Natural Methane Emissions Feedbacks in MAGICC v. 7.6

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

doi:10.5194/egusphere-2025-3873

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

Preprints

05 Sep 2025

| 05 Sep 2025

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

Status: final response (author comments only)

RC1:
'Comment on egusphere-2025-3873', Anonymous Referee #1, 08 Oct 2025
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.
Citation: https://doi.org/10.5194/egusphere-2025-3873-RC1
- AC1: 'Reply on RC1', Trevor Sloughter, 31 Oct 2025
  
  We thank you for your comments and will amend the manuscript accordingly, correct word choice and typos per the suggestions. Additionally, we can clarify some of the broader concerns here:
  
  Point 6: Lines 71-72: This was just a matter of the sentence being unclear, the RCPs were fit simultaneously like the SSPs. It was the same procedure, we'll reword slightly so that is clear.
  
  Point 8: We'll clarify that other studies were unsuitable either due to insufficient coverage or incomplete time-series of temperature and emissions data.
  
  Point 10: We'll clarify that "this setup" in line 79 refers to the choice of mean and standard deviation for parameter distribution.
  
  Point 13: Lines 88-89 refer to the "intercept E0", this was unclear as it was referring to the intercept of Emissions = (temperature sensitivity * Temperature) + E0. This line was saying that E0 didn't need to be parameterised as it was constrained by the historical setup. Emissions are continuous from the concentration-driven years (up to 2015) into the emissions-driven (beyond 2015).
  
  Point 101: Another mistake in our clarity, and we'll correct this so it's clear that the runs all went up to 2105 in order to keep the results at 2100 smooth and conintuous and prevent any numericla errors at the end of century.
  Re Figure 1: The quantiles are plotted for both versions of MAGICC, the overlap is conservable and in hindsight hard to discern at this resolution and with three scenarios in each plot. We'll adjust the plot to make it clearer to read.
  
  Citation: https://doi.org/10.5194/egusphere-2025-3873-AC1
RC2:
'Comment on egusphere-2025-3873', Anonymous Referee #2, 08 Oct 2025

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.

Citation: https://doi.org/10.5194/egusphere-2025-3873-RC2
- AC2: 'Reply on RC2', Trevor Sloughter, 31 Oct 2025
  
  We thank the reviewer for their comments and will update the manuscript to correct the typos and improve clarity, and to address the specific issues highlighted here. In particular, the models were chosen for the period they covered (the full 21st century) and data availability at the time of the study. We can add more discussion about the choices we made for coverage and consistency.
  We will also elaborate on the probabilistic draws of parameters, while the complete details are outline in prior papers on MAGICC we accept that this paper should have better summarised how those 600 draws represent configurations of parameters. Likewise the citation in line 124 to how MAGICC turns emissions into concentrations will be expanded into a short summary.
  
  Citation: https://doi.org/10.5194/egusphere-2025-3873-AC2

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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