Climate and ocean circulation changes toward a modern snowball Earth

Obase, Takashi; Kodama, Takanori; Kawasaki, Takao; Sherriff-Tadano, Sam; Takasuka, Daisuke; Abe-Ouchi, Ayako; Fujii, Masakazu

doi:10.5194/egusphere-2025-1484

Preprints

https://doi.org/10.5194/egusphere-2025-1484

Preprints

04 Apr 2025

| 04 Apr 2025

Climate and ocean circulation changes toward a modern snowball Earth

Takashi Obase, Takanori Kodama, Takao Kawasaki, Sam Sherriff-Tadano, Daisuke Takasuka, Ayako Abe-Ouchi, and Masakazu Fujii

Abstract. In the past, Earth experienced snowball events, where its surface became completely covered with ice. Previous studies used general circulation models to investigate the onset and climate of such snowball events. Using the MIROC4m coupled atmosphere-ocean climate model, this study examined the changes in the oceanic circulation during the onset of a modern snowball Earth and elucidated their evolution to steady states under the snowball climate. Abruptly changing the solar constant to 94 % of its present-day value caused the modern Earth climate to turn into a snowball state after 1300 years and initiated rapid increase in sea ice thickness. During onset of the snowball event, extensive sea ice formation and melting of sea ice in the mid-latitudes caused substantial freshening of surface waters and salinity stratification. By contrast, such salinity stratification was absent if the duration necessary for snowball onset was short because of stronger solar constant forcing. After snowball onset, the global sea ice cover reduced air–sea fluxes and caused drastic weakening in the deep ocean circulation. However, as the ocean temperature and salinity fields approached near constant states, the meridional overturning circulation resumed in the steady-state snowball climate. Although the evolution of the oceanic circulation would depend on model setting, particularly regarding the treatment of air–sea fluxes and the continental distribution, our results highlight the importance of the oceanic circulation and associated biogeochemical changes in the climate system feedback and sequence of snowball events.

Received: 28 Mar 2025 – Discussion started: 04 Apr 2025

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.

Download & links

Preprint (PDF, 2155 KB)

Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
Preprint (2155 KB)

Download & links

The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.

Journal article(s) based on this preprint

20 Apr 2026

Climate and ocean circulation changes toward a modern snowball Earth

Takashi Obase, Takanori Kodama, Takao Kawasaki, Sam Sherriff-Tadano, Daisuke Takasuka, Ayako Abe-Ouchi, and Masakazu Fujii

Clim. Past, 22, 845–859, https://doi.org/10.5194/cp-22-845-2026,https://doi.org/10.5194/cp-22-845-2026, 2026

Short summary

Takashi Obase, Takanori Kodama, Takao Kawasaki, Sam Sherriff-Tadano, Daisuke Takasuka, Ayako Abe-Ouchi, and Masakazu Fujii

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-1484', Yonggang Liu, 04 May 2025

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1484/egusphere-2025-1484-RC1-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2025-1484-RC1
- AC1: 'Reply on RC1', Takashi Obase, 30 Jul 2025
  
  Thank you for providing precise and valuable feedback on our manuscript. Please see the attached pdf for our response.
  
  Citation: https://doi.org/10.5194/egusphere-2025-1484-AC1
RC2:
'Comment on egusphere-2025-1484', Anonymous Referee #2, 09 May 2025
The manuscript by Takashi Obase and colleagues presents simulation results from climate experiments of transitioning from a modern-day climate state to a snowball-Earth state in response to an abrupt reduction in incoming solar radiation. The authors use the MIROC4m atmosphere–ocean general circulation model (AOGCM), which—in principle—seems a good choice for this kind of study and enables them to investigate the evolution of ocean and atmosphere during and after that transition.
I think the study is interesting especially with regard to the ocean dynamics and the results are discussed in an enlightening way. However, I would suggest a few general and a number of specific minor revisions before publication.
Minor revisions (general)
The authors acknowledge that their results differ from previous studies in some important points, which is not a problem in its own right, of course. However, two assumptions appear very critical to me, as outlined below. Ideally, a study would mostly cover the sensitivity of the results to these assumptions, but seeing that the scope of the paper would considerably grow and the discussed results do not need to be “most realistic”, I would suggest an even deeper discussion of the following points.

I think it is a strong assumption and maybe a large perturbation for the model to abruptly replace all land surface with ice sheets once the oceans are fully covered in sea ice. The effect of this assumption on the results (especially regarding the hydrological cycle) should be discussed.

The comparatively strong momentum transfer from the wind to the ocean in the case of very thick sea ice is already discussed. Ideally, one would need a sensitivity run to see the differences in the model. This is not absolutely necessary from my point of view, but it should be stressed at a prominent point of the manuscript that the assumption is probably unrealistic.

That said, it is notable that the authors included a discussion and sensitivity test regarding the minimal thermal diffusion coefficient over the ocean surface, because this is another critical point.
Minor revisions (specific)

L8: I find it not clear what is meant by “necessary”. Maybe replace the formulation by “the duration between the change in solar constant and snowball onset” or something similar.

L17f.: “iron formation” is too general.

L20: “… 94% of its …” without “that”

L27: “… equilibrium solution of the climate" instead of “planet”

L38: “… the same AOGCM[s]” is not correct. E.g., the study by Poulsen et al. (2002) used FOAM, which the studies mentioned before did not employ. Voigt (2013) used an AGCM. The study by Eberhard et al. (2023) differs from the ones mentioned before, as they used a model of intermediate complexity (which is not an AOGCM) and focused on a large set of sensitivity runs instead.

L39: The terms “Marinoan” and “Sturtian” have to be switched. The Marinoan is the younger one.

L65 and other instances: It appears that sometimes the names MIROC and MIROC4m are synonymous. Are they? If not, please make a clearer distinction.

L95: I think this value of the ECS is for doubling CO₂ in a modern climate state, right? If so, please mention this, as for states with more ice the ECS is sometimes expected to be much higher.

L112f.: Is it likely that this nonconservation of global water volume and salinity introduces any artificial long-term salinity trends for “stable” climate states which could be relevant for the snowball simulations? One could, e.g., check the discussed TS100 run for the long-term evolution of total salinity. I ask because sometimes this can happen in the case of nonconservative schemes.

L120: What is the reason for using this value for the solar constant? Basically, I am wondering where the .12 comes from.

L120: “changed from 91 to 100%” or “set to 91–100%”

L135ff.: I realize the difficulty to run the TS096 simulation for even longer, but it is not evident for me that this one will stay in a non-snowball state forever. Sea-ice area and SAT still have considerable trends and might reach the transition point eventually. I suggest to at least mention this in the manuscript and, for example, weaken the statement in L139f: “… was determined to be between 94 and 96%”.

L155: “Fig. 2c blue”—Please check whether you indicated the right color.

L190f.: “The sea ice formation …” is a repetition of a similar statement in L184ff.

L192: It would make sense to add a reference to Fig. 5c, as well.

L196: “maximum sea ice thickness along the western side of the Pacific Ocean”—This is difficult to see in the figure.

L203f.: This last sentence is partly a repetition of a similar earlier statement in L199f.

L246: “.. from that in a multimodel study”, similar in L249f.

L284: “… depends on radiative forcing”—This sounds as if the radiative forcing directly influenced the salinity stratification. Maybe: “depends on the rate of cooling”?

L284f.: Please specify what you mean with “external forcing”, this could even be early in the paper. Some people describe the insolation itself as a forcing, and then it would sound strange to speak of a stronger external forcing for reduced insolation.

L289: “as opposed to” instead of “than”?

L347: “… coupled with an EBM”

L359: Please carefully revise the values given. Liu et al. (2013) find thresholds between 80 and 150 ppm, but thresholds even below 80 ppm for another aerosol parametrization. Feulner and Kienert (2014) find thresholds of 100–110 ppm for the Sturtian and 120–130 ppm for the Marinoan.
Citation: https://doi.org/10.5194/egusphere-2025-1484-RC2
- AC2: 'Reply on RC2', Takashi Obase, 30 Jul 2025
  
  Thank you for providing precise and valuable feedback on our manuscript. Please see the attached pdf for our response.
  
  Citation: https://doi.org/10.5194/egusphere-2025-1484-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-1484', Yonggang Liu, 04 May 2025

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1484/egusphere-2025-1484-RC1-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2025-1484-RC1
- AC1: 'Reply on RC1', Takashi Obase, 30 Jul 2025
  
  Thank you for providing precise and valuable feedback on our manuscript. Please see the attached pdf for our response.
  
  Citation: https://doi.org/10.5194/egusphere-2025-1484-AC1
RC2:
'Comment on egusphere-2025-1484', Anonymous Referee #2, 09 May 2025
The manuscript by Takashi Obase and colleagues presents simulation results from climate experiments of transitioning from a modern-day climate state to a snowball-Earth state in response to an abrupt reduction in incoming solar radiation. The authors use the MIROC4m atmosphere–ocean general circulation model (AOGCM), which—in principle—seems a good choice for this kind of study and enables them to investigate the evolution of ocean and atmosphere during and after that transition.
I think the study is interesting especially with regard to the ocean dynamics and the results are discussed in an enlightening way. However, I would suggest a few general and a number of specific minor revisions before publication.
Minor revisions (general)
The authors acknowledge that their results differ from previous studies in some important points, which is not a problem in its own right, of course. However, two assumptions appear very critical to me, as outlined below. Ideally, a study would mostly cover the sensitivity of the results to these assumptions, but seeing that the scope of the paper would considerably grow and the discussed results do not need to be “most realistic”, I would suggest an even deeper discussion of the following points.

I think it is a strong assumption and maybe a large perturbation for the model to abruptly replace all land surface with ice sheets once the oceans are fully covered in sea ice. The effect of this assumption on the results (especially regarding the hydrological cycle) should be discussed.

The comparatively strong momentum transfer from the wind to the ocean in the case of very thick sea ice is already discussed. Ideally, one would need a sensitivity run to see the differences in the model. This is not absolutely necessary from my point of view, but it should be stressed at a prominent point of the manuscript that the assumption is probably unrealistic.

That said, it is notable that the authors included a discussion and sensitivity test regarding the minimal thermal diffusion coefficient over the ocean surface, because this is another critical point.
Minor revisions (specific)

L8: I find it not clear what is meant by “necessary”. Maybe replace the formulation by “the duration between the change in solar constant and snowball onset” or something similar.

L17f.: “iron formation” is too general.

L20: “… 94% of its …” without “that”

L27: “… equilibrium solution of the climate" instead of “planet”

L38: “… the same AOGCM[s]” is not correct. E.g., the study by Poulsen et al. (2002) used FOAM, which the studies mentioned before did not employ. Voigt (2013) used an AGCM. The study by Eberhard et al. (2023) differs from the ones mentioned before, as they used a model of intermediate complexity (which is not an AOGCM) and focused on a large set of sensitivity runs instead.

L39: The terms “Marinoan” and “Sturtian” have to be switched. The Marinoan is the younger one.

L65 and other instances: It appears that sometimes the names MIROC and MIROC4m are synonymous. Are they? If not, please make a clearer distinction.

L95: I think this value of the ECS is for doubling CO₂ in a modern climate state, right? If so, please mention this, as for states with more ice the ECS is sometimes expected to be much higher.

L112f.: Is it likely that this nonconservation of global water volume and salinity introduces any artificial long-term salinity trends for “stable” climate states which could be relevant for the snowball simulations? One could, e.g., check the discussed TS100 run for the long-term evolution of total salinity. I ask because sometimes this can happen in the case of nonconservative schemes.

L120: What is the reason for using this value for the solar constant? Basically, I am wondering where the .12 comes from.

L120: “changed from 91 to 100%” or “set to 91–100%”

L135ff.: I realize the difficulty to run the TS096 simulation for even longer, but it is not evident for me that this one will stay in a non-snowball state forever. Sea-ice area and SAT still have considerable trends and might reach the transition point eventually. I suggest to at least mention this in the manuscript and, for example, weaken the statement in L139f: “… was determined to be between 94 and 96%”.

L155: “Fig. 2c blue”—Please check whether you indicated the right color.

L190f.: “The sea ice formation …” is a repetition of a similar statement in L184ff.

L192: It would make sense to add a reference to Fig. 5c, as well.

L196: “maximum sea ice thickness along the western side of the Pacific Ocean”—This is difficult to see in the figure.

L203f.: This last sentence is partly a repetition of a similar earlier statement in L199f.

L246: “.. from that in a multimodel study”, similar in L249f.

L284: “… depends on radiative forcing”—This sounds as if the radiative forcing directly influenced the salinity stratification. Maybe: “depends on the rate of cooling”?

L284f.: Please specify what you mean with “external forcing”, this could even be early in the paper. Some people describe the insolation itself as a forcing, and then it would sound strange to speak of a stronger external forcing for reduced insolation.

L289: “as opposed to” instead of “than”?

L347: “… coupled with an EBM”

L359: Please carefully revise the values given. Liu et al. (2013) find thresholds between 80 and 150 ppm, but thresholds even below 80 ppm for another aerosol parametrization. Feulner and Kienert (2014) find thresholds of 100–110 ppm for the Sturtian and 120–130 ppm for the Marinoan.
Citation: https://doi.org/10.5194/egusphere-2025-1484-RC2
- AC2: 'Reply on RC2', Takashi Obase, 30 Jul 2025
  
  Thank you for providing precise and valuable feedback on our manuscript. Please see the attached pdf for our response.
  
  Citation: https://doi.org/10.5194/egusphere-2025-1484-AC2

Peer review completion

AR – Author's response | RR – Referee report | ED – Editor decision | EF – Editorial file upload

ED: Reconsider after major revisions (02 Sep 2025) by Yannick Donnadieu

AR by Takashi Obase on behalf of the Authors (07 Jan 2026) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (19 Jan 2026) by Yannick Donnadieu

RR by Yonggang Liu (13 Feb 2026)

Suggestions for revision or reasons for rejection

This is my second review of this manuscript. The authors have addressed my previous concerns diligently and responsibly, and I believe the paper has been significantly improved. It will be ready for publication once the following two inaccurate expressions are corrected (see below).

Comment:
L1: “In the past, Earth experienced snowball events …”
The hypothesis of a fully ice-covered “snowball Earth” is not yet conclusively proven. To reflect this scientific uncertainty, it is recommended to use a more cautious formulation such as “its surface might have been completely covered with ice”. Moreover, "where" may be replaced with "during which"

L119-120: the influence of sea ice thickness on albedo is actually quite strong, negligence of which might have strengthened the positive sea-ice albedo feedback and rendered the Earth more susceptible to entering a snowball state. This needs to be discussed either here or near the end of the manuscript.

L150-151: a speed of 2 cm/sec is still very large for thick sea ice (or sea glaciers), which moves at a speed of 100 m/year. The possible influence of such high mobility of sea ice should be briefly discussed here.

L210-212: I do not really understand how atmosphere-ocean heat exchange is treated in the model when the ocean is completely covered by ice. In reality, the heat can only be transferred from the ocean into the atmosphere through thermal conduction within ice, which is extremely slow when ice is more than 10 m thick. When sea ice is thin, there can be many cracks or leads through which strong atmosphere-ocean heat exchange can happen, but these leads will presumably disappear once the sea ice is more than 10s of meters thick. The setup of your model could have greatly exaggerated the atmosphere-ocean heat flux and thus the sea-ice formation rate. This high sea-ice formation rate could drive the active MOC after the snowball onset. It is important for the authors to compare the atmosphere-ocean heat exchange rate to the conductive heat flux through ice, and discuss the possible consequence if the former is large.

L295-297: it is not really clear whether the weakening of oceanic heat transport is completely due to the weakening or disappearance of NADW. The change of wind-driven gyre circulation and STC could all have some contribution. Therefore, it is better to soften the statement here.

L331-344: The discussion here is not very helpful because the major problem is there are too few experiments so that we do not know where the threshold is in your model.

L357: "at" should be "about"

L419: "The" -> "the"

L448-449: “Donnadieu et al. (2003) and Pollard and Kasting (2004) used ice sheet
models asynchronously coupled with an AGCM …”
This description is inaccurate. To my understanding, neither of the two studies implemented a real two-way asynchronous coupling. Instead, they used outputs (temperature and precipitation) from an AGCM to drive their ice sheet models in a one-way, offline manner. Therefore, “coupled with” is misleading and should be replaced with phrasing like “...ice sheet models driven by outputs from an AGCM...”.

L471-473: what geological records?

Figure 2: it may be helpful plotting the MOC strength as negative.

Hide

ED: Publish subject to minor revisions (review by editor) (17 Feb 2026) by Yannick Donnadieu

AR by Takashi Obase on behalf of the Authors (27 Feb 2026) Author's response Author's tracked changes Manuscript

ED: Publish as is (11 Mar 2026) by Yannick Donnadieu

AR by Takashi Obase on behalf of the Authors (19 Mar 2026) Manuscript

Journal article(s) based on this preprint

20 Apr 2026

Climate and ocean circulation changes toward a modern snowball Earth

Takashi Obase, Takanori Kodama, Takao Kawasaki, Sam Sherriff-Tadano, Daisuke Takasuka, Ayako Abe-Ouchi, and Masakazu Fujii

Clim. Past, 22, 845–859, https://doi.org/10.5194/cp-22-845-2026,https://doi.org/10.5194/cp-22-845-2026, 2026

Short summary

Takashi Obase, Takanori Kodama, Takao Kawasaki, Sam Sherriff-Tadano, Daisuke Takasuka, Ayako Abe-Ouchi, and Masakazu Fujii

Viewed

Total article views: 1,733 (including HTML, PDF, and XML)

HTML	PDF	XML	Total	BibTeX	EndNote
1,298	388	47	1,733	54	64

HTML: 1,298
PDF: 388
XML: 47
Total: 1,733
BibTeX: 54
EndNote: 64

Views and downloads (calculated since 04 Apr 2025)

Month	HTML	PDF	XML	Total
Apr 2025	136	29	7	172
May 2025	114	13	5	132
Jun 2025	54	38	3	95
Jul 2025	50	23	4	77
Aug 2025	110	7	2	119
Sep 2025	387	13	0	400
Oct 2025	49	8	1	58
Nov 2025	42	22	3	67
Dec 2025	101	70	11	182
Jan 2026	91	57	8	156
Feb 2026	66	52	3	121
Mar 2026	73	48	0	121
Apr 2026	25	8	0	33

Cumulative views and downloads (calculated since 04 Apr 2025)

Month	HTML	PDF	XML	Total
Apr 2025	136	29	7	172
May 2025	114	13	5	132
Jun 2025	54	38	3	95
Jul 2025	50	23	4	77
Aug 2025	110	7	2	119
Sep 2025	387	13	0	400
Oct 2025	49	8	1	58
Nov 2025	42	22	3	67
Dec 2025	101	70	11	182
Jan 2026	91	57	8	156
Feb 2026	66	52	3	121
Mar 2026	73	48	0	121
Apr 2026	25	8	0	33

Viewed (geographical distribution)

Total article views: 1,725 (including HTML, PDF, and XML) Thereof 1,725 with geography defined and 0 with unknown origin.

Country	#	Views	%

Latest update: 30 Apr 2026

Download

The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.

Preprint (2155 KB)
Metadata XML

Short summary

In the past, Earth experienced its surface became completely covered with ice. Using an atmosphere-ocean climate model, we examined the evolution in the ocean circulation from modern to the snowball Earth. We found that the deep ocean ocean circulation experienced drastic weakening before the snowball onset by salinity changes, and after that the ocean circulation resumed. The ocean circulation changes have implications for understanding climate system feedback on the past snowball events.


Total:	0
HTML:	0
PDF:	0
XML:	0