings makes up a significant portion of this study and thus reduces the novelty of the manuscript. The manuscript should be reorganized to have a greater focus on the spatial and temporal extension of the surface observation record.
We will restructure the manuscript to clarify the novelty of the extension in spatial and temporal observations. Our aim was to provide full context to the observations we report and discuss, rather than simply citing previous work. We will make the distinction requested.
In addition, the numerical modeling portion of this manuscript is rather disconnected to the scope and findings of the rest of the paper and is not robust enough to support the authors' conclusions. The first half of the manuscript analyzes short-term and fine-scale changes of features on the floating portion of the Shackleton Ice Shelf System; however, the authors then model the 400-year response of the grounded regions of mainly Denman Glacier to near-instantaneous disintegration of this ice shelf. This disconnect between the focus on observing small-scale ice shelf features across the entire Shackleton Ice Shelf System and modeling grounding line retreat and volume loss of Denman Glacier (without mentioning of the response Scott and Roscoe Glacier) makes following the progression of the manuscript very difficult. If modeling is going to be included in this study, it needs to complement the rest of the manuscript (i.e. model how future ice flow responds to changes in the surface features discussed in the first half of the manuscript). Furthermore, from the analysis of a single model simulation, the authors imply that the Queen Mary and Knox coasts are relatively stable and insensitive to reasonable forcing in the next 400 years (see L313, I am also assuming this is the "implied dynamic stability" referenced in the title). I don't believe the authors can claim stability of the system and make a statement about sensitivity without modeling the system's response to realistic forcing perturbations. Overall, I believe the modeling portion of this manuscript needs to be either redone so that it supports the observational-focus of the paper or separated and made the focus of a secondary manuscript.
It is apparent that the authors have put a lot of effort into the text and figures in the manuscript; however, because of my significant concerns over the scope of the manuscript, its connection to the modelling work, and the key takeaways, I suggest that major revisions (or perhaps a resubmission) are needed before the manuscript can be considered for publication in The-Cryosphere.
We thank Anonymous Referee #1 for their detailed comments on the modelling section of the manuscript, which are echoed by those of Anonymous Referee #2. The authorship team have discussed this at length and appreciate the limitations of the modelling approach highlighted here and in the general comments. The logic in the flow of our original manuscript from observation to modelling lies in the following. As explained further under the 'Title' section below, our observations revealed only minor changes of structural changes in the Shackleton Ice Shelf System over the past 60 years. Our inference is therefore one of a rather stable system (not withstanding previous authors' observations of some dynamic variability) and indeed, this is confirmed by our BISCLES modelling experiments in that even upper limit conditions, i.e. complete loss of all floating ice that may buttress the grounded ice, lead to only minor simulate change relative to those elsewhere in Antarctica.
Nonetheless, given the matching comments by both anonymous referees we feel that -subject to advice by the journal editor, the best approach may be for us to separate the observations from the modelling, with the latter forming the focus of a subsequent manuscript. As suggested by the anonymous referee(s) we will therefore remove the modelling aspects from this manuscript and focus on the observations that already make up the majority of the current manuscript. The modelling proportion will then form the basis of a subsequent manuscript as suggested.

General Comments:
Title: I don't believe the title accurately describes the presented work. I am unsure what the authors mean by "glaciological setting", I think wording that describes the analysis would be better suited to use in the title. I also think it is a bit misleading to claim that the authors are studying the entire Queen Mary and Knox coasts, when only the Shackleton Ice Shelf system is analyzed. Lastly, I am not sure what the authors mean by "implied dynamic stability". Is this stability over the entire 60-year observational period (which would be inaccurate because grounding line retreat (~5 km, Barancato et al., 2020), floating and grounded ice accelerations (Miles et al., 2021;Rignot et al., 2019), and accelerated ice discharge (Rignot et al. 2019) have been observed over this timeframe), over the 400-year modeling period (which, as stated above, I do not think the authors can claim based on the results presented), or between 2018-2021 (following the ice velocity results)? It is difficult to suggest a new title right now because significant changes to the scope of the manuscript need to be made.
The inference of an 'implied stability' is based on our observations of little dynamic change over the past 60 years, as well as reduced modelled sensitivity to even extreme events such as complete removal of all floating ice. It is natural for systems such as the Shackleton Ice Shelf to show some dynamic variability although, as explained in the manuscript, our observations and modelling cannot confirm that the variabilities observed by previous work necessarily herald major future change. We recognise that the same time that field data are too sparse to make this conclusion with confidence and am therefore recommending that focused programs of field observation are urgently needed. Either way, we will reflect on the current title and revise it appropriately.
Ice sheet model validation: As the manuscript presents one of the first regional ice sheet models to make future projections of the Shackleton Ice Shelf system, it is critical that it is properly described and validated. It is not enough to only show the mismatch of observed and modeled surface ice velocity in order to validate your ice sheet model, one also needs to know how the modeled and observed grounding Agree and we will make this the focus of a separate manuscript.

Units of speed: When referencing speeds of both ice and rift/ice front propagation in the main text and figures, the authors switch between m/day and m/year (see lines 258 and line 270 for examples of each). For ice speed, the convention is m/year, so I think it would be best to abide by this convention so that your results can be easily compared to other values in the literature (change in both the text and in figures)
. Also, when referencing the unit "year", please stick to either "year" or "a", as both were used in the manuscript.

May we clarify that m/year is commonly used when the temporal resolution of available data is greater than one year. For this reason, when describing the movement of structural features and ice frontal positions we have used the unit m/year to describe the longer-term trends as the measurements are based on data with annual or multi-annual temporal frequency.
When describing the ice speed data from feature tracking, we have deliberately used the unit m/day because we are using much higher temporal resolution data. While we are happy to include annual trends for the data, ice speed does vary on much shorter timescales and by simply changing the ice speed data to m/year we would lose this valuable information.
Data availability statement: Please add a data availability statement at the end of the manuscript, as to abide by The-Cryosphere's data policy. All of the links in sections 2.1 and 2.2 should be moved to the data availability statement. In addition, a link to the BISICLES ice flow model, as well as links to all datasets used in the simulation, should be added to this statement if the modeling portion is to remain in the manuscript.
We are happy to make this change.

Grammar: When reading through the manuscript, I noticed a fair amount of spelling and grammar mistakes (especially missing commas, which would help the readability of the text). I tried to point them out as I found them in the specific comments, but it is possible I missed a few!
The whole authorship team will all check the manuscript thoroughly for spelling and grammatical errors.

Specific Comments:
L13-L38: In general, I think the abstract is a bit long and should be condensed. Below, I suggested a few sentences that can be removed and/or shortened.
We are happy to make these the changes.

L15: change to ``. . . on understanding the controls driving Denman Glacier's dynamic evolution, although . . .''
We are happy to make these the changes.

L17: Shackleton Ice Shelf (use capitalization because it is a proper name)
We will make the change throughout the manuscript.

L22-L23: Remove "in response to coupled ocean and atmospheric forcing". Coupled forcing suggests that your ice sheet model is coupled to an atmosphere and/or ocean model, which it is not.
This section will be removed from the manuscript as outlined in the response to the general comments above.

L31: I make note of this later in the results section, but the authors should not use real years to describe the output of their modeling work because it is not a realistic simulation.
Instead of saying "in the third century from now", it would be better to say "in approximately 300 years into the model simulation." This section will be removed from the manuscript as outlined in the response to the general comments above.

L31: Please check the computation of the 6 cm of sea level rise, I computed 40 Tt = 40000 Gt / 3600 [Gt/cm] = 11.11 cm sea level rise equivalent ice mass, but it is possible that my math is off! Is this the sea level contribution from just Denman Glacier, or from the entire model domain? I believe this is from the whole domain; however, in the previous sentence, you discuss the grounding line of Denman Glacier, so it is a bit confusing. Please specify.
This section will be removed from the manuscript as outlined above.
L32: I would hesitate to say that 6 cm of global sea level rise equivalent ice volume loss is "small" in comparison to other areas of East Antarctica. First, I don't believe there are any published studies that have run regional transient simulation of the EAIS through 2400, so we cannot compare. Also, 6 cm is on the upper limit of the ISMIP6 projected contribution of the entire Antarctic Ice Sheet to global sea levels by 2100, so this contribution from a single EAIS glacier by 2400 must be fairly significant.
This section will be removed from the manuscript as outlined above.
We are happy to remove the sentence.
L34: Here you conclude that there is potential vulnerability of the system to accelerating retreat, but further along in the manuscript (L313), you say that the modeled domain is relatively stable and insensitive to reasonable forcing in the next 400 years. These statements conflict and left me confused about the message of the manuscript. Perhaps it would be more consistent with the rest of the manuscript to say that these data are needed to improve model initialization and validation.
We agree and will alter the manuscript to reflect the main point that we don't know enough about the system to be able to accurately model potential vulnerabilities.

L34: Insert comma after "accelerating retreat".
Happy to make the change.
L41-L44: These first two sentences can be combined and condensed, which I think would be a bit easier on the reader. Perhaps something like: "It has long been perceived that the East Antarctic Ice Sheet is the stable sector of Antarctica (citations); however, it has now emerged that the Aurora and Wilkes subglacial basins of the EAIS have been contributing to sea level rise since at least the 1980s, with Aurora contributing 1.9 mm and Wilkes contributing 0.6 mm (citations)." We agree that the change increases the clarity and are happy to make the change.

L45: Insert comma after "WAIS"
Happy to make the change.
L45 and L47: You are referencing both BedMachine Antarctica (Morlighem et al., 2020) and Bedmap2 (Fretwell et al., 2013) for your values of sea level potential. As BedMachine is the most up-to-date dataset, I would stick to just using the BedMachine citation throughout the manuscript (unless of course you are using the BedMap2 dataset in the paper).
Bedmap2 was used in the first draft of the manuscript in the current version we updated to BedMachine. The inclusion was an oversight, and we will remove the Fretwell reference.
L52: Change "it is supplied by . . ." to "Major outlet glaciers drain into this ice shelf system, including Denman, Scott, Northcliffe, Roscoe, and Apfel Glaciers." Happy to make the change. L82-L85: Please remove "A satisfactory explanation . . . with the nearby Totten Glacier." I think this interrupts the flow of the introduction and does not serve the rest of the paper, as the focus is not to determine where the high melt rates are being forced from.
Happy to make the change.
L92: What does it mean to put previously observed dynamic changes in the Shackleton system into the wider regional context of the Queen Mary and Knox coasts? The observational and modelling components of this study do not investigate changes beyond the Shackleton Ice Shelf System, so I think that claiming to frame the regional context of the entire Queen Mary and Knox coasts is a bit misleading. As stated above, I think the really exciting science presented here is the extension of the observational record to other sectors of the Shackleton Ice Shelf and to 2021. So I think this sentence should reflect that.
We agree and are happy to make the change.
L95: I do not think we are testing the sensitivity of the domain, as this would require further model runs (such as a control simulation and variance of the ocean forcing).