the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Jun 2022
Abrupt climate change and millennial-scale cycles: an astronomical mechanism
Abstract. Contributing to the poor understanding of abrupt climate change is the lack of a known mechanism for a ~1470-yr quasi-periodicity, leading to debates as to its existence. This oscillation is associated with the controversial Bond cycle, which has been by some as stochastic resonance, and is a harmonic resonating with Heinrich, Dansgaard-Oeschger ice-rafting debris events, as well as millennial-scale ENSO events in the Pacific. Suggestions of a solar link to the Bond cycle were made but there is no known solar periodicity of this length. Here, statistically-significant results of a comparison between TSI reconstructions based on Antarctic 10Be data and the modelled interaction of the solar and lunar cycles that produce a 1470-yr cycle are presented. These results confirm the cycle’s existence, its astronomical mechanism, and the major lunar role in the timing of all these ice-rafting debris events. The associated data show that the occurrence of Bond events coincides with maximum gravitational forcing and peak TSI, both associated with minimum Sun-Earth distance that are influenced by both the perihelion and the Moon. These findings are consistent with previous suggestions by Bond and other researchers that amplified gravitational and solar forcing may be involved. The results also indicate that the Moon’s gravitation influences patterns of cosmogenic isotopes at millennial time-scales.
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RC1: 'Comment on cp-2022-49', Anonymous Referee #1, 16 Sep 2022
The manuscript under consideration is a modified version of the manuscript rejected seven years ago. I did not review the previous version of this manuscript and did not make a detailed comparison between them, but the main results, figures and references are very similar. In particular, I found that practically all cited papers were published more than ten years, which is a serious shortcoming of the manuscript since significant progress in the understanding of Quaternary millennial-scale variability has been achieved during the past decade. The reviewers of the first version of the manuscript mainly criticized the central premise formulated in the first sentence, namely, that “the existence of a 1470-year cycle of abrupt climate change is well-established”. Now the author admits that the Bond cycle is “controversial”. Below I discuss the manuscript in a more general context. The appraisal of the manuscript depends on the answers to three questions:
- Is the 1470-yr cycle really observed in climate records?
- Does millennial-scale Quaternary variability associated with DO events and IRDs (i.e. Heinrich and similar events) require any external forcing?
- Is there any evidence that astronomical factors discussed by the author can influence Quaternary millennial-scale climate variability?
The answer to the first question, as has already been argued by the reviewers of the first version of the manuscript, is definitely no. 1470-yr cycle was popular at the beginning of 2000th primarily thanks to a very influential publication by Bond et al. (1997). However, later this periodicity went out of fashion since the existence of this cycle was not corroborated by other studies.
The answer to the second question is also no. Numerous modelling studies performed during the past decades demonstrated that the millennial-scale variability, such as Dansgaard-Oeschger and Heinrich events) can be fully explained by the internal instabilities emerging in the ocean-atmosphere system and ice sheets under glacial conditions without any external forcing (see, for example, review by Menviel et al, 2020).
Concerning the potential astronomical forcing of millennial-scale variability. The author considers two astronomical processes which together may produce 1470-yr cycle. The first is the well-known solar cycles. However, recently solar cycles were additionally "downgraded", and the current estimates of changes in TSI at multi-centennial time scales is only 0.1% which is too small to have any appreciable impact on climate. How the 133-yr cyclicity in Earth-Sun and Earth-Moon distances can affect climate, the author does not explain. Moreover, this periodicity is not seen in climate records. Thus, the answer to the third question is also no.
This is why I cannot recommend this manuscript for publication in Climate of the Past.
Menviel, L.C., Skinner, L.C., Tarasov, L. and Tzedakis, P.C., 2020. An ice–climate oscillatory framework for Dansgaard–Oeschger cycles. Nature Reviews Earth & Environment, 1(12), pp.677-693.
Citation: https://doi.org/10.5194/cp-2022-49-RC1 -
AC1: 'Reply on RC1', Alison Kelsey, 06 Oct 2022
The author thanks Reviewer 1 (R1) for their comments. However, there are several flaws and oversights in R1’s comments and these issues are highlighted in this author’s response as follows. This reviewer establishes three points on which to judge the merits of this article:
- A question of existence of the 1470-yr quasi-periodicity
- Need for external forcing
- Evidence of astronomical factors that influence Quaternary millennial-scale climate variability
Point 1:
Observation of the ~1470-yr quasi-periodicity in climate records is essentially an argument about whether this periodicity exists or not. On the matter of existence of the ~1470, Wolff (2015) states that, on the 1470-yr quasi-periodicity, the jury is still out and the matter has not been settled, reiterating his 2010 position (Wolff et al. 2010) that the matter was not resolved. One of the previous reviewer’s, Ditlevsen (2015), said “even though I am not convinced about the findings, I still find the paper interesting and worthy of publication” despite the debate over the existence of this quasi-periodicity.
This question really forms part of a complex and lengthy debate, with the issue being that the timing and number of Dansgaard-Oeschger events depends on the time-scale used (Wolff et al., 2010) and how they are defined (Alley et al., 2001; cf. Ditlevsen et al., 2005). Arguments for and against this periodicity are covered in Section 2 of this article, with attention given to countering existing arguments against the ~1470-yr quasi-periodicity (Section 2.1). Additionally, further chronological matters have been discussed in this article (Section 3.5) that have been ignored by R1.
Importantly, the results of this author’s research confirm the existence of this quasi-periodicity through statistical testing of a chronologically-anchored, normalised trigonometric model that is compared to an independent, Be10-based reconstruction of Total Solar Irradiance (TSI). This trigonometric model is underpinned by physical models of gravitation and TSI that are linked by distance (see this author’s response to point 3).
In all instances during the time interval of astronomical data, Bond events occur at times of maximum gravitational forcing (Kelsey 2018:119), associated with minimum Earth-Moon distances (Figure 4) that are magnified by minimum Earth-Sun distances occurring at perihelion. Fluxes in gravitation and TSI are inherently linked because of the reliance of both sets of calculations on distance. These results confirm Bond’s suggestion that the potential cause of Bond IRD events was an ocean-atmosphere link through solar forcing (Bond et al., 2001; cf. Braun et al., 2005; Schulz, 2002).
R1 does not comment on or acknowledge any of this material in justifying their answer to this question. Rather R1 talks about fashions and corroboration of the quasi-periodicity. It is not unusual for science to progress through waves of dominant paradigms, often with intevening decades between times of popularity such as occurred with the Milankovitch cycles (cf. Imbrie 1979; Summerhayes, 2015). This factor also affects referencing and the age of many sources. Lack of corroboration is not equivalent to proof of non existence. Research results presented here corroborate the existence of this quasi-periodicity.
Point 2:
R1’s argument of necessity in regard to point 2, is invalid and does not reflect what was said in the referenced article used to support their statement (cf. Menviel et al. 2020). Whilst R1’s assertion that numerical models fully explain the causes of millennial-scale variability emerging from the ocean-atmosphere system and icesheets, without resorting to external forcing, Menkiel et al 2020 state that “Palaeorecords and numerical studies indicate that the AMOC, with a tight coupling to Nordic Seas sea ice, is central to D–O variability, yet, a complete theory remains elusive.” They further state that “the sequence of events that led to D–O climatic variability is still highly debated.” Whilst several mechanisms have been proposed to account for D-O variability, none can account for all DO’s characteristics.
Menkiel et al highlight that “current Earth system models do not include all necessary components (for example, biogeo-chemistry, ice shelves, ice- sheet dynamics), inadequately represent important processes or cannot be integrated long enough under intermediate glacial conditions to simulate self- sustained D–O cycles. NADW formation in climate models is highly parameterized and not well constrained by observations, so there is little confidence in simulated changes in the strength and location of NADW formation in response to climate change, both past and future.”
An important factor in DO events is the Atlantic Meridional Overturning Circulation (AMOC), which is central to Earth’s climate and internal systems (Menviel et al. 2020). AMOC is part of Earth’s Meridional Overturning Circulation and is the major transporter of heat around the world. Wunsch (2010) suggests that the simple conveyor belt idea [Figure 2.4] is much more complex than often presented and that “only a tiny minority” has attempted to understand the underlying physics.
External forcing cannot be dismissed as unnecessary. Earth is not an isolated unit and must be considered in the context of its celestial neighbourhood (Kelsey 2018:48-83), as its neighbours (primarily the Sun and Moon) influence Earth’s internal systems through gravitational forcing and heat input. These factors influence both atmospheric and oceanic tides and are another piece of the puzzle, acting as a teleconnection between the two hemispheres. Internal systems work alongside external forcing and are not competing interests – they are interlinked.
Point 3:
Regarding evidence of astronomical factors that influence Quaternary millennial-scale climate variability, this author is surprised at R1’s comments. Abundant evidence has been supplied and appears to have been ignored by R1. This includes:
- The model of superimposed 209-yr and 133-yr cycles that emulates the ~1470-yr quasi-periodicity was statistically tested against an independent TSI reconstruction of Be10 data, producing X2, r values. Variance (r2), and significance (p) values (Section 4.2 and Table 4)
- The results of tests in (a) show a significant result with a strong correlation between the model and the Be-10-based TSI reconstruction.
- Visually, there is a striking visual similarity between the model presented here and the TSI reconstruction, as well as the Be10 flux (Section 4.2; see also Kelsey 2018:141).
- The parameters of the chronologically-anchored model and their values were justified and discussed within this article (Sections 3.4 and 4), including the incorporation of the 0.1% solar flux that R1 has deemed “too weak” to have an appreciable impact. Contra R1’s comments, results presented here show otherwise, as it plays an appreciable role in the amplification of the superimposed model’s patterns.
- Data presented here of Earth-Moon distances (Figure 4) show that Bond events of ice-rafted debris occur at minimum Earth-Sun distances that are associated with times of maximum gravitational forcing (cf. Figure 5.13, Kelsey 2018:119). The relationship between distance and gravitation is clearly stated within the article and this relationship is a commonly known scientific axiom.
- Data presented here (Figure 4) also shows pulses of cyclical 133-yr minimal Earth-Sun distances, viz maximum gravitational forcing. Contra R1’s comment that this cycle does not appear in climate records, this cycle is found in numerous climatic records referenced in section 2 of this article.
- The gravitational pull of the Moon influences the tilt of the Earth’s axis and consequently the 133-yr cycle also appears in solar declination data (Section 4.1).
- The strength of the 133-yr cycle is associated with the proximity of the lunation (New Moon) to the perihelion (closest point in the Earth’s orbit to the sun) (Section 4.1), when gravitational forcing and TSI are at their maximum.
- It is also commonly known that atmospheric and oceanic tides are also caused by the Sun and Moon and there is ample evidence of solar and lunar forcing in the palaeoclimatic record.
- Contra R1, an explanation of how distances can affect climate was provided.
On the previous article:
Regarding the general comments made by the reviewer re the previous article, R1 is wrong in inferring it is the same as the current article or that the main issue was about semantics that had only recently been accepted. The two articles are very different but related, forming integral parts of the same unpublished PhD thesis (Kelsey 2018). They share one figure in common, which was referenced in the second article.
The previous article presented research showing that an interacting combination of astronomical variables related to Earth’s orbit may be causally related to the ~1470-yr quasi-periodicity and several associated key isotopic spectral signals. It was a conceptual model and did not rely on statistical tests between the model and data. Its purpose was to present a framework of understanding. These variables were the ~11.4-yrs Schwabe sunspot cycle, the Metonic lunation cycle of 19 yrs, and an anomalistic year forcing at ~104 yrs.
The article presented here presents a different, more complex model that uses different variables as described within this article. This model presents a lower temporal resolution of solar and lunar forcing, which is underpinned by physical models (based on astronomical data) of TSI and gravitational flux (see author’s response to R1’s comments for point 3). Note that aggregation and different temporal resolution influence the appearance of spectral signals and oscillations in the palaeoclimatic record (Kelsey 2018:137-139). The purpose of this article was to describe the astronomical mechanism associated with the millennial-scale climate oscillations, and to provide evidence of the link between the two using statistical testing.
In relation to referencing, there are 10 references to material published in the last decade, and there should be additional references to Kelsey (2018). Also refer to author’s response to point 1 regarding age of references; for advancement in Quaternary millennial-scale variability, refer to author’s response to point 2. The issue regarding well-established and highly-debated/controversial was accepted at the time and is not a recent development.
References:
DITLEVSEN, P., 2015 Past Discuss., 11, C2224–C2226, 2015 www.clim-past-discuss.net/11/C2224/2015/
KELSEY, A. 2018. Astronomical forcing of sub-Milankovitch climate oscillations during the late Quaternary. PhD Thesis, School of Earth and Environmental Sciences, The University of Queensland. https://doi.org/10.14264/uql.2018.186
Other references appear within the currently submitted article.
Citation: https://doi.org/10.5194/cp-2022-49-AC1
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RC2: 'Comment on cp-2022-49', Anonymous Referee #2, 19 Oct 2022
Review of Abrupt climate change and millennial-scale cycles: an astronomical mechanism, by Alison Kelsey
General comments:
In this manuscript Alison Kelsey presents the results of the superimposition of lunar and solar periodic signals and compares it with a reconstruction of total solar irradiance.
This comparison appears to be satisfactory. This, together with the fact that the Earth-moon distance shows a modulation on a ~1400 yr periodicity (among others), allows the author to develop the article upon the idea that it represents a new mechanism to explain abrupt climate changes on millennial time scales.
In my opinion, although the former finding is of interest, the manuscript is addressed in a highly biased manner making it difficult to be published under its current form.
Showing that there is a robust solar irradiance and lunar cycle oscillation around 1500 years would be of major importance to the paleoclimatology research and worth publishing, but the current way on which these results are justified and framed in the paper is, in my opinion, erroneous.
Let me develop:
The first sentence of the abstract already illustrates this flawed approach. It reads: “Contributing to the poor understanding of abrupt climate change is the lack of a known mechanism for a ~1470-yr quasi-periodicity [...]”
Firstly, I guess what you mean here instead of “mechanism” is “forcing”, because even if the solar-lunar cyclicity described here is robust, the author is not contributing to the description of any mechanism that causes the climate to abruptly change on a ~1500 yr periodicity.
Secondly, I guess that what you mean by “a known mechanism for a ~1470-yr quasi-periodicity” refers in reality to a “known climate forcing”, because there are several proposed mechanisms, even in the articles cited here, to explain the climate variability on that time scale.
Finally, the lack of a known mechanism (forcing) does not contribute in any way to the poor understanding of abrupt climate change. The reasons of such a poor understanding lie on the complex character of the climate system together with the sparse character of the climatic proxies which do not fully allow for a robust discrimination of causes and effects. Besides, one could simply conceive that, even if such a forcing on a 1470 yr quasi-periodicity exists, it might be irrelevant in triggering the abrupt climate changes.
This biased approach is recognizable all over the manuscript. (see specific comments)
What the author is showing is that, according to her model and the good agreement with TSI reconstructions, there is a lunar-solar cyclicity around 1500 years that can potentially influence climate on that time scale. But this is far to represent an “astronomical mechanism for abrupt climate change and millennial-scale cycles” as suggested already by the title of the paper. In fact, every time that the author tries to link both aspects it is done in an arguably manner (if not simply wrong). For example: line 499 reads: “Effectively, the interaction between TSI, solar and lunar gravitational forces trigger responses in Earth’s oceans and atmospheres, such as the thermohaline current and ENSO’s atmospheric transport of heat and moisture (Broecker, 2003).” The reader could interpret this sentence as if Broecker, 2003 has shown any relationship between TSI and solar and lunar gravitational forces and the cited climatic processes related to abrupt transitions. He did not. I am not saying that your sentence is false, but nowadays it is just speculative and your research does not further elaborate on the potential connection between lunar-solar cyclicity and the mechanisms causing the climate to abruptly change on millennial time scales.
Again, if your calculations are correct (I am not an expert on solar and lunar variability, so I am giving full credibility to that aspect), it is intriguing and interesting to wonder how the climate system would respond. But the author's contribution to the physical mechanism responsible for such a link is absent in the study. There could be several mechanisms making the climate system to abruptly respond to a ~1500 years solar and lunar forcing, but they are not well addressed in the paper or absent. For example, the work by Arbic (e.g. 2004 and 2008) presents the possibility of triggering Heinrich events as a response to increased tidal amplitude in the Labrador Sea. These papers are not even cited in your manuscript. Moreover, the main way for energy dissipation in the ocean is caused by tides. Current GCMs investigating DO events need to put arguably too high mixing coefficients in order to spontaneously produce the oscillations compatible with the associated abrupt climate changes. Considering a tidal forcing on glacial millennial time scales could eventually facilitate the understanding of the necessary conditions for the models to show a DO-like variability. (This last sentence is a suggestion for the author in the line of what the paper fails to address).
There is also a clear flaw in this study: it overlooks (or simply ignores) a large part of the research on millennial-scale abrupt climate changes carried out in the last years.
Looking at your discussion section, particularly the paragraphs starting at line 503 and 511, the reader is invited to believe that, because the solar-lunar forcing described in the manuscript shares the time scale with the Bond cycle, DO and H events are a “natural” manifestation of the pretended “astronomical mechanism”.
DO and H events are complex and intriguing manifestations of the climate system and the literature describing the attempts of explaining their physical origin is vaste and rich.
If one claims that has discovered the common underlying forcing triggering their existence must explain the mechanisms translating such a forcing in their climatic imprints (see above), and also, must recognize all the previous work on such a subject.
Many important articles are not referenced in the manuscript both for the triggering mechanisms of DOs and Heinrich events. A few are cited, often incorrectly (see specific comments). I am not giving a list of the papers that should be cited. I suggest the author to carefully check the recent (and not that recent) bibliography on Heinrich and DO events.
This having said, I suggest the author to deeply reshape the manuscript. I see two ways of doing so:
- If the author wants to keep the current manner of framing her results (i.e. the implications of a newly described solar-lunar cyclicity on millennial-scale climate transitions), I suggest to address all my concerns and give a detailed, correct and humble context on which her results are presented.
- Perhaps a simpler manner of recycling the study would consist on forgetting the “climate aspect” and focus on the real novelty of the paper (i.e. the description of the solar irradiance and the Earth-moon distance cycles on millennial time scales). In my opinion, the pretension of having an “astronomical mechanism to explain abrupt climate changes on millennial time scales” heavily harms the current study. Even more when there is no need for such an approach to make the work interesting by itself.
Specific and technical comments:
Line 22: What “has the capacity to inform…”?
Introduction section: A whole paragraph describing the internal climatic theories explaining glacial millennial-scale variability is missing. (For example: Sakai and Peltier 1996, Peltier and Vettoretti 2014, Dokken et al, 2013, Alvarez-Solas et al, 2013, Basis et al, 2017 … and many others)
Line 34: “This astronomical mechanism also explains…”. The astronomical mechanism has not been described yet. Only a conjecture on its existence.
Line 34: “...tidal and solar records are synchronised in the paleoclimatic record”. Is that a fact? Or just an interpretation?
Line 44: “Based on a premise that precession causes…” On what is this premise based? So far nothing has been described with a higher temporal resolution than Milankovitch.
Line 59: “... and are therefore linked to..” Why? Sentence not justified.
Line 64: “... the debate is clouded by a poor understanding of ..” Speculative phrases and the citations at the end do not justify the sentence. On what Banderas et al, 2015 (for example) elaborates with respect to what the author wrote? Those papers are just trying to make it less cloudy.
Line 116: “...stochastic resonance is a possibility..” among many others.
Paragraph 116-120 is repeated from the introduction
Lines 127-131: Why is the explicit quote needed?
Line 258: “... using planetary software”. Reference?
Line 268: NOVA is not defined
Legend of Figure 5: “The black solid line is the model”? Or the red one?
Line 470: “... it is now evident lunar gravitational…” Why? Explain and reference please
Line 497: “Such geographical variations…” This sentence is highly speculative
Line 499: Sentence is not appropriate nor the citation (see above)
Line 514: “... 5910 years (the same oscillation as Heinrich events)”. Is that so? I do not think HEs have such a precise timing, and if so it would be a larger periodicity (see papers above).
Line 529: “These results provide a new and important understanding of the physics of climate change…”. That would be nice indeed, but I am afraid this is just an obvious overstatement. Your study does not elaborate newly on the physics of climate change. (And again there is no need of such a thing for author’s results to be framed appropriately and thus be published)
Line 533: “ … and offers a mechanical explanation for abrupt climate change where previously none existed”. Your study does not provide a “mechanical explanation” it would be an “explanation of the timing” at most. Moreover, “none existed”? Was not there any “mechanical explanation for abrupt climate change”?
Citation: https://doi.org/10.5194/cp-2022-49-RC2 -
AC2: 'Reply on RC2', Alison Kelsey, 25 Nov 2022
Author’s response to referee 2:
The author thanks referee 2 for their comments.
1. General
Ignoring the ad hominem arguments, which are both irrelevant and unsubstantiated, some valid points were made that the author acknowledges could improve the manuscript (discussed below). The author also agrees that the character of the climate system is complex and there is a dearth of climate proxies. However, there are many points made by referee 2, with which the author disagrees.
In the course of this response, the author will deliver a response to all of referee 2’s comments. However, the author would like to highlight a couple of other areas of major disagreement with referee 2:
- referee 2’s erroneous redefinition of “mechanism” to mean “forcing”
- the resultant creation of a pseudo competition between external and internal forcing
- lack of a new mechanism or understanding it provides
- accusations of
- bias
- pretentiousness
In places, referee 2’s comments are disorganised and also appear contradictory. On one hand, referee 2 acknowledges real novelty in this paper based on “i.e., the description of the solar irradiance and the Earth-moon distance cycles on millennial time scales”. Referee 2 also recognises the value in the model, describing it as “satisfactory”. Furthermore, referee 2 recognises the worthiness and major importance in establishing a “robust solar irradiance and lunar cycle oscillation around 1500 years”, although disagreeing with the paper’s presentation of that material. Yet, confusingly, the model is described as “pretended” and the author as “pretentious”.
2. Differentiation between mechanism and forcing
Contra referee 2’s redefinition:
A “mechanism” is the combination of components that creates the forcing, whilst “forcing” is a product of the mechanism. As the title of the article indicates, this is astronomical mechanism. This mechanism produces forces that act on all players within the mechanism (i.e., Sun, Earth, and Moon), and is a primary driver of Earth’s climate, impacting significantly on Earth’s internal climate system. Many astronomical forcings are well known, such as those produced monthly, seasonally, annually, and at orbital time scales.
Using their redefinition, referee 2 subsequently bases criticism of this paper on their misunderstanding of this difference, rendering those comments invalid. This includes:
- Alleged competition between internal and external forcing, whereas in reality Earth’s climate is not isolated from its celestial neighbourhood and its influences
- Allegations of a pretended mechanism
- Allegations of bias and lack of humility
- Classification of incomplete models of internal or hypothesised systems as mechanisms of climate change, when it is still unknown as to what triggers these systems or where model variables are missing
Limitations on the extent of current scientific knowledge and potential variables for GCM models (non-exhaustiveness of variables) contribute to an incomplete understanding of the operation and triggers of internal climate systems. Comments in literature, including recent articles, are indicative of these limitations of knowledge. Menviel et al. (2020:689) state “current Earth system models do not include all necessary components” and that the causal mechanisms that trigger D-0 variability are still highly debated. Meanwhile, an astronomical mechanism has been provided here with results confirming its existence, along with direct links to climatic records. This mechanism triggers responses in Earth’s climate system through tidal forcing of ocean and atmosphere, and determines levels of heat input into that system. This astronomical mechanism has the potential to enhance GCMs and inform on missing triggers/variables in current models.
Whilst it would be very nice to be able to provide all the answers at once and evidence of all links between these astronomical mechanics and internal climate systems, this is not possible in one article, which is already lengthy. Being that climate research is such a huge interdisciplinary field, this article simply presents initial results with the view to opening discussions in the future and promoting further research. A significant start has been made with comparison of physical models of gravitation to the occurrence of Bond events, as well as the highly significant correlation between the physically-based and chronologically-anchored model to cosmogenic indicator of climate change in an Antarctic ice core.
3. Addressing allegations of bias
Where is the bias, accusations of which seem to be rather nebulous? Perhaps the referee is referring to internal versus external forcing? Firstly, for bias, there needs to be competing systems. However, this is not the case. Earth’s internal climate is symptomatic of and responsive to external forcing and not the origin of the forcing itself. In the case of the 1470-yr quasi-periodicity, links between the astronomical mechanism described herein and relevant indicators in the paleoclimatic record were provided.
As a competing factor to the astronomical mechanism and external forcing of the 1470-yr quasi-periodicity, referee 2 gives an example of Earth’s internal system: increased tidal amplitudes within the Labrador Sea. Referee 2 also describes the main method of energy dispersion as being caused by the tides, as an internal process. These two tidal factors mentioned are cases in point that show the relationship between external and internal forcing of Earth’s climate.
The solar and lunar influences on Earth’s tides are commonly known and understood. We know that the Sun is the main source of heat and energy in our solar system, and that the effects of solar power are felt through Earth-Sun distance, gravitation, solar irradiance, solar flares, sunspot variability, and the solar wind. Proximity (distance) to the Sun influences the intensity of this influence. We also know of the Moon’s impact:
- steadies Earth’s axial tilt (obliquity) and seasons.
- Influences the earth’s axial tilt
- acts as a brake to Earth’s rotation,
- influences timing of events (such as the perihelion)
- has a greater tidal influence than the Sun on Earth’s oceanic tides due to its proximity to Earth.
It is also commonly known that the Earth’s oceans are strongly influenced by the gravitational influence of both the Sun and Moon, creating Earth’s tides; that the combined influences of the Sun and Moon produce tidal peaks. These combined influences occur at key lunar phases, lunation at perihelion, and are commonly known, e.g., high and low tides, neap tides, spring tides, king tides. It is also known that physical geographic features shape the height of tides.
It is also commonly known that the oceans provide the major storage of Earth’s heat and act as the major transporter of heat around the globe; that the ocean’s distribution of heat is influenced by land mass; that atmospheric transport of heat and moisture is influenced by solar and lunar tides as well as land form.
It is commonly known that the physical geography of the Earth also contributes to the level and timing of these tides, e.g., latitudinal position (in combination with the Moon), shapes/depth of ocean basins, bays etc, path of the thermohaline current, accumulation and loss of heat, deepwater formation, and input of freshwater into the oceanic system. Additionally, gravitation, heating and cooling also contribute to atmospheric dynamics and the functioning of the thermohaline current.
So, it naturally follows that variations in Earth-Sun, Earth-Moon distances, affect all of these above-mentioned factors through variations in gravitational and solar forcing (TSI and insolation). The author does not believe that these factors need to be spelt out for readers, especially in an already lengthy article. This article is about external forcing and the author has provided a background relevant to this research in Section 2 of the manuscript, including referencing to articles of the same age as those suggested by referee 2.
The author also provided responses to existing criticisms against the 1470 year ‘cycle’, yet the only comment referee 2 provides is that this quasi-periodicity could be stochastic and makes no mention of any other point. The real competing ideas were whether this cycle exists or not (Section 2 of the article), which were addressed in Section 2 of the article. The results from this study (Section 4 of the article) show that it does; the referee acknowledges that the comparison is satisfactory.
4. Links between the astronomical mechanism and Earth’s internal systems.
Contra referee 2 that no mechanism or links were provided, both the abstract and introduction briefly describe this mechanism. This mechanism is elaborated upon in the Sections 3, 4, with links established to palaeoclimatic indicators: (1) physical model of lunar gravitation to the occurrence of Bond events and (2) the chronologically-anchored, and astronomically- based model of interacting astronomical cycles that produce a 1470-yr ‘cycle’, which is statistically coherent with cosmogenic indicators of climate change in an Antarctic ice core. The basis for this mechanism in the Milankovitch cycles, as a high frequency expression of the Milankovitch precessional cycle, is well known in the literature, e.g., associations with sea-level change, geomagnetic flux, MIS stages, etc.
In this article, the physical associations of the two cited cycles (209 and 133 yrs) were identified. The 209yr (SdV) cycle, results from the combination of perihelion (anomalistic year) with Metonic lunation and sunspot cycle (solar luminosity cycle) (Lines 93, 110, 254-255, 477-480) to effectively produce a peak in gravitational and solar forcing (TSI/insolation) due to reduced Earth-Sun and Earth-Moon distances. The 133-yr astronomical cycle (lines 104-107), which is also found in climate records, is the result of changes in Earth-Sun and Earth Moon distances (lines 280, 415-435, 508), that directly influence gravitational and solar forcing and the tilt of Earth’s axis. Both cycles influence the patterning of cosmogenic isotopes. A physical basis to the forcing of Earth’s climate has been identified, explained, and shown to correlate to the timing of Bond events and the patterning of TSI reconstruction (14C) (and the 10Be isotopes on which the reconstruction was based). It builds upon the work of Braun, Bond, and others, and confirms the existence of the 1470-yr quasi-periodicity, and their hypotheses as to the potential cause as a combination of periodicities (Lines 72, 75).
5. Points of agreement between author and referee 2
- The reasons of such a poor understanding lie on the complex character of the climate system together with the sparse character of the climatic proxies which do not fully allow for a robust discrimination of causes and effects.
- DO and H events are complex and intriguing manifestations of the climate system and the literature describing the attempts of explaining their physical origin is vast and rich.
- Duplicated paragraph at line 116-120: edit or remove
- Lines 258 and 268. Nova and Skychart III were the planetarium software being used and require referencing: NOVA 2.13, (Hand, 1989-1994) and SkyChart III (DeBenedictis, 1993-2004).
- Legend of Figure 5: “The black solid line is the model”? Or the red one? Typo – should be red
- Line 499: The Broecker 2003 reference should read (cf. Broecker 2003). Disagreement with the rest of the comment (see sections 4, 6 of author’s response)
- Line 514: Referee 2 is correct in identifying this error and the correct length is as the author stated on line 59, that Heinrich events occur on a 11±1 ky ‘periodicity’. This value here is a subharmonic of Heinrich events and can be seen as a phase of that oscillation.
6. Points of disagreement between author and referee 2
In general
One option for an improvement to this paper and its reworking proposed by referee 2 was to forget the climate aspect of this model, whose connection with Earth’s climatic record was confirmed (Sections 3, 4 of the manuscript). This suggestion is unreasonable and illogical. The Earth must be considered in the context of its celestial neighbourhood, especially considering that orbital and obliquity factors are known to affect Earth’s climate and internal systems and the Moon’s influence in these matters. The author does not agree that this manuscript needs a major reshaping.
Referencing: It is not possible to recognise all other work on a subject matter, just key articles that exemplify progress or new ideas. There are other papers of the same age as suggested by the referee that are referenced in Section 2 of the manuscript. Also refer to author’s comments in Sections 1, 4 and 5 of author’s reply to referee 2 (also see comments on Introduction in technical notes).
Introduction: The manuscript’s introduction is brief and introduces broad concepts that are latter developed in the article (Sections 3, 4). Note that the 1470-yr cycle has been dismissed/questioned as real, and samples of main arguments are discussed in Section 2 of the article (background). Regarding missing internal climatic theories explaining millennial-scale variability, see Section 3 of authors’ response.
Line 34: On mechanism description and synchronisation of solar and tidal records
A full explanation of the astronomical mechanism is not provided in the introduction. It is briefly described here and in the abstract and is later elaborated in Section 3. This is not conjecture, as the hypothesis is tested and confirmed (see Sections 3 and 4).
Regarding the synchronisation of solar and tidal records, this is fact. For example, look to the history of the Milankovitch cycles and their calibration to uplifted coral terraces, geomagnetic record, and isotopic variations in deep sea cores. There is a host of literature available on this subject. Also see Bond et al (2001) where the deposition of IRD, due to oceanic influences associated with the thermohaline current (of which AMOC is part), has a close correlation with isotopic flux caused by the Sun (see line 221 of this article).
Line 44: “Based on a premise that precession causes…” On what is this premise based? So far nothing has been described with a higher temporal resolution than Milankovitch (referee 2).
This is a deductive study. A premise does not need to be inductively based but if it is, it needs to be expressed in an objective framework, as is the case here. The premise is that the Bond quasi-periodicity is caused by the same factors that cause the Milankovitch precessional cycle (lines 39-46 of the article). These factors are the solar and lunar gravitational forces acting on the Earth’s equatorial bulge. Previously, nothing of higher temporal resolution has been described, which is why this concept is novel – hence this article. Note that gravitational influences are strongest at perihelion and perigee, and that aggregation of data and level of resolution can mask trends and alter the appearance of oscillation.
Line 59: “... and are therefore linked to..” Why? Sentence not justified. (Referee 2)
Heinrich 1988, referenced in line 59, states the debris from these events shows a strong precessional signal at 11±1ky. This sentence is at the start of a new paragraph and immediately follows on from the preceding paragraph, which also provides the justification. With Bond et al demonstrating that the 1470-yr ‘cycle” is a pulse underlying both DO and Heinrich events. Precessional links at different temporal scales exist:
- Milankovitch precessional cycle
- Heinrich events
- DO events
- Bond cycles
The results (Section 4 of the article) from this research reinforce those findings.
Line 64: This phrase is not speculative and whilst each of these authors try to make the situation “less cloudy” the fact still remains that the physics of abrupt climate change is still poorly understood. The referee also agrees in their opening sentences that it is not well-understood and gives a reason.
At the end of this sentence (line 64), are quotes that reference authors who refer to the poor understanding of the physics of abrupt climate change (Ditlevsen and Ditlevsen, 2009, Banderas et al., 2015, Turney, 2008). Not much has changed since then. Menviel et al 2020 highlights that the cause of DO climatic variability is still highly debated, with no model produced so far being able to fully replicate these events. All of this taken together is indicative that the physics of abrupt climate change is pooly understood. Specifically, the authors listed in this article stated:
- Turney (2008): it was still unclear as to (1) what caused the Heinrich events (2008:95) and (2) in relation to AMOC, the cause of ocean circulation slow down and restart.
- Ditlevsen (2009) (see line 196 of this article): “Dansgaard-Oeschger (DO) events of rapid climate shifts in the glacial climate observed in the Greenland ice cores are still not well understood.”
- Banderas (2015): “the origin of these AMOC reorganizations remains poorly understood.”
Line 116 – whilst stochastic resonance is a possibility, other causes first need to be eliminated. This article, research, deductive study, and results provide an explanation and evidence supporting it (Sections 3, 4 of the manuscript).
Line 470: “... it is now evident lunar gravitational…” Why? Explain and reference please (referee 2)
Refer to author’s response in Section 4 of the author’s response. No need for referencing – this is the author’s own work.
Lines 497 and 499: Regarding referee 2’s comments about the mechanism and links, please refer to Section 4 of this reply. These lines are not highly speculative and Line 497 is based on the one immediately preceding it. The models are physically-based, chronologically anchored, and parameters justified. Statistical testing between the trigonometric model and an independent TSI reconstruction from climatic data (observed versus expected behaviour) confirms the hypothesis (Sections 3, 4 of the article). Additionally, Figure 4 shows the occurrence of Bond events in association with peak lunar forcing associated with the 133-yr cycle. These events are associated with reduced Earth-Sun, Earth-Moon distances, and fluctuations in solar declination and TSI.
This article also elaborates on the lunar role in precession (see author’s response to line 44), which forms part of the Milankovitch precessional cycle and consequently its role in the 1470-yr quasi-periodicity. By virtue of its various gravitational influences, the Moon affects TSI, insolation, and associated patterns through time. With peak gravitational forces caused by perigee (Moon) and perihelion (Sun) (and their combination) (lunations and lunar phases), variations will occur in tides depending on the proximity of the lunation and perigee to the perihelion. Variations in tidal and insolation levels are thus associated with both Milankovitch precessional and Bond cycles.
Regarding the reference to Broecker, there is a comparison to Broecker 2003, where there is a discussion on the debate between the two potential causes for abrupt climate change. In Broecker 2003, there is a description of the influence of physical geography in determining the cause of these events. Furthermore, Broecker points out the deficiencies in both theories: the incapacity of one to change suddenly and the incapacity of the other to be able to lock the mechanism into an alternative state. However, the Broecker 2003 reference on line 500 should read (cf. Broecker 2003).
Line 529: Your study does not elaborate newly on the physics of climate change (referee 2).
These models are unique and new – how do they not elaborate newly on the physics of abrupt climate change? See section 4 of this response.
Line 533: Your study does not provide a “mechanical explanation” it would be an “explanation of the timing” at most. Moreover, “none existed”? Was not there any “mechanical explanation for abrupt climate change”? (Referee 2)
Please refer to Section 4 of author’s response. Additionally, whilst correlation is not causation, a deductive study was undertaken and the hypothesis confirmed. It remains for future scientific studies to falsify it.
7. Other points
- With regard to Earth’s internal climate systems, please refer to author’s response to referee 2 (Section 3)
8. Explanation
- Line 22: “capacity to inform” = To illuminate, to give information
- Line 127-131: reinforces an argument in a lead into discussion on the Ditlevsen paper
9. Referee 2 opinions
Referee 2 holds an opinion that “even if such a forcing on a 1470 yr quasi-periodicity exists, it might be irrelevant in triggering the abrupt climate changes”. However, results and implications provided in Section 4 of this article are sound and “satisfactory” according to referee 2. Whilst correlation is not causation, a deductive study was undertaken and the hypothesis confirmed. It remains for future scientific studies to falsify it.
Citation: https://doi.org/10.5194/cp-2022-49-AC2 - referee 2’s erroneous redefinition of “mechanism” to mean “forcing”
Status: closed
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RC1: 'Comment on cp-2022-49', Anonymous Referee #1, 16 Sep 2022
The manuscript under consideration is a modified version of the manuscript rejected seven years ago. I did not review the previous version of this manuscript and did not make a detailed comparison between them, but the main results, figures and references are very similar. In particular, I found that practically all cited papers were published more than ten years, which is a serious shortcoming of the manuscript since significant progress in the understanding of Quaternary millennial-scale variability has been achieved during the past decade. The reviewers of the first version of the manuscript mainly criticized the central premise formulated in the first sentence, namely, that “the existence of a 1470-year cycle of abrupt climate change is well-established”. Now the author admits that the Bond cycle is “controversial”. Below I discuss the manuscript in a more general context. The appraisal of the manuscript depends on the answers to three questions:
- Is the 1470-yr cycle really observed in climate records?
- Does millennial-scale Quaternary variability associated with DO events and IRDs (i.e. Heinrich and similar events) require any external forcing?
- Is there any evidence that astronomical factors discussed by the author can influence Quaternary millennial-scale climate variability?
The answer to the first question, as has already been argued by the reviewers of the first version of the manuscript, is definitely no. 1470-yr cycle was popular at the beginning of 2000th primarily thanks to a very influential publication by Bond et al. (1997). However, later this periodicity went out of fashion since the existence of this cycle was not corroborated by other studies.
The answer to the second question is also no. Numerous modelling studies performed during the past decades demonstrated that the millennial-scale variability, such as Dansgaard-Oeschger and Heinrich events) can be fully explained by the internal instabilities emerging in the ocean-atmosphere system and ice sheets under glacial conditions without any external forcing (see, for example, review by Menviel et al, 2020).
Concerning the potential astronomical forcing of millennial-scale variability. The author considers two astronomical processes which together may produce 1470-yr cycle. The first is the well-known solar cycles. However, recently solar cycles were additionally "downgraded", and the current estimates of changes in TSI at multi-centennial time scales is only 0.1% which is too small to have any appreciable impact on climate. How the 133-yr cyclicity in Earth-Sun and Earth-Moon distances can affect climate, the author does not explain. Moreover, this periodicity is not seen in climate records. Thus, the answer to the third question is also no.
This is why I cannot recommend this manuscript for publication in Climate of the Past.
Menviel, L.C., Skinner, L.C., Tarasov, L. and Tzedakis, P.C., 2020. An ice–climate oscillatory framework for Dansgaard–Oeschger cycles. Nature Reviews Earth & Environment, 1(12), pp.677-693.
Citation: https://doi.org/10.5194/cp-2022-49-RC1 -
AC1: 'Reply on RC1', Alison Kelsey, 06 Oct 2022
The author thanks Reviewer 1 (R1) for their comments. However, there are several flaws and oversights in R1’s comments and these issues are highlighted in this author’s response as follows. This reviewer establishes three points on which to judge the merits of this article:
- A question of existence of the 1470-yr quasi-periodicity
- Need for external forcing
- Evidence of astronomical factors that influence Quaternary millennial-scale climate variability
Point 1:
Observation of the ~1470-yr quasi-periodicity in climate records is essentially an argument about whether this periodicity exists or not. On the matter of existence of the ~1470, Wolff (2015) states that, on the 1470-yr quasi-periodicity, the jury is still out and the matter has not been settled, reiterating his 2010 position (Wolff et al. 2010) that the matter was not resolved. One of the previous reviewer’s, Ditlevsen (2015), said “even though I am not convinced about the findings, I still find the paper interesting and worthy of publication” despite the debate over the existence of this quasi-periodicity.
This question really forms part of a complex and lengthy debate, with the issue being that the timing and number of Dansgaard-Oeschger events depends on the time-scale used (Wolff et al., 2010) and how they are defined (Alley et al., 2001; cf. Ditlevsen et al., 2005). Arguments for and against this periodicity are covered in Section 2 of this article, with attention given to countering existing arguments against the ~1470-yr quasi-periodicity (Section 2.1). Additionally, further chronological matters have been discussed in this article (Section 3.5) that have been ignored by R1.
Importantly, the results of this author’s research confirm the existence of this quasi-periodicity through statistical testing of a chronologically-anchored, normalised trigonometric model that is compared to an independent, Be10-based reconstruction of Total Solar Irradiance (TSI). This trigonometric model is underpinned by physical models of gravitation and TSI that are linked by distance (see this author’s response to point 3).
In all instances during the time interval of astronomical data, Bond events occur at times of maximum gravitational forcing (Kelsey 2018:119), associated with minimum Earth-Moon distances (Figure 4) that are magnified by minimum Earth-Sun distances occurring at perihelion. Fluxes in gravitation and TSI are inherently linked because of the reliance of both sets of calculations on distance. These results confirm Bond’s suggestion that the potential cause of Bond IRD events was an ocean-atmosphere link through solar forcing (Bond et al., 2001; cf. Braun et al., 2005; Schulz, 2002).
R1 does not comment on or acknowledge any of this material in justifying their answer to this question. Rather R1 talks about fashions and corroboration of the quasi-periodicity. It is not unusual for science to progress through waves of dominant paradigms, often with intevening decades between times of popularity such as occurred with the Milankovitch cycles (cf. Imbrie 1979; Summerhayes, 2015). This factor also affects referencing and the age of many sources. Lack of corroboration is not equivalent to proof of non existence. Research results presented here corroborate the existence of this quasi-periodicity.
Point 2:
R1’s argument of necessity in regard to point 2, is invalid and does not reflect what was said in the referenced article used to support their statement (cf. Menviel et al. 2020). Whilst R1’s assertion that numerical models fully explain the causes of millennial-scale variability emerging from the ocean-atmosphere system and icesheets, without resorting to external forcing, Menkiel et al 2020 state that “Palaeorecords and numerical studies indicate that the AMOC, with a tight coupling to Nordic Seas sea ice, is central to D–O variability, yet, a complete theory remains elusive.” They further state that “the sequence of events that led to D–O climatic variability is still highly debated.” Whilst several mechanisms have been proposed to account for D-O variability, none can account for all DO’s characteristics.
Menkiel et al highlight that “current Earth system models do not include all necessary components (for example, biogeo-chemistry, ice shelves, ice- sheet dynamics), inadequately represent important processes or cannot be integrated long enough under intermediate glacial conditions to simulate self- sustained D–O cycles. NADW formation in climate models is highly parameterized and not well constrained by observations, so there is little confidence in simulated changes in the strength and location of NADW formation in response to climate change, both past and future.”
An important factor in DO events is the Atlantic Meridional Overturning Circulation (AMOC), which is central to Earth’s climate and internal systems (Menviel et al. 2020). AMOC is part of Earth’s Meridional Overturning Circulation and is the major transporter of heat around the world. Wunsch (2010) suggests that the simple conveyor belt idea [Figure 2.4] is much more complex than often presented and that “only a tiny minority” has attempted to understand the underlying physics.
External forcing cannot be dismissed as unnecessary. Earth is not an isolated unit and must be considered in the context of its celestial neighbourhood (Kelsey 2018:48-83), as its neighbours (primarily the Sun and Moon) influence Earth’s internal systems through gravitational forcing and heat input. These factors influence both atmospheric and oceanic tides and are another piece of the puzzle, acting as a teleconnection between the two hemispheres. Internal systems work alongside external forcing and are not competing interests – they are interlinked.
Point 3:
Regarding evidence of astronomical factors that influence Quaternary millennial-scale climate variability, this author is surprised at R1’s comments. Abundant evidence has been supplied and appears to have been ignored by R1. This includes:
- The model of superimposed 209-yr and 133-yr cycles that emulates the ~1470-yr quasi-periodicity was statistically tested against an independent TSI reconstruction of Be10 data, producing X2, r values. Variance (r2), and significance (p) values (Section 4.2 and Table 4)
- The results of tests in (a) show a significant result with a strong correlation between the model and the Be-10-based TSI reconstruction.
- Visually, there is a striking visual similarity between the model presented here and the TSI reconstruction, as well as the Be10 flux (Section 4.2; see also Kelsey 2018:141).
- The parameters of the chronologically-anchored model and their values were justified and discussed within this article (Sections 3.4 and 4), including the incorporation of the 0.1% solar flux that R1 has deemed “too weak” to have an appreciable impact. Contra R1’s comments, results presented here show otherwise, as it plays an appreciable role in the amplification of the superimposed model’s patterns.
- Data presented here of Earth-Moon distances (Figure 4) show that Bond events of ice-rafted debris occur at minimum Earth-Sun distances that are associated with times of maximum gravitational forcing (cf. Figure 5.13, Kelsey 2018:119). The relationship between distance and gravitation is clearly stated within the article and this relationship is a commonly known scientific axiom.
- Data presented here (Figure 4) also shows pulses of cyclical 133-yr minimal Earth-Sun distances, viz maximum gravitational forcing. Contra R1’s comment that this cycle does not appear in climate records, this cycle is found in numerous climatic records referenced in section 2 of this article.
- The gravitational pull of the Moon influences the tilt of the Earth’s axis and consequently the 133-yr cycle also appears in solar declination data (Section 4.1).
- The strength of the 133-yr cycle is associated with the proximity of the lunation (New Moon) to the perihelion (closest point in the Earth’s orbit to the sun) (Section 4.1), when gravitational forcing and TSI are at their maximum.
- It is also commonly known that atmospheric and oceanic tides are also caused by the Sun and Moon and there is ample evidence of solar and lunar forcing in the palaeoclimatic record.
- Contra R1, an explanation of how distances can affect climate was provided.
On the previous article:
Regarding the general comments made by the reviewer re the previous article, R1 is wrong in inferring it is the same as the current article or that the main issue was about semantics that had only recently been accepted. The two articles are very different but related, forming integral parts of the same unpublished PhD thesis (Kelsey 2018). They share one figure in common, which was referenced in the second article.
The previous article presented research showing that an interacting combination of astronomical variables related to Earth’s orbit may be causally related to the ~1470-yr quasi-periodicity and several associated key isotopic spectral signals. It was a conceptual model and did not rely on statistical tests between the model and data. Its purpose was to present a framework of understanding. These variables were the ~11.4-yrs Schwabe sunspot cycle, the Metonic lunation cycle of 19 yrs, and an anomalistic year forcing at ~104 yrs.
The article presented here presents a different, more complex model that uses different variables as described within this article. This model presents a lower temporal resolution of solar and lunar forcing, which is underpinned by physical models (based on astronomical data) of TSI and gravitational flux (see author’s response to R1’s comments for point 3). Note that aggregation and different temporal resolution influence the appearance of spectral signals and oscillations in the palaeoclimatic record (Kelsey 2018:137-139). The purpose of this article was to describe the astronomical mechanism associated with the millennial-scale climate oscillations, and to provide evidence of the link between the two using statistical testing.
In relation to referencing, there are 10 references to material published in the last decade, and there should be additional references to Kelsey (2018). Also refer to author’s response to point 1 regarding age of references; for advancement in Quaternary millennial-scale variability, refer to author’s response to point 2. The issue regarding well-established and highly-debated/controversial was accepted at the time and is not a recent development.
References:
DITLEVSEN, P., 2015 Past Discuss., 11, C2224–C2226, 2015 www.clim-past-discuss.net/11/C2224/2015/
KELSEY, A. 2018. Astronomical forcing of sub-Milankovitch climate oscillations during the late Quaternary. PhD Thesis, School of Earth and Environmental Sciences, The University of Queensland. https://doi.org/10.14264/uql.2018.186
Other references appear within the currently submitted article.
Citation: https://doi.org/10.5194/cp-2022-49-AC1
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RC2: 'Comment on cp-2022-49', Anonymous Referee #2, 19 Oct 2022
Review of Abrupt climate change and millennial-scale cycles: an astronomical mechanism, by Alison Kelsey
General comments:
In this manuscript Alison Kelsey presents the results of the superimposition of lunar and solar periodic signals and compares it with a reconstruction of total solar irradiance.
This comparison appears to be satisfactory. This, together with the fact that the Earth-moon distance shows a modulation on a ~1400 yr periodicity (among others), allows the author to develop the article upon the idea that it represents a new mechanism to explain abrupt climate changes on millennial time scales.
In my opinion, although the former finding is of interest, the manuscript is addressed in a highly biased manner making it difficult to be published under its current form.
Showing that there is a robust solar irradiance and lunar cycle oscillation around 1500 years would be of major importance to the paleoclimatology research and worth publishing, but the current way on which these results are justified and framed in the paper is, in my opinion, erroneous.
Let me develop:
The first sentence of the abstract already illustrates this flawed approach. It reads: “Contributing to the poor understanding of abrupt climate change is the lack of a known mechanism for a ~1470-yr quasi-periodicity [...]”
Firstly, I guess what you mean here instead of “mechanism” is “forcing”, because even if the solar-lunar cyclicity described here is robust, the author is not contributing to the description of any mechanism that causes the climate to abruptly change on a ~1500 yr periodicity.
Secondly, I guess that what you mean by “a known mechanism for a ~1470-yr quasi-periodicity” refers in reality to a “known climate forcing”, because there are several proposed mechanisms, even in the articles cited here, to explain the climate variability on that time scale.
Finally, the lack of a known mechanism (forcing) does not contribute in any way to the poor understanding of abrupt climate change. The reasons of such a poor understanding lie on the complex character of the climate system together with the sparse character of the climatic proxies which do not fully allow for a robust discrimination of causes and effects. Besides, one could simply conceive that, even if such a forcing on a 1470 yr quasi-periodicity exists, it might be irrelevant in triggering the abrupt climate changes.
This biased approach is recognizable all over the manuscript. (see specific comments)
What the author is showing is that, according to her model and the good agreement with TSI reconstructions, there is a lunar-solar cyclicity around 1500 years that can potentially influence climate on that time scale. But this is far to represent an “astronomical mechanism for abrupt climate change and millennial-scale cycles” as suggested already by the title of the paper. In fact, every time that the author tries to link both aspects it is done in an arguably manner (if not simply wrong). For example: line 499 reads: “Effectively, the interaction between TSI, solar and lunar gravitational forces trigger responses in Earth’s oceans and atmospheres, such as the thermohaline current and ENSO’s atmospheric transport of heat and moisture (Broecker, 2003).” The reader could interpret this sentence as if Broecker, 2003 has shown any relationship between TSI and solar and lunar gravitational forces and the cited climatic processes related to abrupt transitions. He did not. I am not saying that your sentence is false, but nowadays it is just speculative and your research does not further elaborate on the potential connection between lunar-solar cyclicity and the mechanisms causing the climate to abruptly change on millennial time scales.
Again, if your calculations are correct (I am not an expert on solar and lunar variability, so I am giving full credibility to that aspect), it is intriguing and interesting to wonder how the climate system would respond. But the author's contribution to the physical mechanism responsible for such a link is absent in the study. There could be several mechanisms making the climate system to abruptly respond to a ~1500 years solar and lunar forcing, but they are not well addressed in the paper or absent. For example, the work by Arbic (e.g. 2004 and 2008) presents the possibility of triggering Heinrich events as a response to increased tidal amplitude in the Labrador Sea. These papers are not even cited in your manuscript. Moreover, the main way for energy dissipation in the ocean is caused by tides. Current GCMs investigating DO events need to put arguably too high mixing coefficients in order to spontaneously produce the oscillations compatible with the associated abrupt climate changes. Considering a tidal forcing on glacial millennial time scales could eventually facilitate the understanding of the necessary conditions for the models to show a DO-like variability. (This last sentence is a suggestion for the author in the line of what the paper fails to address).
There is also a clear flaw in this study: it overlooks (or simply ignores) a large part of the research on millennial-scale abrupt climate changes carried out in the last years.
Looking at your discussion section, particularly the paragraphs starting at line 503 and 511, the reader is invited to believe that, because the solar-lunar forcing described in the manuscript shares the time scale with the Bond cycle, DO and H events are a “natural” manifestation of the pretended “astronomical mechanism”.
DO and H events are complex and intriguing manifestations of the climate system and the literature describing the attempts of explaining their physical origin is vaste and rich.
If one claims that has discovered the common underlying forcing triggering their existence must explain the mechanisms translating such a forcing in their climatic imprints (see above), and also, must recognize all the previous work on such a subject.
Many important articles are not referenced in the manuscript both for the triggering mechanisms of DOs and Heinrich events. A few are cited, often incorrectly (see specific comments). I am not giving a list of the papers that should be cited. I suggest the author to carefully check the recent (and not that recent) bibliography on Heinrich and DO events.
This having said, I suggest the author to deeply reshape the manuscript. I see two ways of doing so:
- If the author wants to keep the current manner of framing her results (i.e. the implications of a newly described solar-lunar cyclicity on millennial-scale climate transitions), I suggest to address all my concerns and give a detailed, correct and humble context on which her results are presented.
- Perhaps a simpler manner of recycling the study would consist on forgetting the “climate aspect” and focus on the real novelty of the paper (i.e. the description of the solar irradiance and the Earth-moon distance cycles on millennial time scales). In my opinion, the pretension of having an “astronomical mechanism to explain abrupt climate changes on millennial time scales” heavily harms the current study. Even more when there is no need for such an approach to make the work interesting by itself.
Specific and technical comments:
Line 22: What “has the capacity to inform…”?
Introduction section: A whole paragraph describing the internal climatic theories explaining glacial millennial-scale variability is missing. (For example: Sakai and Peltier 1996, Peltier and Vettoretti 2014, Dokken et al, 2013, Alvarez-Solas et al, 2013, Basis et al, 2017 … and many others)
Line 34: “This astronomical mechanism also explains…”. The astronomical mechanism has not been described yet. Only a conjecture on its existence.
Line 34: “...tidal and solar records are synchronised in the paleoclimatic record”. Is that a fact? Or just an interpretation?
Line 44: “Based on a premise that precession causes…” On what is this premise based? So far nothing has been described with a higher temporal resolution than Milankovitch.
Line 59: “... and are therefore linked to..” Why? Sentence not justified.
Line 64: “... the debate is clouded by a poor understanding of ..” Speculative phrases and the citations at the end do not justify the sentence. On what Banderas et al, 2015 (for example) elaborates with respect to what the author wrote? Those papers are just trying to make it less cloudy.
Line 116: “...stochastic resonance is a possibility..” among many others.
Paragraph 116-120 is repeated from the introduction
Lines 127-131: Why is the explicit quote needed?
Line 258: “... using planetary software”. Reference?
Line 268: NOVA is not defined
Legend of Figure 5: “The black solid line is the model”? Or the red one?
Line 470: “... it is now evident lunar gravitational…” Why? Explain and reference please
Line 497: “Such geographical variations…” This sentence is highly speculative
Line 499: Sentence is not appropriate nor the citation (see above)
Line 514: “... 5910 years (the same oscillation as Heinrich events)”. Is that so? I do not think HEs have such a precise timing, and if so it would be a larger periodicity (see papers above).
Line 529: “These results provide a new and important understanding of the physics of climate change…”. That would be nice indeed, but I am afraid this is just an obvious overstatement. Your study does not elaborate newly on the physics of climate change. (And again there is no need of such a thing for author’s results to be framed appropriately and thus be published)
Line 533: “ … and offers a mechanical explanation for abrupt climate change where previously none existed”. Your study does not provide a “mechanical explanation” it would be an “explanation of the timing” at most. Moreover, “none existed”? Was not there any “mechanical explanation for abrupt climate change”?
Citation: https://doi.org/10.5194/cp-2022-49-RC2 -
AC2: 'Reply on RC2', Alison Kelsey, 25 Nov 2022
Author’s response to referee 2:
The author thanks referee 2 for their comments.
1. General
Ignoring the ad hominem arguments, which are both irrelevant and unsubstantiated, some valid points were made that the author acknowledges could improve the manuscript (discussed below). The author also agrees that the character of the climate system is complex and there is a dearth of climate proxies. However, there are many points made by referee 2, with which the author disagrees.
In the course of this response, the author will deliver a response to all of referee 2’s comments. However, the author would like to highlight a couple of other areas of major disagreement with referee 2:
- referee 2’s erroneous redefinition of “mechanism” to mean “forcing”
- the resultant creation of a pseudo competition between external and internal forcing
- lack of a new mechanism or understanding it provides
- accusations of
- bias
- pretentiousness
In places, referee 2’s comments are disorganised and also appear contradictory. On one hand, referee 2 acknowledges real novelty in this paper based on “i.e., the description of the solar irradiance and the Earth-moon distance cycles on millennial time scales”. Referee 2 also recognises the value in the model, describing it as “satisfactory”. Furthermore, referee 2 recognises the worthiness and major importance in establishing a “robust solar irradiance and lunar cycle oscillation around 1500 years”, although disagreeing with the paper’s presentation of that material. Yet, confusingly, the model is described as “pretended” and the author as “pretentious”.
2. Differentiation between mechanism and forcing
Contra referee 2’s redefinition:
A “mechanism” is the combination of components that creates the forcing, whilst “forcing” is a product of the mechanism. As the title of the article indicates, this is astronomical mechanism. This mechanism produces forces that act on all players within the mechanism (i.e., Sun, Earth, and Moon), and is a primary driver of Earth’s climate, impacting significantly on Earth’s internal climate system. Many astronomical forcings are well known, such as those produced monthly, seasonally, annually, and at orbital time scales.
Using their redefinition, referee 2 subsequently bases criticism of this paper on their misunderstanding of this difference, rendering those comments invalid. This includes:
- Alleged competition between internal and external forcing, whereas in reality Earth’s climate is not isolated from its celestial neighbourhood and its influences
- Allegations of a pretended mechanism
- Allegations of bias and lack of humility
- Classification of incomplete models of internal or hypothesised systems as mechanisms of climate change, when it is still unknown as to what triggers these systems or where model variables are missing
Limitations on the extent of current scientific knowledge and potential variables for GCM models (non-exhaustiveness of variables) contribute to an incomplete understanding of the operation and triggers of internal climate systems. Comments in literature, including recent articles, are indicative of these limitations of knowledge. Menviel et al. (2020:689) state “current Earth system models do not include all necessary components” and that the causal mechanisms that trigger D-0 variability are still highly debated. Meanwhile, an astronomical mechanism has been provided here with results confirming its existence, along with direct links to climatic records. This mechanism triggers responses in Earth’s climate system through tidal forcing of ocean and atmosphere, and determines levels of heat input into that system. This astronomical mechanism has the potential to enhance GCMs and inform on missing triggers/variables in current models.
Whilst it would be very nice to be able to provide all the answers at once and evidence of all links between these astronomical mechanics and internal climate systems, this is not possible in one article, which is already lengthy. Being that climate research is such a huge interdisciplinary field, this article simply presents initial results with the view to opening discussions in the future and promoting further research. A significant start has been made with comparison of physical models of gravitation to the occurrence of Bond events, as well as the highly significant correlation between the physically-based and chronologically-anchored model to cosmogenic indicator of climate change in an Antarctic ice core.
3. Addressing allegations of bias
Where is the bias, accusations of which seem to be rather nebulous? Perhaps the referee is referring to internal versus external forcing? Firstly, for bias, there needs to be competing systems. However, this is not the case. Earth’s internal climate is symptomatic of and responsive to external forcing and not the origin of the forcing itself. In the case of the 1470-yr quasi-periodicity, links between the astronomical mechanism described herein and relevant indicators in the paleoclimatic record were provided.
As a competing factor to the astronomical mechanism and external forcing of the 1470-yr quasi-periodicity, referee 2 gives an example of Earth’s internal system: increased tidal amplitudes within the Labrador Sea. Referee 2 also describes the main method of energy dispersion as being caused by the tides, as an internal process. These two tidal factors mentioned are cases in point that show the relationship between external and internal forcing of Earth’s climate.
The solar and lunar influences on Earth’s tides are commonly known and understood. We know that the Sun is the main source of heat and energy in our solar system, and that the effects of solar power are felt through Earth-Sun distance, gravitation, solar irradiance, solar flares, sunspot variability, and the solar wind. Proximity (distance) to the Sun influences the intensity of this influence. We also know of the Moon’s impact:
- steadies Earth’s axial tilt (obliquity) and seasons.
- Influences the earth’s axial tilt
- acts as a brake to Earth’s rotation,
- influences timing of events (such as the perihelion)
- has a greater tidal influence than the Sun on Earth’s oceanic tides due to its proximity to Earth.
It is also commonly known that the Earth’s oceans are strongly influenced by the gravitational influence of both the Sun and Moon, creating Earth’s tides; that the combined influences of the Sun and Moon produce tidal peaks. These combined influences occur at key lunar phases, lunation at perihelion, and are commonly known, e.g., high and low tides, neap tides, spring tides, king tides. It is also known that physical geographic features shape the height of tides.
It is also commonly known that the oceans provide the major storage of Earth’s heat and act as the major transporter of heat around the globe; that the ocean’s distribution of heat is influenced by land mass; that atmospheric transport of heat and moisture is influenced by solar and lunar tides as well as land form.
It is commonly known that the physical geography of the Earth also contributes to the level and timing of these tides, e.g., latitudinal position (in combination with the Moon), shapes/depth of ocean basins, bays etc, path of the thermohaline current, accumulation and loss of heat, deepwater formation, and input of freshwater into the oceanic system. Additionally, gravitation, heating and cooling also contribute to atmospheric dynamics and the functioning of the thermohaline current.
So, it naturally follows that variations in Earth-Sun, Earth-Moon distances, affect all of these above-mentioned factors through variations in gravitational and solar forcing (TSI and insolation). The author does not believe that these factors need to be spelt out for readers, especially in an already lengthy article. This article is about external forcing and the author has provided a background relevant to this research in Section 2 of the manuscript, including referencing to articles of the same age as those suggested by referee 2.
The author also provided responses to existing criticisms against the 1470 year ‘cycle’, yet the only comment referee 2 provides is that this quasi-periodicity could be stochastic and makes no mention of any other point. The real competing ideas were whether this cycle exists or not (Section 2 of the article), which were addressed in Section 2 of the article. The results from this study (Section 4 of the article) show that it does; the referee acknowledges that the comparison is satisfactory.
4. Links between the astronomical mechanism and Earth’s internal systems.
Contra referee 2 that no mechanism or links were provided, both the abstract and introduction briefly describe this mechanism. This mechanism is elaborated upon in the Sections 3, 4, with links established to palaeoclimatic indicators: (1) physical model of lunar gravitation to the occurrence of Bond events and (2) the chronologically-anchored, and astronomically- based model of interacting astronomical cycles that produce a 1470-yr ‘cycle’, which is statistically coherent with cosmogenic indicators of climate change in an Antarctic ice core. The basis for this mechanism in the Milankovitch cycles, as a high frequency expression of the Milankovitch precessional cycle, is well known in the literature, e.g., associations with sea-level change, geomagnetic flux, MIS stages, etc.
In this article, the physical associations of the two cited cycles (209 and 133 yrs) were identified. The 209yr (SdV) cycle, results from the combination of perihelion (anomalistic year) with Metonic lunation and sunspot cycle (solar luminosity cycle) (Lines 93, 110, 254-255, 477-480) to effectively produce a peak in gravitational and solar forcing (TSI/insolation) due to reduced Earth-Sun and Earth-Moon distances. The 133-yr astronomical cycle (lines 104-107), which is also found in climate records, is the result of changes in Earth-Sun and Earth Moon distances (lines 280, 415-435, 508), that directly influence gravitational and solar forcing and the tilt of Earth’s axis. Both cycles influence the patterning of cosmogenic isotopes. A physical basis to the forcing of Earth’s climate has been identified, explained, and shown to correlate to the timing of Bond events and the patterning of TSI reconstruction (14C) (and the 10Be isotopes on which the reconstruction was based). It builds upon the work of Braun, Bond, and others, and confirms the existence of the 1470-yr quasi-periodicity, and their hypotheses as to the potential cause as a combination of periodicities (Lines 72, 75).
5. Points of agreement between author and referee 2
- The reasons of such a poor understanding lie on the complex character of the climate system together with the sparse character of the climatic proxies which do not fully allow for a robust discrimination of causes and effects.
- DO and H events are complex and intriguing manifestations of the climate system and the literature describing the attempts of explaining their physical origin is vast and rich.
- Duplicated paragraph at line 116-120: edit or remove
- Lines 258 and 268. Nova and Skychart III were the planetarium software being used and require referencing: NOVA 2.13, (Hand, 1989-1994) and SkyChart III (DeBenedictis, 1993-2004).
- Legend of Figure 5: “The black solid line is the model”? Or the red one? Typo – should be red
- Line 499: The Broecker 2003 reference should read (cf. Broecker 2003). Disagreement with the rest of the comment (see sections 4, 6 of author’s response)
- Line 514: Referee 2 is correct in identifying this error and the correct length is as the author stated on line 59, that Heinrich events occur on a 11±1 ky ‘periodicity’. This value here is a subharmonic of Heinrich events and can be seen as a phase of that oscillation.
6. Points of disagreement between author and referee 2
In general
One option for an improvement to this paper and its reworking proposed by referee 2 was to forget the climate aspect of this model, whose connection with Earth’s climatic record was confirmed (Sections 3, 4 of the manuscript). This suggestion is unreasonable and illogical. The Earth must be considered in the context of its celestial neighbourhood, especially considering that orbital and obliquity factors are known to affect Earth’s climate and internal systems and the Moon’s influence in these matters. The author does not agree that this manuscript needs a major reshaping.
Referencing: It is not possible to recognise all other work on a subject matter, just key articles that exemplify progress or new ideas. There are other papers of the same age as suggested by the referee that are referenced in Section 2 of the manuscript. Also refer to author’s comments in Sections 1, 4 and 5 of author’s reply to referee 2 (also see comments on Introduction in technical notes).
Introduction: The manuscript’s introduction is brief and introduces broad concepts that are latter developed in the article (Sections 3, 4). Note that the 1470-yr cycle has been dismissed/questioned as real, and samples of main arguments are discussed in Section 2 of the article (background). Regarding missing internal climatic theories explaining millennial-scale variability, see Section 3 of authors’ response.
Line 34: On mechanism description and synchronisation of solar and tidal records
A full explanation of the astronomical mechanism is not provided in the introduction. It is briefly described here and in the abstract and is later elaborated in Section 3. This is not conjecture, as the hypothesis is tested and confirmed (see Sections 3 and 4).
Regarding the synchronisation of solar and tidal records, this is fact. For example, look to the history of the Milankovitch cycles and their calibration to uplifted coral terraces, geomagnetic record, and isotopic variations in deep sea cores. There is a host of literature available on this subject. Also see Bond et al (2001) where the deposition of IRD, due to oceanic influences associated with the thermohaline current (of which AMOC is part), has a close correlation with isotopic flux caused by the Sun (see line 221 of this article).
Line 44: “Based on a premise that precession causes…” On what is this premise based? So far nothing has been described with a higher temporal resolution than Milankovitch (referee 2).
This is a deductive study. A premise does not need to be inductively based but if it is, it needs to be expressed in an objective framework, as is the case here. The premise is that the Bond quasi-periodicity is caused by the same factors that cause the Milankovitch precessional cycle (lines 39-46 of the article). These factors are the solar and lunar gravitational forces acting on the Earth’s equatorial bulge. Previously, nothing of higher temporal resolution has been described, which is why this concept is novel – hence this article. Note that gravitational influences are strongest at perihelion and perigee, and that aggregation of data and level of resolution can mask trends and alter the appearance of oscillation.
Line 59: “... and are therefore linked to..” Why? Sentence not justified. (Referee 2)
Heinrich 1988, referenced in line 59, states the debris from these events shows a strong precessional signal at 11±1ky. This sentence is at the start of a new paragraph and immediately follows on from the preceding paragraph, which also provides the justification. With Bond et al demonstrating that the 1470-yr ‘cycle” is a pulse underlying both DO and Heinrich events. Precessional links at different temporal scales exist:
- Milankovitch precessional cycle
- Heinrich events
- DO events
- Bond cycles
The results (Section 4 of the article) from this research reinforce those findings.
Line 64: This phrase is not speculative and whilst each of these authors try to make the situation “less cloudy” the fact still remains that the physics of abrupt climate change is still poorly understood. The referee also agrees in their opening sentences that it is not well-understood and gives a reason.
At the end of this sentence (line 64), are quotes that reference authors who refer to the poor understanding of the physics of abrupt climate change (Ditlevsen and Ditlevsen, 2009, Banderas et al., 2015, Turney, 2008). Not much has changed since then. Menviel et al 2020 highlights that the cause of DO climatic variability is still highly debated, with no model produced so far being able to fully replicate these events. All of this taken together is indicative that the physics of abrupt climate change is pooly understood. Specifically, the authors listed in this article stated:
- Turney (2008): it was still unclear as to (1) what caused the Heinrich events (2008:95) and (2) in relation to AMOC, the cause of ocean circulation slow down and restart.
- Ditlevsen (2009) (see line 196 of this article): “Dansgaard-Oeschger (DO) events of rapid climate shifts in the glacial climate observed in the Greenland ice cores are still not well understood.”
- Banderas (2015): “the origin of these AMOC reorganizations remains poorly understood.”
Line 116 – whilst stochastic resonance is a possibility, other causes first need to be eliminated. This article, research, deductive study, and results provide an explanation and evidence supporting it (Sections 3, 4 of the manuscript).
Line 470: “... it is now evident lunar gravitational…” Why? Explain and reference please (referee 2)
Refer to author’s response in Section 4 of the author’s response. No need for referencing – this is the author’s own work.
Lines 497 and 499: Regarding referee 2’s comments about the mechanism and links, please refer to Section 4 of this reply. These lines are not highly speculative and Line 497 is based on the one immediately preceding it. The models are physically-based, chronologically anchored, and parameters justified. Statistical testing between the trigonometric model and an independent TSI reconstruction from climatic data (observed versus expected behaviour) confirms the hypothesis (Sections 3, 4 of the article). Additionally, Figure 4 shows the occurrence of Bond events in association with peak lunar forcing associated with the 133-yr cycle. These events are associated with reduced Earth-Sun, Earth-Moon distances, and fluctuations in solar declination and TSI.
This article also elaborates on the lunar role in precession (see author’s response to line 44), which forms part of the Milankovitch precessional cycle and consequently its role in the 1470-yr quasi-periodicity. By virtue of its various gravitational influences, the Moon affects TSI, insolation, and associated patterns through time. With peak gravitational forces caused by perigee (Moon) and perihelion (Sun) (and their combination) (lunations and lunar phases), variations will occur in tides depending on the proximity of the lunation and perigee to the perihelion. Variations in tidal and insolation levels are thus associated with both Milankovitch precessional and Bond cycles.
Regarding the reference to Broecker, there is a comparison to Broecker 2003, where there is a discussion on the debate between the two potential causes for abrupt climate change. In Broecker 2003, there is a description of the influence of physical geography in determining the cause of these events. Furthermore, Broecker points out the deficiencies in both theories: the incapacity of one to change suddenly and the incapacity of the other to be able to lock the mechanism into an alternative state. However, the Broecker 2003 reference on line 500 should read (cf. Broecker 2003).
Line 529: Your study does not elaborate newly on the physics of climate change (referee 2).
These models are unique and new – how do they not elaborate newly on the physics of abrupt climate change? See section 4 of this response.
Line 533: Your study does not provide a “mechanical explanation” it would be an “explanation of the timing” at most. Moreover, “none existed”? Was not there any “mechanical explanation for abrupt climate change”? (Referee 2)
Please refer to Section 4 of author’s response. Additionally, whilst correlation is not causation, a deductive study was undertaken and the hypothesis confirmed. It remains for future scientific studies to falsify it.
7. Other points
- With regard to Earth’s internal climate systems, please refer to author’s response to referee 2 (Section 3)
8. Explanation
- Line 22: “capacity to inform” = To illuminate, to give information
- Line 127-131: reinforces an argument in a lead into discussion on the Ditlevsen paper
9. Referee 2 opinions
Referee 2 holds an opinion that “even if such a forcing on a 1470 yr quasi-periodicity exists, it might be irrelevant in triggering the abrupt climate changes”. However, results and implications provided in Section 4 of this article are sound and “satisfactory” according to referee 2. Whilst correlation is not causation, a deductive study was undertaken and the hypothesis confirmed. It remains for future scientific studies to falsify it.
Citation: https://doi.org/10.5194/cp-2022-49-AC2 - referee 2’s erroneous redefinition of “mechanism” to mean “forcing”
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