the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Feb 2021
A Modified Milankovitch theory that reconciles contradictions with the paleoclimate record
Abstract. Based upon research results over the past five decades, there has been a general acceptance that the ice ages were initiated by astronomical phenomenon. Specifically, marine, ice and terrestrial paleoclimate data have supported elements of the Milankovitch astronomical theory of the ice ages. However, there remain unresolved problems between the empirical findings and theory. The 100 thousand year problem
has been the subject of extensive research since a 100 thousand year cycle that matches the Earth orbit eccentricity period dominates the frequencies found in paleoclimate records. Yet, eccentricity produces an insignificant variation in annual solar energy. Other problems include the Stage 11 problem
, the missing interglacials problem
, how glaciation is sustained over multiple tens of thousands of years and synchronous hemispheric glaciation. I shall show these problems are resolved by modification of the prevailing Milankovitch theory. In particular, two elements of the theory need modification. One is the limitation of eccentricity's role and the other assuming that glaciation results only from cool summer conditions. By applying the Solar Energy Invariance law to define e-seasons, how eccentricity provides conditions for glaciation is demonstrated. The results show eccentricity variations provide significant solar energy variations at the top of the earth's atmosphere to produce glaciation that is global. Global glaciation results in colder winter glaciation occurring in one hemisphere simultaneous with cool summer glaciation in the other hemisphere. Analysis with these modifications resolves each of the problems.
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RC1: 'Comment on cp-2021-10', Anonymous Referee #1, 04 Mar 2021
This paper has the ambitious goal of rewriting Milankovitch theory in order to explain the apparent 100 kyr cycles seen in global climate records. It claims that it resolves some issues that exist in reconciling current theory with data.
It is quite hard to review this paper because much of the discussion is conceptual and broad statements are made that seem to be correct at a rhetorical level but that have no basis when pulled apart. The basic idea of the paper is that while the annual insolation received by the Earth varies little during an eccentricity cycle, the daily energy received by Earth when it is at aphelion is less than when it is at perihelion; and that this contrast is large when eccentricity is large and zero when eccentricity is zero. Both these statements are obvious and well-known.
The author then says that this means that at aphelion and with large eccentricity, Earth will experience cool winters in one hemisphere and cool summers in the other – this is also true. The logical leap he then makes is to say that this will promote glaciation in both hemispheres. As a statement taken on its own (and not considering other aspects of orbital theory) this is flawed in several ways:
Firstly while it is well understood (and the basis of Milankovitch theory) that cold summers at high latitude allow glaciation because they turn seasonal snowcover into permanent ice cover, there is no basis for the statement about winter. Although it is a gross simplification, to first order cold winters are likely to lead to less snowfall. However the author seems to be referring to something else, which is the proportion of rain to snow. This is a red herring because at the latitudes where an ice sheet would nucleate it never rains in winter. Finally it is also the case that at the latitudes that ice sheets form there is almost no winter insolation (above 67 degrees none at all in midwinter), so changes in insolation through eccentricity cycles are very small. Thus there is no basis for the assertion that the winter hemisphere would accumulate ice at aphelion more when eccentricity is large.
Even more fundamentally, I could equally plausibly make the opposite argument. At perihelion and with high eccentricity, Earth will experience warm winters in one hemisphere and warm summers in the other. This would therefore promote deglaciation in both hemispheres. This obviously suffers the same flaw for winter as the statement by the author. However it illustrates the fallacy of making a bold statement based on only a part of the story: using just the position at aphelion or perihelion, one could argue that high eccentricity promotes both glaciation and deglaciation. There is actually no substitute for looking at the system in the detail required to tease apart the actual energy received in geographical locations (such as 65N and 65S), as is done in traditional Milankovitch analysis.
These two issues come together when one thinks about what happens to a high latitude point through a year at high eccentricity. When it was NH summer at aphelion, the NH ice sheets persist and grow because of low summer insolation (we agree). But the author would claim that the SH ice sheet grows because of low winter insolation (we disagree because the small change in winter insolation shouldn’t affect growth), but he ignores the fact that the very high summer insolation at aphelion would melt any excess ice and promote SH ice sheet loss. After 10 kyr (half a precession cycle) everything would be reversed.
Although the paper briefly discusses it the author fails to engage with precession which determines which hemisphere is at aphelion in summer. It is of course through its control on the amplitude of the energy variability during a precession cycle that eccentricity does exert an influence on climate. By ignoring this nuance, the author creates an illusion of simplicity that is not justified. The real hemispheres, at times of high eccentricity, experience both low insolation in a particular season (at aphelion) and, 10 kyr later (half a precession cycle) high insolation (at perihelion). The author chooses only to be interested in the former.
These fundamental problems mean that the paper is not making any breakthrough and is flawed. I will not therefore go into detail on other parts, however a few comments are worthwhile.
I am mystified by Fig 2a: what are the numbers on the curve. They seem somehow to refer to the cycle lengths but they don’t seem to be either the distance between adjacent peaks or the result of spectral analysis of the same curve.
An obvious deficit in the paper is that it never shows the climate curves it claims to explain. This is strange.
While the author correctly describes some of the difficulties with Milankovitch theory he does not acknowledge that many of them are satisfactorily explained. For example the change in greenhouse gases (especially CO2) over the glacial cycles offers a very satisfactory way of globalising what should be a hemispheric climate response. There are now several ideas that explain why multiple precession cycles are needed to initiate deglaciation, and these seem plausible (a new idea is welcome but not needed). I’d agree that the strength of MIS 11 is not yet well understood, but the current paper does not address this.
Citation: https://doi.org/10.5194/cp-2021-10-RC1 -
RC2: 'Comment on cp-2021-10', Anonymous Referee #2, 26 Mar 2021
This manuscript attempts to provide a new point of view on the question of Quaternary glaciations and the 100-kyr cyclicity. It is based on rather obvious facts : indeed the Earth receives (globally) less energy when far from the Sun at the aphelion. But it also receives (globally) more energy six months later. Overall the argumentation is confused, to say the least. It seems that the author is « cherry-picking » the processes that corresponds to the desired effect : when at the aphelion, the Earth being far from the Sun we have cold winters (in the winter hemisphere) and cool summers (in the summer hemisphere) therefore a glaciation… But obviously the opposite holds true six months later.
More fundamentally, it appears that the author attempts to give answers to problems that do not even exist. For instance, he states (line 308) that « An issue with the Milankovitch summer insolation curves is that glaciation is asynchronous between hemispheres ». This is not true. The main forcing in the Milankovitch theory (in contrast to Croll’s one) is obliquity, at 41 kyr. Obliquity changes have a symmetric impact on both hemispheres : both are cooled due to low obliquity. This was actually a major difference with previous theories based mostly on precession (on the false premise that Antarctica is currently in a glacial state). Besides, ice-sheets are too cold in winter to allow for any significant precipitations, and there is never « mild rainy winters » over there. This is the key argument of Milankovitch to focus on the summer season, something already strongly discussed in the 19th century.
But at the end, on Figure 4, the author mentions (line 279) that « The precession cycles are responsible for the oscillation of insolation for summer and winter bounded by the ap- and peri-insolation ». Indeed, this is the well-known amplitude modulation of insolation at the basis of most of classical theories explaining the 100-kyr cycles. Nothing new.
Overall, this manuscript is flawed (and very confused) in its climatic mechanisms. I do not recommend its publication.
Detailed comments :
Lines 73-74 :
« to attribute the dominant cycle to multiples of precession or obliquity cycles »
This is certainly not the « standard » explanation. The most common explanation involves the amplitude modulation of the effect of precession on climate (which is precisely excentricity). A non-linear system will in general extract the envelop of the forcing and produce a 100ky oscillation, even for very simplified models. The difficulty is not to identify the origin of the 100-kyr cycles (ie. the amplitude modulation of precession) but to identify the non-linearity of ice-sheets that can extract this envelopp. Many possibilities have been put forward.
Line 80 :
« eccentricity does provide necessary solar energy conditions for global glaciation »
Obliquity is symmetric and explains very easily globally synchronous glaciations. This is in fact one of the main argument for the Milankovitch theory. It could also be argued that teleconnections are also critical : The Quaternary is characterized by northern glaciations (since in Antarctica, the ice-sheet is always close to its maximum extent, the continent being ice-covered today) therefore, concerning ice-sheets, we can neglect Antarctica. Concerning climate, the main (ice) changes are occuring in the North and will probably have global consequences. CO2 can also have a global impact.
Line 160 :
« Which theory is valid has been a long running debate (Imbrie and Imbrie, 1989). »
This is not true for at least a century. Nobody would claim today (except maybe in this current paper) that winter insolation has any consequence on ice sheets. In polar regions, winter is extremely cold and without significant precipitation. The mass balance of the ice sheet does not depend at all of insolation changes. The rôle of summer insolation is widely recognized, and is also consistent with simulations of ice-sheets using detailed numerical models. Since Milankovitch at the beginning of the 20th century, there is no more debate on this point. And again, Milankovitch already has glaciations synchronous in both hemispheres, since the main forcing is obliquity.
Line 163 :
« Annual snow growth for building ice sheets would result from more snow created in the colder winter of one hemisphere and less snowmelt in a cool summer of the other hemisphere ».
No. Winter is irrelevant : there is almost no snow in winter, since there is almost no water vapor in the winter atmosphere.
Line 185 :
« The eccentricity … shown in Fig. 2a reveal three discernable cycle periods of approximately 65 kyr, 100 kyr and 400 kyr »
65 ky does not clearly show up in the spectral analysis…. The vocabulary « cycle period » is not quite relevant, « duration of specific events » would be more appropriate.
Figure 2
In other words, Earth receives less energy per day (less power) when far from the Sun. This seems quite obvious to me.
Line 200 :
« Conditions for glaciation occur during increasing eccentricity resulting in increasing ap-season cooling (more negative) and increasing peri-season warming while conditions for deglaciation occur during diminishing eccentricity »
Why ? I do not understand… ap-season = cold winters (in the winter hemisphere) and cool summers (in the summer hemisphere) but just the opposite 6 months later during the peri-season. So why should « cooling during the ap-season » dominate ?
The reasoning seems basically flawed to me…
Line 205 :
« Global temperature variations are not directly derived from eccentricity deviations indirectly by Earth system responses to solar energy variations including the contraction and expansion of the cryosphere, the corresponding albedo variations and the carbon cycle »
Sso why do we need to define ap-seasons and peri-seasons if this does not explain anything after all ?
Line 244 :
« For this analysis, vernal equinox is defined as the middle of the spring season quartile rather than the start of spring. The other seasons follow suit. »
I do not think « equinox » should be redefined.
Line 262 :
« traditional seasons participating as cooling in the ap season would provide either conditions for snow where rain would have occurred »
It is too cold in winter to rain or snow… This is fully irrelevant for ice-sheet dynamics. The rain/snow is certainly not a simple linear response to insolation forcing.
Line 284 :
« This alternation of glaciation types also explains »
Actually, I do not see any explanation here…
Line 292 :
« Obliquity is prominent when eccentricity approaches zero »
Again, obliquity is dominant in the Milankovitch theory. This probably explains why climate variability was dominated by a 41k periodicity before the MPT (with smaller ice-sheets having a more linear behavior).
Line 308 :
« An issue with the Milankovitch summer insolation curves is that glaciation is asynchronous between hemispheres »
No. This is not true. You should have a look at the « caloric seasons » as defined by Milankovitch at high latitude in both hemispheres, which are closely in phase.
Citation: https://doi.org/10.5194/cp-2021-10-RC2 -
RC3: 'Comment on cp-2021-10', Anonymous Referee #3, 30 Mar 2021
The article introduces and computes a new insolation summary based on the concept of e-season. These e-seasons are defined by dividing the Earth orbit in four quadrants, two of which are centred on the perihelion and the aphelion, respectively. The so-called "daily solar energy anomaly" during the perihelion season is a quantity which closely follows eccentricity (Figure 2) and the author considers that this quantity is a predictor of the occurrence of interglacials. He therefore calls for a revision of our current understanding of the dynamics of glacial interglacial cycles.
As the insolation defined by the author closely follows eccentricity, the potential interest of this contribution would have been to suggest an interpretation or justification for introducing direct eccentricity forcing in models of glacial-interglacial cycles. The current understanding of experts is that such term is not essential. Quite a number of conceptual or semi-mechanistic models, which reproduce pretty convincingly the SPECMAP or LR04 benthic curves (with all terminations), assume a summer insolation forcing, only; the key is that some instability mechanism is needed to trigger terminations when glacial volume is large/sea level is low. There is nowadays no longer a "100-ka mystery" or "MIS 11 mystery", but rather a number of competing hypotheses or plausible mechanisms which all follow a same broad scheme. The variations in eccentricity are important, but the way they impact glacial interglacial cycles dynamics is understood to be mainly through the modulation of summer insolation variations. Likewise, there isn't much mystery about glaciations being "global". Northern hemisphere ice sheets are forced by summer insolation; the response of the rest-of-climate involves temperature, CO2, and Antarctica, the latter being little sensitive to direct insolation changes because it is so cold; it grows and shrinks with sea level. Again, there are many possible variations around this broad scheme, attributing CO2 a more or less active or passive role, giving more or less attention to snow accumulation, but the main idea is clear.
Of course, science evolves by breaking consensus, so let's be open and examine the present proposal. Does it present compelling evidence that the current understanding is flawed, by explaining some element, observation, perhaps a phasing between different observations, that would have remained unnoticed so far an not explained by current theories ? Does it "reconcile contradictions" ? I regret not to have seen any evidence of this here. Once insolation curves are introduced, the text is qualitative, pretty confusing, and doesn't provide any physical, quantitatively-supported argument, beyond speculation. It seems that one invokes either a summer mechanism (for not melting snow, the Milankovitch argument) or a winter one (for bringing snow, the Croll argument) as it fits best, without any regard to knowledge gained by detailed analysis of climate records and physical modelling accumulated over a century since the pioneering works.
Consequently, I regret not to encourage the publication of the current manuscript.Citation: https://doi.org/10.5194/cp-2021-10-RC3
Status: closed
-
RC1: 'Comment on cp-2021-10', Anonymous Referee #1, 04 Mar 2021
This paper has the ambitious goal of rewriting Milankovitch theory in order to explain the apparent 100 kyr cycles seen in global climate records. It claims that it resolves some issues that exist in reconciling current theory with data.
It is quite hard to review this paper because much of the discussion is conceptual and broad statements are made that seem to be correct at a rhetorical level but that have no basis when pulled apart. The basic idea of the paper is that while the annual insolation received by the Earth varies little during an eccentricity cycle, the daily energy received by Earth when it is at aphelion is less than when it is at perihelion; and that this contrast is large when eccentricity is large and zero when eccentricity is zero. Both these statements are obvious and well-known.
The author then says that this means that at aphelion and with large eccentricity, Earth will experience cool winters in one hemisphere and cool summers in the other – this is also true. The logical leap he then makes is to say that this will promote glaciation in both hemispheres. As a statement taken on its own (and not considering other aspects of orbital theory) this is flawed in several ways:
Firstly while it is well understood (and the basis of Milankovitch theory) that cold summers at high latitude allow glaciation because they turn seasonal snowcover into permanent ice cover, there is no basis for the statement about winter. Although it is a gross simplification, to first order cold winters are likely to lead to less snowfall. However the author seems to be referring to something else, which is the proportion of rain to snow. This is a red herring because at the latitudes where an ice sheet would nucleate it never rains in winter. Finally it is also the case that at the latitudes that ice sheets form there is almost no winter insolation (above 67 degrees none at all in midwinter), so changes in insolation through eccentricity cycles are very small. Thus there is no basis for the assertion that the winter hemisphere would accumulate ice at aphelion more when eccentricity is large.
Even more fundamentally, I could equally plausibly make the opposite argument. At perihelion and with high eccentricity, Earth will experience warm winters in one hemisphere and warm summers in the other. This would therefore promote deglaciation in both hemispheres. This obviously suffers the same flaw for winter as the statement by the author. However it illustrates the fallacy of making a bold statement based on only a part of the story: using just the position at aphelion or perihelion, one could argue that high eccentricity promotes both glaciation and deglaciation. There is actually no substitute for looking at the system in the detail required to tease apart the actual energy received in geographical locations (such as 65N and 65S), as is done in traditional Milankovitch analysis.
These two issues come together when one thinks about what happens to a high latitude point through a year at high eccentricity. When it was NH summer at aphelion, the NH ice sheets persist and grow because of low summer insolation (we agree). But the author would claim that the SH ice sheet grows because of low winter insolation (we disagree because the small change in winter insolation shouldn’t affect growth), but he ignores the fact that the very high summer insolation at aphelion would melt any excess ice and promote SH ice sheet loss. After 10 kyr (half a precession cycle) everything would be reversed.
Although the paper briefly discusses it the author fails to engage with precession which determines which hemisphere is at aphelion in summer. It is of course through its control on the amplitude of the energy variability during a precession cycle that eccentricity does exert an influence on climate. By ignoring this nuance, the author creates an illusion of simplicity that is not justified. The real hemispheres, at times of high eccentricity, experience both low insolation in a particular season (at aphelion) and, 10 kyr later (half a precession cycle) high insolation (at perihelion). The author chooses only to be interested in the former.
These fundamental problems mean that the paper is not making any breakthrough and is flawed. I will not therefore go into detail on other parts, however a few comments are worthwhile.
I am mystified by Fig 2a: what are the numbers on the curve. They seem somehow to refer to the cycle lengths but they don’t seem to be either the distance between adjacent peaks or the result of spectral analysis of the same curve.
An obvious deficit in the paper is that it never shows the climate curves it claims to explain. This is strange.
While the author correctly describes some of the difficulties with Milankovitch theory he does not acknowledge that many of them are satisfactorily explained. For example the change in greenhouse gases (especially CO2) over the glacial cycles offers a very satisfactory way of globalising what should be a hemispheric climate response. There are now several ideas that explain why multiple precession cycles are needed to initiate deglaciation, and these seem plausible (a new idea is welcome but not needed). I’d agree that the strength of MIS 11 is not yet well understood, but the current paper does not address this.
Citation: https://doi.org/10.5194/cp-2021-10-RC1 -
RC2: 'Comment on cp-2021-10', Anonymous Referee #2, 26 Mar 2021
This manuscript attempts to provide a new point of view on the question of Quaternary glaciations and the 100-kyr cyclicity. It is based on rather obvious facts : indeed the Earth receives (globally) less energy when far from the Sun at the aphelion. But it also receives (globally) more energy six months later. Overall the argumentation is confused, to say the least. It seems that the author is « cherry-picking » the processes that corresponds to the desired effect : when at the aphelion, the Earth being far from the Sun we have cold winters (in the winter hemisphere) and cool summers (in the summer hemisphere) therefore a glaciation… But obviously the opposite holds true six months later.
More fundamentally, it appears that the author attempts to give answers to problems that do not even exist. For instance, he states (line 308) that « An issue with the Milankovitch summer insolation curves is that glaciation is asynchronous between hemispheres ». This is not true. The main forcing in the Milankovitch theory (in contrast to Croll’s one) is obliquity, at 41 kyr. Obliquity changes have a symmetric impact on both hemispheres : both are cooled due to low obliquity. This was actually a major difference with previous theories based mostly on precession (on the false premise that Antarctica is currently in a glacial state). Besides, ice-sheets are too cold in winter to allow for any significant precipitations, and there is never « mild rainy winters » over there. This is the key argument of Milankovitch to focus on the summer season, something already strongly discussed in the 19th century.
But at the end, on Figure 4, the author mentions (line 279) that « The precession cycles are responsible for the oscillation of insolation for summer and winter bounded by the ap- and peri-insolation ». Indeed, this is the well-known amplitude modulation of insolation at the basis of most of classical theories explaining the 100-kyr cycles. Nothing new.
Overall, this manuscript is flawed (and very confused) in its climatic mechanisms. I do not recommend its publication.
Detailed comments :
Lines 73-74 :
« to attribute the dominant cycle to multiples of precession or obliquity cycles »
This is certainly not the « standard » explanation. The most common explanation involves the amplitude modulation of the effect of precession on climate (which is precisely excentricity). A non-linear system will in general extract the envelop of the forcing and produce a 100ky oscillation, even for very simplified models. The difficulty is not to identify the origin of the 100-kyr cycles (ie. the amplitude modulation of precession) but to identify the non-linearity of ice-sheets that can extract this envelopp. Many possibilities have been put forward.
Line 80 :
« eccentricity does provide necessary solar energy conditions for global glaciation »
Obliquity is symmetric and explains very easily globally synchronous glaciations. This is in fact one of the main argument for the Milankovitch theory. It could also be argued that teleconnections are also critical : The Quaternary is characterized by northern glaciations (since in Antarctica, the ice-sheet is always close to its maximum extent, the continent being ice-covered today) therefore, concerning ice-sheets, we can neglect Antarctica. Concerning climate, the main (ice) changes are occuring in the North and will probably have global consequences. CO2 can also have a global impact.
Line 160 :
« Which theory is valid has been a long running debate (Imbrie and Imbrie, 1989). »
This is not true for at least a century. Nobody would claim today (except maybe in this current paper) that winter insolation has any consequence on ice sheets. In polar regions, winter is extremely cold and without significant precipitation. The mass balance of the ice sheet does not depend at all of insolation changes. The rôle of summer insolation is widely recognized, and is also consistent with simulations of ice-sheets using detailed numerical models. Since Milankovitch at the beginning of the 20th century, there is no more debate on this point. And again, Milankovitch already has glaciations synchronous in both hemispheres, since the main forcing is obliquity.
Line 163 :
« Annual snow growth for building ice sheets would result from more snow created in the colder winter of one hemisphere and less snowmelt in a cool summer of the other hemisphere ».
No. Winter is irrelevant : there is almost no snow in winter, since there is almost no water vapor in the winter atmosphere.
Line 185 :
« The eccentricity … shown in Fig. 2a reveal three discernable cycle periods of approximately 65 kyr, 100 kyr and 400 kyr »
65 ky does not clearly show up in the spectral analysis…. The vocabulary « cycle period » is not quite relevant, « duration of specific events » would be more appropriate.
Figure 2
In other words, Earth receives less energy per day (less power) when far from the Sun. This seems quite obvious to me.
Line 200 :
« Conditions for glaciation occur during increasing eccentricity resulting in increasing ap-season cooling (more negative) and increasing peri-season warming while conditions for deglaciation occur during diminishing eccentricity »
Why ? I do not understand… ap-season = cold winters (in the winter hemisphere) and cool summers (in the summer hemisphere) but just the opposite 6 months later during the peri-season. So why should « cooling during the ap-season » dominate ?
The reasoning seems basically flawed to me…
Line 205 :
« Global temperature variations are not directly derived from eccentricity deviations indirectly by Earth system responses to solar energy variations including the contraction and expansion of the cryosphere, the corresponding albedo variations and the carbon cycle »
Sso why do we need to define ap-seasons and peri-seasons if this does not explain anything after all ?
Line 244 :
« For this analysis, vernal equinox is defined as the middle of the spring season quartile rather than the start of spring. The other seasons follow suit. »
I do not think « equinox » should be redefined.
Line 262 :
« traditional seasons participating as cooling in the ap season would provide either conditions for snow where rain would have occurred »
It is too cold in winter to rain or snow… This is fully irrelevant for ice-sheet dynamics. The rain/snow is certainly not a simple linear response to insolation forcing.
Line 284 :
« This alternation of glaciation types also explains »
Actually, I do not see any explanation here…
Line 292 :
« Obliquity is prominent when eccentricity approaches zero »
Again, obliquity is dominant in the Milankovitch theory. This probably explains why climate variability was dominated by a 41k periodicity before the MPT (with smaller ice-sheets having a more linear behavior).
Line 308 :
« An issue with the Milankovitch summer insolation curves is that glaciation is asynchronous between hemispheres »
No. This is not true. You should have a look at the « caloric seasons » as defined by Milankovitch at high latitude in both hemispheres, which are closely in phase.
Citation: https://doi.org/10.5194/cp-2021-10-RC2 -
RC3: 'Comment on cp-2021-10', Anonymous Referee #3, 30 Mar 2021
The article introduces and computes a new insolation summary based on the concept of e-season. These e-seasons are defined by dividing the Earth orbit in four quadrants, two of which are centred on the perihelion and the aphelion, respectively. The so-called "daily solar energy anomaly" during the perihelion season is a quantity which closely follows eccentricity (Figure 2) and the author considers that this quantity is a predictor of the occurrence of interglacials. He therefore calls for a revision of our current understanding of the dynamics of glacial interglacial cycles.
As the insolation defined by the author closely follows eccentricity, the potential interest of this contribution would have been to suggest an interpretation or justification for introducing direct eccentricity forcing in models of glacial-interglacial cycles. The current understanding of experts is that such term is not essential. Quite a number of conceptual or semi-mechanistic models, which reproduce pretty convincingly the SPECMAP or LR04 benthic curves (with all terminations), assume a summer insolation forcing, only; the key is that some instability mechanism is needed to trigger terminations when glacial volume is large/sea level is low. There is nowadays no longer a "100-ka mystery" or "MIS 11 mystery", but rather a number of competing hypotheses or plausible mechanisms which all follow a same broad scheme. The variations in eccentricity are important, but the way they impact glacial interglacial cycles dynamics is understood to be mainly through the modulation of summer insolation variations. Likewise, there isn't much mystery about glaciations being "global". Northern hemisphere ice sheets are forced by summer insolation; the response of the rest-of-climate involves temperature, CO2, and Antarctica, the latter being little sensitive to direct insolation changes because it is so cold; it grows and shrinks with sea level. Again, there are many possible variations around this broad scheme, attributing CO2 a more or less active or passive role, giving more or less attention to snow accumulation, but the main idea is clear.
Of course, science evolves by breaking consensus, so let's be open and examine the present proposal. Does it present compelling evidence that the current understanding is flawed, by explaining some element, observation, perhaps a phasing between different observations, that would have remained unnoticed so far an not explained by current theories ? Does it "reconcile contradictions" ? I regret not to have seen any evidence of this here. Once insolation curves are introduced, the text is qualitative, pretty confusing, and doesn't provide any physical, quantitatively-supported argument, beyond speculation. It seems that one invokes either a summer mechanism (for not melting snow, the Milankovitch argument) or a winter one (for bringing snow, the Croll argument) as it fits best, without any regard to knowledge gained by detailed analysis of climate records and physical modelling accumulated over a century since the pioneering works.
Consequently, I regret not to encourage the publication of the current manuscript.Citation: https://doi.org/10.5194/cp-2021-10-RC3
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100,000 year problem.