the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 05 Dec 2025
Global Methane Emission Estimates from a Dual-Isotope Inversion: New Constraints from δD-CH4
Abstract. Methane (CH4) is a potent greenhouse gas; however, the causes of its growth since 2006 are a subject of debate. While measurements of CH4 mole fraction and carbon isotopic composition (δ13C-CH4) have been extensively used to investigate the global CH4 budget, the hydrogen isotopic composition (δD-CH4) remains underutilised despite its unique sensitivity to source types and oxidation processes. Here, we assimilate a newly harmonised 35-year dataset of dual isotope measurements from high-latitude monitoring stations in both hemispheres within a two-box Bayesian inversion to quantify global CH4 sources and sinks. The model integrates prior emissions from five source categories based on global bottom-up inventories. Methane removal processes are represented by sink-specific kinetic isotope effects as tropospheric and stratospheric loss, and soil uptake.
We find that the inclusion of δD-CH4 improves the model's ability to constrain emission apportionment between biogenic and thermogenic sources, particularly for fossil fuel emissions during the late 1990s and early 2000s, which affects CH4 lifetime estimate. CH4 increase post-2006 is driven mainly by rising wetland emissions, while fossil-fuel growth is modest, biomass burning declines, and agriculture and waste make smaller, regionalised contributions. The optimised inversion results favour a strong 13C kinetic isotope effect in the total CH4 removal in the troposphere and a net shortening of NH lifetime of CH4 by 0.2 years. This study demonstrates the added value of incorporating δD-CH4 into inverse modelling frameworks and underscores the importance of long-term δD-CH4 measurements for advancing our understanding of CH4 biogeochemistry and its role in the global carbon cycle.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-5571', Anonymous Referee #1, 03 Jan 2026
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RC2: 'Comment on egusphere-2025-5571', Anonymous Referee #2, 01 May 2026
General Comments
This study presents a joint inversion framework that assimilates atmospheric CH4 mole fraction, carbon isotopic composition (δ13C-CH4), and hydrogen isotopic composition (δD-CH4) to better constrain methane source partitioning and sink processes. The main objective was to assess whether combining δ13C-CH4 and δD-CH4 provides stronger constraints on fossil versus biogenic CH4 sources compared to single-isotope or mole-fraction-only inversions. However, several aspects of the methodology, interpretation, and robustness of the conclusions require further clarification and stronger justification before the main claims can be fully supported.
The description of the box-model framework, particularly with respect to sink processes are briefly presented. The governing continuity equations for both CH4 and its isotopes should be explicitly presented. For isotope-based budget analyses, the representation of sinks is critically important, and the current description lacks insufficient details. The assumed interhemispheric exchange time also appears unrealistically short and may strongly influence the inferred hemispheric source partitioning. Previous three-dimensional modeling studies (e.g., TransCom) suggest exchange times on the order of ~1 year. Given the sensitivity of inversion results to this parameter, the use of more realistic values are strongly recommended.
In the comparison of inversion results, the species-specific inversions appear to reproduce their respective observations more closely (e.g., the δ13C-based inversion better matches δ¹³C observations, while the δD-based inversion better reproduces δD-CH4; Fig. 2). However, after approximately 2005, the differences between the dual-isotope inversion and the single-isotope inversions become difficult to distinguish. This raises an important question regarding the added value of the dual-isotope framework and whether it provides a significantly stronger constraint than single-isotope approaches.
Another concern is the large interannual variability (IAV) inferred for agriculture and waste emissions. These sectors are generally expected to exhibit relatively smooth, monotonic trends rather than strong short-term fluctuations. The pronounced variability appears somewhat artificial and may reflect compensatory adjustments among source categories within an underdetermined inversion system. This raises concerns about the robustness of the inferred emissions and the extent to which artifacts from the simplified two-box framework may influence the results.
Furthermore, the inversion results are not fully consistent with the observed isotopic trends in both hemispheres. For example, the dual-isotope simulation suggests a more rapid decline in δ13C over the Southern Hemisphere than indicated by observations (Fig. 3). Similarly, for δD-CH4, all inversion cases show broadly similar temporal behavior, yet the model underestimates the observed trend in the Northern Hemisphere (Fig. 3). These discrepancies suggest that the proposed source attribution may not fully capture the underlying processes.
In particular, the interpretation of increasing microbial emissions—especially from Southern Hemisphere wetlands—appears somewhat overstated given the available evidence. The observed δ13C trend could also be consistent with alternative explanations, such as a slower increase in thermogenic emissions or different sink dynamics. Therefore, the strength of the conclusions regarding source attribution should be moderated, and uncertainties more clearly acknowledged.
Specific Comment on Figure 1
In Figure 1, the second-last pictorial element appears to represent fossil-fuel combustion. However, this may not be the most appropriate visualization for methane emissions, as fossil-fuel CH4 emissions predominantly arise from leakage during extraction, processing, and transport rather than combustion itself. Additionally, the clarity of the figure would be improved by explicitly labeling all source categories.
Citation: https://doi.org/10.5194/egusphere-2025-5571-RC2
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Please find my comments in the attached file.