Status: this preprint is currently under review for the journal ESSD.
Monthly-updated territorial fossil CO2 emissions estimates for Japan
Taro Kunimitsuand Robbie M. Andrew
Abstract. We present a low-lag, monthly dataset of emissions of carbon dioxide from fossil fuel consumption and industrial processes in Japan, derived directly from official activity data and starting in 2007. Such estimates will be crucial for near-real-time monitoring of emissions and making swift policy responses accordingly. When aggregated, our estimates replicate the annual data published through the UNFCCC national inventory report very well, with only small discrepancies which we describe and quantify. The dataset is produced with a lag of two months and can be used to estimate the annual emissions a year ahead of the official estimates submitted to the UNFCCC, and also provides a better starting point for short-term emissions projections.
Received: 31 May 2026 – Discussion started: 15 Jun 2026
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This is a high-quality manuscript that is well suited for publication in Earth System Science Data. It presents an up-to-date monthly CO₂ emission dataset for Japan and provides a comprehensive description of the underlying data sources, accounting methodology, and the differences among alternative estimation approaches. The authors have made commendable efforts to explain the possible origins of discrepancies between different methods, and the entire workflow—from data collection to emission estimates—is presented in a transparent and reproducible manner. I particularly appreciate the authors' careful treatment of the underlying data and the thorough documentation of the inventory development process.
Overall, I recommend publication in Earth System Science Data after only minor revisions.
Comments
1. Discussion and presentation of uncertainties
The authors discuss the sources of uncertainty and report that “The uncertainty for total emissions in each month is estimated to be ±3% at 2 standard deviations (95% confidence interval), given the uncertainties in the NIR are also given at 2 standard deviations.” From my perspective, this uncertainty appears remarkably small, particularly at the monthly timescale.
I encourage the authors to provide a more detailed discussion to justify why such a low uncertainty is reasonable. In addition, it would be valuable to assess the robustness of the uncertainty estimates using approaches other than the conventional root-sum-of-squares method, if possible, so that the results can be cross-validated.
Furthermore, have the authors considered adding uncertainty ranges to some of the emission time series? Displaying the uncertainty ranges directly on the figures would provide readers with a more intuitive understanding of the confidence range and make it easier to judge whether differences among various estimates are statistically meaningful. In my opinion, strengthening the uncertainty analysis and presentation would represent the most important improvement to this manuscript.
2. Comparison with other inventories
In Figure 12, I suggest that the authors first present the absolute emissions from all inventories before comparing their relative differences. Showing only the relative differences makes it difficult for readers to appreciate the baseline magnitude of each inventory.
In addition, the recently released MEIC-global-CO₂ inventory also provides monthly CO₂ emissions with approximately a two-year reporting lag. The authors may consider including this inventory in the comparison.
3. Manuscript organization
Although the detailed methodological descriptions and comprehensive comparisons are valuable, the manuscript is currently rather lengthy. As a reader, it is sometimes difficult to quickly locate the most relevant information.
I therefore suggest moving part of the detailed methodological descriptions and some of the extended discussions to the Supplementary Information or Appendix, allowing the main text to place greater emphasis on the key findings and conclusions. This would substantially improve the readability of the paper.
4. Treatment of international aviation
I would appreciate further clarification on how fuel consumption associated with international aviation is treated in the inventory. Since this can influence the definition of national emissions, a brief explanation of the accounting approach would be helpful.
We present a low-lag dataset of monthly CO2 emissions from fossil fuel consumption and industrial processes in Japan, derived directly from official statistical data. The monthly dataset can be used to estimate the annual emissions a year ahead of the official estimates submitted to the UNFCCC. Such monthly emissions estimates will provide further opportunities for understanding in a timely manner the reasons behind emissions changes, leading to targeted policy changes for climate mitigation.
We present a low-lag dataset of monthly CO2 emissions from fossil fuel consumption and...
General Assessment
This is a high-quality manuscript that is well suited for publication in Earth System Science Data. It presents an up-to-date monthly CO₂ emission dataset for Japan and provides a comprehensive description of the underlying data sources, accounting methodology, and the differences among alternative estimation approaches. The authors have made commendable efforts to explain the possible origins of discrepancies between different methods, and the entire workflow—from data collection to emission estimates—is presented in a transparent and reproducible manner. I particularly appreciate the authors' careful treatment of the underlying data and the thorough documentation of the inventory development process.
Overall, I recommend publication in Earth System Science Data after only minor revisions.
Comments
1. Discussion and presentation of uncertainties
The authors discuss the sources of uncertainty and report that “The uncertainty for total emissions in each month is estimated to be ±3% at 2 standard deviations (95% confidence interval), given the uncertainties in the NIR are also given at 2 standard deviations.” From my perspective, this uncertainty appears remarkably small, particularly at the monthly timescale.
I encourage the authors to provide a more detailed discussion to justify why such a low uncertainty is reasonable. In addition, it would be valuable to assess the robustness of the uncertainty estimates using approaches other than the conventional root-sum-of-squares method, if possible, so that the results can be cross-validated.
Furthermore, have the authors considered adding uncertainty ranges to some of the emission time series? Displaying the uncertainty ranges directly on the figures would provide readers with a more intuitive understanding of the confidence range and make it easier to judge whether differences among various estimates are statistically meaningful. In my opinion, strengthening the uncertainty analysis and presentation would represent the most important improvement to this manuscript.
2. Comparison with other inventories
In Figure 12, I suggest that the authors first present the absolute emissions from all inventories before comparing their relative differences. Showing only the relative differences makes it difficult for readers to appreciate the baseline magnitude of each inventory.
In addition, the recently released MEIC-global-CO₂ inventory also provides monthly CO₂ emissions with approximately a two-year reporting lag. The authors may consider including this inventory in the comparison.
3. Manuscript organization
Although the detailed methodological descriptions and comprehensive comparisons are valuable, the manuscript is currently rather lengthy. As a reader, it is sometimes difficult to quickly locate the most relevant information.
I therefore suggest moving part of the detailed methodological descriptions and some of the extended discussions to the Supplementary Information or Appendix, allowing the main text to place greater emphasis on the key findings and conclusions. This would substantially improve the readability of the paper.
4. Treatment of international aviation
I would appreciate further clarification on how fuel consumption associated with international aviation is treated in the inventory. Since this can influence the definition of national emissions, a brief explanation of the accounting approach would be helpful.