the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Milankovitch Theory “as an Initial Value Problem”
Abstract. The dynamics of large ice sheets is fundamentally defined by the advection of mass and temperature. The timescale of these processes is critically dependent on the surface mass balance. Because of the ice-climate system's nonlinearity, its response to the orbital forcing in terms of engagement of negative and positive feedbacks is not symmetrical. This asymmetry may reduce the effective mass influx, and the resultant advection timescale may become longer, which is equivalent to a longer system’s memory of its initial conditions. In this case the Milankovitch theory becomes an initial value problem: Depending on initial conditions, for the same orbital forcing and for the same balance between terrestrial positive and negative feedbacks, the historical glacial rhythmicity could have been dominated either by the eccentricity period of ~100 kyr, or by the doubled obliquity period of ~80 kyr, or by a combination of both. In fact, empirical records demonstrate that the dominant period of the Late Pleistocene ice ages evolved from ~80-kyr to ~100-kyr rhythmicity. The quantitative similarity of this dominant-period trajectory and the one, made by the long-memory model, suggests that the records of the Late Pleistocene glacial rhythmicity could have been produced by a long-memory initial-value-dependent climate system, or, in other words, the slopes in empirical dominant-period trajectories are signatures of a long memory.
The scaling law of the dominant-period trajectory provides a theoretical insight into the discovered phenomenon. It reveals that this trajectory is dependent on the memory duration that is sensitive to initial conditions. The sensitivity of the memory duration to initial values emerges as the result of system’s incomplete similarity in two similarity parameters colliding into one conglomerate similarity parameter that is the ratio of the advection timescale and the orbital period. The critical dependence of this similarity parameter on poorly defined accumulation-minus-ablation mass balance as well as its dependence on initial values makes ice ages to be hardly predictable and disambiguation of paleo-records to be extremely challenging. The quasi-eccentricity periods produced by the long-memory system in response to pure obliquity forcing make a remarkable example of this challenge because in the time series they may be naively attributed to the eccentricity modulated precession forcing.
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Version 2 | 21 Nov 2024
RC1: 'Comment on egusphere-2024-1255', Anne Willem Omta, 21 Mar 2025- AC1: 'Reply on RC1', Mikhail Verbitsky, 01 Apr 2025
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RC2:
'Comment on egusphere-2024-1255', Anonymous Referee #2, 14 Jul 2025
The authors use a Saltzman-style 3-equation in 3-unknown model of the ice ages. They explore the response to pure obliquity, pure precession, and combined Milankovitch forcing, and examine the dependence on initial conditions. There is no question that the subject is interesting and important. The authors use a relatively simple model, which again is very appropriate in this context. It seems, however, that the paper largely repeats previously published analyses, which the authors do not discuss. I recommend a major revision that will allow the authors to search and examine the related literature, discuss it, and reformulate their analysis and discussion in a way that focuses on some novel elements to be identified.
The authors may want to take a look at Tziperman et al. 2006, "Consequences of pacing the Pleistocene 100 kyr ice ages by nonlinear phase locking to Milankovitch forcing", as well as papers that are cited and cite this paper. This could be a useful entry to the idea that glacial cycles are phase locked to Milankovitch forcing. Figures 1 and 2 in the current paper, and possibly additional ones, seem to repeat analysis in that paper and in related papers that have been written by multiple authors. Phase locking to Milankovitch implies that different initial conditions converge to the same ice volume trajectory. This seems directly relevant to the goals of this paper, yet this draft does not mention this concept.
The initial couple of paragraphs where simple models are advocated seem unnecessary in this research field where simple models have been used for many decades now, including in multiple interesting papers by the lead author.
I am not convinced the analysis of the linear equation (1) and its solution (2) adds a meaningful message. "A model should be as simple as possible, but not simpler". This model may be "simpler", given the sophistication of idealized glacial models in the literature over the past decades, including the work of the lead author over the past decades. This linear model shows no oscillations, does not include Milankovitch forcing, and its relevance to the ice age problem is questionable. I do understand the lesson the authors are trying to derive here, but it seems to me that a sentence or two could make the point without this linear equation. My feeling is that our understanding of ice age dynamics is too sophisticated for this linear equation to provide any new or useful insights.
Citation: https://doi.org/10.5194/egusphere-2024-1255-RC2 - AC2: 'Reply on RC2: "Initial Value Problem” vs phase locking concept', Mikhail Verbitsky, 16 Jul 2025
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Version 1 | 30 Apr 2024
Status: closed
-
Version 2 | 21 Nov 2024
RC1: 'Comment on egusphere-2024-1255', Anne Willem Omta, 21 Mar 2025- AC1: 'Reply on RC1', Mikhail Verbitsky, 01 Apr 2025
-
RC2:
'Comment on egusphere-2024-1255', Anonymous Referee #2, 14 Jul 2025
The authors use a Saltzman-style 3-equation in 3-unknown model of the ice ages. They explore the response to pure obliquity, pure precession, and combined Milankovitch forcing, and examine the dependence on initial conditions. There is no question that the subject is interesting and important. The authors use a relatively simple model, which again is very appropriate in this context. It seems, however, that the paper largely repeats previously published analyses, which the authors do not discuss. I recommend a major revision that will allow the authors to search and examine the related literature, discuss it, and reformulate their analysis and discussion in a way that focuses on some novel elements to be identified.
The authors may want to take a look at Tziperman et al. 2006, "Consequences of pacing the Pleistocene 100 kyr ice ages by nonlinear phase locking to Milankovitch forcing", as well as papers that are cited and cite this paper. This could be a useful entry to the idea that glacial cycles are phase locked to Milankovitch forcing. Figures 1 and 2 in the current paper, and possibly additional ones, seem to repeat analysis in that paper and in related papers that have been written by multiple authors. Phase locking to Milankovitch implies that different initial conditions converge to the same ice volume trajectory. This seems directly relevant to the goals of this paper, yet this draft does not mention this concept.
The initial couple of paragraphs where simple models are advocated seem unnecessary in this research field where simple models have been used for many decades now, including in multiple interesting papers by the lead author.
I am not convinced the analysis of the linear equation (1) and its solution (2) adds a meaningful message. "A model should be as simple as possible, but not simpler". This model may be "simpler", given the sophistication of idealized glacial models in the literature over the past decades, including the work of the lead author over the past decades. This linear model shows no oscillations, does not include Milankovitch forcing, and its relevance to the ice age problem is questionable. I do understand the lesson the authors are trying to derive here, but it seems to me that a sentence or two could make the point without this linear equation. My feeling is that our understanding of ice age dynamics is too sophisticated for this linear equation to provide any new or useful insights.
Citation: https://doi.org/10.5194/egusphere-2024-1255-RC2 - AC2: 'Reply on RC2: "Initial Value Problem” vs phase locking concept', Mikhail Verbitsky, 16 Jul 2025
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Version 1 | 30 Apr 2024
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Dmitry Volobuev
- V1, 30 Apr 2024
See attached file for the review