CO<sub>2</sub> and summer insolation as drivers for the Mid-Pleistocene transition

Scherrenberg, Meike D. W.; Berends, Constantijn J.; van de Wal, Roderik S. W.

doi:https://doi.org/10.5194/cp-2024-57

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https://doi.org/10.5194/cp-2024-57

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22 Aug 2024

| 22 Aug 2024

Status: this preprint is currently under review for the journal CP.

CO₂ and summer insolation as drivers for the Mid-Pleistocene transition

Meike D. W. Scherrenberg, Constantijn J. Berends, and Roderik S. W. van de Wal

Abstract. During the Mid-Pleistocene transition (MPT) the dominant periodicity of glacial cycles increased from 41 thousand years (kyr) to an average of 100 kyr, without any appreciable change in the orbital pacing. As the MPT is not a linear response to orbital forcing, it must have resulted from feedback processes in the Earth system. However, the precise mechanisms underlying the transition are still under debate.

In this study, we investigate the MPT by simulating the Northern Hemisphere ice sheet evolution over the past 1.5 million years. The transient climate forcing of the ice-sheet model was obtained using a matrix method, by interpolating between two snapshots of global climate model simulations. Changes in climate forcing are caused by variations in CO₂, insolation, as well as implicit climate–ice sheet feedbacks.

Using this method, we were able to capture glacial-interglacial variability during the past 1.5 million years and reproduce the shift from 41 kyr to 100 kyr cycles without any additional drivers. Instead, the modelled frequency change results from the prescribed CO₂ combined with orbital forcing, and ice sheet feedbacks. Early Pleistocene terminations are initiated by insolation maxima. After the MPT, low CO₂ levels can compensate insolation maxima which favour deglaciation, leading to an increasing glacial cycle periodicity. These deglaciations are also prevented by a relatively small North American ice sheet, which, through its location and feedback processes, can generate a relatively stable climate. Larger North American ice sheets become more sensitive to small temperature increases. Therefore, Late Pleistocene terminations are facilitated by the large ice-sheet volume, were small changes in temperature lead to self-sustained melt instead.

This concept is confirmed by experiments using constant insolation or CO₂. The constant CO₂ experiments generally capture only the Early Pleistocene cycles, while those with constant insolation only capture Late Pleistocene cycles. Additionally, we find that a lowering of CO₂concentrations leads to an increasing number of insolation maxima that fail to initiate terminations. These results therefore suggest a regime shift, where during the Early Pleistocene, glacial cycles are dominated by orbital oscillations, while Late Pleistocene cycles tend to be more dominated by CO₂. This implies that the MPT can be explained by a decrease in glacial CO₂ concentration superimposed on orbital forcing.

Received: 17 Aug 2024 – Discussion started: 22 Aug 2024

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Meike D. W. Scherrenberg, Constantijn J. Berends, and Roderik S. W. van de Wal

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RC1: 'Comment on cp-2024-57', Anonymous Referee #1, 28 Aug 2024 reply

Review of Scherrenberg et al., “CO₂ and summer insolation as drivers for the Mid-Pleistocene transition,” submitted to Climate of the Past.
The authors present modeling results of the Mid-Pleistocene Transition using a simple ice-climate model. This is a key approach for exploring the mechanisms behind this major climate transition, which remains an unsolved question in paleoclimatology. Their model is forced with caloric summer insolation, and an indirect atmospheric CO₂ reconstruction derived from leaf wax spanning the last 1.5 Ma. In addition to baseline simulations, the authors conducted additional simulations with either constant atmospheric CO₂ concentrations or constant insolation to disentangle the impact of each forcing on the climatic cycles of Pleistocene.
Overall, the manuscript is well-written and structured, although the second part is somewhat challenging to follow due to a mix of the results and discussion. The figures are well chosen and informative but would benefit from more specific description rather than broader summaries. I do have some questions about the way the authors interpret their results. Specifically, I find the boundary between the conclusions derived from their baseline simulations, the ones derived from their constant_ simulations, and the assumption based on paleoclimatic records very blurred. In addition, the use of the Yamamoto’s CO₂ record as a model forcing, that did not register any glacial CO₂ decrease through MPT, is not obvious to link with one of the main conclusions of the authors (i.e. the role of glacial CO₂ concentrations in the trigger of the MPT).

Major comments:
(1) The authors used the Yamamoto CO₂ record as forcing for their model. As it is the only continuous CO₂ record through the MPT, this choice is perfectly understandable. Nevertheless, the results derived from the model are per definition dependent on this record and this should be mentioned. A second point, probably the most important one, is the interpretation made of this record by the authors: “Yamamoto et al. (2022) (…) find a decrease in glacial CO₂ concentrations across the MPT”. I strongly disagree with this statement. Surprisingly, Yamamoto et al. found constant CO₂ glacial concentrations through the MPT, and a gradual increase in interglacial CO₂ concentrations. All along the paper, the authors suggests that a glacial decrease in CO₂ concentrations would have skipped some deglaciations during the MPT and the late Pleistocene: the baseline experiment performed using the Yamamoto’s record seems to demonstrate that it is not necessary to involve a decrease in glacial CO₂ concentrations to « reproduce » the MPT. It would be interesting if the authors could clarify the relation between the use of Yamamoto’s record as a forcing and their conclusion that a decrease in glacial CO₂ concentrations would have cause the MPT.
(2) This second point is related to the first one: it is unclear which conclusion is derived from their baseline simulation, from their constant_ simulations or from literature. For instance, in the case of the CO₂, the two approaches are opposite: in the baseline simulation, the authors forced their model with a glacial-constant CO₂ record while the comparison of constant_simulation would indeed allow to discuss the decrease of glacial CO₂ concentrations (but not only), even if these experiments are theoretical.
(3) The results derived from the constant_insolation are interesting. Nevertheless, I wonder to what extent these results (the absence of climatic cycles in the pre-MPT world) depend on the chosen insolation value (set as present-day by the authors). A comparison of different insolation values, similar to what was done for the CO2_constant experiments, would strengthen these conclusions.
(4) The authors have chosen to use a constant sediment map throughout the MPT, thereby excluding the possibility of testing the regolith hypothesis. While I understand this decision, as testing all MPT hypotheses is challenging and time-consuming, I strongly suggest that the authors exercise greater caution in interpreting their results : “Using these driving forces, we are able to capture the MPT without any change in (…) basal friction”. Simulating the MPT is not a binary process where one either succeeds or fails. While I agree that their simulations effectively capture the change in frequency, there is no guarantee that adding a variable sediment mask wouldn't improve the simulations. In my view, if the study still does not include a varying sediment mask, it cannot address the regolith hypothesis.
(5) I found the organization of the paper, particularly paragraph 3.2 in the results section, somewhat confusing. The discussion within the results section also contributed to this confusion. For example, the section concludes with one of the study's most significant findings: “A gradual decrease in glacial CO2 levels could therefore explain the MPT.” This conclusion is not a direct observation of the result. As a result, I believe the discussion section is somewhat brief and would have benefited from a more thorough comparison of the authors' results with recent studies that have also aimed to simulate the MPT. For instance, Willeit et al. (2019) discuss the role of CO₂ using multiple scenarios of unconstrained CO₂ forcing; Legrain et al. (2023) draw similar conclusions to the authors using conceptual modeling; and Verbitsky et al. (2018) present interesting findings based on a physical approach of the MPT.

Minor comments:
Line 8: Provide the approximate timing of the MPT.
Line 17: Here and after you only mention the frequency. Why don’t you discuss the amplitude change that should be observed during the MPT ?
Line 19: Here and after: when you are talking about low CO₂ levels, are you talking about low glacial or low interglacial CO₂ levels ? Or both ? Please be more specific to help the reader to correctly understand your findings.
Line 21: Considering a glacial climate as a “relatively stable climate” could be questioned.
Line 26: I think this is not surprising as the CO₂ is part of the forcing index. From which simulation do you derived these results ? Baseline or the comparison between the constant_ CO2 ?
Line 29: Be cautious with this kind of assessments: “The MPT can be explained by”, especially in this case: I don’t see which simulations can led you to this conclusion, as Yamamoto’s record does not register any glacial CO₂ decrease and the constant_CO2 simulation does not make any difference between the interglacial and glacial CO₂ decrease. I understand that the abstract is not the appropriate for arguing this, but I think this is a crucial point that deserve more explanation in the discussion.
Lines 45-52: The three concepts (ice sheet, regolith, and CO₂) are nicely explained, but I wouldn’t put the first one at the same level as the two others: regolith and CO₂ would be a primary cause of MPT, and the ice sheets are more a secondary response to an initial shift, of what I understand.
Lines 78-80: Here and after: The most important thing to mention when comparing CO₂ record from ice core to other records is the fact that CO₂ is directly measured in the air trapped in the ice, while other CO₂ records used several transfer function, physical and chemical assumption to go from the initial proxy to the final CO₂ record.
Line 83: I would mention here that it is the only continuous CO₂ record across the MPT, and it is associated with a new and still discussed proxy of CO₂ concentrations. The apparently well correlation over the 800-0 ka with ice core record comes from the fact that the amplitude of CO₂ concentrations was calibrated on the ice core record itself.
Line 87: Why do you only focus on frequency and not on amplitude ? The change of these two parameters is equally crucial in the definition of the MPT.
Line 107: I understand that everything could not be tested in a single study, and thus the authors choose to keep a constant sediment map. But I would not argue that this approach would help you to solve the answer: does the MPT can be captured without any changes in basal friction? Because the way you will capture the MPT is not a yes/no answer. Reversely, I would say that it would be very interesting to quantitatively described what are the results improvement using a variable sediment mask rather than a constant one (as the approach made by Willeit et al. 2019, science Advances). If there is no significant gain using a variable mask, then you can mention that your model suggests that a variable sediment mask is not relevant to better capture the MPT.
Line 167: I appreciate the efforts of the authors to perform an additional baseline_icecore experiment but I am not sure if this experiment is relevant as the two records are similar over the 800-0 ka period, due to the tunning performed in Yamamoto et al. (2019) on the ice core record.
Lines 173-175: There is a structure problem in the sentence.
Line 183: You go a bit too fast in the description of your experiment: The amplitude of climate cycles are not significantly different before and after the MPT. I think it is a limitation that deserve to be discussed further.
Line 204 and after : The section 3.2 is a combination of results and discussion. Understanding what is deduced from a direct observation is difficult.
Line 205: Same remark as for line 107: it is not needed to insist on this point as an argument.
Line 219-220: I do not understand from which result this assumption comes from.
Line 265: Are we still talking about glacial CO₂ levels ? Or is it an averaged CO₂ concentration level over an entire climate cycle ?
Line 276: Why do you chose the present-day insolation value ? Intuitively I would have taken the average insolation value over the past 1.5 Ma.
Line 276 and after: This result is interesting, but how would sensitive is it from your chosen insolation value?
Line 288 – 289: Do you perform several simulations (increasing by 10ppm CO₂ concentrations) and pick up a posteriori three representative ones, or did you design a priori the experiments with these three specific values for any reason ?
Line 304: This assumption clearly could not be part of the result section but should be included in a broader discussion. Also, I would be careful with the use of “explain the MPT”.
Line 320: I am not sure to understand how you came to this conclusion.
Discussion section: Some of the results are not contextualized with recent studies that have attempted to model the MPT (e.g. Willeit et al. 2019, Legrain et al. 2023, Verbistky et al. 2018). Some of these articles would reinforce and support the authors' conclusions, or else highlight interesting nuances and diversity in the MPT modeling results.
Line 332: “CO₂ is high enough”: would you detailed what you are talking about specifically? It is not obvious that CO₂ levels of Early Pleistocene were higher than during late Pleistocene. Especially Yamamoto’s record proposes that interglacials CO₂ levels are higher during late Pleistocene.
Line 333: “CO₂ is too low”. If this observation comes from one of your simulations, please specify which one (baseline or constant_ CO₂). If it comes from the paleodata record, please quote the paper from which you get this information.
Line 334-335: I would be careful about the conclusion coming from these CO₂ _constant simulations. A decreasing CO₂ trend throughout the MPT is not similar to successive simulations with constant CO₂ levels, but at decreasing values.
Line 336: I am lost: “CO₂ levels have continued to decrease”: what are you talking about ? Your simulations ? Which one? Or paleodata records ?
Line 364: Please mention the fact that it is an indirect method to reconstruct CO₂ concentrations and not a direct measurement.
Line 368: I strongly disagree with the statement that Yamamoto et al. (2022) and Hönisch et al. (2009) find a decrease in glacial CO₂ concentrations. Yamamoto et al. (2022) find constant glacial CO₂ concentrations and gradually increasing interglacial CO₂ concentrations through MPT. Regarding Hönisch et al. (2009), the authors conclude their abstract as following: “atmospheric CO₂ did not decrease gradually as would be expected were it to be the driver of the transition.”. Nevertheless, it is true that more recent boron isotopes CO₂ reconstructions propose a gradual decline of glacial CO₂ through the MPT: you would refer to Chalk et al. 2017 (see Fig. 4) rather than Hönisch et al. (2009). The fact that Yamamoto’s CO₂ record does not evidence any decline of glacial CO₂ concentrations is quite problematic for one of the conclusion of this study (a decline of glacial CO₂ concentrations would have trigger the MPT), as it is the record used as a forcing in the baseline experiment of the authors.
Line 369: It is relevant to compare your results to Watanabe, you could do the same for your CO₂ results with other modelling studies (e.g. Willeit et al. 2019).
Line 372: I think there is a grammatical problem in the sentence.
Line 390: I would split the last sentence into two for ease the readability.
Figs. 4 and 8: Why not use choose a sea level reconstruction that spanned the last 1.5 Ma ? Here we can not compare your modelled sea level with other reconstructions in the early Pleistocene.
Fig. 6: Adding a quantitative x-axis would enhance the readability of the figure.

References:

Chalk, T. B., Hain, M. P., Foster, G. L., Rohling, E. J., Sexton, P. F., Badger, M. P., ... & Wilson, P. A. (2017). Causes of ice age intensification across the Mid-Pleistocene Transition. Proceedings of the National Academy of Sciences, 114(50), 13114-13119.
Legrain, E., Parrenin, F., & Capron, E. (2023). A gradual change is more likely to have caused the mid-pleistocene transition than an abrupt event. Communications Earth & Environment, 4(1), 90.
Verbitsky, M. Y., Crucifix, M., & Volobuev, D. M. (2018). A theory of Pleistocene glacial rhythmicity. Earth System Dynamics, 9(3), 1025-1043.

Willeit, M., Ganopolski, A., Calov, R., & Brovkin, V. (2019). Mid-Pleistocene transition in glacial cycles explained by declining CO2 and regolith removal. Science Advances, 5(4), eaav7337.

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Citation: https://doi.org/10.5194/cp-2024-57-RC1
RC2: 'Comment on cp-2024-57', Anonymous Referee #2, 30 Aug 2024 reply

This manuscript studies the causes of the Mid-Pleistocene transition (MPT) with an ice sheet / climate coupled model forced by CO2 variations and orbital variations. The ice sheet model used is IMAU-ICE version 2.1, a vertically integrated model. The transient climate forcing of the ice sheet model is derived using a matrix method by interpolating snapshots of global climate simulations. This method allows us to provide transient climate forcing at a significantly reduced computational time compared to GCM’s or intermediate complexity models, albeit without all involved interactions between climate components taken into account. The CO2 forcing used is the one from Yamamoto et al. (2022) based on a leaf-wax indicator. Both sea level and benthic oxygen-18 variations are simulated.
The authors are able to simulate the change of frequency of climate variations during the MPT with this setup. They argue that, because CO2 is the only forcing which has long term variations, it should be the cause of the MPT in the model. They explain that there are 3 regimes of ice sheet variations. Small ice sheet tend to disappear with a relatively small climate forcing. Middle-size ice sheets tend to grow. And large ice sheets are unstable because of several feedbacks that are discussed. They then perform several sensitivity experiments with constant CO2 levels or constant orbital forcing.
Overall, I find the manuscript interesting and well written. The conclusion that a gradual decrease in CO2 is responsible for the MPT coincides with the conclusion of Legrain et al. (Earth. Com. Env., 2023) from a conceptual model. The three regimes of ice sheet variation correspond to what was first proposed by Paillard (1998) and later by Parrenin and Paillard (2003, 2011) with their conceptual models. The model used is quite complex and the work is quite impressive, but the results are presented in a accessible way.
Specific comments:
l. 207: It should be noted that the model does simulate quite well the change in frequency of sea level variations, but not so well the change in amplitude.
l. 209: There are also long term amplitude modulations in the orbital forcing. Legrain et al. (2023) have a simulation of the MPT with only orbital forcing, which is less realistic than the simulation with a long-term forcing, but which still contain some sort of frequency change.
l. 252: "50 m.s.l.e."
l. 372: "Simulating 1.5 million years requires..."
l. 381: it is a bit surprising to have a discussion section but no conclusion.

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Citation: https://doi.org/10.5194/cp-2024-57-RC2

Meike D. W. Scherrenberg, Constantijn J. Berends, and Roderik S. W. van de Wal

Data sets

IMAU-ICE output data: Northern Hemisphere ice sheet evolution of the past 1.5 million years M. D. W. Scherrenberg, C. J. Berends, and R. S. W. van de Wal https://surfdrive.surf.nl/files/index.php/s/nanSDelvSUtnep7

Model code and software

IMAU-ICE model code M. D. W. Scherrenberg, C. J. Berends, and R. S. W. van de Wal https://surfdrive.surf.nl/files/index.php/s/nanSDelvSUtnep7

Meike D. W. Scherrenberg, Constantijn J. Berends, and Roderik S. W. van de Wal

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Short summary

Glacial cycle duration changed from 41.000 to 100.000 years during the Mid-Pleistocene Transition (MPT), but the cause is still under debate. We simulate the MPT with an ice-sheet model forced by prescribed CO₂ and insolation, and simple ice-climate interactions. Before the MPT, glacial cycles follow insolation. After the MPT, low CO₂ levels may compensate warming at insolation maxima, increasing the length of glacial cycles until the North American ice sheet becomes large and thereby unstable.


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CO2 and summer insolation as drivers for the Mid-Pleistocene transition

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CO₂ and summer insolation as drivers for the Mid-Pleistocene transition