Spatio-Temporal Patterns of Methane Emissions from 2019 Onwards: A Satellite-Based Comparison of High- and Low-Emission Regions

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Summary

This manuscript presents an insightful analysis of global atmospheric methane (CH₄) concentrations using satellite-based remote sensing data from the TROPOspheric Monitoring Instrument (TROPOMI) between 2019 and 2025. The study compares temporal and seasonal patterns of XCH4 across high- and low-emission regions and applies statistical techniques such as principal component analysis (PCA) and clustering to characterize spatial differences. Overall, this is an interesting study and relevant to the scope of Atmosphere. A major revision is needed before the acceptance of this paper.

Major comments:

1) The main issue with this paper lies in the overly superficial analysis of the results, which lacks integration with multi-source data for a more in-depth discussion. It is clear that the foundation of this study is the TROPOMI CH₄ dataset. A critical question, therefore, is: how reliable are TROPOMI’s satellite-based methane observations? How do the authors account for and reduce the uncertainties associated with satellite retrievals, especially considering the substantial data gaps frequently present in TROPOMI measurements?

2) The second issue concerns the depth of the discussion. The global distribution of high and low methane concentrations has already been well established through earlier satellite observations and global simulations such as GEOS-Chem. The key scientific question now is how to explain the recent anomalous trends in methane concentrations—how have methane levels changed in high- and low-emission regions in recent years, and what are the underlying driving factors behind these changes? The authors should consider the key scientific questions that warrant investigation in this work.

3) PCA is, in fact, a rather outdated method. Given the complex factors influencing methane concentration variability, applying PCA should be done with greater caution. How did the authors assess the feasibility and applicability of using PCA in this context?

4) The uncertainty of the TROPOMI CH4 product and limitations of the PCA method should be thoroughly discussed.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

1. If possible, please provide specific parameters and validation steps for Google Earth Engine (GEE) data processing.
2. In the trend analysis of low emission areas, such as the "moderate medians but wide variability" methane concentration in the Canadian Shield, can you explore its potential drivers?
3. If possible, when discussing the trend of methane concentration changes, you can add discussions on the impact of temperature changes on methane emissions.
4. Please provide references for relevant research in the past 3 years.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The paper 'Spatio-Temporal Patterns of Methane Emissions from 2019 Onwards: A Satellite-Based Comparison of High and Low Emission Regions' is devoted to a very relevant topic: the study of variations in the emission of the greenhouse gas methane in different regions of the Earth with various soil and land-use structures.

TROPOMI Sentinel 5P CH₄ observations were chosen as the instrument of research.

The authors analyze spatio-temporal patterns of atmospheric methane concentrations between 2019 and 2025, focusing on comparisons between regions characterized by high and low emission intensities. The paper contains some interesting analyses and results. However, in my opinion, the manuscript contains some inaccuracies and unclear conclusions.

Major remarks.

1. There is no information on the quality, validation and/or comparison of TROPOMI L3 (or L2) data with ground-based, aircraft or balloon measurements for CH₄ (Lindqvist et al., 2024). You could use reference [9] for this purpose. I would also recommend adding a few papers on TROPOMI XCH₄ data validation to the references, for example:

- Sha, M. K., Langerock, B., Blavier, J.-F. L., Blumenstock, T., Borsdorff, T., Buschmann, M., Dehn, A., De Mazière, M., Deutscher, N. M., Feist, D. G., García, O. E., Griffith, D. W. T., Grutter, M., Hannigan, J. W., Hase, F., Heikkinen, P., Hermans, C., Iraci, L. T., Jeseck, P., Jones, N., Kivi, R., Kumps, N., Landgraf, J., Lorente, A., Mahieu, E., Makarova, M. V., Mellqvist, J., Metzger, J.-M., Morino, I., Nagahama, T., Notholt, J., Ohyama, H., Ortega, I., Palm, M., Petri, C., Pollard, D. F., Rettinger, M., Robinson, J., Roche, S., Roehl, C. M., Röhling, A. N., Rousogenous, C., Schneider, M., Shiomi, K., Smale, D., Stremme, W., Strong, K., Sussmann, R., Té, Y., Uchino, O., Velazco, V. A., Vigouroux, C., Vrekoussis, M., Wang, P., Warneke, T., Wizenberg, T., Wunch, D., Yamanouchi, S., Yang, Y., and Zhou, M.: Validation of methane and carbon monoxide from Sentinel-5 Precursor using TCCON and NDACC-IRWG stations, Atmos. Meas. Tech., 14, 6249–6304, https://doi.org/10.5194/amt-14-6249-2021, 2021.

This paper is similar to [9] but is based on validation at a significantly larger number of ground-based stations and validation points.

- Lindqvist, H.; Kivimäki, E.; Häkkilä, T.; Tsuruta, A.; Schneising, O.; Buchwitz, M.; Lorente, A.; Martinez Velarte, M.; Borsdorff, T.; Alberti, C.; et al. Evaluation of Sentinel-5P TROPOMI Methane Observations at Northern High Latitudes. Remote Sens. 2024, 16, 2979. https://doi.org/10.3390/rs16162979

2. In my opinion, Figure 3 is unsuccessful. Both the figure itself and the accompanying text explanation are difficult to understand. Section 3.3 also needs improvement.

Minor remarks

Introduction

1. When discussing the greenhouse potential of methane (CH₄), it is usually meant that one molecule of methane is approximately 30 times more effective than a molecule of carbon dioxide (CO₂).

I’d recommend adding a phrase like this:

“'Methane is the second most significant greenhouse gas (GHG) after CO₂ in regard to the overall greenhouse effect, and approximately 30 times more effective than a molecule of carbon dioxide”

[Myhre et al., 2013; Sonneman & Grygalashvily, 2014; Etminan et al., 2016].

Myhre, G., D. Shindell, F.-M. Bréon, W. Collins, J. Fuglestvedt, J. Huang, D. Koch, J.-F. Lamarque, D. Lee, B. Mendoza, T. Nakajima, A. Robock, G. Stephens, T. Takemura, and H. Zhang, 2013: Anthropogenic and natural radiative forcing. In Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. T.F. Stocker, D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Doschung, A. Nauels, Y. Xia, V. Bex, and P.M. Midgley, Eds., Cambridge University Press, pp. 659-740, doi:10.1017/CBO9781107415324.018.

Sonnemann G. R. and Grygalashvyly M. (2014). Global annual methane emission rate derived from its current atmospheric mixing ratio and estimated lifetime. Ann. Geophys., 32, pp. 277-283.

Etminan, M., G. Myhre, E. J. Highwood, and K. P. Shine (2016), Radiative forcing of carbon dioxide, methane, and nitrous oxide: A significant revision of the methane radiative forcing, Geophys. Res. Lett., 43, 12,614–12,623, doi:10.1002/2016GL071930.

2. It would be useful to add information about typical seasonal variations in CH₄ concentrations, as well as its lifetime and relevant references. Information relating to the lifetime of CH4 can be found in Etminan et al. (2016).
3. L56. 'With a spatial resolution as fine as 5.5 × 7 km [5].' This is incorrect; the paper [5] was published in 2018 when the spatial resolution of XCH₄ data was 7 × 7 km. The spatial resolution of the TROPOMI measurements has been improved by reducing the along-track ground pixel size from 7.0 km to 5.5 km, starting on 6 August 2019 [S5P Mission Performance Centre Methane [L2__CH4___] Readme]. Please provide a more relevant reference.
4. It would be useful to include information on the ratio of the integral power of natural and anthropogenic sources of methane in the introduction.

 

2.2. Satellite Data and Preprocessing

 

5. There is no reference to Level 3 XCH₄ data.

 

6. There is no reference to Google Earth Engine.

 

7. L. 108–109 and 119–120 duplicate the same information.

Why does 'the studied period' start in April 2019 and not January 2019, considering the CH₄ OFFL data is available from April 2018?

 

8. L. 120–121: 'For each region and period, the analysis computes the mean CH₄ concentration' – add 'in atmospheric column' to this sentence.

 

9. L. 117: 'Each polygon was buffered uniformly (50 km radius)...' What does this mean? Please provide an explanation or graphic example of buffering.

 

10. Figure 1. Please specify the 'studied period' and spatial resolution of the distribution in the capture. It would also be useful to provide the confidence interval distribution here, as it is well known that TROPOMI measures XCH₄ over oceans and seas with some limitations. In other words, the number of pixels with CH₄ data over the ocean surface is relatively low.

 

11. Figure 2. I recommend using confidence interval values rather than maximum and minimum values.

 

12. Figure 3. What does “Distance” (at capture) mean?

 

13. Figures 4 and 5 are difficult to understand, especially Fig. 5. I recommend duplicating this information in the corresponding tables, either within the paper text or in the supplementary materials.

Also, I recommend using black font colour in both figures.

 

15.05.2025

 

Reviewer

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

I am satisfied with the revisions that have been made. Thus, I recommend publication as it is.

Author Response

We are grateful for the reviewer’s careful reading and constructive comments throughout the revision process. We applied some refinements.

Reviewer 3 Report

Comments and Suggestions for Authors

Authors significantly improved the paper “Spatio-Temporal Patterns of Methane Emissions from 2019 Onwards: A Satellite-Based Comparison of High and Low Emission Regions”. Now I only have a few minor remarks.

Minor remarks

2.2. Satellite Data and Preprocessing

LL 138-140. In my opinion, the sentence “Additional cloud and solar zenith filters were not applied, since the L3 product is preprocessed with cloud screening and data averaging steps already implemented by the Copernicus Sentinel-5P processing chain” is redundant, because Google Earth Engine allows to prepare L3 with any set of variables, including cloudiness, albedo NIR/SWIR, observed and solar angles, etc. The previous sentence (LL 135-138, “The analysis covered the period from April 2019 through March 2025, and included only data where the qa_value (quality assurance flag) was ≥ 0.5, as recommended by the Sentinel-5P data documentation to ensure retrieval re liability” would be sufficient.

For the information of the authors. The quality parameter (qa_value ≥ 0.5) recommended by the developers the TROPOMI XCH4 products do not completely eliminate cloud scenes, pixels with albedo NIR/SWIR channels ≤0 (and ≥ 1), orbits overlap, etc. (you can check it yourself). But (in my opinion) the lack of additional filtering in your study is not a disadvantage.

4. Discussion

LL 433-437. I’d recommend to mentioning here one more reason for the increase in methane concentrations in arid and northern regions, such as processes the global atmospheric circulation and long-range transport from regions with high emissions and relatively long methane life-time for this.

LL 474-491, from “CH₄ exhibits distinct seasonal patterns primarily driven by variations in surface emissions and atmospheric sink strength, particularly through reactions with hydroxyl radicals (OH).” to “…promote permafrost thawing, leading to increased methane release from previously frozen organic matter [49].”:

I’d recommend to remove this information to “Introduction”.

LL 502-504. “Validation studies [15,16] confirm the reliability of the Level-3 TROPOMI XCH₄ product across a range of 503 conditions when properly filtered…”

I’d recommend to exclude “Level 3” from this sentence, since [15,16] studies were based on Level 2 data.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf