| 18 Jul 2025
Reviews and syntheses: Process-based modeling of the CO2:CH4 production ratio is important for predicting future Arctic methane emissions
Abstract. Thawing permafrost in the Arctic threatens to potentially release large amounts of decomposed organic matter as CO2 or CH4 to the atmosphere. Predicting the ratio of emitted CO2 to CH4 is imperative for reliable future projections. Here, we review the recent literature concerning methanogenesis, and its current representation in both land surface models (LSMs) and the state-of-the-art process-based methane models. We found that the key processes, required to capture the dynamics of the CO2:CH4 production ratio, are: fermentation, hydrogenotrophic methanogenesis, and acetoclastic methanogenesis. Commonly discussed linked processes are Fe(III)-reduction and homoacetogenesis. Environmental factors influencing these processes, as identified in the literature, are: temperature, pH, water table position and alternative electron acceptors. While modern process-based methane models account for most of these factors and processes, the same is not true for the simplified methane formulations in many LSMs, which often opt for pre-set parameters that define a constant share of methane production from anaerobic decomposition. This static approach stands in opposition to the growing amount of lab and in-situ data, which suggest a high degree of spatio-temporal variability concerning this ratio, thus preventing its accurate prediction in a changing future Arctic. The challenge lies in upscaling the data as the environmental factors are barely quantified at the pan-Arctic scale. Additionally, there remains the important challenge of how to model and parameterize the temperature dependence of the individual underlying processes. Going forward, these challenges need to be overcome in order to reliably project the CO2:CH4 production ratio and methane emissions on larger scales. This will require a more process-based approach of methanogenesis in LSMs, for which we suggest a baseline concept here.
The paper by Moser et al. is a review on the representation of methane production in process-based models. I think that in principle this is a decent overview that clearly addresses the overly simplistic way in which some models have traditionally modeled methanogenesis, while also proposing ways forward. Still, I have a number of comments for improvement and some thoughts to reflect upon.
First of all, there are many models discussed in this paper but what I’m missing is an overview table showing all LSMs and process-based methane models, and which processes are included in each. This would be really helpful to show which models are leading or still lacking. Similarly, when discussing equations, it would be good to show them. At the very least the basic Q10 and Arrhenius-type equations used by most models.
Otherwise, given the paper’s title and conclusion, I don’t know why the paper decided to focus only on how the models represent methane production, because this is not the only uncertainty related to modeling Arctic methane emissions. Simply knowing where wetlands are located is perhaps one of the largest uncertainties in calculating Arctic methane emissions, as most recently shown by Ying et al. (2025). Correctly simulating snow cover is also very important to accurately simulate soil temperature and soil water content, which in turn affect soil biogeochemistry (Pongracz et al. 2021). Abrupt thaw processes are only mentioned once, even though they can completely transform landscapes and therefore strongly affect methanogenesis.
I am not suggesting that the authors include a complete overview of the uncertainties for spatial upscaling, but the title claims that the CO2:CH4 production ratio is important to predict future Arctic methane emissions. Is this true when compared to accurately simulating the environmental drivers that govern methane production and consumption? I agree that we need to include the right biogeochemical processes, but if the ecological and environmental boundary conditions are misrepresented in the model then the output will be wrong regardless. I hope the authors can reflect on this, because this paper mostly gives qualitative evidence to support the importance of the CO2:CH4 ratio. Perhaps add some text that delves deeper into how this relates to other types of uncertainty in modeling Arctic methane emissions.
One more nit-pick about the title is that it should be specified that this paper focuses on terrestrial methane emissions, not other methane sources in the Arctic such as lakes, geological sources, wildfires, and rivers and streams (or the ocean, for that matter). Parmentier et al. (2024) showed that these other sources can contribute over 30% of the Arctic methane budget.
In addition, there are a couple of processes that are not discussed but might be important in the Arctic methane budget. In a region where the cold season lasts for most of the year, winter emissions can become quite significant (Zona et al. 2016; Treat et al. 2018), but this is not discussed in the paper. For example, burst-like emissions upon freeze-in are a physical process that locally can lead to very high emissions (Mastepanov et al. 2008). While not directly related to methanogenesis, this is not implemented in any model and also not discussed in the paper. It shows that transport is still highly uncertain, despite what’s claimed on line 66-67. See also Ito et al. (2023) for a nice overview of how the models currently represent cold season fluxes.
High-affinity methanogens are discussed briefly in the manuscript (line 338-341) but I think that these also warrant more discussion. The authors say that this is yet to be explored in most models but neglect to cite Oh et al. (2020) who simulated these methanotrophs and showed that the high latitude methane sink strongly increased as a result, thus reducing net emissions by ~5.5 Tg CH4 yr-1. These numbers are uncertain of course, but they stress the need to also focus on methanotrophy, not just methanogenesis. Btw, Oh et al. used a modified version of the Terrestrial Ecosystem Model (TEM), which I think is not mentioned in this paper despite a long history in modeling northern methane budgets (e.g. Zhuang et al. 2004; 2015), and one of the inspirations for the CLM(4Me) model mentioned in this paper. Might be useful to add.
I know that the authors aim to focus on methanogenesis in particular, and I understand the wish for that focus, but the issues I mention above are also important to predict future Arctic methane emissions. Whether the CO2:CH4 production ratio is more important is unclear from this paper. For example, Sulman et al. (2022) did show an effect of Fe(III) reduction on these ratios, but the overall effect on emissions was relatively minor. Either a quantification of process importance or an uncertainty analysis would have been helpful to know whether including the extra complexity in the model, as suggested in Figure 2, really would lead to an important increase in model performance and reliably predict future methane emissions in the Arctic. If the authors are unable to better quantify this importance, then I suggest that the title and conclusions are adjusted accordingly.
Minor comments:
Line 42-44: are these both weight and molar ratios? Since C-CO2:C-CH4 and CO2:CH4 are both mentioned. I recommend converting these to the same unit for better comparison.
Line 90: please show examples of a Q10 and Arrhenius-type equation in the text. Preferably with an example plot of how they differ.
Line 144: very minor comment: maybe change “current” to “latest” (since it’s been a couple of years)
Line 161: which CMIP models use the scheme from CLM(4Me)?
Line 194-195: can you name these models here, and not just the references? Good to add to an overview table.
Line 229: which process-based model?
Line 292-293: is this 40% of permafrost thaw emissions in the form of CO2 or CH4?
Line 320: typo: “Arcitc”
References
Ito, A., Li, T., Qin, Z., Melton, J. R., Tian, H., Kleinen, T., et al. (2023). Cold-Season Methane Fluxes Simulated by GCP-CH4 Models. Geophysical Research Letters, 50(14), e2023GL103037. https://doi.org/10.1029/2023GL103037
Mastepanov, M., Sigsgaard, C., Dlugokencky, E. J., Houweling, S., Ström, L., Tamstorf, M. P., & Christensen, T. R. (2008). Large tundra methane burst during onset of freezing. Nature, 456(7222), 628–630. https://doi.org/10.1038/nature07464
Parmentier, F.-J. W., Thornton, B. F., Silyakova, A., & Christensen, T. R. (2024). Vulnerability of Arctic-Boreal methane emissions to climate change. Frontiers in Environmental Science, 12. https://doi.org/10.3389/fenvs.2024.1460155
Pongracz, A., Wårlind, D., Miller, P. A., & Parmentier, F.-J. W. (2021). Model simulations of arctic biogeochemistry and permafrost extent are highly sensitive to the implemented snow scheme in LPJ-GUESS. Biogeosciences, 18(20), 5767–5787. https://doi.org/10.5194/bg-18-5767-2021
Treat, C. C., Bloom, A. A., & Marushchak, M. E. (2018). Nongrowing season methane emissions–a significant component of annual emissions across northern ecosystems. Global Change Biology, 44, 163. https://doi.org/10.1111/gcb.14137
Ying, Q., Poulter, B., Watts, J. D., Arndt, K. A., Virkkala, A.-M., Bruhwiler, L., et al. (2025). WetCH4: a machine-learning-based upscaling of methane fluxes of northern wetlands during 2016–2022. Earth System Science Data, 17(6), 2507–2534. https://doi.org/10.5194/essd-17-2507-2025
Zona, D., Gioli, B., Commane, R., Lindaas, J., Wofsy, S. C., Miller, C. E., et al. (2016). Cold season emissions dominate the Arctic tundra methane budget. Proceedings of the National Academy of Sciences, 113(1), 40–45. https://doi.org/10.1073/pnas.1516017113
Zhuang, Q., Melillo, J. M., Kicklighter, D. W., Prinn, R. G., McGuire, A. D., Steudler, P. A., et al. (2004). Methane fluxes between terrestrial ecosystems and the atmosphere at northern high latitudes during the past century: A retrospective analysis with a process-based biogeochemistry model. Global Biogeochemical Cycles, 18(3), GB3010. https://doi.org/10.1029/2004GB002239
Zhuang, Q., Zhu, X., He, Y., Prigent, C., Melillo, J. M., McGuire, A. D., et al. (2015). Influence of changes in wetland inundation extent on net fluxes of carbon dioxide and methane in northern high latitudes from 1993 to 2004. Environmental Research Letters, 10(9), 095009. https://doi.org/10.1088/1748-9326/10/9/095009