Re-review of "biogeodyn-MITgcmIS (v1): a biogeodynamical tool for exploring
climate steady states with a new global-scale ice sheet model" by Moinat et al.
Overall comments
Having reviewed an earlier version of this manuscript, I can confirm that important updates have been implemented and the text has been considerably improved. I particularly appreciate the inclusion of the Permian-Triassic test case. Nevertheless, I find a large number of remaining issues that I think need to be addressed to bring the paper to a point where it can be published in GMD.
The most important point is how the new couplings influence the behaviour of MITgcm (see my comment l382-461). A detailed comparison should be included for the pre-industrial. Results in Table 3 for the Permian-Triassic may indicate limited influence. If this is also the case for the pre-industrial, it could be that the whole 6-page comparison l382-461 is basically a repetition of MITgcm validation exercises already done before. Otherwise, the comparison should focus on differences between biogeodyn-MITgcmIS and MITgcm.
Please see further detailed comments below.
L1
Suggest "Modelling the climate system on multi-millennial timescales" to be clear about the timescale of interest from the start.
L8 "we added asynchronous couplings with a vegetation model (BIOME4), a hydrological model (pysheds)"
According to Ragon 2024, coupling with BIOME4 already exists. Be clear about what is new in the current work.
l13 "initialized from bedrock topography" is confusing. Maybe "initialized with zero ice thickness".
l16 "we discuss also a Permian-Triassic simulation."
Add a brief summary of the results in the abstract.
l19-33
I am still not convinced about opening the introduction with focus on tipping. I think a better beginning would build on material l33- "We are interested in a modeling setup ..."
l40 "coarse spatial resolutions and simplified parameterizations"
At least the first part arguably also applies to your model. Maybe "reduced complexity and simplified parameterizations".
l42 "Nowadays, there are few CMIP6 models with interactive ice sheets"
Either open reference list with "e.g." or include all CMIP6 models that have interactive ice sheets (MPI-ESM, NorESM). You should also cite https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021PA004272, which uses CESM2 with an asynchronous coupling to BIOME4.
l49- "Here, we propose a simulation tool, called biogeodyn-MITgcmIS"
Why not list vegetation and ice sheets already in this first sentence?
l87 "KPP scheme"
What does KPP stand for?
l108 "a smoothing procedure is needed"
This calls for a reference or further explanation of the smoothing procedure.
l114 "surface air temperature (SAT)"
Is this the 2m air temperature?
l114 "Additional inputs are soil depth and texture"
Not clear where these come from. Explain.
l117 "The albedo"
Is this the same as "bare-surface albedos" on line 92? Clarify (singular vs plural?).
l120 "BIOME4 follows the principle ..."
You may consider moving this general model description before information about the re-coupling "The albedo and the vegetation map obtained as outputs ..."
l142 "spatial resolutions of around 2◦"
Not quite the same as 2.8◦. Maybe "Since we are interested in simulating ice sheet dynamics at relatively coarse spatial resolution"
l168
Maybe "Ice deformation can be simplified by neglecting basal sliding, by considering that the only important type of deformation is vertical shearing, and by assuming a power-law shear thinning viscous rheology"
Equ. 4
I think gradient(Z_B) is not defined.
l201 "Ta is the temperature at z= h"
Is this the same SAT in l114? Make consistent.
l208 "temperature Ta at h= 2 m"
Is this the same as SAT?
Remove "surface" to remove ambiguity with specification 2m.
"The required input is the air temperature Ta at h= 2 m"
l210 "During each model iteration"
What is the time step between iterations? It does not make sense to include the lapse rate update on a daily basis.
L214 "MITgcm land module does not include accurate snow physics"
I miss an explanation here or elsewhere how snow and ice albedo or handled in the coupled setup.
l217
"to obtain the accumulation rate in [m s−1 ice equvivalent].
l218 "the surface air temperature"
Rather "the 2 m air temperature"
l220
Already mention briefly here what happens with rain over the ice sheet? Since this is not part of the surface mass balance (as opposed to some PDD approaches where rain enters potential refreezing in the firn layer), how is it handled further (routing, combined with runoff, ...)? Extended description should follow in section 2.5.
l223 "Isostatic" --> "Isostasy"
l235 "where τa is typically set to 3000 years"
I still don't understand why you need this time delay. If you are interested in the ice sheet steady state, you can speed up convergence by just setting τa to zero.
l238 "where Htopo is given by the topography file"
What does the topography in the file represent? I suppose for ETOPO you assume balance with the current load. How do you initialise for an unknown ice sheet configuration? Explain!
l239 "also the SAT"
Same as Ta in 208?
l240 "by estimating the lapse rate dT/dz"
I think with Equ 12 you are estimating the temperature correction, not the lapse rate itself. Clarify!
l242 "where Tnew is the temperature after the lapse rate correction"
What is Told? Also coming from The MITgcm output of the previous run? Where the previous run is the run that provides that forcing for the ice sheet run? Clarify.
l242 "The lapse rate is computed at each ice-sheet grid point using the
MITgcm output of the previous run"
Is the previous run the same as the previous iteration (as used in l210)? Clarify.
l244 "based on the corresponding altitude"
How? Maybe using bounding pressure level altitudes around the current surface elevation?
l245 "since some new ice sheets"
--> "since some new ice sheet grid points may have"
l245 "the amount of water that has been exchanged with the ocean is estimated"
Explain how this is done in detail!
l246 "as it is done in North Atlantic hosing experiments"
There are a few different approaches documented in the Jackson paper. What exactly are you doing? In my understanding, the ice sheet is presented to the climate model as a steady state boundary condition. This means that no additional freshwater fluxes should be coming from the ice sheets in the climate iteration other than routing total precipitation over the ice sheets similar to any other land surface. Is this the case?
l248 "updated coastlines defining new topography (including ice sheet height)"
The other way around makes more sense to me:
updated topography (including ice sheet height and sea-level) defining new coastlines"
l256 "(unless elevation reaches values below−2000 m)"
Set in parentheses to avoid interpreting this as "such cases" in the next sentence.
l256 "we re-assign the elevation"
Important to mention if this changes the ocean volume and sea-level, and if so, how.
l279 "redirected into the ocean via the runoff"
How is energy conservation handled? Is the energy needed to melt the snow/ice taken from the ocean? Where?
l281 "in the first steps of the coupling procedure"
Not clear why this is only needed in the first step. Explain.
l281 "precipitation storage area"
It is not clear to me what the precipitation storage area is and how it can be used to correct for a mass flux imbalance. Explain.
l284 "Pitot is the total rain precipitation"
Is that maybe total precipitation? Otherwise, why is snowfall always larger than rainfall? We will also need to know how rain is treated over ice-covered grid cells. Is it just routed without interaction?
l287 "Coupled MITgcm atmosphere-ocean-sea ice-land Setup"
This was defined as "Coupled MITgcm Setup"
l296 "the ocean point becomes a land point"
Does this imply a change in the ocean volume and sea-level?
l296 "(up to−20 m of depth)"
Consider moving this to after "advancing ice flow over shallow ocean" in l295.
Fig1
The numbering 1., 2., ... is confusing as it interferes with the iteration count (initial, 1) and with BIOME version number. Are you using these numbers at all. Remove?
"Coupled MITgcm setup" --> "Coupled MITgcm Setup" in box 1 and 5.
l310 "salinity and sea temperature at all ocean levels are averaged over the last 30 yr"
Is this ocean model output from the first MITgcm iteration? Why do they need to be averaged? Why not restart the model from the end of the last iteration? Are there other changes applied to these fields, like new masks?
l312 "are given back to the MITgcm to run the whole coupling process at least twice, so that the GCM has time to adjust to the new input files"
A bit confusing. You could run MITgcm long enough so that it "has time to adjust to the new input files". But I think what you are saying is that you iterate at least twice (with updated input files each time) so that the whole asynchronously coupled system converges.
How does "at least twice" go together with your own experience for the present day in l372: "Five iterations of the procedure illustrated in Fig. 1 were necessary"?
l320 "3.1.1 biogeodyn-MITgcmIS initial conditions"
I feel many aspects presented here are not well placed in a "Results" section. Description of the initialisation and tuning may be better placed in a section about experimental setup before discussing results.
l321 "To start the first run of our simulations"
Maybe you have to first explain what simulations are planned in a section called "Experimental setup" or similar?
l327 "A smoothing procedure"
Details needed. Describe it in the modelling part.
l330 "with the addition of an isostatic correction as in Paxman et al."
Are you using the Paxman procedure with your LLRA model, or are you using their corrections derived with another Earth model? If the latter, then there is an inconsistency expected between the re-loaded bed and observations. Please explain.
l335 "PANALESIS or other reconstructions directly provide bedrock elevations"
Do they typically provide unloaded or loaded bedrock elevation?
l338 "albedo values for [..] snow and ice have been fixed"
I missed where this was explained. How are they chosen?
Does that mean that your tuning happens after the first iteration? If not, what is the impact of the coupling on the uncoupled climate state?
Figure 2
We don't know what run1 is. Please describe the experimental setup in a separate section.
l343 "Evaluating the surface mass balance produced for Antarctica"
Why is that only relevant for Antarctica and not for Greenland? Are you inheriting values for Greenland from the AIS calibration?
l349 and l352 "the value obtained in our simulation is slightly lower"
I would argue that more than 25% off is not "slightly lower". Rewording needed.
L357 and Table A1
Please check the units are correct for 'a'. The typical rate factor 'a' in Glen's flow law in the literature is 10^{-15} - 10^{-13} kPa^{-3} a^{-1} or
10^{-24} - 10^{-22} kPa^{-3} s^{-1}.
Since basal sliding is not included in the model, one would expect a need to compensate for it by increased deformation. Does that make sense for the choice of parameter compared to typical values of a?
Table 1.
Could specify the type of grid in first row ('Resolution') instead of NA.
l377 "MITgcmIS needs around 1 hour of CPU time"
Specify for how many model years the ISM is run.
l378 "for all land points."
I thought there was a procedure to limit the calculation to ice-covered points? Why not describe this in the manuscript?
l382-461 "Atmosphere" and "Ocean"
What still doesn't become clear in these sections is how the new couplings influence the behaviour of MITgcm. This should be shown by comparing the pre-industrial MITgcm (without coupling) to biogeodyn-MITgcmIS, e.g. with additional lines and panels in figures 3-8 where possible. It is important to understand how behaviour and sensitivity of the model changes with the new couplings.
l385 "close to the real data of 13.7◦C"
"real" --> "measured"?
Maybe this value should be added to the table?
Clarify that this the the 1850 value.
l386 "the global mean SAT of 15.9◦C"
Clarify that this is now for another period.
l387 "Earth Climate Sensitivity (ECS)"
Typically "Equilibrium Climate Sensitivity (ECS)"
l395 "higher temperatures than observations and CMIP models"
We need to know if this due to the ice sheet representation in biogeodyn-MITgcmIS or already in the Coupled MITgcm Setup.
l396 "where the ice sheet elevation is fixed to observed values"
What happens in biogeodyn-MITgcmIS when you fix the ice sheet to observed values?
l490 "The total volume is 24.5·106 km3"
You should comment on the absence of other glaciations (e.g. high-mountain Asia, Canadian Arctic, Alaska, ...), possibly not captured due to the coarse model resolution?
l492 "after smoothing to the same spatial resolution"
How is that done? To compare total volumes, you should integrate the ice thickness, which can be done independently on the different grids. For local comparison on the grid cell level, you should conservatively interpolate to a common grid.
l493 "This overall agreement with observations [...] is reflected in"
There are two different types of agreement. The matching total volume, which you have basically tuned for and the match with spatial distribution, which you are testing with the correlation. The first cannot be "reflected" in the second.
l506 "and measurements of the ice sheet thickness."
Uncertainties in ice thickness are not relevant when comparing surface elevation in your model against BedMachine. Surface elevation of the ice sheets can be measured with very small uncertainties.
l507 "we can conclude that our model reproduces the first-order characteristics of the ice sheets reasonably well"
I fully agree with this statement.
l524 "slightly lower SAT and higher sea ice extent"
Can you identify the reason for these changes? Is it ice sheet elevation, albedo, runoff? Is there a similar influence in the present day?
Figure 13 and Figure 14. Can these be plotted like Figure 12, to get a more familiar and comparable view?
Figure A1.
Consider inserting the full ERA5 as middle panel in lower row, to get a similar look as what I suggest for Figure A2.
Fix gaps across the dateline?
Figure A2.
On first view, all panels look the same, which is probably the idea. But it doesn't help much to identify the differences. Add a difference plot (against ERA) in the remaining panel to reveal where run2 disagrees.
Figure A4. The Antarctic panels are flipped in the vertical (compare to correct version in Figure A7.).
Also consider to rotate all panels for a more recognisable view (North pointing up in the Arctic, South America on the left for the Antarctic)
Something is strange with the colorscale. It is difficult to see where ice-free ocean is. Why is colorscale clipped to 0.2? Consider a color that does not appear in the colorscale to represent ice-free ocean.
Figure A7. Consider to rotate Antarctic panel by 90 degrees CCW for a more recognisable view (Antarctic Peninsula on the left). Since it is hard to make out the shape of the ice sheets (especially for Greenland), consider to show 1. the observed ice sheets, 2. the modelled and 3. the differences in 3 panels per ice sheet. Or at least overlay the outlines of the observed ice sheets. What is the isolated red grid point on the left of Greenland? If it is part of Canada, you may want to remove it from the comparison.
All Figures
Increase all label sizes on axes, legend, colorbar, titles. |
Summary
Moinat et al. present a modelling tool that consists of the MITgcm augmented with additional offline components for vegetation, runoff and ice sheets. The model is run at intermediate resolution and mainly intended for deep time steady state simulations. Simulations under pre-industrial and present-day forcing are compared to other models and observations.
General comments
I have struggled quite some time to understand what this modelling tool is supposed to be used for. It is not until page 6 that this is clearly spelled out ("Since our purpose is to investigate climatic steady states in which ice sheets are in balance with the ocean, atmosphere, and biosphere"). If this is true, the statement should already be present in the abstract and the introduction should be geared towards that purpose. However, it is important to understand that in the real world, an ice sheet is never really in balance with the prevailing climate. While there is also mention of transient simulations, I believe in practice the model can only be used for time slice simulations, and only for those where assuming ice sheets in steady state with that climate is an acceptable assumption. The premise of the presented modelling (operating with climate and ice sheet steady states) excludes the application of the tool to interactions between climate and ice sheets that happen on timescales shorter than the relaxation time of the slowest component (the ice sheets), which is 40-100 kyr (l289). This may e.g. be illustrated by the Greenland ice sheet losing large amounts of its volume in run2 as it is relaxed to a steady state in the present climate, rather than attaining a transient state.
I can see two options for the manuscript to continue, both involving considerable work:
1. There may be value in this tool being applied to simulating deep time climate states (as hinted at e.g. in the title and model name) with steady state ice sheet configurations. In that case, the manuscript has to be strongly refocussed, aspects of transient simulations removed, comparison focussed on the preindustrial and a deep time use case added and discussed.
2. It may be possible to modify the model setup to allow for transient ice sheet simulations. It is common practice to run asynchronously coupled simulations between climate and ice sheet models where the climate forcing is accelerated. In that case the ice sheet component is the 'pacemaker' of the experiment and determines the physical time evolution. In that case more work would have to go into improving the modelling. Adding another example for a climate state different than the PI/PD would also be needed here.
The following comments are mostly independent of this choice.
What I miss in the introduction is a clarifying discussion of limiting factors in ESM modelling: spatial resolution, complexity and timescale. With limited HPC resources compromises are typically made on all three of them with different weight depending on the specific application of interest. What is the benefit of the specific choices made here? There is a relatively high resolution climate component compared to traditional EMICs but very low resolution of the ice sheet components. I don't understand the latter choice, since ice sheet models are typically the least expensive component and it would be feasible to run at a reasonable resolution without slowing down the system considerably. If the ice sheet resolution is not improved, it should at least be made clear that the current setup has a spatial resolution of the ice sheet models that is well below of what is considered appropriate to resolve ice sheet dynamics.
I think it should be demonstrated that the model can be set up for a considerably different climate. For a tool that supposedly can run multi-millennial simulations with ice sheet components it would be reasonable to at least be able to show an LGM configuration. Ideally I would also like to see a warmer climate configuration in addition like the last interglacial or the Pliocene. The setup of the tool may be in principle the same for any other time slice, but as modellers we know that there are always more complications once it is actually done. As is, the pre-industrial and present-day climate are too similar to serve as good validation experiments alone.
The analysis is heavily focussed on comparison to other CMIP models and observations, (where the latter is questionable due to the steady state nature of the modelling). Since I imagine that the uncoupled MITgcm has been validated extensively before, I believe there should be more focus in 3.2.1 and 3.2.2 on comparison to a pre-industrial state of the reference MITgcm without the additional components. This would show how the presented developments change the model behaviour.
Specific comments
Title and model name "a biogeodynamical tool"
I am a bit confused about the term "geodynamical". According to the Wikipedia definition (https://en.wikipedia.org/wiki/Geodynamics), this should involve mantle convection and plate tectonics. If a deep time application with different continent distribution and climate could be shown, this would make a bit more sense.
l2 "climate tipping elements"
The mention of climate tipping elements early in the abstract and in the introduction is confusing. Studying climate tipping points most people are concerned about requires full physical coupling between different climate components, which involves specific time scales, not steady states.
l7 "a complexity level intermediate between EMICs and CMIP-class models"
Consider another description of the complexity level of your model.
EMIC already contains the term intermediate. I would say your model is by definition an EMIC, albeit maybe of relatively high resolution of the climate component compared to many other EMICs (add references).
l9 "offline couplings"
Depending on where the modelling goes, I would suggest to use the term "asynchronous coupling" instead. "Offline" may be read as one-way coupling without feedback on the core climate model.
l10 "(MITgcmIS)"
The naming is a bit confusing. If MITgcm in itself is the climate model, MITgcmIS sounds like it could be the coupled climate-ice sheet model.
l10 "The latter is implemented on the same cubed-sphere grid"
This should be motivates at some point. It also has to be noted somewhere that this resolution is well below of what is considered appropriate to resolve ice sheet dynamics.
l12 "the new ice sheet model and the coupling procedure"
Focus is on the ice sheet component, which is not represented in the title. Consider modifying the title accordingly.
##l12 "We evaluate biogeodyn-MITgcmIS" and l14 "biogeodyn-MITgcmIS successfully reproduces the large-scale climate and its major components"
Offline coupling of the new components (l8) would suggest that the base climate state of biogeodyn-MITgcmIS is identical to the core MITgcm. I suppose MITgcm has been validated before. Clarify what is evaluated here specifically and why another response is expected.
l13 "pre-industrial period and the 1979-2009 period"
Difficult to imagine how to evaluate climate-ice sheet feedbacks for these rather steady periods. How does the model perform when the ice sheets are changing considerably?
l18 "CO2 concentration raises under the present-day climate crisis"
Strange combination in this sentence of future perspective and "present-day". Reformulate.
l32 "simulations cannot reach stationarity in slow deep-ocean and ice sheet dynamics"
Focus only on stationarity seems like an insufficient or even problematic goal when interest is in tipping, feedbacks and dynamics.
l35 "the nonlinear interaction among climatic components"
In your setup, this could only be addressed with high enough frequency of the asynchronous coupling and transient ice sheets.
l46 Add reference to Smith et al. 2021 (UKESM)
l56 "complexity that is in between EMICs and CMIP-like models"
See comment l7
l59 "We will provide a complete description of all the components"
A complete description of all the components of a coupled ESM does not fit in this paper. Maybe you meant a brief description of the climate components and a full description of the ice sheet component?
l74 "SPEEDY is in the intermediate complexity class"
See comment l7
l80 "planetary boundary layer"
Is this level used to provide boundary conditions for the ice sheet model? Usually PDD models ingest 2m air temperature. Discuss this as caveat of the coupling.
l90 "runoff map"
Is this runoff mapping, like a runoff routing scheme? It prescribed the pathway but not the amplitude? Clarify.
l90 "salinity and sea temperature for all the ocean levels"
Sounds like salinity and temperature are prescribed rather than dynamically calculated. Is that the case? Or is this just for the initial state? Clarify.
l90 "Orbital parameters can be set by specifying the obliquity, the duration of the day and the radiative influx from the sun"
Orbital parameters are typically obliquity, precession and eccentricity. I don't think "duration of the day" and "the radiative influx from the sun" are strictly speaking orbital parameters. Is it maybe possible to say that the effect of the orbital configuration can be set by specifying ...?
l91 "200 years per CPU day using 25 cores."
Does this mean you can run 200 years in 24h or 5000 years? Please present two numbers: 1) how fast is the model practically, i.e. how many years per day do you run with your typical configuration, 2) what is the efficiency of the model, e.g. how many CPU hours does a 100 year run consume.
l93 "as well as deep-time climates"
Since the focus of the section is not different climates but different palaeogeographies, I would reformulate here.
l107 BIOME4 is described here as a purely diagnostic component forced by output of MITgcm. Is there any aspect of the vegetation model that would feed back on climate? I am mainly thinking about albedo, but also runoff, since you mention water holding capacity.
l120 "28 including land ice"
What does that imply for the interaction between BIOME4 and the land ice component? You do not mention ice sheet extent as input to the vegetation model. What happens when the ice sheet retreats or expands? Can vegetation grow on land that has newly become ice free? What if vegetation is overrun by expanding ice sheets?
l130 "interested in horizontal resolutions of the order of 2◦ or coarser"
A bit strange to state that you are "interested" in such low resolutions. I think you should aim for a setup that resolves the processes you are interested to represent.
l130 "we can neglect basal melting and other fine-scale processes, as calving and ice streams"
I would rather say that you neglect those processes because they cannot be represented at the coarse resolution you are modelling. I still do not understand why you chose such a low resolution for the ice sheet components.
l132 "STREAMICE"
Could you explain why you are not using this module?
l135 "improved Positive Degree Day"
Is your method improved compared to Braithwaite (1977) or compared to other more recent PDD implementations?
l135 "as we also want to apply our coupled framework to paleoclimates."
Not sure to understand the reasoning. Is this an argument for using PDD instead of prescribing a given SMB?
Also here, I would like to see how the PDD results would look like for a considerably different climate.
l137 "We choose an improved PDD approach"
Same question as l135
l139 "This is an important limiting factor to consider that makes it difficult to apply SEB to paleoclimate simulations"
I am not convinced about the reasoning here. Similar limitations (too coarse resolution, missing important elements of the surface energy budget) certainly also apply to the PDD method, except that it is not as obvious/explicit as for an SEB method.
l163 "where τd = ρigHα, α= −∇zS"
I don't see the purpose of defining alpha here. It is also easily confused with "a" defined in l160
l173 "Since our purpose is to investigate climatic steady states in which ice sheets are in balance with the ocean, atmosphere, and biosphere"
This should be declared much earlier in the manuscript!
I am trying to make sense of the implications of this statement: an ice sheet fully adjusts to new climate conditions on a 100 kyr time scale. In the real world, an ice sheet is therefore never really in balance with the prevailing climate. I suppose this means that you are not interested in the coupled interaction between ice sheets and climate, which involves specific time scales?
l174 "we choose not to represent these higher-order processes"
In ice sheet terms, "higher-order processes" has a specific meaning. Please use another description.
l192 "ODE"
Introduce abbreviation.
l198 "The required input is the air surface temperature"
See comment l80
l199 "significant improvement in capturing early-season melting"
After reading l173 and in the light of all the other simplifications in the model this seems like a quite unimportant refinement to me. But I guess it is good to use recent developments. I would be concerned if the tuning of PDD model parameters presumably done at much higher resolution by Tsai and Ruan and e.g. with a geometry that resolves the ablation areas is translating to your setup. Have you considered retuning the PDD model?
Maybe more importantly for the deep-time application you envision, the assumption that the PDD with present day parameters translates to other orbital configurations is highly problematic
l209 "In summary,"
This summary concerns both 2.4.2 and 2.4.3 but appears in 2.4.3. I would suggest to describe ablation and accumulation in a joint section.
l212 "accumulation rates"
"accumulation" and "accumulation rate" are used interchangeably. Please be consistent in terminology.
l215 "Taking into account the isostatic correction"
Why "correction"? Maybe "isostatic adjustment"?
l216 "where a time delay is included"
Why not use instantaneous adjustment in line with your interest in steady states? This does not seem consistent with the approach in other components.
l232
Define Tnew and Tpickup
l233 "lapse rate is computed at each ice-sheet grid point using the MITgcm output"
Can you explain how this is done?
l235 "freshwater flux to or from the ocean is computed and included at restart at the ocean boundary of the ice sheet"
I am confused about how that works. Does the freshwater flux enter the ocean instantaneously? What determines the time scale of release?
l236 "To guarantee the conservation of salt, a compensation is performed at the global scale"
How does that work precisely? Is the model operating with physical freshwater fluxes or with (negative) salt fluxes?
l283 "the other with daily frequency (for MITgcmIS)"
It seems counterintuitive that the ice sheet component needs a higher frequency than the vegetation model. In some PDD implementations the seasonal cycle is parameterised to reduce the input to monthly or even annual. This should be mentioned and you should consider it to improve the performance of your model.
l285 "if the sea ice thickness"
Do you mean "ice sheet thickness"?
l289 "MITgcmIS is run for an equivalent of 40-100 thousand years"
Why "equivalent"? Isn't it run for an actual 40-100 kyr?
l290 "isostatic, lapse-rate and sea level corrections (Sec. 2.4.4) are included at this stage"
Are these corrections applied after the ice sheet model has run in MITgcm? Clarify. If so, how are feedbacks between isostasy and ice sheet SMB incorporated in this way?
L291 "pysheds is applied using these files"
What are "these files"?
l299 "the pickup files"
What is that?
l303 "describes in a consistent way the evolution of all the climatic variables"
I do not I agree with your notion of "consistent" evolution. The resulting evolution is not physically consistent in that the time scales of various processes are modified or removed. In the real world, e.g. the ice sheets do not have 40-100 kyr time to adjust to a new climate condition.
l304 Figure 1 "Transient simulations: repeat the same procedure by changing the forcing each N years"
See point l303. This setup is not made to run transient simulations with time steps shorter then 40-100 kyr.
l317 "with the addition of an isostatic correction"
Could you give a short summary of how that works? I am mostly interested if this is in line with the LLRA model you are using.
l321 "The resulting isostatic map"
What does "isostatic map" mean? Is this the bedrock adjusted for unloading of the ice sheets? If so, it would be interesting to see the difference to the reference bedrock in addition.
l324 "PANALESIS or other reconstructions directly provide bedrock elevations"
Not sure about the ice sheet loading in that case. Is that always included?
l325 "pre-industrial conditions at 280 ppm"
I am confused about combining present-day ice sheet reconstructions with pre-industrial climate. Is that the idea here?
l331 "set to a= 1.2·10−15 Pa−3 s−1"
What was the tuning procedure/target to find this value?
l333 Figure 2. I find this projection unusual and not very intuitive to interpret. Could you give additional figures with traditional lat-lon projection for global views and polar stereographic projections for the ice sheets?
l335 "The second simulation (run2)"
Can you give more details on how this run is set up? What is exactly the difference to run1 other than another CO2 concentration? Is this also a steady state run.
l336 "This run will be evaluated against the reanalysis and observational data"
Are you assuming that the real climate is in equilibrium with a 360 ppm CO2 forcing over this period? Are the ice sheets? How is it conceptionally possible to compare the steady state model agains transient observations?
l352 "5 hours of CPU time due to the daily stepping of the ODEs"
That seems very expensive for such a low resolution ice sheet model. Consider optimizing the SMB calculations.
l352 "for all land points"
Maybe you can exclude some points from the daily calculations when no ice is present?
l351 "one week to reach equilibrium"
Could you describe further how many years have been run for the different components?
l353 Table 2.
Specify meaning of variables in first column in the caption.
l356 Figure 4
Add latitude tick labels for NorESM2-LM
l361 "Excluding the ice sheet formation"
It would be interesting to see an analysis how the inclusion of the new components (vegetation, ice sheets) changes the model behaviour compared to the reference MITgcm.
l369 "which underestimates the elevation of Antarctica and Greenland"
Maybe this should be verified with a biogeodyn-MITgcmIS run where the ice sheet elevations are prescribed. Having that option in the model would anyhow be a good development.
l371 "the model capability to correctly reproduce the Hadley cells"
In line with comment l361, I seem to understand that MITgcm has been validated extensively. If true, this suggests strongly to focus the present analysis on how including the new components changes the model behaviour compared to the uncoupled reference in addition to comparison with other models.
l398 "the energy used for ice sheet growth is not included in this diagnostics"
Could it be? Shouldn't it be? Does that mean energy is not conserved? What is the result when you include it?
l412 "due to higher surface temperatures"
Could this be related to equilibrium climate vs transient climate. That's an important shortcoming of this comparison that should be discussed.
l438 "3.2.3 Vegetation"
I seem to understand that the offline vegetation model BIOME4 has been extensively validated. In addition you state "Offline coupling between the coupled MITgcm atmosphere-ocean-sea ice-land setup and BIOME4 has been already success-fully applied in Ragon et al. (2024, 2025)". I am wondering about the added value of section 3.2.3 in that context. Instead, it would be interesting to learn if including the ice sheet and runoff components changes the model behaviour. Is there an interaction between ice sheet change and vegetation? What vegetation grows in a much warmer climates where the ice sheets retreat?
l465
Why is there no evaluation of the runoff component here? That should be added.
l466 "3.2.4 Ice sheet"
I find it very difficult to justify the presented detailed comparison with observations of ice sheet models at such low resolution. This is particularly true for Greenland, which consist of only 25 grid points in the PI simulation.
l469 "evaluating the SMB produced for Antarctica is a prerequisite to calibrate the Glen’s law parameter"
The text until line 484 is not well placed in the Results section. I think this should be described earlier in the manuscript.
l471 Figure 10
Very unusual projection to display ice sheet results. Please consult some recent publications for inspiration. The standard are polar stereographic projections EPSG:3413 for Greenland and EPSG:3031 for Antarctica.
l486 "the Greenland ice sheet decreases strongly in height and extent"
This may be explained by the ice sheet being relaxed to a steady state in your model, while it is in a transient state in the real world. This just shows once more that the model is not meant to simulate transients with time scales of interest lower than ~50 kyr. In addition, I am concerned if the remaining 18 grid cells do a great job in simulating the dynamics of the ice sheet correctly.
l489 "after smoothing to the same spatial resolution"
How does smoothing change the total volume?
l514 "further improvements are possible"
There are quite a few that should be discussed here.
l527 "In future iterations of MITgcmIS, sliding and basal heat balance could easily be implemented"
In my view, the most important improvements are to increase the spatial resolution of the ice sheet models and make the SMB model more efficient.
l528 "this would allow study of nonlinear processes"
Before any of that, your model setup has to be modified to be able to run transient ice sheet simulations and not only steady states.
References:
Smith, R. S., Mathiot, P., Siahaan, A., Lee, V., Cornford, S. L., Gregory, J. M., Payne, A. J., Jenkins, A., Holland, P. R., Ridley, J. K., and Jones, C. G.: Coupling the U.K. Earth System Model to Dynamic Models of the Greenland and Antarctic Ice Sheets, Journal of Advances in Modeling Earth Systems, 13, e2021MS002520, https://doi.org/10.1029/2021MS002520, 2021.