Alternative climatic steady states for the Permian-Triassic paleogeography

Ragon, Charline; Vérard, Christian; Kasparian, Jérôme; Brunetti, Maura

doi:https://doi.org/10.5194/egusphere-2023-1808

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https://doi.org/10.5194/egusphere-2023-1808

Preprints

05 Sep 2023

| 05 Sep 2023

Alternative climatic steady states for the Permian-Triassic paleogeography

Charline Ragon, Christian Vérard, Jérôme Kasparian, and Maura Brunetti

Abstract. Because of spatial scarcity and uncertainties in sedimentary data, initial and boundary conditions in deep-time climate simulations lack of constraints. On the other hand, climate is a nonlinear system with a multitude of feedback mechanisms, which compete and balance in a different way that depends on the initial and boundary conditions, opening the possibility, in numerical experiments, to obtain multiple steady states under the same forcing. Here, we use the MITgcm with a coupled atmosphere-ocean-sea ice-land configuration to explore the existence of such alternative steady states around the Permian-Triassic Boundary (PTB). We construct the corresponding bifurcation diagram accounting for processes on a timescale of thousands of years, in order to identify the stability range of the steady states and tipping points in regard to atmospheric CO₂ content. We find three alternative steady states with a difference in global mean surface air temperature of around 10 °C. We also investigate how these climatic steady states are modified when feedbacks acting over comparable or longer time scales are included, in particular vegetation dynamics and air-sea carbon exchange. Our findings for multistability provide a useful framework for explaining climatic variations observed in Early Triassic geological records, and some discrepancies between numerical simulations in the literature and geological data at the PTB and its aftermaths.

Received: 08 Aug 2023 – Discussion started: 05 Sep 2023

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Charline Ragon, Christian Vérard, Jérôme Kasparian, and Maura Brunetti

Status: closed

RC1:
'Comment on egusphere-2023-1808', Yonggang Liu, 04 Oct 2023

Ragon et al. spent lots of effort in finding all the possible steady states for the Permian-Triassic paleogeography using a relatively sophisticated Earth system model. The interesting finding, in my opinion, is that they found a 'warm' state in between the 'cold' and 'hot' states. This warm state cannot be reached from the either the cold or hot state by increasing or decreasing greenhouse gas forcings. The other findings are less interesting but worth being published on the journal EGUsphere.
The major reason that the results may be less important than they seem to is that the multiple steady states found here could disappear when a fully coupled state-of-the-art climate model is used. To my own experience, the cold and hot climate states (even the cold state is still quite warm) presented in this manuscript could not coexist at the same forcing in the NCAR model family. Especially, the sea ice used in the model of this manuscript is thermodynamic only. When the NCAR model was run in such mode, a so-called Jormungand state could be found (Abbot et al., 2011; https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2011JD015927) but never found in the fully coupled mode. Therefore, I think the authors should point out in the abstract or conclusion that the multiple steady states found in their study may depend on the specific configuration of their model, especially the neglect of sea-ice dynamics.
Moreover, I think a snowball Earth branch should exist in their model if the initial condition is cold enough. They do not need to explore the full branch but just confirming their existence is necessary in this kind of study.
L164: "stronger" than what? This makes the sentence hard to understand.
Fig. 6: It has been pointed in the literature that the strength of annual mean Hadley circulation may not be meaningful, can the authors please confirm that the strength of seasonal Hadley circulation has a similar trend?
Figs. 7 and 8: I think these two figures can be combined into one

Citation: https://doi.org/10.5194/egusphere-2023-1808-RC1
- AC1: 'Reply on RC1', Charline Ragon, 28 Nov 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1808/egusphere-2023-1808-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-1808-AC1
RC2:
'Comment on egusphere-2023-1808', Anonymous Referee #2, 05 Oct 2023

Ragon and others explore climate of the Permian-Triassic with the MITGCM. They find three equilibrium climate states with an initial CO2 of 320 ppm. From these initial states, they explore the range of stability and the importance of vegetation and carbon cycle feedbacks.
I found this to be an interesting study. The experiment design was well thought out. The text and figures were easy to follow. Here are some comments that I think could improve the manuscript.
A lot of steps went into the model spin up. They are well documented in the text, which I appreciate. Although the text is generally clear, I think a schematic of the spin up procedure would help the reader understand what was done.
There was a lot of work put into creating realistic boundary conditions. This is worthwhile since most previous MITGCM multiple equilibria studies used idealized configurations. One of the benefits of realistic boundary conditions is model-proxy comparison. The authors speculate that the mismatch between HadCM3 simulations and a proxy reconstruction might be the result of multiple equilibria. However, the authors do not make any model-proxy comparisons themselves. I do not think many temperature records of the Permian-Triassic exist, but it is still worth doing.
On the point of realism, the range of CO2 for these equilibrium states is quite small from a geologic perspective. What are the implications of this? Also, a few sentences of discussion about the biomes of the Permian-Triassic versus the biomes in BIOME4 would be worthwhile.
Line 81-82: Can you provide a bit more information about how the model conserves energy?
Line 84: I do not think Foster et al. (2017) is the original source. Maybe Gough (1981)?
Line 106: Is it possible the tipping points could occur with less forcing if the simulations were run for more than 100 years?
Line 119: Is there any transpiration component in the MITGCM?
Line 123: “run BIOME4 again”
Line 143: 0.1 ppm per year seems like a large drift on geologic time scales. Does it look like the carbon is heading towards an equilibrium or continually drifting?
Line 203: “to simulate larger pCO2 values”
Line 204: To clarify, the model boundary conditions are largely responsible for the high temperature sensitivity?
Line 212: Based on Figure B1, it does not seem like the cold state can be reached from the warm state, so the loop is not closed. What are the implications of this with respect to real climate evolution?
Line 216: I do not follow this argument. HadCM3, like other higher complexity models, does not produce multiple equilibria. CO2 and proxy reconstruction uncertainties seem more probably explanations for the mismatch. Are you arguing that HadCM3 is somehow stuck in a cold equilibrium solution? Also, is it OK to cite an EGU presentation?
Table 3: I am confused by the differences in energy balance between the ocean surface and TOA. Where is the energy going?
Some of the figures would be easier to interpret with difference plots. I think figures 7, 8, and 9 in particular.
Figure D1: I do not understand the temperature response with the carbon cycle. Why does the sensitivity change, especially in the warm state? Also, I am surprised that turning on the carbon cycle did not lead to large changes in atmospheric CO2. Any explanation?

Citation: https://doi.org/10.5194/egusphere-2023-1808-RC2
- AC2: 'Reply on RC2', Charline Ragon, 28 Nov 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1808/egusphere-2023-1808-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-1808-AC2
RC3:
'Comment on egusphere-2023-1808', Anonymous Referee #3, 12 Oct 2023

The presented manuscript describes how up to three different climatic states exist under the same forcing in a climate model of the Permian-Triassic paleogeography. Multistability is further shown to exist also when vegetation and carbon cycle feedbacks are activated and the existence of multiple stable states is proposed as an explanation for the observed high climate variability of the PTB, as well as discrepancies between numerical simulations and geological data. The authors use MITgcm, a coupled general circulation model with a coarse resolution that properly resolves the ocean circulation, but only has a relatively simple representation of atmospheric dynamics (e.g. only 5 vertical layers).
The manuscript is generally well-structured and easy to follow. The language and grammar probably need a revision. The finding of several (non-snowball Earth) stable climate equilibria at the time of the PTB is interesting and, to my knowledge, has not been shown before. However, I have a major concern about the validity of the findings, coming from some of the presented simulation results:
i.)The "hot" state has a global mean temperature of 30.9°C at a CO2 concentration of 320 ppm and a solar forcing that is ~2% weaker than the present-day forcing. To put this in other words, even though the CO2 concentration is 20-25% lower than the modern values and the sun is 2% weaker than today (equivalent to roughly another halving of the CO2 concentration), the global climate of the hot state is simulated to be approximately 16°C warmer than the present-day climate. I am aware that a different continental distribution can lead to very different global mean temperatures, but this contrast to the present-day climate is so extreme that I am really wondering where this is coming from? An explanation for this extreme state is not given in the manuscript. In general, all three states ("cold", "warm" and "hot") have pretty high global mean temperatures. Is it because the land surface has a comparably low albedo value? Does the continental distribution lead to a much smaller cloud coverage? Or is there a very strong water vapour feedback in the MITgcm atmosphere component? A closer investigation of this aspect would, in my opinion, not just be interesting, but is actually crucial, as otherwise the validity of the model results is very questionable.
ii.)The plot of global mean temperatures in Fig. 11 and the values in Tab. 5 highlight another extreme (maybe even unrealistic?) aspect of the simulated model results: the climate sensitivity seems to be extremely high. The slopes of the lines in Fig. 11 indicate that the climate sensitivity of this setup is 15-18 °C of warming per doubling of CO2. An increase of just 8 ppm in the "warm" state led to a temperature increase of 1.45 °C (Tab. 5). If the modern climate would be anywhere near this sensitivity, humanity would be doomed already (we added ~ 140 ppm carbon to experience a comparable warming). The most recent IPCC estimate of modern climate sensitivity is ~2.5-4 °C (likely range). Again, a different continental setup might explain some of the differences to the modern state, but the climate sensitivity in this study is so extreme that it requires a convincing explanation. Otherwise the model results cannot be viewed as reliable.
iii.) I find it quite surprising that the deserts are smallest in the "hot" state. The authors claim that this is because there is more precipitation in that state. However, the annual mean surface air temperatures are 40-50°C (daily temperature extremes should then be around 70-80°C) in tropical and subtropical regions, where their model is simulating a vegetation cover of "forbland and dry shrubland". I am no expert in vegetation cover, but could any plant survive temperatures of >70°C, even if it is just for a few hours a day? Additionally, even though there might be more precipitation in the hot state, this would probably be more localized in individual extreme events and not fall evenly. I see a potential discrepancy here, because the vegetation model is only fed with long-term monthly mean values, which don't capture this variability.
iv.) It is also quite unusual that the meridional overturning circulation (MOC) of the ocean is much stronger in the hot state, especially since there is a weakening of the MOC, when going from the cold to the warm state. Furthermore, in the hot state there seems to be a cell near the equator where the water transport is out of the bounds of the colorbar, i.e. has a transport of >100 Sv, which is extremely high and - to my knowledge - unrealistic for a supposed equilibrium state. Where is the energy that drives a constant massive ocean overturning coming from?

At this point, I am having a hard time believing the outcome of the simulation results, especially with respect to the high climate sensitivity and the generally unusual characteristics of the "hot" state. When seeing these results, my first guess would be that the whole"hot" state is a numerical artefact and that the atmosphere component of MITgcm is not doing a decent job here in general. In order to oppose my concern, the authors could present results of a reference simulation of 1850-today, to show that this version of MITgcm (with the necessary adaptions to the modern state) is able to get the historical warming roughly right (or one pre-industrial simulation and one with a more recent CO2 concentration at ~400 ppm, if that is easier). Additionally, I really need to see a discussion of how the extreme and unusual results of the hot state can be explained physically.
As the existence of the "hot" state at such low CO2 concentrations is almost impossible in my eyes and given the other strange features mentioned above, I highly doubt the reliability of the presented results. Since the whole point of this manuscript is based on the existence of multistability, I have no other option than to suggest a rejection of the manuscript, unless the authors provide an elaborate and convincing explanation of why these extreme results are realistic.

Specific comments

- The language/grammar of the manuscript needs a revision. Very often I have the feeling that a "the" or similar article is missing (as an example in line 184 the sentence should be "In the hot state,...", right?).
- The whole discussion neglects the fact that there should be another stable climate: the snowball Earth. This should be mentioned at least once at some point.
- I would appreciate some more introduction as to why the Permian-Triassic Boundary is an important/interesting period to study? Right now the introduction is only about multistability and tipping points.
- The model has no sea-ice dynamics, which would strongly impact individual climate states that have some sea ice. Given this shortcoming, also the small range in which the "warm" state exists is questionable.
- the choice of colors in the upper panel of Fig. 2, using darker colors for a higher albedo, seems odd. I would reverse the colorbar
- line 143: averaged over which time scale?
- a short description of how the carbon cycle model works would be very helpful. Right now, I cannot judge whether this model is sufficient to provide a proper representation of the carbon cycle during the PTB
- lines 239-248: how do the plants survive these extreme temperatures in the warm state? Also, there is more carbon stored in vegetation in the hot state. This also means there is a lot of fuel for fires, which should occur very often given the extremely high temperatures. Is this self-consistent?
- Figure captions could include more information to make the figures a bit more self-evident (e.g. Fig. 7-9)
- adding some contour lines in e.g. Fig. 3-4 would really help for readability
- The first paragraph of the conclusion is rather a repetition of the introduction
- I don't understand the way that uncertainty in numbers is represented. For example in Tab.4: in the initial state of the hot state, the second iteration of the warm state and the fourth iteration of the cold state the uncertainty of SAT is much larger (9 instead of 1) than in the other states. Do these three cases actually have an uncertainty of 0.9°C and not 9°C? Otherwise the results seem to suggest some kind of instability. Do I understand those numbers correctly?
- The river map in Fig. A1 could simply be added to Fig. 1. The Appendix A is then obsolete, as the content is also already mentioned in the model description section.
- what about other greenhouse gases? Are they held fixed at some values? Which values were used?

Some technical corrections:

- labelling subfigures in Fig. 2 with (a) and (b)
- Fig. 10: It says that the line thickness varies with horizontal velocity, but it is not. Maybe make the thickness more sensitive to velocity or just drop the velocity scaling.

Citation: https://doi.org/10.5194/egusphere-2023-1808-RC3
- AC3: 'Reply on RC3', Charline Ragon, 28 Nov 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1808/egusphere-2023-1808-AC3-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-1808-AC3

Status: closed

RC1:
'Comment on egusphere-2023-1808', Yonggang Liu, 04 Oct 2023

Ragon et al. spent lots of effort in finding all the possible steady states for the Permian-Triassic paleogeography using a relatively sophisticated Earth system model. The interesting finding, in my opinion, is that they found a 'warm' state in between the 'cold' and 'hot' states. This warm state cannot be reached from the either the cold or hot state by increasing or decreasing greenhouse gas forcings. The other findings are less interesting but worth being published on the journal EGUsphere.
The major reason that the results may be less important than they seem to is that the multiple steady states found here could disappear when a fully coupled state-of-the-art climate model is used. To my own experience, the cold and hot climate states (even the cold state is still quite warm) presented in this manuscript could not coexist at the same forcing in the NCAR model family. Especially, the sea ice used in the model of this manuscript is thermodynamic only. When the NCAR model was run in such mode, a so-called Jormungand state could be found (Abbot et al., 2011; https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2011JD015927) but never found in the fully coupled mode. Therefore, I think the authors should point out in the abstract or conclusion that the multiple steady states found in their study may depend on the specific configuration of their model, especially the neglect of sea-ice dynamics.
Moreover, I think a snowball Earth branch should exist in their model if the initial condition is cold enough. They do not need to explore the full branch but just confirming their existence is necessary in this kind of study.
L164: "stronger" than what? This makes the sentence hard to understand.
Fig. 6: It has been pointed in the literature that the strength of annual mean Hadley circulation may not be meaningful, can the authors please confirm that the strength of seasonal Hadley circulation has a similar trend?
Figs. 7 and 8: I think these two figures can be combined into one

Citation: https://doi.org/10.5194/egusphere-2023-1808-RC1
- AC1: 'Reply on RC1', Charline Ragon, 28 Nov 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1808/egusphere-2023-1808-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-1808-AC1
RC2:
'Comment on egusphere-2023-1808', Anonymous Referee #2, 05 Oct 2023

Ragon and others explore climate of the Permian-Triassic with the MITGCM. They find three equilibrium climate states with an initial CO2 of 320 ppm. From these initial states, they explore the range of stability and the importance of vegetation and carbon cycle feedbacks.
I found this to be an interesting study. The experiment design was well thought out. The text and figures were easy to follow. Here are some comments that I think could improve the manuscript.
A lot of steps went into the model spin up. They are well documented in the text, which I appreciate. Although the text is generally clear, I think a schematic of the spin up procedure would help the reader understand what was done.
There was a lot of work put into creating realistic boundary conditions. This is worthwhile since most previous MITGCM multiple equilibria studies used idealized configurations. One of the benefits of realistic boundary conditions is model-proxy comparison. The authors speculate that the mismatch between HadCM3 simulations and a proxy reconstruction might be the result of multiple equilibria. However, the authors do not make any model-proxy comparisons themselves. I do not think many temperature records of the Permian-Triassic exist, but it is still worth doing.
On the point of realism, the range of CO2 for these equilibrium states is quite small from a geologic perspective. What are the implications of this? Also, a few sentences of discussion about the biomes of the Permian-Triassic versus the biomes in BIOME4 would be worthwhile.
Line 81-82: Can you provide a bit more information about how the model conserves energy?
Line 84: I do not think Foster et al. (2017) is the original source. Maybe Gough (1981)?
Line 106: Is it possible the tipping points could occur with less forcing if the simulations were run for more than 100 years?
Line 119: Is there any transpiration component in the MITGCM?
Line 123: “run BIOME4 again”
Line 143: 0.1 ppm per year seems like a large drift on geologic time scales. Does it look like the carbon is heading towards an equilibrium or continually drifting?
Line 203: “to simulate larger pCO2 values”
Line 204: To clarify, the model boundary conditions are largely responsible for the high temperature sensitivity?
Line 212: Based on Figure B1, it does not seem like the cold state can be reached from the warm state, so the loop is not closed. What are the implications of this with respect to real climate evolution?
Line 216: I do not follow this argument. HadCM3, like other higher complexity models, does not produce multiple equilibria. CO2 and proxy reconstruction uncertainties seem more probably explanations for the mismatch. Are you arguing that HadCM3 is somehow stuck in a cold equilibrium solution? Also, is it OK to cite an EGU presentation?
Table 3: I am confused by the differences in energy balance between the ocean surface and TOA. Where is the energy going?
Some of the figures would be easier to interpret with difference plots. I think figures 7, 8, and 9 in particular.
Figure D1: I do not understand the temperature response with the carbon cycle. Why does the sensitivity change, especially in the warm state? Also, I am surprised that turning on the carbon cycle did not lead to large changes in atmospheric CO2. Any explanation?

Citation: https://doi.org/10.5194/egusphere-2023-1808-RC2
- AC2: 'Reply on RC2', Charline Ragon, 28 Nov 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1808/egusphere-2023-1808-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-1808-AC2
RC3:
'Comment on egusphere-2023-1808', Anonymous Referee #3, 12 Oct 2023

The presented manuscript describes how up to three different climatic states exist under the same forcing in a climate model of the Permian-Triassic paleogeography. Multistability is further shown to exist also when vegetation and carbon cycle feedbacks are activated and the existence of multiple stable states is proposed as an explanation for the observed high climate variability of the PTB, as well as discrepancies between numerical simulations and geological data. The authors use MITgcm, a coupled general circulation model with a coarse resolution that properly resolves the ocean circulation, but only has a relatively simple representation of atmospheric dynamics (e.g. only 5 vertical layers).
The manuscript is generally well-structured and easy to follow. The language and grammar probably need a revision. The finding of several (non-snowball Earth) stable climate equilibria at the time of the PTB is interesting and, to my knowledge, has not been shown before. However, I have a major concern about the validity of the findings, coming from some of the presented simulation results:
i.)The "hot" state has a global mean temperature of 30.9°C at a CO2 concentration of 320 ppm and a solar forcing that is ~2% weaker than the present-day forcing. To put this in other words, even though the CO2 concentration is 20-25% lower than the modern values and the sun is 2% weaker than today (equivalent to roughly another halving of the CO2 concentration), the global climate of the hot state is simulated to be approximately 16°C warmer than the present-day climate. I am aware that a different continental distribution can lead to very different global mean temperatures, but this contrast to the present-day climate is so extreme that I am really wondering where this is coming from? An explanation for this extreme state is not given in the manuscript. In general, all three states ("cold", "warm" and "hot") have pretty high global mean temperatures. Is it because the land surface has a comparably low albedo value? Does the continental distribution lead to a much smaller cloud coverage? Or is there a very strong water vapour feedback in the MITgcm atmosphere component? A closer investigation of this aspect would, in my opinion, not just be interesting, but is actually crucial, as otherwise the validity of the model results is very questionable.
ii.)The plot of global mean temperatures in Fig. 11 and the values in Tab. 5 highlight another extreme (maybe even unrealistic?) aspect of the simulated model results: the climate sensitivity seems to be extremely high. The slopes of the lines in Fig. 11 indicate that the climate sensitivity of this setup is 15-18 °C of warming per doubling of CO2. An increase of just 8 ppm in the "warm" state led to a temperature increase of 1.45 °C (Tab. 5). If the modern climate would be anywhere near this sensitivity, humanity would be doomed already (we added ~ 140 ppm carbon to experience a comparable warming). The most recent IPCC estimate of modern climate sensitivity is ~2.5-4 °C (likely range). Again, a different continental setup might explain some of the differences to the modern state, but the climate sensitivity in this study is so extreme that it requires a convincing explanation. Otherwise the model results cannot be viewed as reliable.
iii.) I find it quite surprising that the deserts are smallest in the "hot" state. The authors claim that this is because there is more precipitation in that state. However, the annual mean surface air temperatures are 40-50°C (daily temperature extremes should then be around 70-80°C) in tropical and subtropical regions, where their model is simulating a vegetation cover of "forbland and dry shrubland". I am no expert in vegetation cover, but could any plant survive temperatures of >70°C, even if it is just for a few hours a day? Additionally, even though there might be more precipitation in the hot state, this would probably be more localized in individual extreme events and not fall evenly. I see a potential discrepancy here, because the vegetation model is only fed with long-term monthly mean values, which don't capture this variability.
iv.) It is also quite unusual that the meridional overturning circulation (MOC) of the ocean is much stronger in the hot state, especially since there is a weakening of the MOC, when going from the cold to the warm state. Furthermore, in the hot state there seems to be a cell near the equator where the water transport is out of the bounds of the colorbar, i.e. has a transport of >100 Sv, which is extremely high and - to my knowledge - unrealistic for a supposed equilibrium state. Where is the energy that drives a constant massive ocean overturning coming from?

At this point, I am having a hard time believing the outcome of the simulation results, especially with respect to the high climate sensitivity and the generally unusual characteristics of the "hot" state. When seeing these results, my first guess would be that the whole"hot" state is a numerical artefact and that the atmosphere component of MITgcm is not doing a decent job here in general. In order to oppose my concern, the authors could present results of a reference simulation of 1850-today, to show that this version of MITgcm (with the necessary adaptions to the modern state) is able to get the historical warming roughly right (or one pre-industrial simulation and one with a more recent CO2 concentration at ~400 ppm, if that is easier). Additionally, I really need to see a discussion of how the extreme and unusual results of the hot state can be explained physically.
As the existence of the "hot" state at such low CO2 concentrations is almost impossible in my eyes and given the other strange features mentioned above, I highly doubt the reliability of the presented results. Since the whole point of this manuscript is based on the existence of multistability, I have no other option than to suggest a rejection of the manuscript, unless the authors provide an elaborate and convincing explanation of why these extreme results are realistic.

Specific comments

- The language/grammar of the manuscript needs a revision. Very often I have the feeling that a "the" or similar article is missing (as an example in line 184 the sentence should be "In the hot state,...", right?).
- The whole discussion neglects the fact that there should be another stable climate: the snowball Earth. This should be mentioned at least once at some point.
- I would appreciate some more introduction as to why the Permian-Triassic Boundary is an important/interesting period to study? Right now the introduction is only about multistability and tipping points.
- The model has no sea-ice dynamics, which would strongly impact individual climate states that have some sea ice. Given this shortcoming, also the small range in which the "warm" state exists is questionable.
- the choice of colors in the upper panel of Fig. 2, using darker colors for a higher albedo, seems odd. I would reverse the colorbar
- line 143: averaged over which time scale?
- a short description of how the carbon cycle model works would be very helpful. Right now, I cannot judge whether this model is sufficient to provide a proper representation of the carbon cycle during the PTB
- lines 239-248: how do the plants survive these extreme temperatures in the warm state? Also, there is more carbon stored in vegetation in the hot state. This also means there is a lot of fuel for fires, which should occur very often given the extremely high temperatures. Is this self-consistent?
- Figure captions could include more information to make the figures a bit more self-evident (e.g. Fig. 7-9)
- adding some contour lines in e.g. Fig. 3-4 would really help for readability
- The first paragraph of the conclusion is rather a repetition of the introduction
- I don't understand the way that uncertainty in numbers is represented. For example in Tab.4: in the initial state of the hot state, the second iteration of the warm state and the fourth iteration of the cold state the uncertainty of SAT is much larger (9 instead of 1) than in the other states. Do these three cases actually have an uncertainty of 0.9°C and not 9°C? Otherwise the results seem to suggest some kind of instability. Do I understand those numbers correctly?
- The river map in Fig. A1 could simply be added to Fig. 1. The Appendix A is then obsolete, as the content is also already mentioned in the model description section.
- what about other greenhouse gases? Are they held fixed at some values? Which values were used?

Some technical corrections:

- labelling subfigures in Fig. 2 with (a) and (b)
- Fig. 10: It says that the line thickness varies with horizontal velocity, but it is not. Maybe make the thickness more sensitive to velocity or just drop the velocity scaling.

Citation: https://doi.org/10.5194/egusphere-2023-1808-RC3
- AC3: 'Reply on RC3', Charline Ragon, 28 Nov 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1808/egusphere-2023-1808-AC3-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-1808-AC3

Charline Ragon, Christian Vérard, Jérôme Kasparian, and Maura Brunetti

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Short summary

Deep-time climate simulations are challenging since sedimentary data are too sparse and uncertain to fully constrain initial and boundary conditions. By exploring a wide range of initial conditions, we find three alternative steady states with a temperature difference of around 10 °C in simulations using the Permian-Triassic paleogeography. This opens the possibility of explaining climatic variations in Early Triassic geological records through tipping mechanisms between steady states.


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