the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Mar 2023
Winter sensitivity of glacial states to orbits and ice sheet heights in CESM1.2
Abstract. The changing climate system between icehouse and greenhouse states during the Quaternary Period were dramatic, yet the magnitude of these changes are still uncertain due to unconstrained ice sheets and a lack of transition mechanisms. In this study, we investigate the individual and combined impact of ice sheet heights, orbital configurations, and greenhouse gas changes for a range of Quaternary climate states to assess the linearity or non-linearity of the climate system’s response. To this end, we conduct two sets of sensitivity experiments: a series on the Preindustrial (PI) climate and experiments on Quaternary glacial states. First, modifying the PI conditions with respect to orbit, Greenland ice sheet height, and greenhouse gasses, we find that the climate system’s response to the individual factors do not superimpose on the combined response of the climate system. Thus, already under PI conditions the climate system responds in a non-linear fashion. Second, our results on Quaternary glacial states show that changing ice sheet height is the primary cause of changes in climate systems, regardless of orbit. But the subtle regional effects that orbit has are not always explained by ice sheet height changes. As for the PI simulations we find a strong, non-linear behavior in combined orbital-ice sheet height effect, where the response of the atmospheric circulation plays an important role. Therefore, orbit, ice sheets, and greenhouse gasses evolve through time by specific pathways and imply a theoretical constraint on the real climate state. As the spatial and temporal resolution of the Quaternary proxy data improves, combined with these modeled climates, we expect to generate substantial constraints on the number of realistic Quaternary climate states.
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RC1: 'Comment on egusphere-2023-324', Anonymous Referee #1, 19 Apr 2023
General comments: This preprint presents a suite of simulations with CESM under various forcings and boundary condition combinations to investigate the sensitivity and non-linearity of the climate system to these conditions. However, the specific aims of the work are not clear, and no notable conclusions appear. The presentation of background material is incomplete, the methods section is unnecessarily confusing, and the results are presented superficially. The headline conclusion is that the climate system is non-linear, but this is not novel in any way, and the specific mechanisms through which relevant nonlinearities occur are not investigated. There also appear to be some methodological issues that prevent a full evaluation of the robustness of the results.
Specific comments:
1. There is a tendency to cite only the latest papers in a given topic; for example, in discussing intercomparisons/PMIP, only a few recent examples are used, when instead there is an ample literature from PMIP2 and PMIP3 worth mentioning. Curiously, also, the recent PMIP4 LGM results paper (Kageyama et al., 2021), is not cited, despite the significant focus on the LGM.Relevant papers worth mentioning could include (in no particular order):
Roberts et al. (2019)
Cook and Held (1988)
Bartlein et al. (1998)
Manabe and Broccoli (1985)
Broccoli and Manabe (1987)
Amaya et al. (2022)
Kageyama et al. (2021)
Kutzbach and Wright (1985)
Yanase and Abe-Ouchi (2010)
Lofverstrom and Lora (2017)
Ullman et al. (2014)2. Section 2.2 is confusingly presented and requires an overhaul. Numbers of simulations are mentioned without appropriate context (“7 sensitivity experiments”, “the 5 experiments”), types of changes are labeled strangely (Last Glacial Maximum greenhouse gases is called PI_GH?), and some things are unclear (what exactly defines the LH and RCP8.5 simulations, just land cover as implied by line 141?). Why include a transient simulation here?
I would suggest instead a concise and clear overview of the specific, individual changes imposed, a summary paragraph as in the current one starting on l170, and then a *single* clear table with all simulations listed along with their configurations and names.
3. In changing the orbit, has the calendar effect been taken into account? DJF is not exactly comparable across simulations with different orbits, and this may prove to be a main reason for some of the differences identified. In general, the importance of seasonality is barely mentioned, yet, again, it might be central to some of the results as shown.
4. There is no discussion of sea ice and how this is involved in the changes observed, despite the focus on the northern hemisphere regions where sea ice is important (the polar North Atlantic, for example). This is an obvious candidate mechanism for some of the temperature nonlinearity, at least.
5. Significance appears not to be quantified. 20 years is a relatively short interval, and the manuscript makes no mention of internal variability or how that is accounted for. It is therefore impossible to know whether any of the signals discussed are actually forced changes rather than some manifestation of variability.
6. Model biases are not discussed, yet they are likely important for many of the mentioned patterns of changes, particularly in section 3.1. In combination with the above points, it is impossible to determine which, if any, of the presented changes are robust.
7. Section 3.2: Many previous studies have undertaken comparisons of this sort, including for CESM (indeed, this is mentioned later in the preprint). Therefore, the purpose of this section is not particularly obvious, and the cursory presentation does not add substantially to the rest of the results. This section should be removed.
Technical corrections
l23–24: this is ambiguous; “change” in what? And how are “future climates” being defined here?
l30: “Quaternary orbits” needs revision
l30: What model is being referred to here?
l51: TraCE21ka and iTraCE efforts should be cited here, as well as last deglaciation PMIP efforts?
l55-56: Sentence needs revision
l60: This sentence is unnecessary
l61-79: This section has a few relevant science points, and many relevant citations, but the “historical” description is superfluous and should be reduced considerably. Focus on the relevant scientific background. Also, the focus on CESM is artificial, and other results also need to be included (i.e., Roberts et al., 2019)
l60: “showing the history and versatility of the model family” is unnecessary…
l81: What ice sheet? A southward displacement where?
l86: “More recent” than what? Merz et al. (2015) is cited both here and above…
l93: But this selection is artificial; there have been coupled-model studies, including DiNezio et al. (2018), Roberts et al. (2019), Amaya et al. (2022), and the transient simulations of TraCE
l101: “one, the explicit…” sentence is not grammatically correct
l124: Remove the sentences “The simulation is scientifically… and glacial states.” They are unnecessary.
l134: What “tests”? This sentence can probably be removed as well.
l198: The main driver of what? polar amplification is defined as a stronger-than-average response of temperature
l218: This first sentence is unnecessary.
l242-243: I do not understand the point of this sentence.
l250-252: Could this be related to biases in the model? See, for example, Menemenlis et al. (2021).
l303: This sentence does not make sense; temperature changes *are* cooling or warming.
l343-344: This is not associated with adiabatic lifting; it is due to the environmental lapse rate
l349: Adiabatic compression is the wrong mechanism
l349: Why counter intuitive?
l444-447: This is entirely speculative, and the extended sentence is superfluous.
l450-451: What exactly is this referring to? The vague wording (“all these effects”, “a range of variances”) makes it hard to interpret, and there are no references provided.
l453: What is “primary” in “primary paleo modeling intercomprison projects”?
l456: Precipitation does not increase at the C-C rate…Citation: https://doi.org/10.5194/egusphere-2023-324-RC1 -
AC1: 'Reply on RC1', Jonathan Buzan, 25 Apr 2023
Thank you for your comments on our manuscript. We will address the comments in detail when all reviews are in. However, we will briefly address the major comments below:
General:
One critique point concerns the novelty. To our knowledge nobody has performed 24 fully coupled simulations with a state-of-the-art Earth System Model for different glacial periods and under different sensitivities. The set of simulations is unique and enables us to answer comprehensively the research questions: what constraints are imposed on the Modern and Quaternary climate system from combined ice sheet height and orbit configurations. Maybe the purpose and the unique set of simulations was not fully clear, and we will try to be more precise in the revised version analysis of the simulations. Clearly, we focus on mean changes first in this manuscript to give the reader a broader overview and accessibility of our research. With 24 simulations, we broke our analysis into two parts, Preindustrial and Quaternary sensitivity experiments, followed by applying the same set of variables between both sets of experiments for analysis. The sophistication comes from experimental design that decomposes ice sheet and orbits on the Preindustrial framework and expands those concepts to a 4x4 grid of interchangeable ice sheets and orbit configurations. With this analysis, it is novel to demonstrate that the ideas and processes that have come out of discontinuous experiments with slab ocean and fixed SST simulations in the CCSM/CESM model family are represented in the fully coupled model, and not a spurious result of decoupled model frameworks. Clearly, we see the point that statistical significance needs to be included. Given our experience so far with the simulations of similar kind we are rather certain that the major anomalies are statistically significant and thus the main conclusions of the manuscript will not be affected We can extend the analysis to 100 years. We certainly encourage researchers interested in this battery of simulations to work with us to evaluate complex processes.
1. We thank the reviewer for their suggested literature to add and discuss in our manuscript.
2. We will look into tables 1-3 to address the reviewers concerns about simulation names and simulation setup.
3-5. We will take into consideration the suggested changes regarding drawing sea ice lines, peak winter season analysis, and significance (see also general). For peak winter season we can search which month of the climatology has the minimum temperature and select the two surrounding months for seasonal comparison. Sea ice could be the 50th and/or 90th percentile for that same season. For significance test we will include a T test when using the 100 yr long parts of the simulations.
6. The reviewer is interested in the model biases and patterns of change under PI. As mentioned above we can evaluate a longer time series for robustness characterization. Model biases are already well described in the literature. We will be cautious in interpreting our results taking model biases into account. But by showing differences between the different sensitivity simulations systematic biases should be removed on the long term means. Please note, the modifications made in our experiments are large structural changes and are not intended to correct any biases within CESM1.2.
7. We will consider removing the reanalysis data comparison section 3.2 or moving it to the supplement material.Citation: https://doi.org/10.5194/egusphere-2023-324-AC1
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AC1: 'Reply on RC1', Jonathan Buzan, 25 Apr 2023
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RC2: 'Comment on egusphere-2023-324', Anonymous Referee #2, 25 Apr 2023
General comments:
In this manuscript, the authors aim to investigate nonlinear responses and feedbacks of climate system to changes in greenhouse gases, orbital forcing and ice sheet height under glacial and interglacial climate backgrounds by performing a set of sensitivity experiments in CESM1. Similar work has been done by other climate models (e.g. GFDL, COSMOS, etc.), hence it would be better to specify this work to CESM1 and re-edit the introduction accordingly. The target appears to be of great importance for a comprehensive understanding of CESM performance and perhaps instructive for hypothesis-driven experimental design by this model in the future. However, it remains unclear whether this target can be effectively fulfilled by the current design of sensitivity runs. For instance, orbital parameters chosen for glacial states (e.g. LGM, MIS4, MIS6, MIS8) cannot represent the full range of orbital changes and the ice-sheet sensitivity runs are thus not representative of climate responses regarding a full spectrum of orbital settings. The quasi-equilibrium time scale issue should also be considered when comparing different simulations (see below). Most importantly, an in-depth investigation on the dynamics of the nonlinear responses (e.g. ocean circulation) is missing in this descriptive version (Please refer to existing publications to strengthen this point). Overall, the content of this manuscript fits the journal Climate of the Past. However, given the existing issues in the current version, a major revision is required for further consideration of publication.
Specific comments:
P1L5: Please rephrase the sentence “a series …”
P1L6: “orbit”=> “Earth’s orbit”
P1L19: what’s the definition of “greenhouse state” in the past 1.8Myr? Please clarify.
P2L23-24: it is not clear to me how the glacial-interglacial ice sheet change can be an analog for the future. Please clarify.
P3L56: “These intercomparison …”: you mean transient simulation of the last glacial cycle? But this is contradicted to your previous statement regarding computational coast of these ESMs. Assessing the performance of models on mimicking glacial and interglacial climate state would be more appropriate.
P3L59: please specify the model version, e.g. version 1.2, etc.
P3L61-79: I did not think this lengthy part is of specific relevance in this section and even for this study. Please shorten it and perhaps move it to 2.2.
P5L143: “850CE” => “1850CE”
P5L147: it is not clear to me the designed glacial runs can well explore the influence of orbital forcing. There are three main orbital parameters, i.e. eccentricity, precession and obliquity. The designed runs that are characterized by different values of three parameters can only capture some but not a wide range of orbital settings. Based on the face values of the parameters used, it is not straightforward and even not possible to pinpoint the influence of a single orbital parameter on climate response to ice sheet height change. This might be the reason why this manuscript is mainly about describing modeling outcomes instead of mechanism understanding. I urge the authors re-streamline their runs for a clearer and more feasible target.
P8L171: Please provide initial ocean state information for each sensitivity run. Note that quasi-equilibrium time scale for the same forcing change might be different due to different initial ocean states. Therefore, it is not convincing that integration of up to 400 years ensures a quasi-equilibrium state for all the simulations here. This further undermine their comparability and hence the robustness of the conclusions. In addition to TOA radiation balance, global mean surface temperature or mean ocean temperature above thermocline might also be helpful to evaluate the quasi-equilibrium of the system. By definition, a climatology mean state can be represented by an average of no less than 30 years. Obviously, the average over 20 years is too short.
P8L179-180: why the focus of this study is winter is unclear and shall be elaborated beforehand.
P9L186-188: Please rephrase this sentence and perhaps split it to two.
P9L192-193: changes in Earth’s orbit from LH to PI might be comparable to the changes in GHG, which questions the statement “the temperature changes to fall in al inear series to account for the logarithmic response of greenhouse gases.” as putting LH into the comparison. An assessment is hence required to support this statement.
P9L200-217: since significance of the differences between sensitivity runs is of great importance for the conclusions, t-test or other similar kind of significant test is recommended for the anomaly analysis.
P13L265 Section3.2: Given the focus of this paper as well as previous published CESM LGM simulations, I would rather suggest merge this section to methodology to ensure a smooth flow of the demonstration.
P14L291: see my previous comments on evaluating the influence of orbital forcing.
P16L337: given northern ice sheet geometry during Marine Isotope Stage 3, a reduced Laurentide ice sheet is along with a shrinked extent. Therefore, the experiments conducted here are sort of idealized for the impact of ice sheet height. This point shall be mentioned in the text. In addition, some discussion about the role of ice extent is also helpful, because the thermal effect of ice extent might be of different and even opposite role in comparison to the kinetic role of ice sheet height.
Section 4&5: The discussion about the nonlinear outcomes is not sufficient for an in-depth understanding of the associated underlying dynamics. This gap shall be filled in the new version for potential consideration of publication
Citation: https://doi.org/10.5194/egusphere-2023-324-RC2 -
AC2: 'Reply on RC2', Jonathan Buzan, 26 Apr 2023
We thank the reviewer for their comments on our article. We plan a detailed response when all reviews have come in. In the meantime, we address the larger concerns below.
General:
The reviewer focuses the orbital parameters that are used in our experiments and the quasi-equilibrium time scales. We acknowledge that our simulations are not an exhaustive analysis of eccentricity versus obliquity versus procession. As our title states, we are focused primarily on the glacial states (i.e. maximums) and the orbital parameters are extracted from known glacial maximum Marine Isotopic Stages. Nor are our ice sheet sensitivities exhaustive in their exploration. But we are focused on the maximum state and focus on these conditions. We will make this clearer in then revised version. We are a bit confused by the reviewer’s comment that we were not clear in our focus on CESM1.2, which is explicitly stated in our title, background, and methodology. The reviewer also mention that similar work was undertaken with other models (GFDL and COSMOS simulations). It would be helpful to have the references. If the reviewer can point to literature on this scale, we’ll gladly add them to our manuscript background. The reviewer would like to see more analysis of the ocean. Considering the largest forcing is from the ice sheet, the dominate impact of which is on the atmosphere, thus we focused our analysis on the atmosphere. Future studies using the 24 fully coupled simulations will analyze ocean processes. The reviewer’s question about quasi equilibrium and whether our simulations have small drift in the deep ocean: we don’t expect the ocean to be fully equilibrized, but all of the glacial maximum states were initialized from a glacial maximum state and do not require 1000s of model years to reach a quasi-equilibrium state.
Summarized major points:
- The reviewer asks that we ‘re-streamline’ our runs for a clear more feasible target. We don’t believe the reviewer understands the computational requirements to execute a state-of-the-art general circulation model. No modeling group, as far as we know, has executed 24 fully coupled simulations for different glacial times, and given the computational expense, we are still limited in our selection of orbital configurations. Our experimental design focuses on glacial maximum states; thus, the orbital parameters reflect those situations.
- The initial state of the ocean for the Quaternary simulations are initialized from a 2100-year LGM simulation in quasi-equilibrium. As our methods discuss, this simulation was extended an additional 100 years to confirm quasi-equilibrium state. Then the ice sheet heights were adjusted and executed. Then each of these ice sheet heights, 125, 100, 67, and 33 had their orbit configurations changed to MIS4,6, and 8, thus each change is systematic and always initialized from a cold glacial maximum state. With this strategy it was not necessary to run simulation for thousand of years to reach a quasi-equilibrium given our definition.
- The reviewer suggest that our choice of climatological size is inappropriate. The use of number of years used in climate definitions is arbitrary. For example, the standard that we are using is derived from the IPCC reports. The IPCC has used 20-year climatologies for the past decade in both CMIP5 and CMIP6 analysis (CMIP5’s sub-daily datasets are only 20 years long). Thus, we disagree with the reviewer’s statement. However, we understand that the reviewer is concerned with robustness, and we can used 100-year climatologies and their suggested T-Test as a robustness characterization.
- We will consider removing the climate reanalysis section 3.2 or moving it to the supplemental materials to allow the manuscript to flow from the Preindustrial experiments directly to the Quaternary experiments.
- We agree with the reviewer that the adjustment of the ice sheet height vertically is an idealized case, that in reality ice sheets likewise changed their spatial extent. We will amend the text to make that clear.
Citation: https://doi.org/10.5194/egusphere-2023-324-AC2
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AC2: 'Reply on RC2', Jonathan Buzan, 26 Apr 2023
Status: closed
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RC1: 'Comment on egusphere-2023-324', Anonymous Referee #1, 19 Apr 2023
General comments: This preprint presents a suite of simulations with CESM under various forcings and boundary condition combinations to investigate the sensitivity and non-linearity of the climate system to these conditions. However, the specific aims of the work are not clear, and no notable conclusions appear. The presentation of background material is incomplete, the methods section is unnecessarily confusing, and the results are presented superficially. The headline conclusion is that the climate system is non-linear, but this is not novel in any way, and the specific mechanisms through which relevant nonlinearities occur are not investigated. There also appear to be some methodological issues that prevent a full evaluation of the robustness of the results.
Specific comments:
1. There is a tendency to cite only the latest papers in a given topic; for example, in discussing intercomparisons/PMIP, only a few recent examples are used, when instead there is an ample literature from PMIP2 and PMIP3 worth mentioning. Curiously, also, the recent PMIP4 LGM results paper (Kageyama et al., 2021), is not cited, despite the significant focus on the LGM.Relevant papers worth mentioning could include (in no particular order):
Roberts et al. (2019)
Cook and Held (1988)
Bartlein et al. (1998)
Manabe and Broccoli (1985)
Broccoli and Manabe (1987)
Amaya et al. (2022)
Kageyama et al. (2021)
Kutzbach and Wright (1985)
Yanase and Abe-Ouchi (2010)
Lofverstrom and Lora (2017)
Ullman et al. (2014)2. Section 2.2 is confusingly presented and requires an overhaul. Numbers of simulations are mentioned without appropriate context (“7 sensitivity experiments”, “the 5 experiments”), types of changes are labeled strangely (Last Glacial Maximum greenhouse gases is called PI_GH?), and some things are unclear (what exactly defines the LH and RCP8.5 simulations, just land cover as implied by line 141?). Why include a transient simulation here?
I would suggest instead a concise and clear overview of the specific, individual changes imposed, a summary paragraph as in the current one starting on l170, and then a *single* clear table with all simulations listed along with their configurations and names.
3. In changing the orbit, has the calendar effect been taken into account? DJF is not exactly comparable across simulations with different orbits, and this may prove to be a main reason for some of the differences identified. In general, the importance of seasonality is barely mentioned, yet, again, it might be central to some of the results as shown.
4. There is no discussion of sea ice and how this is involved in the changes observed, despite the focus on the northern hemisphere regions where sea ice is important (the polar North Atlantic, for example). This is an obvious candidate mechanism for some of the temperature nonlinearity, at least.
5. Significance appears not to be quantified. 20 years is a relatively short interval, and the manuscript makes no mention of internal variability or how that is accounted for. It is therefore impossible to know whether any of the signals discussed are actually forced changes rather than some manifestation of variability.
6. Model biases are not discussed, yet they are likely important for many of the mentioned patterns of changes, particularly in section 3.1. In combination with the above points, it is impossible to determine which, if any, of the presented changes are robust.
7. Section 3.2: Many previous studies have undertaken comparisons of this sort, including for CESM (indeed, this is mentioned later in the preprint). Therefore, the purpose of this section is not particularly obvious, and the cursory presentation does not add substantially to the rest of the results. This section should be removed.
Technical corrections
l23–24: this is ambiguous; “change” in what? And how are “future climates” being defined here?
l30: “Quaternary orbits” needs revision
l30: What model is being referred to here?
l51: TraCE21ka and iTraCE efforts should be cited here, as well as last deglaciation PMIP efforts?
l55-56: Sentence needs revision
l60: This sentence is unnecessary
l61-79: This section has a few relevant science points, and many relevant citations, but the “historical” description is superfluous and should be reduced considerably. Focus on the relevant scientific background. Also, the focus on CESM is artificial, and other results also need to be included (i.e., Roberts et al., 2019)
l60: “showing the history and versatility of the model family” is unnecessary…
l81: What ice sheet? A southward displacement where?
l86: “More recent” than what? Merz et al. (2015) is cited both here and above…
l93: But this selection is artificial; there have been coupled-model studies, including DiNezio et al. (2018), Roberts et al. (2019), Amaya et al. (2022), and the transient simulations of TraCE
l101: “one, the explicit…” sentence is not grammatically correct
l124: Remove the sentences “The simulation is scientifically… and glacial states.” They are unnecessary.
l134: What “tests”? This sentence can probably be removed as well.
l198: The main driver of what? polar amplification is defined as a stronger-than-average response of temperature
l218: This first sentence is unnecessary.
l242-243: I do not understand the point of this sentence.
l250-252: Could this be related to biases in the model? See, for example, Menemenlis et al. (2021).
l303: This sentence does not make sense; temperature changes *are* cooling or warming.
l343-344: This is not associated with adiabatic lifting; it is due to the environmental lapse rate
l349: Adiabatic compression is the wrong mechanism
l349: Why counter intuitive?
l444-447: This is entirely speculative, and the extended sentence is superfluous.
l450-451: What exactly is this referring to? The vague wording (“all these effects”, “a range of variances”) makes it hard to interpret, and there are no references provided.
l453: What is “primary” in “primary paleo modeling intercomprison projects”?
l456: Precipitation does not increase at the C-C rate…Citation: https://doi.org/10.5194/egusphere-2023-324-RC1 -
AC1: 'Reply on RC1', Jonathan Buzan, 25 Apr 2023
Thank you for your comments on our manuscript. We will address the comments in detail when all reviews are in. However, we will briefly address the major comments below:
General:
One critique point concerns the novelty. To our knowledge nobody has performed 24 fully coupled simulations with a state-of-the-art Earth System Model for different glacial periods and under different sensitivities. The set of simulations is unique and enables us to answer comprehensively the research questions: what constraints are imposed on the Modern and Quaternary climate system from combined ice sheet height and orbit configurations. Maybe the purpose and the unique set of simulations was not fully clear, and we will try to be more precise in the revised version analysis of the simulations. Clearly, we focus on mean changes first in this manuscript to give the reader a broader overview and accessibility of our research. With 24 simulations, we broke our analysis into two parts, Preindustrial and Quaternary sensitivity experiments, followed by applying the same set of variables between both sets of experiments for analysis. The sophistication comes from experimental design that decomposes ice sheet and orbits on the Preindustrial framework and expands those concepts to a 4x4 grid of interchangeable ice sheets and orbit configurations. With this analysis, it is novel to demonstrate that the ideas and processes that have come out of discontinuous experiments with slab ocean and fixed SST simulations in the CCSM/CESM model family are represented in the fully coupled model, and not a spurious result of decoupled model frameworks. Clearly, we see the point that statistical significance needs to be included. Given our experience so far with the simulations of similar kind we are rather certain that the major anomalies are statistically significant and thus the main conclusions of the manuscript will not be affected We can extend the analysis to 100 years. We certainly encourage researchers interested in this battery of simulations to work with us to evaluate complex processes.
1. We thank the reviewer for their suggested literature to add and discuss in our manuscript.
2. We will look into tables 1-3 to address the reviewers concerns about simulation names and simulation setup.
3-5. We will take into consideration the suggested changes regarding drawing sea ice lines, peak winter season analysis, and significance (see also general). For peak winter season we can search which month of the climatology has the minimum temperature and select the two surrounding months for seasonal comparison. Sea ice could be the 50th and/or 90th percentile for that same season. For significance test we will include a T test when using the 100 yr long parts of the simulations.
6. The reviewer is interested in the model biases and patterns of change under PI. As mentioned above we can evaluate a longer time series for robustness characterization. Model biases are already well described in the literature. We will be cautious in interpreting our results taking model biases into account. But by showing differences between the different sensitivity simulations systematic biases should be removed on the long term means. Please note, the modifications made in our experiments are large structural changes and are not intended to correct any biases within CESM1.2.
7. We will consider removing the reanalysis data comparison section 3.2 or moving it to the supplement material.Citation: https://doi.org/10.5194/egusphere-2023-324-AC1
-
AC1: 'Reply on RC1', Jonathan Buzan, 25 Apr 2023
-
RC2: 'Comment on egusphere-2023-324', Anonymous Referee #2, 25 Apr 2023
General comments:
In this manuscript, the authors aim to investigate nonlinear responses and feedbacks of climate system to changes in greenhouse gases, orbital forcing and ice sheet height under glacial and interglacial climate backgrounds by performing a set of sensitivity experiments in CESM1. Similar work has been done by other climate models (e.g. GFDL, COSMOS, etc.), hence it would be better to specify this work to CESM1 and re-edit the introduction accordingly. The target appears to be of great importance for a comprehensive understanding of CESM performance and perhaps instructive for hypothesis-driven experimental design by this model in the future. However, it remains unclear whether this target can be effectively fulfilled by the current design of sensitivity runs. For instance, orbital parameters chosen for glacial states (e.g. LGM, MIS4, MIS6, MIS8) cannot represent the full range of orbital changes and the ice-sheet sensitivity runs are thus not representative of climate responses regarding a full spectrum of orbital settings. The quasi-equilibrium time scale issue should also be considered when comparing different simulations (see below). Most importantly, an in-depth investigation on the dynamics of the nonlinear responses (e.g. ocean circulation) is missing in this descriptive version (Please refer to existing publications to strengthen this point). Overall, the content of this manuscript fits the journal Climate of the Past. However, given the existing issues in the current version, a major revision is required for further consideration of publication.
Specific comments:
P1L5: Please rephrase the sentence “a series …”
P1L6: “orbit”=> “Earth’s orbit”
P1L19: what’s the definition of “greenhouse state” in the past 1.8Myr? Please clarify.
P2L23-24: it is not clear to me how the glacial-interglacial ice sheet change can be an analog for the future. Please clarify.
P3L56: “These intercomparison …”: you mean transient simulation of the last glacial cycle? But this is contradicted to your previous statement regarding computational coast of these ESMs. Assessing the performance of models on mimicking glacial and interglacial climate state would be more appropriate.
P3L59: please specify the model version, e.g. version 1.2, etc.
P3L61-79: I did not think this lengthy part is of specific relevance in this section and even for this study. Please shorten it and perhaps move it to 2.2.
P5L143: “850CE” => “1850CE”
P5L147: it is not clear to me the designed glacial runs can well explore the influence of orbital forcing. There are three main orbital parameters, i.e. eccentricity, precession and obliquity. The designed runs that are characterized by different values of three parameters can only capture some but not a wide range of orbital settings. Based on the face values of the parameters used, it is not straightforward and even not possible to pinpoint the influence of a single orbital parameter on climate response to ice sheet height change. This might be the reason why this manuscript is mainly about describing modeling outcomes instead of mechanism understanding. I urge the authors re-streamline their runs for a clearer and more feasible target.
P8L171: Please provide initial ocean state information for each sensitivity run. Note that quasi-equilibrium time scale for the same forcing change might be different due to different initial ocean states. Therefore, it is not convincing that integration of up to 400 years ensures a quasi-equilibrium state for all the simulations here. This further undermine their comparability and hence the robustness of the conclusions. In addition to TOA radiation balance, global mean surface temperature or mean ocean temperature above thermocline might also be helpful to evaluate the quasi-equilibrium of the system. By definition, a climatology mean state can be represented by an average of no less than 30 years. Obviously, the average over 20 years is too short.
P8L179-180: why the focus of this study is winter is unclear and shall be elaborated beforehand.
P9L186-188: Please rephrase this sentence and perhaps split it to two.
P9L192-193: changes in Earth’s orbit from LH to PI might be comparable to the changes in GHG, which questions the statement “the temperature changes to fall in al inear series to account for the logarithmic response of greenhouse gases.” as putting LH into the comparison. An assessment is hence required to support this statement.
P9L200-217: since significance of the differences between sensitivity runs is of great importance for the conclusions, t-test or other similar kind of significant test is recommended for the anomaly analysis.
P13L265 Section3.2: Given the focus of this paper as well as previous published CESM LGM simulations, I would rather suggest merge this section to methodology to ensure a smooth flow of the demonstration.
P14L291: see my previous comments on evaluating the influence of orbital forcing.
P16L337: given northern ice sheet geometry during Marine Isotope Stage 3, a reduced Laurentide ice sheet is along with a shrinked extent. Therefore, the experiments conducted here are sort of idealized for the impact of ice sheet height. This point shall be mentioned in the text. In addition, some discussion about the role of ice extent is also helpful, because the thermal effect of ice extent might be of different and even opposite role in comparison to the kinetic role of ice sheet height.
Section 4&5: The discussion about the nonlinear outcomes is not sufficient for an in-depth understanding of the associated underlying dynamics. This gap shall be filled in the new version for potential consideration of publication
Citation: https://doi.org/10.5194/egusphere-2023-324-RC2 -
AC2: 'Reply on RC2', Jonathan Buzan, 26 Apr 2023
We thank the reviewer for their comments on our article. We plan a detailed response when all reviews have come in. In the meantime, we address the larger concerns below.
General:
The reviewer focuses the orbital parameters that are used in our experiments and the quasi-equilibrium time scales. We acknowledge that our simulations are not an exhaustive analysis of eccentricity versus obliquity versus procession. As our title states, we are focused primarily on the glacial states (i.e. maximums) and the orbital parameters are extracted from known glacial maximum Marine Isotopic Stages. Nor are our ice sheet sensitivities exhaustive in their exploration. But we are focused on the maximum state and focus on these conditions. We will make this clearer in then revised version. We are a bit confused by the reviewer’s comment that we were not clear in our focus on CESM1.2, which is explicitly stated in our title, background, and methodology. The reviewer also mention that similar work was undertaken with other models (GFDL and COSMOS simulations). It would be helpful to have the references. If the reviewer can point to literature on this scale, we’ll gladly add them to our manuscript background. The reviewer would like to see more analysis of the ocean. Considering the largest forcing is from the ice sheet, the dominate impact of which is on the atmosphere, thus we focused our analysis on the atmosphere. Future studies using the 24 fully coupled simulations will analyze ocean processes. The reviewer’s question about quasi equilibrium and whether our simulations have small drift in the deep ocean: we don’t expect the ocean to be fully equilibrized, but all of the glacial maximum states were initialized from a glacial maximum state and do not require 1000s of model years to reach a quasi-equilibrium state.
Summarized major points:
- The reviewer asks that we ‘re-streamline’ our runs for a clear more feasible target. We don’t believe the reviewer understands the computational requirements to execute a state-of-the-art general circulation model. No modeling group, as far as we know, has executed 24 fully coupled simulations for different glacial times, and given the computational expense, we are still limited in our selection of orbital configurations. Our experimental design focuses on glacial maximum states; thus, the orbital parameters reflect those situations.
- The initial state of the ocean for the Quaternary simulations are initialized from a 2100-year LGM simulation in quasi-equilibrium. As our methods discuss, this simulation was extended an additional 100 years to confirm quasi-equilibrium state. Then the ice sheet heights were adjusted and executed. Then each of these ice sheet heights, 125, 100, 67, and 33 had their orbit configurations changed to MIS4,6, and 8, thus each change is systematic and always initialized from a cold glacial maximum state. With this strategy it was not necessary to run simulation for thousand of years to reach a quasi-equilibrium given our definition.
- The reviewer suggest that our choice of climatological size is inappropriate. The use of number of years used in climate definitions is arbitrary. For example, the standard that we are using is derived from the IPCC reports. The IPCC has used 20-year climatologies for the past decade in both CMIP5 and CMIP6 analysis (CMIP5’s sub-daily datasets are only 20 years long). Thus, we disagree with the reviewer’s statement. However, we understand that the reviewer is concerned with robustness, and we can used 100-year climatologies and their suggested T-Test as a robustness characterization.
- We will consider removing the climate reanalysis section 3.2 or moving it to the supplemental materials to allow the manuscript to flow from the Preindustrial experiments directly to the Quaternary experiments.
- We agree with the reviewer that the adjustment of the ice sheet height vertically is an idealized case, that in reality ice sheets likewise changed their spatial extent. We will amend the text to make that clear.
Citation: https://doi.org/10.5194/egusphere-2023-324-AC2
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AC2: 'Reply on RC2', Jonathan Buzan, 26 Apr 2023
Data sets
Code and Data for Winter sensitivity of glacial states to orbits and ice sheet heights in CESM1.2 J. R. Buzan, E. Russo, W. M. Kim, and C. C. Raible https://doi.org/10.5281/zenodo.7665583
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- High-resolution LGM climate of Europe and the Alpine region using the regional climate model WRF E. Russo et al. 10.5194/cp-20-449-2024
- Subglacial hydrology from high-resolution ice-flow simulations of the Rhine Glacier during the Last Glacial Maximum: a proxy for glacial erosion D. Cohen et al. 10.5194/egqsj-72-189-2023