the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Simulating the Laurentide ice sheet of the Last Glacial Maximum
Jorge Alvarez-Solas
Javier Blasco
Marisa Montoya
Alexander Robinson
Abstract. In the last decades, great effort has been made to reconstruct the Laurentide Ice Sheet (LIS) during the Last Glacial Maximum (LGM, ca. 21,000 years before present, 21 kyr ago). Uncertainties underlying its modelling have led to large differences in fundamental features such as its maximum elevation, extension and total volume. However, the uncertainty in ice dynamics and thus in ice extension, volume and ice-stream stability remains large. We herein use a higher-order three-dimensional ice-sheet model to simulate the LIS under LGM boundary conditions for a number of basal friction formulations of varying complexity. Their consequences on the Laurentide ice streams, configuration, extension and volume are explicitly quantified. Total volume and ice extent generally reach a constant equilibrium value that falls close to prior LIS reconstructions. Simulations exhibit high sensitivity to the dependency of the basal shear stress on the sliding velocity. In particular, a regularized-Coulomb formulation appears to be the best choice in terms of ice volume and ice-stream realism. Notable differences are found when the stress balance is thermomechanically coupled: the LIS volume is lower than for a purely mechanical friction scenario and the base remains colder. Thermomechanical coupling is fundamental for producing rapid ice streaming, yet it leads to a similar distribution of ice overall.
Daniel Moreno et al.
Status: final response (author comments only)
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RC1: 'Comment on tc-2022-215', Julien Seguinot, 21 Dec 2022
Summary
The physical representation of glacier sliding (or basal friction) in numerical ice-sheet models remain a source of uncertainty and discrepancy. Some studies tune an empirical friction coefficient to velocity observations, while others use a more physics-based, albeit heavily parametrised, approach. In paleoglacier modelling the latter is typically preferred in the absence of velocity measurements.
This manuscript provide a strong case for the validity of this approach. The authors use basal friction physics of increasing complexity and assess modelled basal velocities. While most previous studies on the Laurentide ice sheet (and Cordilleran and Innuitian ice sheets) have focused on reproducing known ice-sheet extents, the authors introduce a novel approach using paleo-ice streams inventories as validation data instead. Expectedly, the different friction laws tested have little effect on ice-sheet extent, more effect on ice thickness, and a major effect on glacier velocities, demonstrating the value of complementary geomorphological datasets.
The authors make an assumption of steady-state. This is a logical choice, but it would deserve more discussion, I think (see general comment). Besides this point, I found the article very well redacted. The introduction is concise but exhaustive, and model physics are described in just enough detail to follow stand-alone without familiarity of the ice-sheet model Yelmo. Parts of the discussion could be shortened or improved (see specific comments) but overall, I enjoyed reading the paper and felt relief that the more advanced physics (water-dependent friction, similar to what I have used in my work) produced the most realistic velocity fields!
I fully support publication, and adress further comments directly to the authors.
General comment
My only general comment is about your steady-state assumption. I think this is a reasonable choice, but it also implies simplifications which I think should be discussed. Specifically:
- As mentioned in the introduction, the LGM ice-sheet configuration was likely transient. How representative is your modelled steady state of this transient configuration, what are the potential shortcomings?
- You mention several times that the simulations attain a steady state. How was this assessed? Are glacier velocities stable? Would you consider adding a line plot showing ice volume, area or thickness (maybe velocity or discharge if relevant)?
- Perhaps most importantly, how were timing inconsistencies taken into account in the validation step? Your target period is 21 ka. The LGM extent span a larger interval of 29-17 ka (numbers from your introduction), and ice streams inventories likely cover an even larger period including pre-LGM remnants and deglacial ice streams.
Specific comments
Some of this will be redundant with the above general comment.
- l. 120-122 "a minimum value N = δP0": which value was chosen for δ? This is an important parameter strongly influencing the results.
- l. 195-197 "we tuned the ice-sheet model to obtain a fully-developed ice stream network (e.g., Margold et al., 2015)": as this step influences results presented thereafter, I think it should be described in much more detail already here or in the methods. Is this a visual or quantitative comparison, please explain how it was done. Also, please explain precisely which data (implied by "e.g.") was used for validation.
- l. 275 "a quantitative comparison": do you mean "qualitative"? If not, please explain how the data-model comparison was computed.
- l. 296-302 "we find an interesting behaviour": I could not really understand the interesting behaviour and the role of SSA here.
- l. 361-364 "Figure 5 depicts... " I find that this paragraph largely repeats the results and is perhaps not necessary
- l. 365-368 "an interesting behaviour [...]": again, I think this is mostly just a description of the results (and I did not really understand this part).
- l. 371-372 "For an idealized scenario [...] the behaviour imposed by Eq 7": this statement repeats the methods parts.
- l. 389-391 and 396-397 "a fully-developed ice-stream network was simulated [...] without any thermomechanical coupling requirements.", "hydrological processes are necessary to achieve physical realism". These two statements appear contradictory. It would help if you can be more explicit about the added benefits of water-dependent friction (also in the discussion part).
- l. 398 "Notably, ice volume above flotation reached a constant equilibrium value for the three cases under consideration.": This conclusion is not supported by the results. Please remove, or add a figure.
Technical corrections
- In the title, and throughout the text, you use "Laurentide ice sheet" to refer to what many paleoglaciologists would call the "North American ice sheets" or "North American ice-sheet complex" (including the Laurentide, Cordilleran, and Innuitian ice sheets). In paleoclimate literature though, "Laurentide" generally includes all of North America. Of course, please use the terms you find most appropriate to your target audience. But I wanted to let you know that to some readers, it almost sounds like you are downplaying your results and it could be worth clearly mentioning the scope of your study somewhere early on.
- l. 38-42, 343-345, Table 2, etc. While you give s.l.e. estimates in the intro, you use ice volumes in cubic kilometres in the rest of the text. Personally I found the s.l.e. values very useful as they are easier to compare to each other and to the known global total of ca. 120 metres, and would recommend sticking to them throughout the text.
- l. 98 "Here, *Q* is the net heat flow": *Q* is missing from the preceding equation.
- l. 99-100 "Bueler and van Pelt (2015)": brackets are missing around the reference.
- l. 218 "coulomb" : should have a capital C.
- l. 267, etc "Thermomechanical coupling": this phrase is used throughout the manuscript, but it can be a bit counter-intuitive. In glaciological literature "thermomechanical coupling" is often used in e.g. "thermomechanically coupled SIA, to describe the thermal feedback on ice rheology, which is included in all your simulations. I would recommend using a different phrase or introduce regular reminders of what you mean with "thermomechanical coupling", otherwise a reader only skimming through figure captions and conclusions could get it wrong.
- l. 280 "Fig 2f as compared to Fig 2": should this be "Fig 8a as compared to Fig 2f"?
- l. 296 "this idea": this is a result, more than an idea. :-)
- l. 313-314 "Quantitatively": is it "qualitatively"? Same commend as above.
- l. 335 "a purely thermodynamic perspective": I can't understand what you mean with these words.
- l. 341 "ICE-6": ICE-6G
- l. 356-357 "surface elevation...our ∼ 4.5 kilometres thickness.": in this statement you compare surface altitude to thickness, which are different things. Besides, which run does the second number refer to?
Apologies for the very late review, happy celebrations, and good luck with your future endeavours!
Citation: https://doi.org/10.5194/tc-2022-215-RC1 - AC1: 'Reply on RC1', Daniel Moreno-Parada, 13 Feb 2023
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RC2: 'Comment on tc-2022-215', Niall Gandy, 21 Dec 2022
Review of “Simulating the Laurentide ice sheet of the Last Glacial Maximum” by Moreno et al.
Moreno et al. present a series of simulations of the North American Ice Sheets, exploring the resulting ice sheet volume, area, and velocity pattern from varying the ice sheet sliding law. While the results show only limited variation in the ice sheet volume and area, the ice sheet velocity pattern is sensitive to the sliding law used.
This manuscript is well presented, with clear text and figures. Most importantly, the work as been clearly and comprehensively described, and the results are presented and discussed in good detail. I recommend that the manuscript is published following minor corrections/clarifications (detailed below).
I hope you have a good Christmas break, and I hope to reading a revised or published version of the manuscript in the new year.
General Comments
A direct visual comparison between the ice stream dataset from Margold et al and your results here would be useful. Essentially, it saves the reader flicking between browser tabs, and I expect it would show clearly the match you have described in the text. Within this it would be good to discuss the potentially transient nature of some ice streams, and how this might effect the empirical mapping, your modelling results, and the comparison between the two.
I think it is reasonable that you have run your simulations to equilibrium, but it will probably have an effect on your results, given ice stream sensitivity to climate forcing. This should be discussed in the text.
Minor points
Title: While I would often refer to the ice simulated here as the “Laurentide”, more formally I would opt for “North American Ice Sheets”. Laurentide is neater, North American Ice Sheets is clearer. If you stick with Laurentide consider a very brief mention in the Introduction.
Ln 25: “Strictly speaking”… This sentence isn’t clear to me, please rephrase
Ln 28: References for the initial assertion? Perhaps Calov et al., 2002, Tarasov and Peltier, 2004, or others?
Ln 36: Extension > extent – and other uses later in the manuscript
Ln 36: “largely differ” > “differ largely”
Ln 44: If the variable ice thickness is through a surging/instability mechanism say this explicitly. This paragraphs touches on the idea that ice stream instabilities could significantly influence the ice sheet configuration, but more detail/references would be appreciated.
Ln 71: Please provide some further justification for these parameter values.
Ln 95: It’s pretty typical to ignore horizontal water transport, but not always (e.g. Gowan et al., 2018). It’s worth justifying this simplification.
Ln 104: What happens to excess water beyond the 2 m limit? Does it accumulate but is ignored, or disappear?
Ln 173: By averaging 11 PMIP simulations you remove the consistent climatology provided by a single model. Is this important?
Ln 173: Are all 11 PMIP simulations using the same ice sheet reconstruction?
Ln 196: As your climate forcing in Figure 1a and b inherently contains a previous ice sheet reconstruction which broadly matches the empirical reconstruction, how surprising is it that your simulated extents are okay?
Figure 2: It would be good to see a direct visual comparison to the Margold ice stream reconstruction. It would also like to see one section of the ice sheet in more detail to show the nature of ice streaming at the margin. Maybe there could be a separate plot of the Hudson Bay and surrounding ice streams?
Ln 256: A table summarising key statistics of Linear, plastic, and coulomb simulations might be helpful to compliment to description in the text. A quick lookup for the equilibrium fluxes, mean/min/stdev velocities would be appreciated, perhaps an extension to Table 2?
Ln 275: Quantitative or Qualitative?
Figure 6: This is a very useful figure. There seems to be an edge effect stripe in panel b and c (around 100m Hice). Do you know what is causing this?
Figure 7: This figure is good at showing the model’s behaviour in general, but the visual comparison between sliding laws is tricky? Perhaps you could experiment with plotting curves from multiple simulations on the same panel.
Ln 341: ICE-6G
Ln 410: Where will the data from the simulations be available?
Citation: https://doi.org/10.5194/tc-2022-215-RC2 - AC2: 'Reply on RC2', Daniel Moreno-Parada, 13 Feb 2023
Daniel Moreno et al.
Daniel Moreno et al.
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