the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Atmospheric methane since the LGM was driven by wetland sources
Thomas Kleinen
Sergey Gromov
Benedikt Steil
Victor Brovkin
Abstract. Atmospheric methane (CH4) has changed considerably in the time between the last glacial maximum (LGM) and the preindustrial period (PI). We investigate these changes in transient experiments with an Earth System Model, focusing on the rapid changes during the deglaciation, especially pronounced in the Bølling Allerød (BA) and Younger Dryas (YD) periods. We consider all relevant natural sources and sinks of methane and examine the drivers of changes in methane emissions as well as in the atmospheric lifetime of methane. We find that the evolution of atmospheric methane is largely driven by emissions from tropical wetlands, while variations in atmospheric lifetime are not negligible but small. Our model reproduces most changes in atmospheric methane very well, with the exception of the mid-Holocene decrease in methane, though the timing of ice sheet meltwater fluxes needs to be adjusted slightly in order to exactly reproduce the variations of the BA and YD.
Thomas Kleinen et al.
Status: final response (author comments only)
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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
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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
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AC1: 'Reply on RC1', Thomas Kleinen, 10 Jan 2023
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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
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AC2: 'Reply on RC2', Thomas Kleinen, 10 Jan 2023
Thomas Kleinen et al.
Thomas Kleinen et al.
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