Reconstructing the evolution of ice sheets, sea level, and atmospheric CO<sub>2</sub> during the past 3.6 million years

Berends, Constantijn J.; de Boer, Bas; van de Wal, Roderik S. W.

doi:https://doi.org/10.5194/cp-17-361-2021

Articles | Volume 17, issue 1

https://doi.org/10.5194/cp-17-361-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Special issue:

Oldest Ice: finding and interpreting climate proxies in ice...

https://doi.org/10.5194/cp-17-361-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 17, issue 1

Research article

|

01 Feb 2021

Research article |

| 01 Feb 2021

Reconstructing the evolution of ice sheets, sea level, and atmospheric CO₂ during the past 3.6 million years

Constantijn J. Berends, Bas de Boer, and Roderik S. W. van de Wal

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Final revised paper (published on 01 Feb 2021)
Preprint (discussion started on 24 Apr 2020)

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Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Review „Reconstructing the evolution of ice sheets,...” by Berends et al.', Anonymous Referee #1, 25 May 2020
- AC1: 'Rebuttal to Anonymous Reviewer #1', Tijn Berends, 07 Jul 2020
RC2: 'Review', Andrey Ganopolski, 19 Jun 2020
- AC2: 'Rebuttal to review by A. Ganopolski', Tijn Berends, 07 Jul 2020

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (24 Jul 2020) by Ed Brook

AR by Tijn Berends on behalf of the Authors (17 Aug 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (24 Sep 2020) by Ed Brook

RR by Andrey Ganopolski (15 Oct 2020)

Suggestions for revision or reasons for rejection

I appreciate the efforts which the authors made to respond to my criticism and suggestions and in general I am satisfied with their response. The only my major remaining concern I still have is the treatment of the uncertainties of CO2 reconstructions. In response to my comment, the authors explained that under “conservative” estimate of uncertainties they understand the estimate only of one but the main source of uncertainties. If so, the authors believe that their errors in CO2 during the entire Quaternary are not higher than ca 5 ppm and thus they have nearly 100% confidence that during the early Quaternary CO2 never exceeded 300 ppm. However, some reconstructions, including the cited in the manuscript Martínez-Botí et al. (2015), suggest that at the beginning of Quaternary, CO2 most of time was well above 300 ppm (somewhere between 300 and 400 ppm). Needless to say, this is an important difference. I do not expect from the authors to perform “objective” (instead of “conservative”) estimate of their method uncertainties because this is impossible without using a comprehensive water-isotope enabled Earth system model. However, it would be useful to inform potential readers that reported uncertainty is nothing more than the method sensitivity to 0.1 promille change in the benthic d18O (which is likely to be over-optimistic estimate for LR04 stack uncertainties, at least, for the early Quaternary) while the real errors of the method can be much larger but, unfortunately, cannot be properly evaluated at this stage.

Other comments.

p. 31, l. 12. I do not understand why “a narrower range of sea levels for each CO2 indicating that their ice sheets respond faster to change in climate” (whatever faster means). The close relationship between CO2 and sea level seen also in the data only indicates the existence of strong climate-ice sheets-carbon cycle feedbacks but tells nothing about the rate of ice sheet response to climate change.

P. 31, l.13. I cannot see on this figure “a relatively narrow range of sea level in the 180-250 ppm CO2 range” in Willeit et al. (2019). The significance of this fact is also unclear to me.

p. 32, l. 1. It is true that in our previous studies (but not in Willeit at al., 2019) we have shown that the threshold for glacial inception depends on both insolation and CO2 and we have shown in Ganopolski et al. (2016) that the true CO2 threshold for glaciation is ca. 350 ppm because for higher CO2 glacial inception would be possible only with such low summer insolation which cannot be achieved with the realistic orbital parameters. This funding not only explains fig. 8f but is also consistent with the data (fig. 8a). However, during most of Quaternary, the interglacial CO2 level was in the range 240-280 ppm and this is why it is natural that any appreciable sea level drop can only be observed when CO2 was below these values and this is true for all figures in fig. 8 except for Stap et al. (2017). In short, the fact that most of sea level drop during Quaternary occurred for CO2 below 250 ppm does not imply that ice sheets cannot grow under significantly higher CO2 level.

p. 34. l.1 It is not clear to me what “downwelling regions” means

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ED: Publish subject to minor revisions (review by editor) (24 Nov 2020) by Ed Brook

AR by Tijn Berends on behalf of the Authors (26 Nov 2020) Author's response Manuscript

ED: Publish as is (15 Dec 2020) by Ed Brook

AR by Tijn Berends on behalf of the Authors (16 Dec 2020)

Short summary

For the past 2.6 million years, the Earth has experienced glacial cycles, where vast ice sheets periodically grew to cover large parts of North America and Eurasia. In the earlier part of this period, this happened every 40 000 years. This value changed 1.2 million years ago to 100 000 years: the Mid-Pleistocene Transition. We investigate this interesting period using an ice-sheet model, studying the interactions between ice sheets and the global climate.

Reconstructing the evolution of ice sheets, sea level, and atmospheric CO2 during the past 3.6 million years

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Reconstructing the evolution of ice sheets, sea level, and atmospheric CO₂ during the past 3.6 million years