| 30 Jan 2026
Progressive Structural Weakening of the Northern Pine Island Ice Shelf, West Antarctica
Abstract. The Pine Island Ice Shelf can be analysed within three distinct sectors: northern, southern, and central. Dynamics within the Northern Pine Island Ice Shelf (N-PIIS) remain less understood than those of the Central Pine Island Ice Shelf. Using satellite imagery, reanalysis datasets, and in situ observations, we examined changes in ice-front position, velocity, thickness, rift propagation, and polynya activity across the N-PIIS since 1973. Overall, the ice-shelf area has decreased, and surface elevation data indicate ongoing thinning. Grounded ice velocities have remained largely stable, although localized accelerations occurred near the floating portion of the N-PIIS and the grounded section of Velasco Glacier. Several former pinning points have disappeared, and three polynyas have developed around the ice shelf, suggesting enhanced ocean-ice interaction. Our observations document continued thinning, frontal retreat, pinning-point loss, and episodic polynya formation at the N-PIIS. While large-scale dynamic acceleration is limited, these changes suggest a progressive reduction in structural stability and increased sensitivity to oceanic and atmospheric forcing. This study provides critical new insights into the evolving stability of the N-PIIS and highlights the importance of continued monitoring to anticipate potential disintegration.
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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