the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Mar 2024
Melt sensitivity of irreversible retreat of Pine Island Glacier
Abstract. In recent decades, glaciers in the Amundsen Sea Embayment in West Antarctica have made the largest contribution to mass loss from the entire Antarctic Ice Sheet. Glacier retreat and acceleration have led to concerns about the stability of the region and the effects of future climate change. Coastal thinning and near-synchronous increases in ice flux across neighbouring glaciers suggest that ocean-driven melting is one of the main drivers of mass imbalance. However, the response of individual glaciers to changes in ocean conditions varies according to their local geometry. One of the largest and fastest flowing of these glaciers, Pine Island Glacier (PIG), underwent a retreat from a subglacial ridge in the 1940s following a period of unusually warm conditions. Despite subsequent cooler periods, the glacier failed to recover back to the ridge and continued retreating to its present-day position. Here, we use the ice-flow model Ua to investigate the sensitivity of this retreat to changes in basal melting. We show that a short period of increased basal melt was sufficient to force the glacier from its stable position on the ridge and undergo an irreversible retreat to the next topographic high. Once high melting begins upstream of the ridge, only near-zero melt rates can stop the retreat, indicating a possible hysteresis in the system. Our results suggest that unstable and irreversible responses to warm anomalies are possible, and can lead to substantial changes in ice flux over relatively short periods of only a few decades.
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Status: open (until 09 May 2024)
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RC1: 'Comment on egusphere-2024-673', Anonymous Referee #1, 09 Apr 2024
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Review of “Melt sensitivity of irreversible retreat of Pine Island Glacier”
The manuscript “Melt sensitivity of irreversible retreat of Pine Island Glacier” by B. Reed and co-authors investigates possible conditions that caused the retreat of Pine Island Glacier from a ridge about 40 km from its present-day conditions where the glacier was likely grounded until the 1940s. Using the Úa ice flow model and starting from an initial state close to steady-state for a cold ice shelf cavity and with an advanced grounding position on the ridge, they test the impact of warming conditions happening over a given period (about a decade) before the cavity becomes cold again. They found that once the glacier is destabilized from its ridge because of warmer conditions, it cannot restabilize on this ridge and continues retreating even if the cavity becomes cold again. They further investigate the time needed to destabilize the glacier depending on the degree of warming and its duration period.
This paper is well written, clear and very interesting. It addresses the important question of understanding the conditions that triggered past grounding line retreats. My main comment concerns the role of uncertainties in geometry and other parameters that are unknown for the 1940s should be better acknowledged and their impact discussed. The other ones are relatively minor, including a few sections that could be clarified for example to explain better what adjustments were made to the ice shelf thickness or how the friction was calibrated under the ice part that is currently floating but is grounded in the initial configuration since this cannot be inferred with the inversion. Finally, the text generalizes some conclusions without demonstrating them in a few cases.
Main comments:
l.101: What adjustments where made to the ice shelf thickness? What part of the ice shelf was impacted? And what was the magnitude of this correction? Some overall numbers should be added to the text.
l.104: Additional information should be added here to understand what is done to the friction in the part of the domain that was grounded in 1940 (and therefore a friction coefficient is needed) but is not grounded during the satellite area (and thefore cannot be inferred with observations). How was this friction chosen? Also, given there is a large uncertainty in these values, what is the impact on the results presented?
l.133: why was a period of 1000 years chosen to simulated the grounding line advance? And why was the melt set-up to zero instead of cold conditions? I am sure a number of conditions could lead to a more or less similarly advanced position, so I am curious why such conditions were chosen?
l.143: similarly to my question about the grounding line advance, why was a period of 100 years chosen for the initial grounding line retreat?
Fig.6: the small inset showing the thermocline depth and its timing is very useful to understand the experiments and should be provided earlier (for example in the description of experiments) for improved clarity.
l.267: the experiments performed show this behavior for Pine Island Glacier but not that it could happen to other places at the same time, so this statement should be toned down.
l.321: How does the retreat and possible readvance from this ridge compare to the present day conditions investigated in previous studies such as Favier et al. 2014 or Seroussi et al. 2014?
Technical comments:
l.40: there was not much slow down reported in Mouginot et al. 2014 outside of the Eastern Thwaites ice shelf, maybe less acceleration or relatively stagnant conditions, but not really a sector wide slowdown and the discharge kept acceleration at least remained constant.
l.83: why not use the actual velocity at the divide instead of zero? It is unlikely the velocity changed much during this period.
l.92: What is the refinement? It would be good to put an actual number to get at least the order of magnitude in the text.
Fig.1: the blue line for A-B on panel 1 is hard to see
Fig.2 caption: Why grounding line is displayed on panel c?
Table 1 and text lines 172-179 : it would be great to add the total number of experiments performed as part of the WARMvar and CODLvar cases. Were all the possible combinations tested? If not which ones were tested and how was that decided?
Fig.3: it would be good to add the years at the top of the corresponding columns
Fig.4 caption: for which experiement are the vertical black dashed lines?
l.220: “there is A continued retreat”
l.300: It looks like the glacier continues to lose mass, so what does “stabilized” mean in this context?
Citation: https://doi.org/10.5194/egusphere-2024-673-RC1
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