The Global Methane Budget 2000–2017

<jats:p>Abstract. Understanding and quantifying the global methane (CH4) budget is important for assessing realistic pathways to mitigate climate change. Atmospheric emissions and concentrations of CH4 continue to increase, making CH4 the second most important human-influenced greenhouse gas in terms of climate forcing, after carbon dioxide (CO2). The relative importance of CH4 compared to CO2 depends on its shorter atmospheric lifetime, stronger warming potential, and variations in atmospheric growth rate over the past decade, the causes of which are still debated. Two major challenges in reducing uncertainties in the atmospheric growth rate arise from the variety of geographically overlapping CH4 sources and from the destruction of CH4 by short-lived hydroxyl radicals (OH). To address these challenges, we have established a consortium of multidisciplinary scientists under the umbrella of the Global Carbon Project to synthesize and stimulate new research aimed at improving and regularly updating the global methane budget. Following Saunois et al. (2016), we present here the second version of the living review paper dedicated to the decadal methane budget, integrating results of top-down studies (atmospheric observations within an atmospheric inverse-modelling framework) and bottom-up estimates (including process-based models for estimating land surface emissions and atmospheric chemistry, inventories of anthropogenic emissions, and data-driven extrapolations). For the 2008–2017 decade, global methane emissions are estimated by atmospheric inversions (a top-down approach) to be 576 Tg CH4 yr−1 (range 550–594, corresponding to the minimum and maximum estimates of the model ensemble). Of this total, 359 Tg CH4 yr−1 or ∼ 60 % is attributed to anthropogenic sources, that is emissions caused by direct human activity (i.e. anthropogenic emissions; range 336–376 Tg CH4 yr−1 or 50 %–65 %). The mean annual total emission for the new decade (2008–2017) is 29 Tg CH4 yr−1 larger than our estimate for the previous decade (2000–2009), and 24 Tg CH4 yr−1 larger than the one reported in the previous budget for 2003–2012 (Saunois et al., 2016). Since 2012, global CH4 emissions have been tracking the warmest scenarios assessed by the Intergovernmental Panel on Climate Change. Bottom-up methods suggest almost 30 % larger global emissions (737 Tg CH4 yr−1, range 594–881) than top-down inversion methods. Indeed, bottom-up estimates for natural sources such as natural wetlands, other inland water systems, and geological sources are higher than top-down estimates. The atmospheric constraints on the top-down budget suggest that at least some of these bottom-up emissions are overestimated. The latitudinal distribution of atmospheric observation-based emissions indicates a predominance of tropical emissions (∼ 65 % of the global budget, &lt; 30∘ N) compared to mid-latitudes (∼ 30 %, 30–60∘ N) and high northern latitudes (∼ 4 %, 60–90∘ N). The most important source of uncertainty in the methane budget is attributable to natural emissions, especially those from wetlands and other inland waters. Some of our global source estimates are smaller than those in previously published budgets (Saunois et al., 2016; Kirschke et al., 2013). In particular wetland emissions are about 35 Tg CH4 yr−1 lower due to improved partition wetlands and other inland waters. Emissions from geological sources and wild animals are also found to be smaller by 7 Tg CH4 yr−1 by 8 Tg CH4 yr−1, respectively. However, the overall discrepancy between bottom-up and top-down estimates has been reduced by only 5 % compared to Saunois et al. (2016), due to a higher estimate of emissions from inland waters, highlighting the need for more detailed research on emissions factors. Priorities for improving the methane budget include (i) a global, high-resolution map of water-saturated soils and inundated areas emitting methane based on a robust classification of different types of emitting habitats; (ii) further development of process-based models for inland-water emissions; (iii) intensification of methane observations at local scales (e.g., FLUXNET-CH4 measurements) and urban-scale monitoring to constrain bottom-up land surface models, and at regional scales (surface networks and satellites) to constrain atmospheric inversions; (iv) improvements of transport models and the representation of photochemical sinks in top-down inversions; and (v) development of a 3D variational inversion system using isotopic and/or co-emitted species such as ethane to improve source partitioning. The data presented here can be downloaded from https://doi.org/10.18160/GCP-CH4-2019 (Saunois et al., 2020) and from the Global Carbon Project.                     </jats:p>

section_content: und für die Berichterstattung an die Fakultäten der Universität und den Kanton Bern verwendet. (if necessary after consultation with the submitters, cf. below, 6. 4). Members of the Editorial Board are appointed by the Open Science Team of the University Library of Bern. 

• Roles of researchers: Submitters can pick the roles of researchers whose work they submit to BORIS Portal. A role is a combination of name, institute/department, and duration of employment. The role of a researcher is necessary to unambiguously identify contributors to research outputs. • Eligibility: authorization on a technical level to perform certain actions in BORIS Portal (e. g., submit or review items). The eligibility criteria are determined by the Open Science Team of the University Library of Bern. On a technical level, the system administrator can authorizes users (granting of rights). • Open -The datasets are accessible without restrictions and can be downloaded. 

• Embargo -The records are freely accessible after the expiration of an embargo period. The embargo end date is determined by the submitter. 

• Restricted -Access to records must be requested from a contact person. 

• Closed -Records cannot be uploaded to the repository but metadata can be entered. 

• Metadata Only -If data are already uploaded to another repository, the metadata on BORIS Portal can be used to verify the existence of the dataset.

section_content: Reasons to restrict access to records may include legal or ethical constraints, contractual obligation, or other compelling reasons.
section_name: Access restriction for datasets

section_content: Interested persons may request access to datasets that are under embargo, restricted or closed access. Such requests will be forwarded to the contact person that was designated when the dataset was submitted. The contact person can then grant or deny access. Submitters can specify terms of access by means of a Data Transfer Agreement (DTA).
section_name: Access to records on request

section_content: • BORIS Portal offers standard licenses for datasets. Depending on the content type, submitters can add other licenses as needed. • Should the submitter choose "restricted" or "closed" access, there is no dataset license to select. In this case, a Data Transfer Agreement (DTA) can regulate access. • Supplementary materials and Data Transfer Agreements in BORIS Portal are public domain (CC0), publicly accessible and downloadable. 5. 5. If not stated otherwise, content from BORIS Portal should be cited appropriately for subsequent use and reuse, at least by indicating author, title, date, repository (BORIS Portal), DOI. 

6. Submission 6. 1. Only designated members (including scientific staff) of the University of Bern or their authorized representatives can submit documents and metadata to BORIS Portal. 6. 2. The quality, validity, and authenticity of BORIS Portal content are the sole responsibility of the submitter. 6. 3. The compliance of content shared on BORIS Portal with legal, ethical, and similar regulations and requirements is the sole responsibility of the submitter. 6. 4. BORIS Portal editorial staff check submissions only for the authorization of the author/submitter, valid layout and format, and ensures the exclusion of content that is not relevant to the scope of BORIS Portal or that is inappropriate (spam). 6. 5. After review (and if necessary after consultation with the submitters), the BORIS portal editorial team publishes the entered metadata and makes the content accessible according to the access type chosen by the submitter. 

7. Storage and availability 7. 1. Metadata will be retained for the operational life of BORIS Portal. Should BORIS Portal cease operation, all metadata and content will be transferred to another repository. 7. 2. Submitted items will be retained for a minimum of 10 years. 7. 3. File formats may be converted if necessary to preserve the readability of files. 7. 4. It may not be possible to preserve the readability of files. BORIS Portal reserves the right to work with external partners to convert or migrate file formats, to use software emulations for old file formats, and/or to store metadata for preservation purposes. 7. 5. The content of the repository is regularly backed up by the university's IT services according to current best practice. 

8. Withdrawal of published items 8. 1. Published items may be withdrawn only for compelling reasons. Reasons for withdrawal are:

• Proven copyright infringement or plagiarism;

• Legal provisions or their violation; • National security;

• Falsified research. 

If BORIS Portal receives proof that one of these reasons applies, the corresponding item will be removed as soon as possible. BORIS Portal does not assume any responsibility for breaches of law in the aforementioned sense (see section 3. 2) by uploaded content. 8. 2. Withdrawn items may not be deleted, but will be withdrawn from public visibility. They may be retained for verification purposes. 8. 3. Withdrawn items may be deleted completely in case of legal violations. 8. 4. Digital Object Identifiers (DOIs) of withdrawn items will continue to be retained for verification purposes. BORIS Portal can then indicate the status of the relevant submissions as "deleted" in harvesting requests. 8. 5. DOIs continue to be forwarded to so-called "tombstone" references that display the metadata of the deleted item. This is done to prevent broken links and to retain item histories. 8. 6. The metadata of withdrawn items will not be searchable or findable. 8. 7. Lists of errors (errata) and corrections (corrigenda) can be added to the original entry as needed. 9. Data of members of the University of Bern, data protection, and confidentiality 9. 1. BORIS Portal processes data about members of the University of Bern. Processing data means in particular storing, linking, disclosing, and deleting data, as necessary for the purposes mentioned in section 2 above. 9. 2. Eligible employees of the university administration have direct access to BORIS Portal. For eligibility, see above, section 0 (definitions). 9. 3. All those involved in the submission or review of metadata and data receive instructions on the confidential and secure handling of the entered information. 9. 4. The following information will be checked against data from the personnel administration system of the University of Bern for the purpose of identification:

1. Name The data under 1-4 can be displayed or viewed publicly. The data under 5 are only visible during submission as well as during review by authorized members of the university administration (see above, 9. 2). The data under 6 are treated as confidential and can only be viewed by authorized members of the university administration. 9. 5. BORIS Portal will also process other data entered. These include:

• Externally provided identifiers such as ORCiD numbers;

• Information on research projects such as participating researchers and organizational units;

• Research project funding information (data submitter can remove this from public visibility);

• Documents that can be uploaded for internal purposes (e. g. project management) of the researchers. These documents will be visible only to the owners as well as to the system administrator, unless the owners grant permission to other persons.
section_name: Licenses

section_content: In accordance with legal provisions, BORIS Portal attaches great importance to the accuracy and timeliness of the stored data. However, BORIS Portal does not guarantee that the stored information is correct, up-to-date, complete, and of adequate quality. BORIS Portal reserves the right to correct or delete information. BORIS Portal shall not be liable for any loss or damage of any kind incurred as a result of accessing, using, or not using the published information. BORIS Portal has not reviewed third party websites, i. e. those that are not on the servers of BORIS Portal or within its sphere of influence, and that are hyperlinked to this website, and assumes no responsibility for their content. 

11. Review of these guidelines These guidelines will be reviewed at least annually. Date of last review: August 18, 2021.
section_name: Disclaimer of warranty and liability