Reply on RC2

Review: This manuscript uses a suite of remote sensing data to map changes in ice extent, structure and velocity across the wider Shackleton system. The manuscript compliments recently published work showing recent grounding line retreat and acceleration of Denman Glacier, but also includes detailed and novel observations across the wider understudied Shackleton system. The authors use these observations to conclude that there has been limited change across the Shackleton system across the observational time period. They also then simulate the response the Shackleton system to a hypothetical loss of floating ice to demonstrate the systems sensitivity to any future ice shelf loss.

Review: This manuscript uses a suite of remote sensing data to map changes in ice extent, structure and velocity across the wider Shackleton system. The manuscript compliments recently published work showing recent grounding line retreat and acceleration of Denman Glacier, but also includes detailed and novel observations across the wider understudied Shackleton system. The authors use these observations to conclude that there has been limited change across the Shackleton system across the observational time period. They also then simulate the response the Shackleton system to a hypothetical loss of floating ice to demonstrate the systems sensitivity to any future ice shelf loss.
Overall, I think the manuscript contains some interesting and novel observations of the Shackleton system that are worthy of publication. In particular I think the 50 year record of rifting evolution across the Shackleton Ice Shelf is a very nice contribution. The background here is while there has been some recent studies focussing on Denman Glacier, we know very little about the recent behaviour of the many other glaciers that feed the wider Shackleton system, so these results are valuable. However, at the moment I think these interesting results are somewhat lost in the manuscript. It seems like quite a jump and a distraction from discussing these detailed annual scale observations, to discussing the response of the Shackleton system to the hypothetical loss of all floating ice 400 years in the future, which then turns into a discussion as to how the deep trough of Denman may favour vigorous channelization of the subglacial meltwater system close to the grounding line. At the moment I am not sure what the main focus of the manuscript is. I think the manuscript would benefit from being more streamlined, with a greater focus on the novel observations. I have included some more detailed comments below.
These comments echo those of Anonymous Referee #1 and have inspired us to remove the modelling part of the manuscript and make it the focus on a followup manuscript instead. In making appropriate revisions to the manuscript as a whole this will allow us to re-focus the manuscript on the novel observations, as suggested by the Anonymous Referee #2.

Observations: The authors state that there have been no significant annual variations in ice flow speed across the Shackleton system. I would argue that the use of 'significant' is not appropriate, what is 'significant' variations greater than 50 m yr -1 100 m yr -1 etc?.
In this context we are using significant to mean greater than the uncertainty and in this case, we do not find that any changes are greater than the uncertainty. We will clarify this use in the text.
The plots in Figure 9 give a good overview of the longer-term changes in ice flow speed across the region. However, because they are in m/day and the scales are somewhat stretched it is difficult to determine if there has or has not been any annual variations in ice flow speed. A variation of 0.3 m/day equates to around 100m yr, which would be larger than the uncertainty of the velocity products and would be an interesting result. Are there similar scale variations, particularly in the faster flowing sections of the Shackleton system, to my eyes it looks that there could be, but I could be wrong, I really cannot tell from the plot alone?
May we refer to our corresponding answer to Anonymous Referee #1. The use of both m/year and m/day is deliberate as it appropriately reflects the temporal resolution of the available data. With all due respect, in our experience it is usually incorrect to state that "0.3 m/day equates to around 100m yr" because our experience with tracking glacier flow speeds over several decades shows that velocities can change on a daily basis.
In Figure 8  We would be happy to add time periods, however this time-period was carefully chosen as it is the period where the multiple velocity maps allow the uncertainties to be much lower than the difference. Other time-periods are generally rather less clear. We are however happy to add such requested figures if desirable.

One of the most striking observations is the migration of the shear margin near Chungunov Island over just a few years. This is a somewhat unique observation. The authors have collated this wide range of velocity data, while they have tended to focus of ice speed, could they also focus on the velocity directional data? Is this change in shear margin caused by the whole Denman ice tongue 'wobbling' or a more localised change to do with Scott?
We can see from Figure 3b that Denman tongue does not appear to be 'wobbling' and all of the evidence we see indicates that changes are confined to Scott, likely a combination of ice front calving close to Mill Island, the rift opening from Chugunov Island towards Mill Island and the speed increases. While we are happy to calculate a flow direction difference between 2019 and 2020, we know from previous experience plotting directional arrows onto individual ice speed maps would provide a much clearer illustration of any changes in velocity direction.
Modelling: My expertise lies in remote sensing, so I am not in a position to comment on the methodological details of the modelling. But I did find the description of the modelling experiment carried out to be lacking. In the discussion it is stated that: 'The upper limit scenario of forcing in our BISICLES model runs suggests that noticeable grounding line retreat occurs in the Denman Glacier over the simulated 400-year time period' But in the methods section there is no mention of an upper or lower limit scenario, nor any mention of the timescales of the simulation. Aside from the basic description of model, there is only a very limited description of simulation. It is essential that the details here are expanded.
In wider point, while I think it could be a useful contribution to repeat the experiment in Martin et al., but with BedMachine, I did feel the modelling appeared as somewhat left field in the manuscript and appears as a bit of a jump in the discussion from the main body of observations. The general tone of the manuscript is that very detailed remote sensing observations have shown limited changes in the floating ice in the Shackleton system... But then the manuscript jumps to.. 'now we simulate the unrealistic loss of all floating ice in the Shackleton system'... The scientific rationale for this is unclear to me? Of course, it is entirely up to the authors, but I would point out that I think the detailed observations have the potential to be a nice contribution alone.
The term upper limit was used to indicate the upper limit of change in the system by removing all of the floating ice, rather than the upper limit of a series of scenarios. May we refer Anonymous Referee #2 to our response to Anonymous Referee #1 for an explanation of the anticipated logic of connection between observations and modelling in our original manuscript. Following up on these matching anonymous referee comments, and also explained in that response, we consequently propose to split the observational and modelling components into two manuscripts, removing the modelling from this manuscript which already largely focuses on observations.

Specific comments:
Line 59: I would not describe the acceleration of Denman Glacier since the 1970s as a short-term fluctuation Whilst an increase in ice flow speed of the Denman Glacier clearly occurred between the 1970s and 2017 detailed examination of the timing of this change (as matched by our own feature-tracking based inferences) reveals that almost all of it happened sometime between 1972 and 1989, with very little change since. We will review this aspect of the manuscript and make appropriate revisions to emphasise the more recent stability in ice flow speed.