the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Jul 2026
Relative contributions of CO2 and ice sheets on the Mid-Pleistocene Transition
Abstract. The Mid-Pleistocene Transition (MPT) represents one of the most prominent and debated climate reorganizations within the long-term cooling trend of the last 3.6 million years. Although the role of ice sheets, greenhouse gases and their combined effects have all been implicated, their individual contributions to the MPT remain unresolved.
Here, we assess their respective roles using a unique set of four series of fully coupled equilibrium palaeoclimate model simulations spanning the last 3.6 Ma at 4-ka intervals, constrained by realistic boundary conditions. These simulations differ in their prescribed insolation, greenhouse gases and ice sheet forcings where a realistic baseline time evolving scenario is analysed against a series where each forcing is kept static throughout to constrain their individual impact on the MPT.
First, we evaluate the performance of the four sets of simulations including the combination of the three changing forcings against geological data. We find that the model captures the main characteristics of temperature variations identified by geological records, such as the global cooling trend over the past 3.6 Ma, acceleration of cooling at the MPT, with amplification of the glacial-interglacial cycles along with a change of pace from a 40-ka to a 100-ka cyclicity. Then, we compare the four simulated timeseries to untangle the individual role of each forcing through time and show that greenhouse gases exert a direct and dominant role on both the long-term global cooling trend and the shift in glacial-interglacial cyclicity associated with the MPT. Ice sheets primarily modulate the amplitude of glacial-interglacial variability through their influence on sea-ice formation and ocean circulation.
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Status: open (until 02 Sep 2026)
- RC1: 'Comment on egusphere-2026-3719', Anonymous Referee #1, 04 Aug 2026 reply
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RC2: 'Comment on egusphere-2026-3719', Anonymous Referee #2, 06 Aug 2026
reply
Review of 'Relative contribution of CO2 and ice sheets on the Mid-Pleistocene Transition'Â by Jeanne Millot-Weil et al.
In this manuscript the authors present a comprehensive new set of snapshot climate model simulations covering the last 3.6 million years, sampled at 4 kyr resolution. They run 4 sets of simulations with different combinations of time-dependent external forcings (orbital parameters, GHGs and continental ice sheet) that allow to separate the role of different forcings on the simulated climate trends and variability. The analysis is focused on global and hemispheric temperature changes and the results show that, when all forcings are included, the model generally reproduces long-term global temperature trends and glacial-interglacial variability reasonably well. The results provide new insights into the impact of different forcings in shaping the temperature response across the Mid-Pleistocene Transition. The paper is generally well written and the methods and results clearly presented. The labelling and the quality of the figures need to be improved.
I support publication of the paper in Climate of the Past after the comments below have been addressed.Â
Main comments
The title and the abstract of the paper give the impression that the paper explores the role of CO2 and ice sheets in driving the change in periodicity of the global climate from ~40 to ~100 kyrs across the Mid-Pleistocene transition, which is not supported by the main paper content, where the authors clearly state that conclusions about the origin of the MPT can not be drawn from the present set of simulations. Please rephrase the abstract to make this clear. In particular, the sentence ending with ‘…show that greenhouse gases exert a direct and dominant role on both the long-term global cooling trend and the shift in glacial-interglacial cyclicity associated with the MPT’ can be interpreted as stating that GHGs (CO2) caused the MPT. Also, I suggest reformulating the title to something like ‘Relative contribution of CO2 and ice sheets to temperature trends and variability across the Mid-Pleistocene Transition’.
The abstract could instead better summarise the main findings:
- GHGs dominate long-term temperature trends
- Ice sheets and GHGs have a comparable impact on glacial-interglacial variations, with ice sheets being more important for the pre-MPT and GHGs for the post-MPT
- The prescribed changes in the periodicity of the GHGs and ice sheets forcings across the MPT contribute roughly equally to the transition from 40 to 100 kyrs cyclicity in global temperature
- Something on the difference in NH vs SH response in ST and SST at the 40 and 100 ka scales
The last sentence in the abstract is about the amplitude of the glacial-interglacial cycles and seems to suggest that the appearance/disappearance of ice sheets (mainly over the NH land) have no direct effect on (global) temperature, only indirectly through sea ice and ocean circulation. I would expect at least the ice sheet surface albedo to have an impact. Moreover, sea level changes due to changes in land ice volume could also impact the carbon cycle and atmospheric CO2.
How non-linearly the different forcings combine to generate the All_forcings response is an interesting aspect of the analysis. However, the analysis is not cleanly done, as it accounts multiple times for orbital forcing. This is particularly relevant for the case of the glacial-interglacial amplitudes (e.g. Fig. 7b). A more appropriate decomposition to test non-linearities could be: All_forcings ~=(?) OrbGhg + OrbIce - OrbOnly. On a related note, why does Orb show up only in panel c of Fig. 7a?
For the trend analysis in Fig. 7a, could you elaborate a bit more on why the post-MPT is qualitatively different?Â
Minor comments
L. 51-53: This sentence is not very clear. You have 3 different time-dependent forcings that are combined in 4 different sets of simulations.
L. 59: ‘stability of the ice sheet’ -> ‘ice sheet dynamics’?
L. 91: Fig.1 (a) A) etc. is not standard figure panel indexing.
L. 104: ‘…forcings, here,…’ -> ‘…forcings. Here,…’
L. 105: ‘parts’ -> ‘sections’?
L. 110-111: Not everyone might be familiar with the concept of CO2 equivalent. I would suggest to add one sentence to explain what it is.
L. 111: ‘first’ or ‘last’?
L. 125: ‘all orbital scale parameters’ is not very clear to me. Maybe ‘all time-dependent forcings’ instead?
In Table 1, specify that ‘Constant’ means ‘Constant pre-industrial’
L. 140: Not just the orbital forcing, but also the other forcings were accelerated.
In Fig. 1 (b) A) Â the unit is wrong and the global ice volume is not the sum of the NH and SH components
L. 169: ‘before and after’ -> ‘before’
L. 170: L. 78: °C/ka is indicated as the unit for the temperature trends, but that can’t be right.
L. 288: Why ‘comparable’?
L. 306-307: I don’t fully understand this sentence.
L. 332: ‘…ice sheet variations show greater amplitude of variations than the greenhouse gases’; is this true? And how does this affect the long-term trends discussed here?
L. 333-342: I have to admit that I couldn’t really follow the reasoning here.
Fig. 8 shows normalized power spectra, which give the impression that OrbOnly shows the largest response to e.g. precession. It would possibly be more useful to see the un-normalized spectra, to make them directly comparable.
L. 406: ‘…the obliquity pacing diminishes when the 100-ka period of the MPT emerges’; where can this be seen?
L. 523: ‘sense’?
The sign convention on the trends in Fig. A2 is opposite to the one used in the main text.
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Citation: https://doi.org/10.5194/egusphere-2026-3719-RC2
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This manuscript presents a large ensemble of fully coupled climate model simulations to investigate the effects of greenhouse gases and ice sheets on Pliocene–Pleistocene temperature evolution and the Mid-Pleistocene Transition. The four sets of 919 snapshot simulations represent a substantial computational effort, and the resulting dataset has clear potential value. The results show that greenhouse gases play a direct and dominant role in both the long-term global cooling trend and the shift in glacial-interglacial cyclicity associated with the MPT. Ice sheets primarily modulate the amplitude of glacial-interglacial variability through their influence on sea-ice formation and ocean circulation. However, the current manuscript is dominated by statistical comparisons and does not yet provide a sufficiently detailed physical explanation of the simulated responses. My major concerns are as follows:
1. The central weakness of the manuscript is that its conclusions are based primarily on statistical contrasts among the simulated temperature time series. Trends, glacial–interglacial amplitudes and spectral characteristics are useful for describing the model response, but they do not identify the physical pathways through which greenhouse gases and ice sheets affect temperature.
2. The manuscript argues that ice sheets influence temperature indirectly through changes in sea ice and ocean circulation. This could be an important result, but the analyses presented do not yet establish these mechanisms. Moreover, if changes in ocean circulation are invoked as a central mechanism, the authors need to demonstrate that the 500-year snapshot simulations are sufficiently equilibrated.
3. The effects of greenhouse gases and ice sheets on temperature may depend on the background climate state. The manuscript would therefore benefit from comparisons among several representative climate states, including a warm Pliocene state and pre- and post-MPT glacial and interglacial states. For each state, the authors should examine not only the magnitude of the temperature response but also the underlying physical processes. Such analyses would help determine whether similar changes in greenhouse gases or ice sheets produce different temperature responses under different background conditions and would identify the mechanisms responsible for any state-dependent climate sensitivity.