Atmospheric methane since the last glacial  maximum was driven by wetland sources

Kleinen, Thomas; Gromov, Sergey; Steil, Benedikt; Brovkin, Victor

doi:https://doi.org/10.5194/cp-19-1081-2023

Articles | Volume 19, issue 5

https://doi.org/10.5194/cp-19-1081-2023

© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/cp-19-1081-2023

© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 19, issue 5

Research article

|

01 Jun 2023

Research article |

| 01 Jun 2023

Atmospheric methane since the last glacial maximum was driven by wetland sources

Thomas Kleinen, Sergey Gromov, Benedikt Steil, and Victor Brovkin

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Final revised paper (published on 01 Jun 2023)
Preprint (discussion started on 17 Oct 2022)

Interactive discussion

Status: closed

RC1:
'Comment on cp-2022-80', Anonymous Referee #1, 24 Nov 2022

The comment was uploaded in the form of a supplement: https://cp.copernicus.org/preprints/cp-2022-80/cp-2022-80-RC1-supplement.pdf

Citation: https://doi.org/10.5194/cp-2022-80-RC1
- AC1: 'Reply on RC1', Thomas Kleinen, 10 Jan 2023
  
  The comment was uploaded in the form of a supplement: https://cp.copernicus.org/preprints/cp-2022-80/cp-2022-80-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/cp-2022-80-AC1
RC2:
'Comment on cp-2022-80', Anonymous Referee #2, 13 Dec 2022

Review of cp-2022-80, entitled “Atmospheric methane since the LGM was driven by wetland sources” by Kleinen et al.

Summary:

This paper by Kleinen et al. makes use of a CMIP6-generation Earth System Model, MPI-ESM, with an interactive methane cycle to investigate changes in the methane budget between the last glacial maximum and the pre-industrial period. The model includes interactive emission schemes for many of the natural emission sources relevant for the time period of interest and includes a parametrised approach for the atmospheric methane sink. Using novel, and for the first time, transient simulations, they focus particularly on the rapid changes in the methane cycle occurring during deglaciation.

The paper is well organised, with clear and sufficient detail for the reader to understand the model set up and the results. It is well written and made for an enjoyable and interesting read. More importantly, this study represents a significant step change in model capability, model setup, and an advancement in the state-of-the-art, particularly in relation to running transient simulations from the last glacial maximum to the present day. To date, other studies addressing changes in the methane cycle over this time period have either used simple models or timeslice simulations.

Below, I have some minor general and/or specific comments. However, I would unreservedly recommend that the manuscript be published.

General comments:

You say that the tropical wetland extent is overestimated in the model. Can you comment on how much that overestimate influences your conclusions regarding the role of tropical wetlands in driving the changes?

As a scientist with an interest in the more contemporary period and future projections, I’d be keen for the manuscript to include some discussion on the implications of this study for future projections of methane and the role of tropical wetland sources.

You say that for the purpose of accounting for the soil uptake, you prescribe the atmospheric concentration of methane. Can you comment on the potential impact on model performance that would arise if soil uptake was coupled to the modelled concentration?

Specific comments:

Page 3, line 89: Change “is produces” to “is produced”

Line 161: For use of “CI” in the first instance, please write out in full with abbreviation. Thereafter, CI is okay to use.

Line 195: On first use, please write out “AMOC” in full with abbreviation

Lines 196 and 199: As a reviewer whose main expertise is more in the contemporary period, it would be useful to explain what is meant by “1a” and “1b” when referring to the meltwater pulse. If they are simply referring to the different transitions in the AMOC which occur, perhaps these could either be labelled in the figure (and with addition to figure caption) or made more explicit in the text.

Caption for Table 1: Suggest that you change “timeslices” to “time periods”

Citation: https://doi.org/10.5194/cp-2022-80-RC2
- AC2: 'Reply on RC2', Thomas Kleinen, 10 Jan 2023
  
  The comment was uploaded in the form of a supplement: https://cp.copernicus.org/preprints/cp-2022-80/cp-2022-80-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/cp-2022-80-AC2

Peer review completion

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

ED: Reconsider after major revisions (11 Jan 2023) by Alberto Reyes

AR by Thomas Kleinen on behalf of the Authors (03 Mar 2023) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (07 Mar 2023) by Alberto Reyes

RR by Anonymous Referee #1 (10 Apr 2023)

Suggestions for revision or reasons for rejection

Kleinen et al. have done a great job addressing my previous comments in a satisfactory manner. They also done a great job in elaborating and adding several important discussion points to their paper. This study provides a novel first attempt to model the transient evolution of methane during the last deglaciation. From modeling approach, it brought forward many important results and ideas, mainly the importance of tropical wetlands in driving the bulk of deglacial methane rise, the role of methane emissions from shelf areas, the evolution of atmospheric methane lifetime during the deglacial transition. As such I would highly recommend this manuscript for publications.

Here are some minor notes I have that might be useful in polishing up the paper:

Page 1 line 13: “Between the last glacial […] doubling in concentration during those 11000 yrs.” I would cite Figure 2 here just to orient readers who are not extremely familiar with deglacial methane evolution.

Page 2 line 27: “Or they used a more detailed […].” Here ‘they’ refer to non-box model (so non transient) time slice studies but it is not immediately clear. To clarify and draw the contrast against the box model studies I would just say something like “An alternative approach to box models would be … “

Page 3 line 60-65: This opening paragraph is quite redundant as every aspect is elaborated further in details below. I would consider removing it entirely.

Page 4 line 103: “assuming no human population before 12 ka BP”. I’m sure with regards to methane emissions, the effect of no human population vs. small human population at the time is fairly negligible, but this statement jumps out as obviously untrue as humans were clearly around in the LGM. Maybe just explicitly say something like “we assume no human population because the effect of ~2 million stone age people on fire emissions is small” to avoid confusion. I’m not an expert in human population during the LGM, just happen to stumble across the 2 million number from Gautney and Holliday (2015).

Page 6 line 178-181: This part I think fit better with the previous section, maybe just add it to section 2.1 and add “geological emissions” to the title of section 2.1

Page 11 line 247: “when dust accumulation”. I think the word ‘concentration’ fits better here than ‘accumulation.’

Page 11 line 255: “Finally, the tropical methane concentration […] throughout the experiment.” Might be worth mentioning that this result is in contrast to some earlier time slice studies like for example very clearly shown in figure 2b of Valdes et al. (2005) where methane concentration during the LGM is highest in the tropics.

Page 11 line 257-274. This is arguably the crux of the paper. Methane is driven by wetlands, which is driven by AMOC. Here the authors have a very nice description of the model results where methane emissions from wetlands respond to AMOC. What is a bit lacking (I understand this is later elaborated in section 3.3) is maybe a brief explanation of why (process-wise) wetland emissions are coupled so strongly to AMOC.

Page 19 line 355 to page 21 line 370. This section again pertains mostly to AMOC, Bolling-Allerod and YD transitions. I personally think this should be on Section 3.3 rather than a section about Holocene.

References
Gautney, J. R. and Holliday, T. W.: New estimations of habitable land area and human population size at the Last Glacial Maximum, Journal of Archaeological Science, 58, 103–112, https://doi.org/10.1016/j.jas.2015.03.028, 2015.
Valdes, P. J., Beerling, D. J., and Johnson, C. E.: The ice age methane budget, Geophys. Res. Lett., 32, L02704, https://doi.org/10.1029/2004GL021004, 2005.

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ED: Publish subject to minor revisions (review by editor) (10 Apr 2023) by Alberto Reyes

AR by Thomas Kleinen on behalf of the Authors (24 Apr 2023) Author's response Author's tracked changes Manuscript

ED: Publish as is (24 Apr 2023) by Alberto Reyes

AR by Thomas Kleinen on behalf of the Authors (26 Apr 2023)

Short summary

We modelled atmospheric methane continuously from the last glacial maximum to the present using a state-of-the-art Earth system model. Our model results compare well with reconstructions from ice cores and improve our understanding of a very intriguing period of Earth system history, the deglaciation, when atmospheric methane changed quickly and strongly. Deglacial methane changes are driven by emissions from tropical wetlands, with wetlands in high northern latitudes being secondary.