Overview

The manuscript by Chien and others reports on remotely sensed observations on changes in the dynamics of the Northern Pine Island Ice Shelf and the glaciers that feed the ice shelf. The Northern Ice Shelf detached from the main ice shelf in 2015 and is now its own independent ice shelf. The Northern Ice Shelf is small and is fed by some very small glaciers in the context of the Antarctic Ice Sheet, thus the future of the ice shelf its self is rather inconsequential for sea level rise. That said, it is located in a very important region and observations of recent changes may help to improve regional understanding of ice-ocean interaction. While there are some nice observations, I feel substantial revisions are required before publication in The Cryosphere can be considered. At times this manuscript is difficult to follow because the dates don’t match up i.e. the authors mention that an analysis is carried from x date, but then the results show a different date. I also found the section on the polynyas confusing and It is still a little unclear to me what you are trying to convey to the reader with these observations. I have included some more detailed comments below.

Major Comments

Pinning points/structural weakening: I am missing an overview figure showing the structural evolution of the northern ice shelf over time. Imagery exists since 1973 and I think it would be beneficial if you could show some snapshots through time. This way the reader can visualize these structural changes and pinning point changes that are sometimes visible in optical imagery. Part of the problem with this manuscript as the moment is that you conclude that the Northern Ice Shelf is undergoing progressive structural weakening, but don’t actually show any evidence of this change through time. The DINSAR fringes are nice, but only show a small part of the ice shelf at one period in time. Also could the apparent ungrounding be linked to tides?

Ice velocity: what are the large spikes on the ice velocity record. Are these erroneous results or do they reflect something more geophysical? This needs to be addressed?

Ice front: You mention since 1973 in some parts of the manuscript, but you do not provide any observations since 1973? The methods says 1997-2025, but figure 1 goes from 1989-2023? Please sort out these inconsistencies.

Polynyas: Overall I found this analysis confusing and difficult to follow. This section is written as if the authors are arguing that the occurrence of polynya means that something has changed and there is now more basal melt.  I agree that the polynyas could be formed by channels of basal melt, this is a well known process and has been studied in detail – see Alley et al 2016 – an important paper in the context of this analysis. However, I do not understand how any temporal changes can be reliably linked to any type of forcing with the observations the authors present. Firstly, it is not clear how you are actually measuring the size/occurrence of these polynyas. You mention in the methods you only track polynya development in 2021, but then highlight some going back to 2003 – presumably there are other occurrences you simply have not been able to observe, for example in the winter? You also mention that there are more polynya activity when there is strong fast ice, this makes sense, by nature sea-ice is required for polynya formation. Presumably the underlying cause of the polynya formation (basal melt) is still happening when the sea ice is not present.

Methods: Please make the methods section consistent with what you actually show. There are numerous instances of the dates not matching up which creates some confusion.

Line comments

Line 12: There are no data from 1973 in this manuscript???

Line 15: ‘three polynyas have developed around the ice shelf, suggesting enhanced ocean-ice interaction’ – the manuscript shows some polynya developing sporadically since 2021. But this is not enough to say ‘enhanced ocean-ice interaction’ those polynya could have been there for decades.

Line 18: it is unclear how the ice shelf can have an increased sensitivity to ocean and atmospheric forcing

Line 30 – could you show the basal melt rates, is there are dataset available for this ice shelf?

Line 31 – the ice shelf lost 25% of its area? An Ice shelf can only retreat by distance, not a percentage.

Line 50: if they are increasingly recognized then you need to provide citations to show this.

Line 51: ocean circulation – please delete. The ocean circulates everywhere to some degree!

Line 66-67: Elsewhere in the manuscript 1973 is mentioned. In Figure 1 there is an ice front position from 1989. This is confusing. If the imagery in 1973 is available, which I think it is, then I think you should present it.

Line 73-75: You present other observations of polynyas later in the manuscript going back to 2004

Line 81: Please use ±0.25

Line 90: were they corrected for firn content?

Line 110: again 1973 mentioned?

Line 143: How are you observing these polynyas in 2003, 2005 etc?

Line 201: I do not think the maximum buttressing values in Figure 1 are valid given that the system has changed so much since it was computed.

Line 203: How do you get a delayed dynamic response? What is the physical reasoning behind this?

Line 252: 1973 mentioned again…

Line 256: I am not sure what the term ‘fully stable’ means. It is either stable or not stable, there is no in-between. Perhaps you mean ‘weakening’

Line 257: ‘Episodic atmospheric forcing’ – what does this mean? Do you mean anonymously  warm summers? (is there any evidence surface melt is doing anything in this region?) Snowfall drought?

Figure 1: I do not think the maximum buttressing values add anything to this figure. They were computed under a very different set of geometries to the current state, you can see this in the image where there is ocean along the shear margins of the ice shelf. Aside from this, the overlay make it very difficult to see the pinning points and structure of the northern ice shelf. I would suggest removing the maximum buttressing.

Figure 2: The ice speed estimates look very noisy with all the spikes. Are they seasonal in origin? What is causing this?

Figure 4: Looks like E3 is still there. Nice observation of a new rift following grounding of E9 – this is an interesting observation

This study seeks to investigate the stability of the Northern Pine Island Ice Shelf, particularly in the context of the drivers of stability changes, including loss of pinning points and polynyas. The study presents observations to suggest that these factors have been driving observed changes to N-PIIS (e.g. velocity increases, ice shelf thinning). The observations the authors show are quite interesting and the connections between ocean conditions, atmospheric conditions, and ice shelf weakening is a nice, comprehensive look at the system. However, I found the study to be missing some rigorous links between the factors that the authors seek to establish and I think some further analysis and figures may be needed to enhance the takeaways for the reader.

Structural Weakening

The paper (as evidenced by the title) seeks to make the argument that N-PIIS is experiencing structural weakening. While I certainly agree that this is the case, I think there is more evidence that needs to be shown here to make this case. When I hear “structural” weakening, I generally think this implies changes to the stress state and the load-bearing ability of the ice shelf. However, as far as I can tell, the only evidence the authors show to suggest there is weakening is ice front retreat (and even so, the retreat on the N-PIIS is fairly small; would maybe be useful to quantify or contextualize this to the rest of the ice shelf), calving events (but is this increasing?) and the formation of a new rift (what is the impact on the ice shelf, though?). I am intrigued by the estimates of buttressing shown in Fig 1 and wonder if this could be used to strengthen this argument – can you show how buttressing ability of N-PIIS changes in time? This would make a possibly clearer and more persuasive argument that there is real weakening happening. As it stands, I’m not sure what the calving events, loss of pinning points, and rift formation means for the overall ice shelf except that as those things are happening, there is also acceleration (which I’m sure is related, but there is no link established here).

Connection to Atmosphere/Ocean Conditions

This is a nice addition that makes this paper stand out from other PIIS-focused papers. However, I do think more needs to be done to convince readers of the link between these external conditions and the weakening of the ice shelf. I believe the authors seek to argue that the existence/persistence of the polynyas allows basal melt to more directly impact the ice shelf, thus driving some of the weakening. Can the authors actually mark on Fig. 5 and 6c where the polynyas are open, so we can see the link between the winds and the polynya existence? I see that surface elevations reduced near the polynyas but is there evidence that this had implications for the rest of the ice shelf? The link between the polynya existence and the ice shelf stability could be made more clearly and persuasively. Some more clarity on the polynya tracking would also be useful – the methods say that they are tracked in 2021, 2024, 2025 but the manuscript later discusses polynya evolution from 2003 (line 143).

Other Comments

• I think you can remove the second sentence, as it doesn’t have much impact on the rest of the introduction
• Line 23: what does “change in ice-shelf grounding” mean?
• Methods section – what specifically about the polynyas was tracked? Existence, size, shape, etc.? Why just consider rift propagation in 2025 and thinning in 2024?
• Line 239: “The observed combination…more vulnerable structural state” – what evidence is there for this?