Natural Methane Emissions Feedbacks in MAGICC v. 7.6

Sloughter, Trevor Martin; Nicholls, Zebedee; Tang, Gang; Kleinen, Thomas; Zhang, Zhen; Rogelj, Joeri

doi:https://doi.org/10.5194/egusphere-2025-3873

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https://doi.org/10.5194/egusphere-2025-3873

Preprints

05 Sep 2025

| 05 Sep 2025

Status: this preprint is open for discussion and under review for Geoscientific Model Development (GMD).

Natural Methane Emissions Feedbacks in MAGICC v. 7.6

Trevor Martin Sloughter, Zebedee Nicholls, Gang Tang, Thomas Kleinen, Zhen Zhang, and Joeri Rogelj

Abstract. Literature estimates of natural methane emissions, particularly from wetlands, have a wide range of uncertainty. Meanwhile, few Earth System Models (ESMs) explicitly model wetlands as a potential source of methane. As a result, Simple Climate Models that aim to emulate the behaviour of ESMs have little to constrain their present and future contributions. MAGICC, as of version 7.5.3, fixed natural methane concentrations as constant after the historical period. Two studies that model wetland methane emissions over the 21^st century both find a relationship between those emissions and global temperature, though disagree on the extent of this temperature sensitivity. An updated version of MAGICC has been created that uses this evidence to include a linearised representation of the relationship between wetland methane emissions and global temperature. The temperature-sensitivity parameter in this relationship has been parametrised in a way such that its distribution encompasses the uncertainty in both modelling literature and carbon budget studies, reflecting the currently high degree of uncertainty in wetlands emissions. Our results show how incorporating a temperature feedback in methane emissions leads to both higher temperature projections for all scenarios used here, and a widening of the uncertainty in global temperature response.

Received: 08 Aug 2025 – Discussion started: 05 Sep 2025

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Trevor Martin Sloughter, Zebedee Nicholls, Gang Tang, Thomas Kleinen, Zhen Zhang, and Joeri Rogelj

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RC1:
'Comment on egusphere-2025-3873', Anonymous Referee #1, 08 Oct 2025 reply
The authors present the impact of including a temperature-dependent wetland methane feedback in the Model for the Assessment of Greenhouse-gas-Induced Climate Change (MAGICC) v.7.6 instead of using constant natural methane concentrations after the historical period as was done in MAGICC v.7.5.3.
Few Earth System Models explicitly model wetland methane emissions and uncertainty of methane emissions over the 21^st century is high. Including temperature-dependent methane emissions from wetlands in MAGICC resulted in higher temperature projections for future scenarios and also increased the uncertainty in global temperature response.

General comments
This paper should be publishable after below-mentioned revisions are made. It is generally well written, clear and understandable. It provides an important contribution to providing boundary conditions for CMIP7 simulations. In some places, sentences could be clarified and some typographical errors should be corrected.
See the specific comments below for details.

Specific comments
51: “For” should be replaced with “Due to”.

55: You mention Figure A3 before Figure A1 and A2, so A3 should be the first figure.

60: “evidence” should be “evident”.

61: “only taking” should maybe be rephrased to “which only includes”.

67: I’d suggest removing “then”, as it’s not needed. Maybe add a reference for the “Nelder-Mead method”.

71-72: Why did you simultaneously fit the five SSPs, but you separately fit the RCPs?

74: Regarding “Values from Kleinen et al. (2021a) were assumed to be on the high end of sensitivity”: Did you mention what the values are yet? That might make this easier to follow.

76: Could you maybe make it clearer why those studies were not usable for calibration?

78: Maybe say “close to” instead of “near”, as it seems to fit better in this context.

79: “This setup” -> It's not exactly clear from the previous sentence what setup you're referring to here. Maybe rephrase this.

84: Instead of “wetlands emissions” I’d say “wetland emissions”.

85: The end of the sentence is missing a colon.

88-89: It isn’t completely clear to me what you mean with “This also serves to fix the intercept E₀ …”. Maybe explain this a bit more to make it clearer.

100: “(ibid)” should be removed or corrected.

101: Why was 2105 chosen here and not 2100 like elsewhere?

127: Remove the unnecessary “ after (under higher temperature).

127-129: This seems like a repetition of what you said in the previous sentences, so why repeat it here? If there is additional information here, maybe rephrase it to make it clearer.

Figure 1: It’s difficult to see, if the quantiles are also shown for MAGICCv7.5.3. If they are included, maybe make them more visible.
In the figure caption: “CH4 emissions between in MAGICC” should not have the “in”.
Figure 2 caption: l. 3: “lotted” should be “plotted”
6: “for for” should be “for”

152: It should be “Figure 3 shows” instead of “Figure 3 plot”.

154: “The C1 and C1 scenarios” should probable be “The C1 and C2 scenarios”.

158-159: I would rephrase this either saying “so too” or “is also increased”, not both.

195-196: Maybe explain a bit more what kind of progress or new information you expect to obtain from CMIP7 compared to CMIP6 models and why.

Reply
Citation: https://doi.org/10.5194/egusphere-2025-3873-RC1
RC2: 'Comment on egusphere-2025-3873', Anonymous Referee #2, 08 Oct 2025 reply

The manuscript introduces the Model for the Assessment of Greenhouse-gas-Induced Climate Change (MAGICC) v.7.6, which includes a temperature-dependent wetland methane feedback in the instead of using constant natural methane concentrations after the historical period as was done in MAGICC v.7.5.3. The manuscript is well-written and objectives and arguments are well-justified. I find the paper suitable for publication after the below comments, which are mainly clarification in methodology and choices, are addressed:
1) Section 2.1: It should be better described and justified why only the two models are used for calibration? Is it the extent of the periods they cover?
2) Line 76: Why are other models/studies mentioned not suitable for calibration? They could at least be used in fitting the early half of the current century? In addition to Im et al. (2025), which horse until 2050, the same ESM is also used with interactive wetland in CMIP6 that extents to 2100, and with different sets of anthropogenic emissions, which may impact the feedbacks.
3) Lines 101-102: The probabilistic approach and the choice of 600 draws should be better described and justified, not only by referring to Sloughter and Nicholls (2025).
4) Line 124: The processes should be briefly mentioned. Especially, a short discussion would improve with respect to CH4 oxidation as it will be different under different SSPs and RCPs due to differences in chemical composition and impacts on OH.
5) Line 154: "The C1 and C1....", the second C1 should be corrected to C2.

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Citation: https://doi.org/10.5194/egusphere-2025-3873-RC2

Trevor Martin Sloughter, Zebedee Nicholls, Gang Tang, Thomas Kleinen, Zhen Zhang, and Joeri Rogelj

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Short summary

High resolution models of the earth system exhibit some disagreement and uncertainty on future methane emissions from natural sources, in particular wetlands, with some studies predicting wetlands alone could be very significant sources over the 21st century. Modelling these emissions as a response to global temperature is one option for simple models to approximate the climate impact of wetlands. The effect is a small increase in overall temperatures and a widening of the uncertainty range.


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