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| 24 Sep 2025
Weakened and Irregular Miocene Climate Response to Orbital Forcing compared to the modern day
Abstract. Orbital forcing is a well-established driver of Pleistocene glacial-interglacial cycles, but its role in warmer climates remains less clear. Using climate model simulation, we assess temperature response to maximum and minimum boreal summer insolation during the Miocene and pre-industrial (PI) time. Both exhibit broadly anti-phased responses, but the Miocene shows weaker and less coherent patterns. Three notable differences emerge: (1) Boreal land regions respond less strongly in the Miocene due to dampened albedo feedbacks from altered vegetation; (2) Tropical Africa experiences stronger cooling under high insolation, driven by an intensified hydrological cycle with a broader Tethys Ocean under warm climate; (3) The Southern Ocean warms unexpectedly under low insolation, linked to sea ice involved albedo feedback. Lower internal temperature variability in the Miocene suggests enhanced climate stability and weaker orbital pacing. These findings highlight the importance of background climate state in shaping orbital-scale climate and interpreting proxy records.
Competing interests: Some authors are members of the editorial board of journal Climate of the Past.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: open (until 19 Nov 2025)
- RC1: 'Comment on egusphere-2025-4485', Anonymous Referee #1, 11 Oct 2025 reply
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RC2: 'Comment on egusphere-2025-4485', Anonymous Referee #2, 13 Oct 2025
reply
Review: Zhang et al., Weakened and Irregular Miocene Climate Response to Orbital Forcing compared to the modern day
Summary
This manuscript explored the impact of orbital forcing during the Miocene. The authors provided a comparison between a preindustrial and a high CO2 middle Miocene simulation and various sensitivity experiments with orbital min and max configuration. The authors suggest a weaker seasonality response to orbital configuration primarily due to the weak response of surface albedo feedback. Although the results presented are interesting, the current version of the manuscript presents more questions than answers. This is mostly due to insufficient analysis being presented.
Major
From a first-principle standpoint, why does your middle Miocene run have a weaker seasonality? Is it CO2, paleogeography, or ice-sheet configuration? The author raised all of these things in the introduction, but doesn’t really provide any answers.
It seems apparent in Figure 2 that your baseline MCO run shows an overall weaker seasonality, so in turn, your other sensitivity experiments also have a similar response to orbital changes. This leads to the question, is it because your PI run have lower CO2 that is leading to a stronger seasonality? Is it a general statement that warm climate intervals have weak seasonality or is it unique to the MCO?
Although it is interesting to see a weaker surface albedo response in the MCO simulations, it should be noted that this feedback is inherently linked to the prescribed vegetation and land ice. It is really only sea-ice and potentially cloud feedback that’s responding to the orbital changes. The authors should show which parameter is causing the large albedo change. I assume from Figure S6 that sea-ice in the PI is responding much more readily, where your MCO runs most likely do not have any sea ice.
It would be useful to see how the SST, deep ocean, and various MOC respond to the orbital changes. I suspect this could be one of the reasons why you have such a weak climate response. For example, the PI run would most likely have a strong AMOC and could be easily impacted by orbital changes, while your MCO 3x simulation does not. Also, the authors primarily use ocean proxy evidence to indicate a weaker orbital response; its only appropriate the authors should supply some type of ocean analysis.
The author should modify the use of the general term “Miocene” to either middle Miocene or MCO since the boundary condition utilized in the experiments does not represent paleogeography, vegetation, ice sheet and etc changes in the late Miocene.
Minor
The title is a bit misleading since regardless of the orbital changes with or without your MCO runs have a weak seasonality; nothing about it is irregular.
Line 57 extra parenthesis
Line 66 vague sentence. Mechanism for what? Also, plenty of examples of Miocene modeling targeting specific mechanisms including orbital forcing. A generic statement is a bit disingenuous.
Lines 106-112 TOA imbalance of .34 suggest not fully equilibrated I would suggest modifying “reach equilibrium” to quasi-equilibrium.
Line 190 citation needs to be fixed.
Line 269 I’m not sure what you mean by “less stable anti-phased behavior” ? Please provide a timeseries that shows fluctuation or instability in mean climate. From your results overall weaker seasonality would suggest much more stable climate.
Citation: https://doi.org/10.5194/egusphere-2025-4485-RC2
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The authors conducted two sets of experiments: one for the pre-industrial (PI) period serving as a control, and another for the Middle Miocene. Within each set, they examined two orbital configurations (orbmin and orbmax), analyzing seasonal and spatial patterns of air temperature across these different configurations. While the modeling results are interesting, the study's motivations remain somewhat unclear. I recommend major revisions to ensure the model results are discussed more appropriately and the research objectives are better articulated.
In the introduction, the authors review a range of literature documenting global mean climate changes across G-IG cycles during the (Late) Pleistocene. Similarly, in the third paragraph, they cite several benthic δ¹⁸O reconstructions (Holbourn et al., 2007; Tian et al., 2013; Westerhold et al., 2020, etc.) to highlight how global climate also oscillated in response to orbital forcing during the Miocene. It is important to note that these studies primarily examine how global mean climate varies in relation to boreal summer insolation (i.e., changes in the seasonal distribution of insolation). However, the analyses presented in this manuscript do not address the mean and magnitude of global climate variability; instead, the discussions focus exclusively on seasonal and spatial patterns. In fact, the manuscript does not provide any estimate of global mean surface temperature under different orbital configurations. As a result, the motivations outlined in the introduction appear largely disconnected from the rest of the paper.
Another drawback, in my opinion, lies in the underlying assumption of the experiments. This is illustrated by lines 85–86: “A cold-orbit simulation with minimum Northern Hemisphere (NH) summer insolation (orbmin), and a warm-orbit simulation with maximum NH summer insolation (orbmax), were performed for both the preindustrial (PI) and the Miocene.”
Why should one expect a warmer equilibrium climate in response to stronger Northern Hemisphere summer insolation (NHSI) during the Miocene? The phase relationship between NHSI and global (mean) climate described here is primarily based on observations from the Pleistocene and is likely influenced by boundary conditions specific to that period—such as the presence of large Northern Hemisphere ice sheets and the way ocean circulation modulates the global carbon cycle. There is no evidence that these conditions existed during the Miocene.
In order to determine the phase relationship between Northern Hemisphere summer insolation (NHSI) and global climate, we need a precise orbital-scale chronology that is independent of astronomical tuning—something that is currently unavailable for the Miocene interval. In other words, one could equally hypothesize that the Miocene was warm during periods of weak NHSI (and thus stronger Southern Hemisphere summer insolation) due to a reduced continental ice sheet and/or higher pCO2, potentially resulting from, for example, enhanced deep-ocean ventilation. In this scenario, the spatial climate responses could also differ significantly from those simulated in this study.
That said, if this paper is published without a major redesign of the experiment, the author may wish to revise the introduction to clearly articulate and define the scope of the research, clarify the underlying assumptions, and highlight the major limitations—ensuring readers are aware of the study’s potential biases, constraints, and motivations.
Some minor points:
Line 26: The Southern Ocean warms unexpectedly
Why “unexpectedly”
Line 27: “Lower internal temperature variability in the Miocene”
What do you mean by “Lower internal temperature variability”?
Line 37: “glacial-interglacial cycle through climate feedbacks (Milanković, 1969).”
The original work was published in the 1940s. Are you certain Milankovitch had any understanding of climate feedbacks at that time?
Line 70-72 “it explores how the absence of NH ice sheets, expanding Southern Ocean sea ice and strengthening monsoon rainfall shape Miocene orbital-scale climate variability on orbital scale.”
What evidence suggests that there was expanding Southern Ocean sea ice?
Line 80: “MioMIP2 protocol”
What is the MioMIP2 protocol? What is its source? How does it differ from the MioMIP1 protocol?
Line 242: rather than the 40 ka and 10 ka cycle of the Pleistocene
10 ka cycle?