the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Decreasing seasonal cycle amplitude of methane in the northern high latitudes being driven by lower latitude changes in emissions and transport
Chris Wilson
Martyn P. Chipperfield
Emanuel Gloor
Alistair Manning
Ruth Doherty
Abstract. Atmospheric methane (CH4) concentrations are rising which is expected to lead to a corresponding increase in its global seasonal cycle amplitude (SCA), the difference between its seasonal maximum and minimum values. Spatially-varying changes in the SCA could indicate long-term persistent variations in the seasonal sources and sinks but such SCA changes have not been investigated. Here we use surface flask measurements and a 3-D chemical transport model (TOMCAT) to diagnose changes in the SCA of atmospheric CH4 between 1995–2020 and attribute the changes regionally to contributions from different sectors. We find that the observed SCA decreased by 4 ppb (7.6 %) in the northern high latitudes (NHL, 60° N–90° N), whilst globally the SCA increased by 2.5 ppb (6.5 %) during this time period. TOMCAT reproduces the change in the SCA at observation sites across the globe and therefore we use it to attribute regions which are contributing to the changes in the NHL SCA. We find that well-mixed background CH4, likely from emissions originating in, and transported from, more southerly latitudes has the largest impact on the decreasing SCA in the NHL (56.5 % of total contribution to NHL). In addition to the background CH4, recent emissions from Canada, the Middle East and Europe contribute 16.9 %, 12.1 % and 8.4 %, respectively, to the total change in the SCA in the NHL. The regional contributions are driven by increases in summer emissions from the Boreal Plains in Canada, decreases in winter emissions across Europe, and a combination of increases in summer emissions and decreases in winter emissions over the Arabian Peninsula and Caspian Sea in the Middle East. These results highlight that changes in the observed seasonal cycle can be an indicator of changing emission regimes in local and non-local regions, particularly in the NHL where the change is counter-intuitive.
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Emily Dowd et al.
Status: final response (author comments only)
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RC1: 'Comment on egusphere-2023-132', Anonymous Referee #1, 16 Mar 2023
The manuscript by Dowd et al., entitled "Decreasing seasonal cycle amplitude of methane in the northern high latitudes being driven by lower latitude changes in emissions and transport," presents an interesting new study that analyzes changes in the seasonal cycle of methane concentration. The authors use an atmospheric chemistry-transport model with an inverse model to explore how and why the seasonal cycle amplitude (SCA) in methane concentration has changed from 1995-2020. The topic is of interest to scientists from a wide range of disciplines and backgrounds in the CH4 community. The model description is detailed and clear, but I have a few questions about the results and conclusions.
General Comments:
- The manuscript's structure is not well-balanced, with the first half discussing global patterns of SCA and the second half focusing only on changes in the 60N-90N region. The lack of in-depth discussion or explanation for why the region below 60N is not interpreted is also reflected in the abstract, which suggests a global increase in SCA by 2.5 ppb without further explanation.
- The manuscript does not discuss how the model bias of the transport model affects interpretation. The evaluation shows that the BRW site is an outlier with a large overestimation of the decrease in SCA. With only four sites in the Northern High Latitudes (NHL), it is unclear how well the model captures the seasonal amplitude in the NHL and how model errors affect the results.
- The manuscript does not discuss the role of Russia in the change in SCA, despite having the largest wetland emissions in the northern high latitudes, high oil and gas emissions, and severe biomass burning events in Siberia. Is that partly because there are no in-situ sites for that region so you underestimate the regional contributions from Russia with the taggers?
- The attribution of regional contribution is confusing, especially since more than 50% of the changes in SCA in the NHL come from unknown regions where the emissions originate. It is unclear how accurately the authors can rank regional contributions.
Specific Comments:
- In the abstract, it would be helpful to add an explanation of why higher methane concentration leads to a corresponding increase in seasonal amplitude.
- The total contribution in the abstract is not 100%, and the regions or processes missing need to be specified.
- Regarding initialization, it would be beneficial to remove the beginning period to exclude the initialization effect (Line 245-248).
- The paragraph over results (Line 280) is confusing. Please rewrite it to be more clear about whether positive means the emissions lead to an increase in DeltaSCA or a decrease in DeltaSCA.
- There is a typo in Line 284.
- It would be beneficial to have a conclusion sentence at the beginning of Line 286.
- Section 3.4 title needs to clarify whether it discusses DeltaSCA for the NHL.
- If Figure 5 is for the NHL, it needs to be described clearly in the figure caption.
- Line 398. ‘Emissions’ or ‘anthropogenic emissions’?
Citation: https://doi.org/10.5194/egusphere-2023-132-RC1 -
RC2: 'Comment on egusphere-2023-132', Anonymous Referee #2, 20 Mar 2023
Dowd et al first use NOAA measurements of methane (CH4) concentration to quantify the changes in the observed seasonal cycle amplitude (SCA) at available surface sites throughout the globe over the past ~25 years. The northern high latitude (NHL) sites show a large reduction in SCA over this time, while most other sites show a range of increases. They then compare the observed changes with those from a chemical transport model driven by optimized CH4 fluxes. Various model set-ups are used to evaluate the skill of the model and determine the causes of the negative dSCA at the NHL sites. The authors find that the NHL sites are mostly impacted by transported well-mixed background and more recent emissions from Canada, Europe, and the Middle East. The study highlights how impacts on the NHL can be used to isolate both local and non-local CH4 emissions changes.
Overall, the paper contains content and is of a quality and significance appropriate for publication in ACP. Both the observational and extensive modeling components are well done. My main concerns relate to the description of the results and inclusion of appropriate caveats within the discussion. Streamlining certain sections and providing clarifications (in text and in figure captions) will improve the readability of the paper and increase its impact for the reader.
Specific comments, questions, and suggestions follow below:
Line 16: It is not obvious from the abstract alone why the negative change in SCA is counter-intuitive (oxidation of increasing CH4 concentration leading to increasing SCA is only mentioned later).
Line 42: “the observed CH4 seasonal cycle” Do the authors mean cycle of concentrations or flux/emissions? Check to make sure this distinction is clear throughout the paper. If “CH4” alone implies “CH4 concentration”, state this early on.
Line 63: I was confused to see 80 sites mentioned here after only 22 were depicted in Fig 2 (directed to in line 60). The reason for this difference is mentioned later but expanding the Fig 2 caption to state that only the sites shown were used to calculate the SCA would reduce initial confusion.
Line 79: How does the choice of the specific regions and their boundaries impact the results?
Line 142: Are the results impacted by running the inverse model over each calendar year when the seasonal cycle in the southern hemisphere spans two calendar years? or is more important only that the inversion is run consistently over each year?
Line 154: Is this the top-down or bottom-up value from Saunois et al., 2020?
Line 205: “the SCA is increasing globally” should more precisely be “the global mean SCA at available sites is increasing”.
Fig 4: Site labels should be manually adjusted to avoid over-plotting. Expand caption to clarify that 4a shows the mean SCA and interannual variability at each site over a certain range of years.
Line 235: Do the NOAA sites perform better because they were also used to determine the optimized fluxes?
Line 236: A bit of clarification is needed here. Only 2 of the 4 NHL sites match with the correct negative sign, but yes, they are all consistently on the relatively lower end - for both observed and model.
Line 248: Should this be four sites in the NHL? Or mention more specifically the two sites meant here?
The ICE site is included with the NHL grouping but does not have a large negative observed dSCA. Why is this? Despite the NHL focus, ICE is not mentioned or discussed anywhere in the paper. Similarly, MHD (in upper northern mid-latitudes) is not discussed but does have a larger negative dSCA. Should MHD be grouped with the NHL sites?
Lines 236-252: In addition to addressing the questions directly above, this paragraph would benefit from being refocused on the main point, which seems to be the last sentence.
Sect 3.3: Perhaps more explicitly state that this result forms the intuition (referenced in abstract and elsewhere) for what would be expected absent any transport or emissions changes.
Fig 5a: Ideally, resize to be same scales as regions in 5b for a better comparison. Perhaps make 5a only 0.25 of the figure width to expand region bars. Also better differentiate that BKGRD is not a region – maybe remove “Region Code” from x-axis title.
Lines 296-306: The explanation of the offset between the concentration and emission seasonal cycle here is confusing and should be rewritten for clarity. Does a positive ISR always lead to a decreasing SCA? What about if transport is accounted for?
Figs 6-9: Including the yearly SCA values and trend for each region would help with interpreting the CH4 concentration and ISR changes already shown.
Lines 307-335: Reorganize and condense the several regional paragraphs to focus on the main point – that emissions from these regions are decreasing the SCA in the NHL. Then describe how each region is different, what about the regional emissions is changing, and the corresponding uncertainties. What impact does the relative proximity of Canada and Europe to the NHL sites have on the results?
Sect 3.5: Which of these sensitivity experiments is more realistic? Are there any examples from the literature that 9 months is an appropriate choice?
Sec 4: It seems like there is little information overall to verify the seasonality of emissions in various regions, which is critical in determining the SCA. Expand on the discussion of this uncertainty. Where did the assumed initial seasonal emission cycles in the model come from, and what were they based on?
Line 420: Do these numbers refer to the p-values of the trends?
Figure captions should be expanded to include years of data shown.
Citation: https://doi.org/10.5194/egusphere-2023-132-RC2
Emily Dowd et al.
Emily Dowd et al.
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