the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Methane emissions responsible for record-breaking atmospheric methane growth rates in 2020 and 2021
Liang Feng
Robert J. Parker
Mark F. Lunt
Hartmut Boesch
Abstract. The global atmospheric methane growth rates reported by NOAA for 2020 and 2021 are the largest since systematic measurements began in 1983. To explore the underlying reasons for these anomalous growth rates we use newly available methane data from the Japanese Greenhouse gases Observing SATellite (GOSAT) to estimate methane surface emissions. Relative to baseline values in 2019 we see the largest annual increases in methane emissions during 2020 over Eastern Africa (13 Tg), tropical Asia (4 Tg), tropical South America (3 Tg), and temperate Eurasia (3 Tg), and the largest reductions over China (-6 Tg) and India (-2 Tg). We find comparable emission changes in 2021, relative to 2019, except for tropical and temperate South America where emissions increased to 9 Tg and 5 Tg, respectively, and tropical Asian emissions increased to 8 Tg. The elevated contributions we saw in 2020 over the western half of Africa (-5 Tg) and Europe (-3 Tg) are substantially reduced in 2021, compared to our 2019 baseline. We find statistically significant positive correlations between anomalies of tropical methane emissions and groundwater, consistent with recent studies that have highlighted a growing role for microbial sources over the tropics. Emission reductions over India and China are expected in 2020 due to the Covid-19 shutdown but continued in 2021, which we do not currently understand. Based on a sensitivity study for which we assume a conservative 5 % decrease in hydroxyl concentrations in 2020, due to reduced pollutant emissions during the Covid-19 shutdown, we find that the global increase in our a posteriori emissions in 2020 is ~22 % lower than our control calculation. We conclude therefore that most of the observed increase in atmospheric methane during 2020 and 2021 is due to increased emissions.
Liang Feng et al.
Status: final response (author comments only)
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RC1: 'Reviewer comment on Feng et al', Anonymous Referee #1, 18 Jun 2022
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AC1: 'Reply on RC1', Paul Palmer, 24 Jun 2022
The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2022-425/acp-2022-425-AC1-supplement.pdf
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RC2: 'Reply on AC1', Anonymous Referee #1, 24 Jun 2022
This reviewer appreciates the quick response from the authors. Although I would like to clarify one point in case it was not well articulated in my initial review.
It is this reviewers understanding that previous work arguing for a role of chemistry claims that chemistry could explain some large fraction of the methane growth. My reading of this manuscript is that the central conclusion is that methane emissions are responsible for for the methane growth. The former claim that a process can explain a phenomena is justified through forward simulations. In this reviewer's opinion, arguing that a process is responsible for a phenomena requires a higher burden of proof. This reviewer was unconvinced that the numerical experiements presented in the inital manuscript meet the standard to justify the latter claim. This is why I suggested either reframing the conclusions to include approriate caveats or presenting additional numerical experiements to fully justify the central claims. Apologies for any confusion in my inital review.
Citation: https://doi.org/10.5194/acp-2022-425-RC2 -
AC2: 'Reply on RC2', Paul Palmer, 24 Jun 2022
This reviewer is correct that studies proposing that OH played a large role in the atmospheric growth in 2020 used forward simulations. Point well taken. However, these studies were effectively blind to the methane emission hotspots that we can see from space, drawing attention to the disconnect between different modelling approaches used by different parts of the community. Also, these studies tended to make their claims a little more robustly than suggested by this reviewer.
We sincerely thank the reviewer for keeping us honest and challenging us to provide additional evidence to support our claim. In our submitted response, we have suggested expanding our caveats and including additional experiments. Our preliminary follow-on experiments are consistent with our original claim, which we will detail in the revised manuscript.
Citation: https://doi.org/10.5194/acp-2022-425-AC2
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AC2: 'Reply on RC2', Paul Palmer, 24 Jun 2022
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RC2: 'Reply on AC1', Anonymous Referee #1, 24 Jun 2022
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AC1: 'Reply on RC1', Paul Palmer, 24 Jun 2022
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CC1: 'Comment on acp-2022-425', Janne Hakkarainen, 01 Jul 2022
I noticed that the second formula in Appendix B has an extra k in the denominator.
Citation: https://doi.org/10.5194/acp-2022-425-CC1 -
RC3: 'Comment on acp-2022-425', Anonymous Referee #2, 06 Aug 2022
Feng et al attempts to attribute the stunning increases in the methane growth rate from 2020-2021 using satellite observations of methane in conjunction with surface data. The subject is very timely and worth getting out into the community for further analysis and discussion. However, there are a number of issues that the authors need to address, which are incorporated into the annotated text of the paper.  In particular, there are no uncertainty estimates of the fluxes, no real explanation of the Eastern African increase, and an unrealistic description of OH change. Spending more time on these elements will strengthen the paper and adding credibility to the results.Â
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AC3: 'Integrated and updated author responses to reviewer comments of acp-2022-425', Paul Palmer, 20 Dec 2022
The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2022-425/acp-2022-425-AC3-supplement.pdf
Liang Feng et al.
Liang Feng et al.
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