the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Jul 2021
A theory of glacial cycles: resolving Pleistocene puzzles
Abstract. Since the summer surface air temperature that regulates the ice margin is anchored on the sea surface temperature, we posit that the climate system constitutes the intermediary of the orbital forcing of the glacial cycles. As such, the relevant forcing is the annual solar flux absorbed by the ocean, which naturally filters out the precession effect in early Pleistocene but mimics the Milankovitch insolation in late Pleistocene. For a coupled climate system that is inherent turbulent, we show that the ocean may be bistable with a cold state defined by the freezing point subpolar water, which would translate to ice bistates between a polar ice cap and an ice sheet extending to mid-latitudes, enabling large ice-volume signal regardless the forcing amplitude so long as the bistable thresholds are crossed. Such thresholds are set by the global convective flux, which would be lowered during the Pleistocene cooling, whose interplay with the ice-albedo feedback leads to transitions of the ice signal from that dominated by obliquity to the emerging precession cycles to the ice-age cycles paced by eccentricity. Through a single dynamical framework, the theory thus may resolve many long-standing puzzles of the glacial cycles.
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RC1: 'Comment on cp-2021-94', Anonymous Referee #1, 25 Aug 2021
The article claims to resolve "long-standing puzzles" with a new dynamical system model presented as the combination of an ocean bistable system coupled with an ice accumulation model. In essence, switches between two ocean circulation modes, emerging by application of the maximum entropy principle, and triggered by the astronomical forcing, control the growth and melt of continental ice. The direct response of ice sheets to insolation changes (Milankovitch theory) is neglected: everything is mediated by the ocean dynamics. The mid-Pleistocene transition is obtained by a reduction of the average convective flux and the 'long-standing puzzles' resolved here are (a) the absence of 400-ka signal (dominating eccentricity), the gain in 100-ka strength while eccentricity decreases, a so-called 'variable termination problem' associated with the variable length of ice age cycles, and another so-called 'polar synchronisation problem'.
The issue at stake here is that many dynamical system models may 'resolve' these puzzles, either with a synchronised oscillator or with non-linear resonance forced by the astronomical forcing. For convincing us of an actual 'resolution', the present model should have a clearly superior mechanical background compared to other models.
Neglecting entirely the direct ice sheet response to astronomical forcing is a provoking proposal, because there is so much evidence of direct insolation forcing of the net ice balance. The originality of the present setup is to use (following Ou, 2018) the maximum entropy production principle as a better way to capture the emerging heat transport by turbulent eddies. MEP is a fascinating but controversial topic. There is indeed a series of articles dating from the 2000-2010 decade that suggests a good success of MEP in predicting heat fluxes in systems with many degrees of freedom. However, some of its main proponents, including Deware and Jupp, follow the Jaynesian interpretation of statistical mechanics: they use it more as an inference than as a prediction principle. A match between observed macro-trajectories and MEP predictions is a suggestion that the right effective constraints on the flow have been identified (see, e.g. Jupp and Cox 2010).
This is important as for example, l. 170--172, the authors argue that sea-ice presence at the LGM would imply heat loss and weakened entropy production, thus "in contradiction with the MEP". Not necessarily. If it were to happen, it would not be a contradiction with MEP. It would imply that an additional constrain (here, the fact that sea-ice can actually develop) needs to be taken into account. MEP does no magic, alas !
Hence, my preliminary assessment is that although it is plausible that ocean dynamics have gone through some distinct states during the Pleistocene and that switches between these states have been somehow paced by the astronomical forcing, the proposal remains too speculative for claiming to have resolved "long standing puzzles".
Line by line comments
line 8: climate system is presumably "ocean"
line 33: 'should emerge from fundamental physics... which is yet to be delineated'. Not sure exactly what the sentence claims. Of course a model that refers to so-called fundamental principles (of physics and, perhaps, of biology ?) is to be preferred to a statistical fit. But the attempt here is not the only one to refer to such fundamental principles. Verbitsky et al. 2018 ESD claims also to provide a model rooted in fundamental principles and scaling laws, but with a focus on ice sheet dynamics and basal heat flux.ll. 57-60 : It is correct that we can't tell for sure what the pCO2 before 800 ka, but the claim of a long-term decrease in CO2 is reasonable given Pliocene proxies for CO2, even if they are very uncertain. The "no evidence" seems excessive. Atmospheric CO2 having "only a minor effect on the temperature" is a strange sentence. Yes, the radiative forcing of a 20 or 30 ppm change is small compared to the radiative effect of large ice sheet swings, but there are enough numerical simulations to claim that it is still likely to be enough to make a difference between a deglaciation terminating a 40-ka cycle, and an aborted termination that merely produces an interstadial, eventually leading to an 80-120ka cycle.
l. 79 'bistability has been demonstrated by coupled models'. Demonstrated is certainly too strong. That particular experiment by Manabe and Stouffer used the controversial freshwater flux adjustment.
Yin and Stouffer, Journal of Climate 2007, for example saw CM2.1 having no stable 'off state' (though whether two states could be obtained with a different freshwater flux background is another question). The Rahmstorf et al. 2015 is an authoritative intercomparison that remains citable today and indeed shows hysteresis for all models, but only EMICS.
This said, the recent article by Alkhayuon et al. 2019, Proceedings of the Royal Society A, presents a nice bifurcation structure for a box ocean model that may be of interest to the author.l. 102: 'glacial cycles are dominated by the subpolar temperature' : please expand on this.
l. 136 - 137: see above
l. 141: admittance could be better defined.
l. 155 : "veritable generalization" : see introductory comments
l. 172 : see introductory commentssection 3.1 and l. 401 : Numbers are not quite right (though order of magnitude are ok).
The range of mid-June insolation at 65N over the last million years is 435W - 559W/m2, so 123.2 W/m2.
The range of, e.g., mean insolation over the summer season (JJA, defined astronomically) is about 80W/m2.
The range of annual mean insolation at that latitude is 7 W/2m.l. 302 - 304: references on the stability of the Greenland ice sheet and its future fate definitely need an update (see, e.g. Van Breedam et al. 2020, Payne et al., 2021). Clarify also whether we speak of local temperatures or global averages. There is consensus for long-term commitment to melt Greenland for globally-averaged temperatures above around 2 deg C. Is the author disputing this claim ?
l. 316 : if the 'cooling is tectonic in origin', what would be the mechanism ? Generally tectonically-forced cooling implicitly refers to a tectonically-forced decrease in pCO2, though I concede there could be other mechanisms.
l. 319 : the ocean convective flux does not need to balance changes in net IR if the atmosphere heat flux divergence absorbs some of this change
l. 343 : the physical interpretation of the cause of a reduction in convective activity remains elusive.
l. 369 : "differing physics" : in what sense other models require differing physics ? Clearly the state of the ice sheet differs near full glaciation from early glaciation state, and it is therefore natural to expect different effects of the forcing. This does not require 'differing physics' but merely accounting for 'different states'.
l. 421 and ll. 442 - 443: The lack of figure with a simulated time series covering the last 800 ka is disturbing.l. 502 : The argument would be convincing if an alternative explanation was used to justify the change in q'_c.
l. 496-497 : The tri-state was certainly overly schematic, but there are some explanations to the unstable character of a deeply glaciated state; glaciological interpretations evoke bedrock depletion and basal flow, and proposals giving a role to the circulation in the southern ocean / carbon cycle have also been made (Bouttes, CPast, 2012, Paillard and Parennin 2004 ,EPSL).l. 539 : Are Antarctic volume fluctuations driven by sea-level not a well-accepted resolution of this so-called polar synchronisation problem ? Kawamura et al. does not actually mention a 'synchronisation problem'. They made their best to accurately date terminations and confirmed indeed a northern hemisphere trigger to southern hemisphere variations.
All that considered, it seems to me that the article makes no convincing case of a plausible alternative to the more classical approach focusing on the direct insolation forcing of non-linear ice sheet dynamics.
Citation: https://doi.org/10.5194/cp-2021-94-RC1 - AC1: 'Reply on RC1', Hsien-Wang Ou, 31 Aug 2021
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RC2: 'Comment on cp-2021-94', Anonymous Referee #2, 20 Sep 2021
In this paper the authors present and elaborate on a conceptual box model of the ocean-atmosphere coupled to a simple ice sheet model, where ice sheet mass balance is tied to ocean temperature. The ocean-atmosphere model is bistable and the different circulation modes result in growing or melting of the ice. Switches between different ocean states can be triggered by changes in orbital forcing.
The model is then applied to explain some of the major transitions in Quaternary climate dynamics by arguing that the global convective flux changed with Pleistocene cooling, giving rise to different model dynamics.The argument that continental ice sheet dynamics is controlled by ocean dynamics is new and goes against the rather well-established Milankovitch theory that postulates that glacial cycles are controlled by summer insolation through its effect on ice melt. This paper still represents an interesting, although highly controversial, piece of work, but should not be sold as having solved the major ‘Pleistocene puzzles’. Conceptual climate models (and this is just one of many out there) can be useful to understand particular features of a complex system but I don’t think that those are the tools that will allow us to resolve all ‘Pleistocene puzzles’. Physically based, spatially resolved, coupled climate-ice sheet models, whose parameters can be directly inferred from observations, are required for that.
General comments
Changes in the AMOC state do affect summer temperatures over North America and Europe (e.g. Jackson et al., 2015), but the magnitude of these changes is comparable or smaller than the direct changes in surface air temperature over land induced by changes in orbital configuration. Also, incoming solar radiation in summer is undoubtably important for the mass balance of an ice sheet. It therefore seems unlikely that the thermal state of the ocean alone should determine the position of the southern ice sheet margin, as assumed in the conceptual model presented in this paper. Also, there are some reconstructions of AMOC variations over glacial cycles. So the question is: should this different MOC states, implied by the conceptual model as drivers of the glacial cycles, not be reflected in proxy reconstructions of the AMOC? Is this not an ‘observable’ that would allow to test the model?
I find the discussion about role of CO2 for Quaternary glacial cycles on Lines 53-62 misleading and incomplete. To better understand the causes of the MPT it would be fundamental to know how CO2 changed across this transition. There are large uncertainties in CO2 reconstructions for the pre-ice core era, and a gradual CO2 decrease over the Pleistocene is still a possible scenario (e.g. Fig.6 in Berends et al., 2021). A step forward in this respect will be represented by the planned drilling of Antarctic ice cores with ice as old as 1.5 Myr. Also, the fact that a possible long-term decrease in CO2 is a consequence of the higher amplitude glacial cycles rather than the opposite, as suggested by the author, is highly speculative. The claim that CO2 variations have only a minor effect on global temperature and on glacial cycles in general is not corroborated by evidence and no references are provided to support this claim. Simplified coupled climate-ice sheet models forced with observed CO2 do produce realistic variations of global temperature between glacial and interglacial states (e.g. Fig. 8 in Ganopolski et al., 2010).
The author mentions several times the need for tuning diapycnal diffusivity in ocean GCMs, and how that makes these models lose credibility. I can’t see how all the assumptions and approximations made in deriving the simple model described in the paper are better than tuning a single parameter like diffusivity in an ocean general circulation model.
The long-term changes in the global convective flux play a crucial role in the explanation of the various Pleistocene climate regime shifts in the model. A more quantitative description of how this parameter is supposed to change under global cooling would be desirable. It is mentioned that it would depend on downward LW radiation, but then a 10°C (!) mean Pleistocene cooling is assumed, which, considering that temperature difference between glacials and interglacials is ~6°C, seems unreasonable. Furthermore, I imagine e.g. clouds, wind etc would also play a role.
A continuous 3 Myr long time series of the results of the model would be interesting to see. In principle such simulation should be possible to perform by simply prescribing a scenario for the convective flux evolution over time.
Minor comments
Lines 221-223: sentence is not clear
Lines 308-310: the warming threshold for Greenland melt is probably at ~2°C (Gregory et al., 2020; Robinson et al., 2012)
Lines 318-320: ‘This is because the cooling implies a drier air hence a smaller downward LW flux, which then requires a smaller global convective flux for the ocean heat balance --- all else being equal.’ Could the author elaborate on this? It seems far from obvious to me. How reasonable is the assumption of ‘all else being equal’?
References
Berends, C. J., De Boer, B., & Van De Wal, R. S. W. (2021). Reconstructing the evolution of ice sheets, sea level, and atmospheric CO2during the past 3.6 million years. Climate of the Past, 17(1), 361–377. https://doi.org/10.5194/cp-17-361-2021
Ganopolski, A., Calov, R., & Claussen, M. (2010). Simulation of the last glacial cycle with a coupled climate ice-sheet model of intermediate complexity. Climate of the Past, 6(2), 229–244. https://doi.org/10.5194/cp-6-229-2010
Gregory, J., George, S., & Smith, R. (2020). Large and irreversible future decline of the Greenland ice-sheet. The Cryosphere Discussions, 1–28. https://doi.org/10.5194/tc-2020-89
Jackson, L. C., Kahana, R., Graham, T., Ringer, M. A., Woollings, T., Mecking, J. V., & Wood, R. A. (2015). Global and European climate impacts of a slowdown of the AMOC in a high resolution GCM. Climate Dynamics, 45(11–12), 3299–3316. https://doi.org/10.1007/s00382-015-2540-2
Robinson, A., Calov, R., & Ganopolski, A. (2012). Multistability and critical thresholds of the Greenland ice sheet. Nature Climate Change, 2(6), 429–432. https://doi.org/10.1038/nclimate1449
Citation: https://doi.org/10.5194/cp-2021-94-RC2 -
AC4: 'Reply on RC2', Hsien-Wang Ou, 23 Sep 2021
The comment was uploaded in the form of a supplement: https://cp.copernicus.org/preprints/cp-2021-94/cp-2021-94-AC4-supplement.pdf
-
AC4: 'Reply on RC2', Hsien-Wang Ou, 23 Sep 2021
-
AC8: 'Comment on cp-2021-94', Hsien-Wang Ou, 27 Sep 2021
I want to applaud the editor for finding two highly qualitative reviewers, who have read the paper carefully and made constructive comments. They have raised a key issue about the role of the ocean, and I like to take this opportunity of my final response to further clarify my view.
- Both reviewers have invoked the well-established link of the glacial cycles to the summer insolation in questioning the ocean role. As stated in the first paragraph of my paper, I am fully subscribed to such link in late Pleistocene, which is simply an observational fact; my difference is that this link cannot be through the direct radiative forcing but through the ocean. That the ocean plays a central role in the glacial cycles is not “new”, but has been strongly argued by Broecker and Denton more than thirty years ago, but unfortunately it remains overlooked.
- It is true that the direct forcing is implicated in Milankovitch theory and widely assumed to this date, but it runs into incontrovertible obstacle from observations. The direct forcing of the ablation is primarily through summer SAT, but the latter basically tracks the SST for the glacial cycles with similar amplitude (of order 10 C). Not only is it greater than that can be induced by direct forcing, but if it were, then SAT would be driving SST --- against the second law.
- Given the above observation, the question becomes why the direct forcing is still widely ascribed. One possible explanation is that an interactive MOC of a turbulent ocean, which regulates the SST, remains out of reach of numerical calculations since they need to resolve eddies while carrying out integration over orbital periods. This does not immunize them against replicating the observed SST and SAT --- even though the direct linkage can be boosted by tuning to produce realistic simulations.
- While it is beyond the reach of numerical calculations, a turbulent ocean can be incorporated in a theory via MEP. Despite its initial guesswork, MEP has entered the mainstream in recent years given the books, special volumes and symposia dedicated to the subject and it has been successfully applied in climate theories. Just because the present paper may represent the first foray of MEP into paleoclimate arena does not make it “speculative”, and it certainly is well justified as a working hypothesis. On the other hand, the demonstrated potency of MEP in resolving diverse glacial puzzles suggests its possible utility, which may lead to its further exploration by paleoclimate researchers. Indeed, in my current research on abrupt climate changes, I found that MEP may explain their salient features as well, further strengthening its relevance.
- Above arguments only crystallize while I was contemplating the reviewers’ comments, for which I am thankful, and they point to the need for a thorough overhaul of the paper, which I will undertake regardless of your decision. Logistically, I do not know if the referees have the opportunity to provide additional input after digesting my original response, which would further aid my revision effort.
Citation: https://doi.org/10.5194/cp-2021-94-AC8
Status: closed
-
RC1: 'Comment on cp-2021-94', Anonymous Referee #1, 25 Aug 2021
The article claims to resolve "long-standing puzzles" with a new dynamical system model presented as the combination of an ocean bistable system coupled with an ice accumulation model. In essence, switches between two ocean circulation modes, emerging by application of the maximum entropy principle, and triggered by the astronomical forcing, control the growth and melt of continental ice. The direct response of ice sheets to insolation changes (Milankovitch theory) is neglected: everything is mediated by the ocean dynamics. The mid-Pleistocene transition is obtained by a reduction of the average convective flux and the 'long-standing puzzles' resolved here are (a) the absence of 400-ka signal (dominating eccentricity), the gain in 100-ka strength while eccentricity decreases, a so-called 'variable termination problem' associated with the variable length of ice age cycles, and another so-called 'polar synchronisation problem'.
The issue at stake here is that many dynamical system models may 'resolve' these puzzles, either with a synchronised oscillator or with non-linear resonance forced by the astronomical forcing. For convincing us of an actual 'resolution', the present model should have a clearly superior mechanical background compared to other models.
Neglecting entirely the direct ice sheet response to astronomical forcing is a provoking proposal, because there is so much evidence of direct insolation forcing of the net ice balance. The originality of the present setup is to use (following Ou, 2018) the maximum entropy production principle as a better way to capture the emerging heat transport by turbulent eddies. MEP is a fascinating but controversial topic. There is indeed a series of articles dating from the 2000-2010 decade that suggests a good success of MEP in predicting heat fluxes in systems with many degrees of freedom. However, some of its main proponents, including Deware and Jupp, follow the Jaynesian interpretation of statistical mechanics: they use it more as an inference than as a prediction principle. A match between observed macro-trajectories and MEP predictions is a suggestion that the right effective constraints on the flow have been identified (see, e.g. Jupp and Cox 2010).
This is important as for example, l. 170--172, the authors argue that sea-ice presence at the LGM would imply heat loss and weakened entropy production, thus "in contradiction with the MEP". Not necessarily. If it were to happen, it would not be a contradiction with MEP. It would imply that an additional constrain (here, the fact that sea-ice can actually develop) needs to be taken into account. MEP does no magic, alas !
Hence, my preliminary assessment is that although it is plausible that ocean dynamics have gone through some distinct states during the Pleistocene and that switches between these states have been somehow paced by the astronomical forcing, the proposal remains too speculative for claiming to have resolved "long standing puzzles".
Line by line comments
line 8: climate system is presumably "ocean"
line 33: 'should emerge from fundamental physics... which is yet to be delineated'. Not sure exactly what the sentence claims. Of course a model that refers to so-called fundamental principles (of physics and, perhaps, of biology ?) is to be preferred to a statistical fit. But the attempt here is not the only one to refer to such fundamental principles. Verbitsky et al. 2018 ESD claims also to provide a model rooted in fundamental principles and scaling laws, but with a focus on ice sheet dynamics and basal heat flux.ll. 57-60 : It is correct that we can't tell for sure what the pCO2 before 800 ka, but the claim of a long-term decrease in CO2 is reasonable given Pliocene proxies for CO2, even if they are very uncertain. The "no evidence" seems excessive. Atmospheric CO2 having "only a minor effect on the temperature" is a strange sentence. Yes, the radiative forcing of a 20 or 30 ppm change is small compared to the radiative effect of large ice sheet swings, but there are enough numerical simulations to claim that it is still likely to be enough to make a difference between a deglaciation terminating a 40-ka cycle, and an aborted termination that merely produces an interstadial, eventually leading to an 80-120ka cycle.
l. 79 'bistability has been demonstrated by coupled models'. Demonstrated is certainly too strong. That particular experiment by Manabe and Stouffer used the controversial freshwater flux adjustment.
Yin and Stouffer, Journal of Climate 2007, for example saw CM2.1 having no stable 'off state' (though whether two states could be obtained with a different freshwater flux background is another question). The Rahmstorf et al. 2015 is an authoritative intercomparison that remains citable today and indeed shows hysteresis for all models, but only EMICS.
This said, the recent article by Alkhayuon et al. 2019, Proceedings of the Royal Society A, presents a nice bifurcation structure for a box ocean model that may be of interest to the author.l. 102: 'glacial cycles are dominated by the subpolar temperature' : please expand on this.
l. 136 - 137: see above
l. 141: admittance could be better defined.
l. 155 : "veritable generalization" : see introductory comments
l. 172 : see introductory commentssection 3.1 and l. 401 : Numbers are not quite right (though order of magnitude are ok).
The range of mid-June insolation at 65N over the last million years is 435W - 559W/m2, so 123.2 W/m2.
The range of, e.g., mean insolation over the summer season (JJA, defined astronomically) is about 80W/m2.
The range of annual mean insolation at that latitude is 7 W/2m.l. 302 - 304: references on the stability of the Greenland ice sheet and its future fate definitely need an update (see, e.g. Van Breedam et al. 2020, Payne et al., 2021). Clarify also whether we speak of local temperatures or global averages. There is consensus for long-term commitment to melt Greenland for globally-averaged temperatures above around 2 deg C. Is the author disputing this claim ?
l. 316 : if the 'cooling is tectonic in origin', what would be the mechanism ? Generally tectonically-forced cooling implicitly refers to a tectonically-forced decrease in pCO2, though I concede there could be other mechanisms.
l. 319 : the ocean convective flux does not need to balance changes in net IR if the atmosphere heat flux divergence absorbs some of this change
l. 343 : the physical interpretation of the cause of a reduction in convective activity remains elusive.
l. 369 : "differing physics" : in what sense other models require differing physics ? Clearly the state of the ice sheet differs near full glaciation from early glaciation state, and it is therefore natural to expect different effects of the forcing. This does not require 'differing physics' but merely accounting for 'different states'.
l. 421 and ll. 442 - 443: The lack of figure with a simulated time series covering the last 800 ka is disturbing.l. 502 : The argument would be convincing if an alternative explanation was used to justify the change in q'_c.
l. 496-497 : The tri-state was certainly overly schematic, but there are some explanations to the unstable character of a deeply glaciated state; glaciological interpretations evoke bedrock depletion and basal flow, and proposals giving a role to the circulation in the southern ocean / carbon cycle have also been made (Bouttes, CPast, 2012, Paillard and Parennin 2004 ,EPSL).l. 539 : Are Antarctic volume fluctuations driven by sea-level not a well-accepted resolution of this so-called polar synchronisation problem ? Kawamura et al. does not actually mention a 'synchronisation problem'. They made their best to accurately date terminations and confirmed indeed a northern hemisphere trigger to southern hemisphere variations.
All that considered, it seems to me that the article makes no convincing case of a plausible alternative to the more classical approach focusing on the direct insolation forcing of non-linear ice sheet dynamics.
Citation: https://doi.org/10.5194/cp-2021-94-RC1 - AC1: 'Reply on RC1', Hsien-Wang Ou, 31 Aug 2021
-
RC2: 'Comment on cp-2021-94', Anonymous Referee #2, 20 Sep 2021
In this paper the authors present and elaborate on a conceptual box model of the ocean-atmosphere coupled to a simple ice sheet model, where ice sheet mass balance is tied to ocean temperature. The ocean-atmosphere model is bistable and the different circulation modes result in growing or melting of the ice. Switches between different ocean states can be triggered by changes in orbital forcing.
The model is then applied to explain some of the major transitions in Quaternary climate dynamics by arguing that the global convective flux changed with Pleistocene cooling, giving rise to different model dynamics.The argument that continental ice sheet dynamics is controlled by ocean dynamics is new and goes against the rather well-established Milankovitch theory that postulates that glacial cycles are controlled by summer insolation through its effect on ice melt. This paper still represents an interesting, although highly controversial, piece of work, but should not be sold as having solved the major ‘Pleistocene puzzles’. Conceptual climate models (and this is just one of many out there) can be useful to understand particular features of a complex system but I don’t think that those are the tools that will allow us to resolve all ‘Pleistocene puzzles’. Physically based, spatially resolved, coupled climate-ice sheet models, whose parameters can be directly inferred from observations, are required for that.
General comments
Changes in the AMOC state do affect summer temperatures over North America and Europe (e.g. Jackson et al., 2015), but the magnitude of these changes is comparable or smaller than the direct changes in surface air temperature over land induced by changes in orbital configuration. Also, incoming solar radiation in summer is undoubtably important for the mass balance of an ice sheet. It therefore seems unlikely that the thermal state of the ocean alone should determine the position of the southern ice sheet margin, as assumed in the conceptual model presented in this paper. Also, there are some reconstructions of AMOC variations over glacial cycles. So the question is: should this different MOC states, implied by the conceptual model as drivers of the glacial cycles, not be reflected in proxy reconstructions of the AMOC? Is this not an ‘observable’ that would allow to test the model?
I find the discussion about role of CO2 for Quaternary glacial cycles on Lines 53-62 misleading and incomplete. To better understand the causes of the MPT it would be fundamental to know how CO2 changed across this transition. There are large uncertainties in CO2 reconstructions for the pre-ice core era, and a gradual CO2 decrease over the Pleistocene is still a possible scenario (e.g. Fig.6 in Berends et al., 2021). A step forward in this respect will be represented by the planned drilling of Antarctic ice cores with ice as old as 1.5 Myr. Also, the fact that a possible long-term decrease in CO2 is a consequence of the higher amplitude glacial cycles rather than the opposite, as suggested by the author, is highly speculative. The claim that CO2 variations have only a minor effect on global temperature and on glacial cycles in general is not corroborated by evidence and no references are provided to support this claim. Simplified coupled climate-ice sheet models forced with observed CO2 do produce realistic variations of global temperature between glacial and interglacial states (e.g. Fig. 8 in Ganopolski et al., 2010).
The author mentions several times the need for tuning diapycnal diffusivity in ocean GCMs, and how that makes these models lose credibility. I can’t see how all the assumptions and approximations made in deriving the simple model described in the paper are better than tuning a single parameter like diffusivity in an ocean general circulation model.
The long-term changes in the global convective flux play a crucial role in the explanation of the various Pleistocene climate regime shifts in the model. A more quantitative description of how this parameter is supposed to change under global cooling would be desirable. It is mentioned that it would depend on downward LW radiation, but then a 10°C (!) mean Pleistocene cooling is assumed, which, considering that temperature difference between glacials and interglacials is ~6°C, seems unreasonable. Furthermore, I imagine e.g. clouds, wind etc would also play a role.
A continuous 3 Myr long time series of the results of the model would be interesting to see. In principle such simulation should be possible to perform by simply prescribing a scenario for the convective flux evolution over time.
Minor comments
Lines 221-223: sentence is not clear
Lines 308-310: the warming threshold for Greenland melt is probably at ~2°C (Gregory et al., 2020; Robinson et al., 2012)
Lines 318-320: ‘This is because the cooling implies a drier air hence a smaller downward LW flux, which then requires a smaller global convective flux for the ocean heat balance --- all else being equal.’ Could the author elaborate on this? It seems far from obvious to me. How reasonable is the assumption of ‘all else being equal’?
References
Berends, C. J., De Boer, B., & Van De Wal, R. S. W. (2021). Reconstructing the evolution of ice sheets, sea level, and atmospheric CO2during the past 3.6 million years. Climate of the Past, 17(1), 361–377. https://doi.org/10.5194/cp-17-361-2021
Ganopolski, A., Calov, R., & Claussen, M. (2010). Simulation of the last glacial cycle with a coupled climate ice-sheet model of intermediate complexity. Climate of the Past, 6(2), 229–244. https://doi.org/10.5194/cp-6-229-2010
Gregory, J., George, S., & Smith, R. (2020). Large and irreversible future decline of the Greenland ice-sheet. The Cryosphere Discussions, 1–28. https://doi.org/10.5194/tc-2020-89
Jackson, L. C., Kahana, R., Graham, T., Ringer, M. A., Woollings, T., Mecking, J. V., & Wood, R. A. (2015). Global and European climate impacts of a slowdown of the AMOC in a high resolution GCM. Climate Dynamics, 45(11–12), 3299–3316. https://doi.org/10.1007/s00382-015-2540-2
Robinson, A., Calov, R., & Ganopolski, A. (2012). Multistability and critical thresholds of the Greenland ice sheet. Nature Climate Change, 2(6), 429–432. https://doi.org/10.1038/nclimate1449
Citation: https://doi.org/10.5194/cp-2021-94-RC2 -
AC4: 'Reply on RC2', Hsien-Wang Ou, 23 Sep 2021
The comment was uploaded in the form of a supplement: https://cp.copernicus.org/preprints/cp-2021-94/cp-2021-94-AC4-supplement.pdf
-
AC4: 'Reply on RC2', Hsien-Wang Ou, 23 Sep 2021
-
AC8: 'Comment on cp-2021-94', Hsien-Wang Ou, 27 Sep 2021
I want to applaud the editor for finding two highly qualitative reviewers, who have read the paper carefully and made constructive comments. They have raised a key issue about the role of the ocean, and I like to take this opportunity of my final response to further clarify my view.
- Both reviewers have invoked the well-established link of the glacial cycles to the summer insolation in questioning the ocean role. As stated in the first paragraph of my paper, I am fully subscribed to such link in late Pleistocene, which is simply an observational fact; my difference is that this link cannot be through the direct radiative forcing but through the ocean. That the ocean plays a central role in the glacial cycles is not “new”, but has been strongly argued by Broecker and Denton more than thirty years ago, but unfortunately it remains overlooked.
- It is true that the direct forcing is implicated in Milankovitch theory and widely assumed to this date, but it runs into incontrovertible obstacle from observations. The direct forcing of the ablation is primarily through summer SAT, but the latter basically tracks the SST for the glacial cycles with similar amplitude (of order 10 C). Not only is it greater than that can be induced by direct forcing, but if it were, then SAT would be driving SST --- against the second law.
- Given the above observation, the question becomes why the direct forcing is still widely ascribed. One possible explanation is that an interactive MOC of a turbulent ocean, which regulates the SST, remains out of reach of numerical calculations since they need to resolve eddies while carrying out integration over orbital periods. This does not immunize them against replicating the observed SST and SAT --- even though the direct linkage can be boosted by tuning to produce realistic simulations.
- While it is beyond the reach of numerical calculations, a turbulent ocean can be incorporated in a theory via MEP. Despite its initial guesswork, MEP has entered the mainstream in recent years given the books, special volumes and symposia dedicated to the subject and it has been successfully applied in climate theories. Just because the present paper may represent the first foray of MEP into paleoclimate arena does not make it “speculative”, and it certainly is well justified as a working hypothesis. On the other hand, the demonstrated potency of MEP in resolving diverse glacial puzzles suggests its possible utility, which may lead to its further exploration by paleoclimate researchers. Indeed, in my current research on abrupt climate changes, I found that MEP may explain their salient features as well, further strengthening its relevance.
- Above arguments only crystallize while I was contemplating the reviewers’ comments, for which I am thankful, and they point to the need for a thorough overhaul of the paper, which I will undertake regardless of your decision. Logistically, I do not know if the referees have the opportunity to provide additional input after digesting my original response, which would further aid my revision effort.
Citation: https://doi.org/10.5194/cp-2021-94-AC8
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