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| 01 Aug 2025
The role of Antarctic sea ice in the Earth system: Perspectives informed by 130,000 years of sea ice records
Abstract. Antarctic sea-ice cover reached historically low levels in 2023, consistent with the simulated decrease in sea-ice extent in response to anthropogenic warming. Antarctic sea ice is closely linked to multiple components of the Earth system, thus its demise could precipitate widespread, cascading changes across the cryosphere, atmosphere, and ocean. However, the nature and strength of these interconnections are poorly understood, and they are often inadequately represented in models. In this review paper we use modern observations, models and paleoclimate archives covering the last glacial cycle to gain insights into how reductions in sea ice may affect other components of the Earth system. We review how Antarctic sea ice interacts with ocean and atmosphere circulation, ice sheets and ice shelves, marine productivity, and the carbon cycle over the last glacial cycle, for which we have the most robust sea-ice reconstructions. The review finds strong evidence from theory and models for impacts of Antarctic sea ice on the Earth system. Paleo-proxy reconstructions provide examples where changes in sea ice co-occur with changes in the carbon cycle, marine productivity, and ocean circulation. However, challenges remain in isolating the impact of sea ice in a highly interconnected system.
Competing interests: Some authors are members of the editorial board of Climate of the Past.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-3504', Anonymous Referee #1, 01 Sep 2025
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RC2: 'Comment on egusphere-2025-3504', Anonymous Referee #2, 02 Oct 2025
Review: Chase et al. “The role of Antarctic sea ice in the Earth system: Perspectives informed by 130,000 years of sea ice records”
This paper is an interesting summary of how Antarctic sea-ice has interacted with several components of the climate system in the last glacial cycle, and would be potentially a very useful contribution to CP. I particularly like that it covers such a broad range of processes, and that these are brought together at various points. To me this wide scope is also the main weakness – I have come away with the view that most sections seem a little superficial / lacking depth – this paper covers so much ground (or, rather, water!) that I think it needs to be considerably longer to do justice to a very substantial body of past work. In particular, the need to review modern processes to provide context for paleo studies, means that the modern part is lacking detail and the paleo examples are perhaps too few or lacking sufficient discussion of their important nuances (especially uncertainties / limitations). More figures are also needed – both schematics showing processes/feedbacks, and also to show results of key studies that are discussed in more depth.
Some more specific comments follow.
L91 “Sea-ice presence along the Antarctic coast buttresses ice shelves, stabilising the ice shelf and associated glaciers (Christie et al., 2022).”
Whether or not sea-ice can buttress an ice shelf is still uncertain & under debate. For just one example see Surawy-Stepney et al. (2024) who found negligible buttressing of Larsen B by sea-ice.
Please revise to present a more balanced view.
L113: subheading here? Since the subsequent text suddenly dives from a general discussion into more detailed discussion of LIG sea ice and SST.L122-128 explanation of marine isotope stages & glacial cycles: seems unnecessary for CP.
L129 “ Sea salt sodium measurements in ice cores”. Only one ice core (EDC) is implied in Fig 2c. Are there some others to include?
L137-140. Ranges of LIG SST: what are they? Are these the ranges between different studies?
Fig 2. The small text on the map is hard to read, so a legend would be helpful for the different sea-ice extents.
L160 I think the detail in this paragraph is not really justified by the uncertainties. To me it’s only really the Scotia sector that stands out as responding differently, in Chadwick et al. 2022 Fig 4.
L174 Now we’re back to a general view and paper objectives. Should this be moved to before L113? Or move the material in Lines 113 to 173 to dedicated sections later on.
L196 Bathymetry might play a role in addition to Ekman divergence. See Tamsitt et al. 2017
L236 Nice comparison with warming scenarios, but if you’re wanting to compare with a warming scenario, it might be better to pick more than one study. What do the CMIP6 models predict?
L244 / Fig 4c. The %AAIW reconstruction seems to be very noisy, and so not very convincing. Unless you can show the uncertainties are tiny, I suggest not to include this record & related discussion.
L281: Puzzle of where AABW formed in LGM. If we suppose dense water from brine rejection sinks to the seafloor, does it really matter whether it formed on the shelf or in the open ocean during the LGM? The end result would still be the same, i.e., more AABW filling the abyssal Southern Ocean?
L286: Foram deposits could presumably represent short episodes with polynyas, they don’t have to be continuous?
L302 “Weakening of NADW” do you mean weakening of NADW production, or export, or somehow weakening of its physical properties?
L335 “This unexpected behaviour… “ I’m not convinced that this study alone (Shub et al. 2024) from the Brazil margin in TII is itself sufficient to question a direct link between sea-ice production/extent and AABW volume.
L365 / Section 3.1 (Sea ice as a buffer for ice shelves). As noted above (L91) I think this issue is still debated in the glaciology community. This section should be expanded considerably to reflect other opinions, or perhaps omitted.L398 “Increased stratification is also observed at the Totten Glacier…” Increased when and relative to what?
L418 “… during glacial climates when grounded ice expanded across the continental shelves
419 around Antarctica…” It’s really not certain that the ice sheet advanced to the shelf edge everywhere. See the Bentley paper you’ve cited, or Klages et al. 2017.L428, attributing sea-ice increase to greater ice sheet melt. This logic might have sounded reasonable prior to 2016, and indeed motivated a whole suite of hosing studies. But the ice sheet melting hasn’t slowed down as far as we know, yet the Antarctic sea ice is now at its lowest extent – so this argument is now seeming weaker.
L431-437 These are common features across many other studies too. See Swart et al. 2023 & refs therein. Or, Chen et al. 2023 for an ensemble view.
L446-449, Subduction of water that has been heated by insolation: this was already covered earlier (L414 approx) but different references are cited.
L442-444, L456-456: first it’s stated that increasing stratification can allow warmer CDW to access ice shelf cavities, then it’s later stated that increasing stratification prevents CDW mixing onto continental shelves. This seems contradictory, and overall I found this section (3.3) is struggling to summarise concisely a large body of research on the environmental drivers of ice shelf basal melt.
L485-487 “This process may also have occurred westward in Prydz Bay where a similar sea-ice increase was observed”. Due to increasing super-cooled ice shelf water from where? Also not sure where do you mean by “Westward in Prydz Bay”, are you meaning west of Wilkes Land?
L497-L499, use past tense here.
L512. An increase in IRD at one site could have many different interpretations that are not necessarily indicative of ice sheet retreat. Certainly the next sentence (“This sequence of events supports the link between sea ice as a buffer for ice shelves and the potential role of decreased sea-ice cover in facilitating basal melt and ice shelf disintegration”) isn’t justified by the evidence presented here. If sea-ice provided zero buttressing to ice shelves, you would still probably end up with a time series very similar to Fig 5.
L537 Should salinity be in this list of drivers?
L562 Why are ghost flux experiments the most compelling approach?
L583 “… did nothing”. Probably it did something, do you mean “had a negligible impact” or “not statistically significant” or etc.
L613 “However, recent proxy records suggest the sea-ice effect on the SHW is of second
Order.” That’s not supported by a following sentence “They [Kohlfield2013] concluded that making inferences about wind shifts based on paleo-evidence alone is difficult, as the data are consistent with multiple scenarios.”L621 “… a poleward shift of the SHW during the last deglaciation from the latitude of SST gradients in the Southern Ocean” Not sure what is meant by the latitude of an SST gradient.
L647-650 If the ssNa signal in 5 cores examined by Buizert et al. 2018 is “weak and not observed in all cores” then how is the next statement justified (“Yet, the sign and timing of the sea-ice signal, and its relationship to the inferred wind changes, do not support a strong link between changes in sea ice and changes in atmospheric circulation over millennial timescales.” I’ve lost track of the logic here.
L663 proxy record – should that be plural?
L670 Rather than new modelling efforts, I would start by looking at the wealth of existing climate simulations to see if any pattern can be found beyond the SHW.
L700 Interesting that sea ice supplies iron to the ocean. Where does that iron come from originally? How does it provide a link between glaciers/sediments and the ocean (L721)? Is this iron flux greater than the flux from icebergs and subglacial melt discharge?
L731 “the length of the ice-free season determines the upper limit of opal burial in the underlying sediments”… not sure I follow this sentence, what is the upper limit of opal burial?
L758-786. There’s no discussion here about the potential contribution of changes in iceberg nutrient fluxes to past changes in Southern Ocean productivity. Should be mentioned, even if to say it’s a negligible part of the budget.
L833 Can you quantify what is “long term”?
L849-L852 This seems very speculative!
L867-869. Have there been Earth system modelling studies of this process? They would provide some useful insights.
L911. This is an idealised experiment – but in reality would there be much upwelling under sea ice, or instead would you expect that either (1) the region of Ekman-driven upwelling would move further north, beyond the sea-ice edge, in tandem with a northwards shift in SHW, or (2) if SHW haven’t moved, then the presence of sea ice reduces Ekman divergence and upwelling. Either way there’s less upwelling under the ice.
L944 “The lag between the expansion of sea ice and the decrease in CO2” That lag (of 3ka) is really not clear to me in Fig 4 and is the dating of the marine sea-ice records really good enough to resolve it?
L979-L981 “While we focus here on modern studies, as well as paleo-records spanning the last glacial cycle (150 – 0 ka), processes described are also applicable to more ancient, recent, and future conditions” What is the difference, in this paper’s context, between modern and recent?
L993-995 “To increase our knowledge and further improve our understanding, we need paleo-records with higher temporal resolution and better chronologic control in more areas of the Southern Ocean” I don’t think any reader of CP would disagree with that, but obviously that’s a big task… Can you help by recommending some particular regions or time periods that should be targeted? I also wonder about the chronological control – yes this is an important issue, particularly when comparing multiple sites or data vs models, but it was really skipped over in this review.
Table 1: Not referred to anywhere in the text, and not very helpful as it over-simplifies processes that are already covered only briefly in the text. Suggest to delete it.
ReferencesChadwick et al. 2022, Clim Past, https://doi.org/10.5194/cp-18-1815-2022.
Chen et al. 2023, GRL, https://doi.org/10.1029/2023GL106492.
Klages et al. 2017, PLOS One, https://doi.org/10.1371/journal.pone.0181593.
Li et al. 2021, GRL, https://doi.org/10.1029/2021GL094317.
Shub et al. 2024, PP, https://doi.org/10.1029/2023PA004635.
Swart et al. 2023, GMD, https://doi.org/10.5194/gmd-16-7289-2023.
Tamsitt et al. 2017, Nat Comms, https://doi.org/10.1038/s41467-017-00197-0.
Trystan Surawy-Stepney et al. 2024, https://doi.org/10.5194/tc-18-977-2024.Citation: https://doi.org/10.5194/egusphere-2025-3504-RC2
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- 1
[General comments]
In this manuscript, the authors comprehensively survey numerous papers on the role of sea ice in the Southern Ocean in the climate system, and succeeds in collecting vast amounts of information on the topic. The authors' attempt to provide an excellent overview on the issue surely has a potential to be appreciated by scientists of diverse research fields, and in particular, their effort to discuss how sea ice has interacted with other subsystems during past large-scale climate changes from a paleoclimatological perspective should be a novel and scientifically worthwhile conception. However, I have to raise serious concern that the specific methods used to organize the research findings and the manner to present them to readers are not necessarily optimal. The manuscript tries to cover vast amounts of phenomena and information; of necessity, the authors should pay close attention to methods for organizing and communicating them. I am certain that this manuscript will be dramatically improved and will realize its potential with a proper methodology to convey the authors' arguments in a clear and accessible manner. In addition, I also point out that the manuscript would be further enhanced by including descriptions on interactive feedback between Antarctic sea-ice and other Earth system components and a discussion on the role of paleo studies in the improvement of our knowledge about the Earth system.
[Major issues]
1. Each section is poorly structured.
In each of the main sections, descriptions based on varying methodology (e.g. theory, modelling, modern observation, and paleo evidence) appear in a semi-random way, and besides, they have different structures (e.g. some sections have subsections, but others not). Therefore, it would be troublesome for readers to grasp the contents smoothly. To clarify the contributions from different methodologies, I would suggest several ideas for improvement.
- It would be helpful that each section has a table that summarizes the survey corresponding to the section. The potential table would have a similar structure to that of current Table 1; namely, columns for hypotheses based on theory and/or modelling, modern observation, paleo evidence, and so on. Then, rows would be allotted to important processes (rather than sub-components) that are relevant to the section topic.
- All the main sections should have a common subsection structure. The structure might be in line with the section's table that is suggested above.
- I would point out that the current manuscript only has insufficient visualization. I believe it will be certainly beneficial if the manuscript has more schematic illustrations that show not only the name of components but also key mechanisms or processes discussed in the main text. Fig.1 of Lannuzel et al. (2020) might be a good example of such a schematic.
2. Table 1 should be updated accordingly.
I would suggest that the general summary should have more visualized expression that conveys the information in a more intuitive way. For example, direction of changes, correlation strength, uncertainty/reliability, and so forth would be depicted as symbols with different sizes, colors, thickness, etc. An example of such an illustration might be found in Table 1 of Henson et al. (2022; https://doi.org/10.1038/s41561-022-00927-0).
Apart from this, Table 1 also raises some conceptual concerns (see item #5).
3. The manuscript contains a huge number of acronyms.
I would suggest that the authors should show a list of the acronyms that are used in the text.
4. "Antarctic sea-ice .... as drivers of substantial Earth system changes." (Lines 113-115)
As represented by this sentence, the authors' only discuss "one-way" influences of Antarctic sea ice on other components of the Earth system. However, the relationship between the sea ice and other components should be interactive: what determines the sea-ice extent? or which controls which? In other words, not only a correlation between them but also their causal relationship should be discussed, which may be based on theoretical inferences and/or fact-based (e.g. timing analyses) discussion.
5. What is the significance of paleo studies?
In some rows of Table.1, especially in the Atmosphere row, (some of) the paleo observations are treated as counter evidence, rather than positive proof. What is the background "philosophy" of this manuscript that gives more weight to other methodologies, although it is defined as "perspectives informed by 130,000 years of sea ice records"? Of course, it is well known that extracting meaningful signals from paleo indicators is not always straightforward, and that non-negligible uncertainty often arises in their interpretation. In a general sense, when reconstructions from paleoenvironmental research conflict with understandings based on other methods, what criteria should be used to judge, and how should we move on? If this manuscript handle and discuss this fundamental issue, it will make a major contribution to research communities.
6. Section 5.2: dimethyl sulphide
It seems to be somewhat peculiar that the authors suddenly start a topic about a particular chemical compound, although they deal with large components or subsystems in the Earth system in other (sub)sections. Is there any particular reasons for that? Otherwise, the section 5.2 would look anomalous.
[A comparatively minor issue]
- l.68 regime shift
What exactly does this "regime" indicate? Do we find any evidence or trace of a similar regime shift in the paleo-record? If yes, descriptions about recorded shifts should be added. If not, the reasons why such a shift is not recorded in the history of large-scale climate changes.