Review of Saunois et al. Global Methane Budget 2000-2020.

This is a most important paper that makes a major contribution not just in terms of scientific discovery, but more generally to the wider effort of protecting the well-being of our planet.  The work is extremely well documented and carefully presented.

I strongly recommend publication.

That said, I do have a number of comments that the authors may wish to take into account for minor revisions prior to final publication.

General comments

Quantifying the rapidly changing global methane budget is of the highest importance in tracking climate forcing, and of major societal value, particularly in the Global Methane Pledge, now signed by 155 nations, and in the UN Framework Convention on Climate Change, signed by all nations.

This paper is the most recent update in a series of authoritative and very influential assessments of the global methane budget, that have been produced by the Global Carbon Project over the past decade. Throughout the series of papers, the work of this team has been detailed, very well-documented and of high academic quality. 

The work in the paper is comprehensive and generally appears accurate, with thoughtful evaluations of uncertainties and gaps in knowledge.  Once published, the budget assessment will be immediately valuable both to the very active scientific methane community, and in the wider climate debate.

However, I do have a number of specific questions that the authors may wish to consider.

In particular, it would be useful to add at least a comment on the need for isotopic balance. There is enough ¹³C isotopic information available to provide an independent test of the budget, and thereby to help to constrain uncertainties.

In the concluding sections it might also be interesting to compare the findings to the official UNFCCC declarations and other important assessments (e.g. IEA), and perhaps to say a little more about the remarkable changes that have taken place in the 2020s.

As a matter of presentation, it would be a lot easier on the tired-eyed reader to break up some of these enormously long paragraphs! Also, as is inevitable in such a comprehensive paper, some sections seem a bity elderly and could do with updating and a few more recent references.

Conclusion

This is a major paper of great importance to society as a whole. It is a credit to the authorship team and should be published after minor revision. The biggest weakness is the absence of any effort to use the isotopic constraints. Perhaps that should wait until the next update in a few years’ time, but it should be considered or at least mentioned here, if only in a brief section.

Specific Comments

Abstract: Line 155.The abstract will be widely cited, and many will look at it but not read the paper in detail (sadly). Thus it would help if the abstract could include more source-specific numbers detailing inputs like fossil fuels, agriculture, biomass burning and also sink estimates. If that makes the abstract too long, then maybe move lines 130-135 (which are very important, but not necessarily needed in the abstract) to the introduction?

L190 – GMP now has 155 signatures.

L208 – maybe somewhere mention Anderson, D. C., et al. (2024). Trends and interannual variability of the hydroxyl radical in the remote tropics during boreal autumn inferred from satellite proxy data. Geophysical Research Letters, 51,

L251 – perhaps mention Yu, X, et al. (2023) A high-resolution satellite-based map of global methane emissions reveals missing wetland, fossil fuel, and monsoon sources. Atmospheric Chemistry and Physics 23): 3325-3346.

L306 – EDGAR v6? – See also L424.v7, and now v8. I guess in a big synthesis exercise like this it is hard to keep up with changes to inventories. Maybe mention new work by one of the co-authors. Crippa, M., et al. (2023) Insights on the spatial distribution of global, national and sub-national GHG emissions in EDGARv8. 0. Earth System Science Data Discussions 2023: 1-28.

L327 – good to have N2O and CH4 regions comparable.

L348 – maybe could add a few general words on how to assess heavily human-dominated floodplain deltas like the Nile and Ganges and the Yangtse and Mekong (Tonlé Sap) floodplains.  

L371 – soil uptake – major uncertainty.

L424 – see earlier comment on EDGAR v 6,7,8. (L306).

L439 – abatement coefficient - how much uncertainty does this introduce?

L507 – Methane mitigation challenge – maybe mention Nisbet et al. 2020 Rev. Geophys.

L510 – ‘well within the range of scenarios’ -– the paper seems to suggest the recent climb is OK and expected, so we can be complacent. But the past few years have been much more like 5-8.5. Scary! See also fig. 6  in Nisbet et al  Phil Trans Roy Soc A 379 (2021): 20200457.

L530 – FF 34% - maybe a comment on how this is changing?

L563 – underground fires – real guesswork.

L592-594 Low fracking losses - My gut feeling is to concur with this, but it might be a bit higher. The reference list is pretty old here - maybe cite more recent studies both by the EDF supported and other teams. Examples include Zhang, Y, et al. Science advances 6 (2020): eaaz5120. Or Li,  et al.   Science of The Total Environment 912 (2024): 169645. Or Williams, et al. EGUsphere 2024 (2024): 1-31. Shen, Lu, et al.   Nature Communications 14 (2023): 4948.

L611 – 613 – again, elderly references in a fast moving field…..but Lavaux ref is good.

L624 – extra 8 Tg….perhaps – but ultra-emitters ted to be short lived.

L640-642 90/10 ratio of eructation vs flatulence – give a reference for this? Or Johnson?

L669 – enteric 114-124 Tg – my instinct is that the uncertainty is wider than this?

L737 and L769 – maybe cite some direct measurements? For example Barker, P., et al. (2023) Airborne measurements of fire emission factors for African biomass burning sampled during the MOYA campaign. ACP 20s 15443-15459.

L834 - 842 – 853 There’s a problem in depending on land surface models. Note the bad failure of land surface models in tropical African and Bolivian wetlands – see especially Fig 8 and discussion in Shaw J.T. et al. (2022) Large methane emission fluxes observed from tropical wetlands in Zambia. Global Biogeochemical Cycles 36: e2021GB007261. Also see France et al. France, J. L., et al. (2022) Very large fluxes of methane measured above Bolivian seasonal wetlands. PNAS 119  e2206345119. Also note models in Zhang, Z. et al. Nat. Clim. Change https://doi.org/10.1038/s41558-023-01629-0 (2023) and comment by Nisbet 2023 Nature Climate Change 13 421-422.

L957 – note that Shaw et al (section 3.1.1) found low emissions over the wide shallow lake Bangweulu, even though the lake is shallow, warm and there is a lot of organic input – instead, the high emissions were dominated by the wetlands beside the lake. Although I have often observed warm afternoon ebullition events (sometimes dramatic) in lakes and open water bodies,  as a general but intuitive impression (i.e. personal observation not backed up by quantitative study) my ‘feel’ is that where water is more than about 3 to 4m deep and where plant stems (e.g. reeds, papyrus) do not project to entrain methane up from mud to air in the sap, then lake emissions are fairly low. Any bubbles are taken up in the aerobic conditions in oxic upper water.

L975 – see comment on lakes – these numbers are pretty high and based on few observations - may be higher than reality.

L1020 – run-off of agricultural Nitrogen fertilizer and manure to nearly wetlands?

L1068. Interesting discussion of this vexed problem. Good.

L1084 – may still be too high? Very much guesswork!

L1116 – The Petrenko group’s evidence is compelling –  theirs are very high quality measurements.  Maybe also cite Dyonisius, Michael N., et al. (2020) Science 367(2020): 907-910. I’m not convinced by the Thornton et al (2021) arguments.

L1127 – typo – Schwietzke.

L1133 – Termites – interesting how little work has been done since Pat Zimmerman’s work in the 1980s. The 10 Tg estimate looks like a placeholder! Maybe cite Chiri, E., et al (2020). Termite mounds contain soil-derived methanotroph communities kinetically adapted to elevated methane concentrations. The ISME journal, 14, 2715-2731. And Chiri, E, et al. Termite gas emissions select for hydrogenotrophic microbial communities in termite mounds. PNAS 118 (2021): e2102625118.

L1205 – I strongly suspect this 2 Tg number is a serious underestimate. Even in cattle monocultures there are dense populations of small nocturnal antelope (in Africa) and small deer (in EU and N America), in bush and scrub forest, etc as well as larger tree clumps and forests. For Africa see for example Hempson, G.P., Archibald, S. and Bond, W.J., 2017. The consequences of replacing wildlife with livestock in Africa. Scientific reports, 7, p.17196.

L1216 – gas plumes – see Fig 1 in Westbrook, Graham K., et al. (2009) Escape of methane gas from the seabed along the West Spitsbergen continental margin. Geophysical Research Letters 36. Plumes disappear quickly and even very large plumes don’t manage to rise through more than about 250m of water.

L1275 – the in-situ oceanic source from phosphonates is real. It may be small, but it’s essentially unknown. This 5 Tg number looks a bit like the ‘placeholder’ in Cicerone and Oremland 1988.

L1314 – see also L 1116 – seems too high given the Petrenko team’s results.

L1374 – does this also include features like the Yamal blowout collapse structures? Bogoyavlensky, Vasily, et al. (2021) Permanent gas emission from the Seyakha Crater of gas blowout, Yamal Peninsula, Russian Arctic. Energies 14: 5345.

L1381 – Nisbet et al 2009 demonstrated transport of methane via sap – i.e. plant methane comes from underlying anaerobic soil methanogens, not in situ aerobic processes in the leaf. In other words the plants were acting as ‘straws’ (see Line1385). UV-caused emissions are tiny (Bloom et al.). Note the important work of Gauci et al and Pangala et al. (see Line 1388): are their emissions encapsulated in the other flux categories (Line 1401).

L1433 some of this OH section is a bit elderly. Maybe add some new references?  E.g. (see also above L208) Anderson, D. C., et al. (2024); and  Stevenson, David S., et al. (2020) Trends in global tropospheric hydroxyl radical and methane lifetime since 1850 from AerChemMIP. Atmospheric Chemistry and Physics 20 12905-12920. Or Naus, S., et al  (2021). A three-dimensional-model inversion of methyl chloroform to constrain the atmospheric oxidative capacity. Atmospheric Chemistry and Physics, 21, 4809-4824.

L1459-1461 and 1465-1466 – maybe these numbers should be in the abstract?

L1504 – interesting this number is low – the Cl sink has a strong isotopic leverage, so if the low number is correct that leverage is small.

L1507 soil uptake. This is a major uncertainty in the budget because there are so few measurements from tropical seasonal rainfall woodlands and savannas. It’s a huge unknown, linked to the termite unknown. (see Chiri papers L1133). I suspect the already wide uncertainty range on L1535  and L1543 is low!

L1546 – lifetime definition – burden/sink - maybe make clear that this is not the same as the 12 yr perturbation lifetime cited in some IPCC chapters. Media people get very confused between the two sorts of lifetime. Maybe cite Prather here (as well as on L1619)

L1683 608 Tg – see also L 1836. maybe this number should be in the abstract also? It’s ~10% higher than  2010’s 554 Tg. See also Nisbet et al 2023 for discussion of post 2020 changes.  

L1824 – I suspect this 3 Tg number is a major underestimate!

L1883 regional budgets – maybe mention how these numbers contrast with recent studies like Yu, Xueying, et al. (2023) A high-resolution satellite-based map of global methane emissions reveals missing wetland, fossil fuel, and monsoon sources. Atmospheric Chemistry and Physics 23: 3325-3346.

L1983 ? Mention the current d13C(CH4) isotopic plunge – and concerns (e.g. Nisbet et al 2023).

L2076 – FINALLY!!! A mention of isotopes!!! That’s the big missing factor in the whole study!!!! The isotopes must balance – if they don’t the budget needs to be re-examined. Obviously it is too late to add a full isotopic analysis to this paper, but the need for isotopic constraint surely should be mentioned. I would suggest that a short section on isotopes should be added here at this stage in the paper, including maybe a brief one-paragraph attempt at balance, and a promise to do something about it next time.

L2190 – maybe mention Nisbet et al 2023 here and/or in Line 2194?

TABLES – these are extremely valuable and will be very heavily used. The Table 'callouts' could maybe be betterl integrated in the text and perhaps need to have more mentions: they’ll be very prominent on publication.

FIGURES – these are excellent and again the 'callouts' maybe need some more prominence in the text. For Fig 2 see comparison with Nisbet et al 2021 Phil Trans. In Fig 3 wetlands is heavily biassed by the Congo peatlands – but somewhat misses/downplays the upper Congo wetlands and also downplays big emitters like the Pantanal and Mamore R. and the Sudd, despite lots of recent evidence for big emissions. I accept these reagions are very difficutl to study by remote sensing, as cloud cover is so pervasive in wet season. I've flown over many and seen only cloud! Also I’m very sceptical of the 'lowish' biomass burn in India where crop waste burning is horrendous (or is crop waste burning classed under ‘waste’?)  Also India has a huge coal mining industry and giant landfills. Fig 5 'Geological' seems improbable that central Asia is less prominent than Romania (e.g. see MEMO2 / ROMEO measurements).

This manuscript represents a lot of work by a large number of co-authors.  The work is massive and I admit to being unable to seriously review the entire document.  It contains a lot of valuable material plus a number of errors or misguided recommendations.    Given the number of authors, it would be nice if more of them spent time carefully reading/editing.  I begin to wonder if the massive tomes generated so frequently by the Global Carbon Project are a benefit to the community.  I suspect it is too late to raise this, but I do not think this work is ready for publication.

A number of the sections seem to be repeats of the same old ideas and references in Saunois 2020.  Why is this not an update of what is new?

Section 7 appears to be a personal wish list of the authors' personal research goals, rather than a critical review.  Many of the references are to a single paper, often involving one or more of the authors.

Abstract:

L115:  "maintaining CH4 as…" is incorrect.  Even if CH4 emissions remained constant, it would maintain CH4 as the 2^nd GHG for a long while!

L117: "importance of CH4 EMISSIONS compared to THOSE OF CO2…" The atmospheric abundance is what it is, and is observed!  Here I think you want to emphasize the emissions, not the abundance. 

L119: "in reducing uncertainties in the factors.." is very contorted, try: "in quantifying the factors responsible for the observed atmospheric.."  we do not really need 'well' here either.

L130: this 2024 edition (help remind the reader of which edition they are reading)

L133: either the double counting is "potential" in which case it MAY exist, or it is not 'potential'  but an estimate of the REMAINING double accounting.

Introduction

L172ff:  No, this is simply incorrect.  CH4 is not a stronger absorber in the current atmosphere, there is more CO2 and hence CO2 is the stronger absorber (witness the ERF of each).  Anyhow, what you are talking about is GWP>1 meaning stronger impact on climate over the next 100 years per kg emitted. Anyway, this paper does not have the background in radiative transfer and forcing and you should not be talking about 'absorbers', stick to climate forcing and GWPs.

L173ff:  It is not 'climate feedbacks' that matter here, but 'climate and chemical composition changes' (not feedbacks!).  Chemical feedback are included in the GWP.

L174ff:  Since you are insisting on quoting all the GWPs for methane (20 & 100 yr, fossil and non-fossil) let us get it right.  OK, you are quoting GWPs from last IPCC, but it is about time we correct the maths that were done in Forster et al 2021.  I know this is not the job of this CH4 budget review to fix IPCC problems, but we should avoid further propagating errors. 

(1) The difference in fossil vs non-fossil GWP is 2.75 based on the molecular wt of CH4 and CO2.  IPCC rounds to 2.8 – OK.

(2) This assumes that the fossil 1 kg CH4 releases immediately 2.75 kg CO2 (stoichiometric) and that the CO2 is there for the full GWP time scale.

(3) In reality the CO2 is released as the CH4 decays and thus the difference from non-fossil to fossil must be smaller than 2.75.

(4) We agree that a tropospheric CH4 pulse decays as exp(-t/11.8) since a perturbation lifetime is 11.8 yr per IPCC Table 7.15.  For 20 years, the average CH4 is about 9.63 yr or about 48% of the 20-yr period.  Thus, the duration of the CO2 is 0.518*20 or only 10.37 years (i.e., add +1.4 to the GWP-20).  Thus the fossil CH4 GWP-20 should be corrected to 81.1 not 82.5. This is a minor correction, but since you are quoting decimal places, we should all correct the value.

(5) For 100 year GWP, this correction is smaller, 29.4 not 29.8.

L175:  "CH4-non fossil" is very poor English it is more in the French style with the noun first.  I love the French language, but here try "non-fossil CH4" and "fossil CH4"

L1467FF

This section on the stratosphere is remarkably out of date and misguided. 

The idea that strat loss could be 10 Tg/y is ridiculous (I know that models may calculate this, but why report absurd values).  This gives a strat lifetime of 500 years!! Then it would be well mixed to the mesosphere.  Scaling to N2O and other known species it should be ~120 yr (100-200?) = ~40 Tg, and it is uncertain to what level, maybe 30-50, but not 10-70. The large percentages for O(1D) can Cl loss must refer to regions of the stratosphere and not to total stratospheric loss?  If so, drop them because they are misleading and  it does not matter what drives CH4 loss at 48 km.

L1617ff:  This text looks like the previous 2020 edition that made the mistake of assuming that the fluctuations in CH4 abundance caused by emissions bumps would follow the budget lifetime rather than the perturbation lifetime (12 yr).  Thus, the excellence of your fit to 9 yr should give you pause and admit that the variability is a mix of time scales for varying emissions and OH.  The IPCC clearly discusses the different lifetimes and prominently displays the perturbation lifetime (11.8 yr) in the Table 7.15 from which you took the GWPs.

L1690:  "In addition, most of the top-down models use the same OH distribution from the TRANSCOM experiment (Patra et al., 2011),.." If this is so, then the top-down model mean of 575 Tg/y is really fixed by the assumed OH values.  It is hardly a chemistry model, since the chemistry is fixed.  This makes all the top-down numbers useless as they are fixed by Patra's choice of OH in 2011.  Can you explain?

L2052:  I think we need to improve the chemistry-transport models and chemistry-climate models using full chemistry rather than try to replace interactive chemistry with a ML approximation so you can use tracer-transport models.  (There is a confusion here that CTMs are tracer models with fixed OH, they are not.)  It is more important to find out why the CCMs and CTMs (with interactive chemistry) may be wrong.  That should be a higher recommendation than the Zhao approach that the lead author here is recommending. 

L2063:  If you recommend studying the reactivity of air parcels, then you really should reference the major effort on that done by the NASA ATom mission (e.g., 2023 ESSD, doi: 10.5194/essd-15-3299-2023).

L2067:  You can recommend satellite-constrained OH and that is a very attractive area for some, but it lacks the vertical resolution needed for CH4 studies and it seems to be overhyped here.

L2153:  Again, this whole list of recommended directions seems like the co-authors personal research agenda and hardly a critical review of where advances might come form. The idea of porting transport models to GPUs to reach sub-one-degree will not improve the overall transport characteristics of these models.  Many CTM/CCMs are already running one-degree simulations.  Much of the error is in the large-scale flows.  Further, who is going to generate and store the petabytes needed to run the tracer models? Already we have full up CCMs running refined mesh at 3 km scales embedded in ESMs.  If you want hi-res, that is where you must go.  We already have CTM/CCMs with hybrid coordinates, what is so special here (L2155).  The choice of odd grids will come from the dynamics of the underlying climate/forecast model rather than being designed by the tracer-transport operator.