Reconciliation of observation- and inventory- based methane emissions for eight large global emitters
Authors: Petrescu Ana Maria Roxana , Peters Glen P. , Engelen Richard , Houweling Sander , Brunner Dominik , Tsuruta Aki , Matthews Bradley , Patra Prabir K. , Belikov Dmitry , Thompson Rona L. , Höglund-Isaksson Lena , Zhang Wenxin , Segers Arjo J. , Etiope Giuseppe , Ciotoli Giancarlo , Peylin Philippe , Chevallier Frédéric , Aalto Tuula , Andrew Robbie M. , Bastviken David , Berchet Antoine , Broquet Grégoire , Conchedda Giulia , Gütschow Johannes , Haussaire Jean-Matthieu , Lauerwald Ronny , Markkanen Tiina , van Peet Jacob C. A. , Pison Isabelle , Regnier Pierre , Solum Espen , Scholze Marko , Tenkanen Maria , Tubiello Francesco N. , van der Werf Guido R. , Worden John R. 
Published: January 2024

Abstract. Monitoring the spatial distribution and trends in surface greenhouse gas (GHG) fluxes, as well as flux attribution to natural and anthropogenic processes, is essential to track progress under the Paris Agreement and to inform its Global Stocktake. This study updates earlier syntheses (Petrescu et al., 2020, 2021, 2023) and provides a consolidated synthesis of CH4 emissions using bottom-up (BU) and top-down (TD) approaches for the European Union (EU) and seven additional countries with large anthropogenic and/or natural emissions (USA, Brazil, China, India, Indonesia, Russia, and the Democratic Republic of Congo (DR Congo)). The work utilizes updated National GHG Inventories (NGHGIs) reported by Annex I Parties under the United Nations Framework Convention on Climate Change (UNFCCC) in 2023 and the latest available Biennial Update Reports (BURs) reported by non-Annex I Parties. The NGHGIs are considered in an integrated analysis that also relies on independent flux estimates from global inventory datasets, process-based models, inverse modeling and, when available, respective uncertainties. Whenever possible, it extends the period to 2021. Comparing NGHGIs with other approaches reveals that differences in the emission sources that are included in the estimate is a key source of divergence between approaches. A key system boundary difference is whether both anthropogenic and natural fluxes are included and, if they are, how fluxes belonging to these two sources are grouped/partitioned. Additionally, the natural fluxes are sensitive to the prior geospatial distribution of emissions in atmospheric inversions. Over the studied period, the total CH4 emissions in the EU, USA, and Russia show a steady decreasing trend since 1990, while for the non-EU emitters analyzed in this study, Brazil, China, India, Indonesia, and DR Congo, CH4 emissions have generally increased. In the EU, the anthropogenic BU approaches are reporting relatively similar mean emissions over 2015 to 2020 of 18.5 ± 2.7 Tg CH4 yr-1 for EDGAR v7.0, 16 Tg CH4 yr-1 for GAINS and 19 Tg CH4 yr-1 for FAOSTAT, with the NGHGI estimates of 15 ± 1.8 Tg CH4 yr-1. Inversions give higher emission estimates as they include natural emissions. Over the same period, the three high-resolution regional inversions report a mean emission of 21 (19–25) Tg CH4 yr-1, while the mean of six coarser-resolution global inversions results in emission estimates of 24 (23–25) Tg CH4 yr-1. The magnitude of BU natural emissions (peatland and mineral soils, lakes and reservoirs, geological and biomass burning) accounts for 6.6 Tg CH4 yr-1 (Petrescu et al., 2023a) and explains the differences between the TD inversions and the BU estimates of anthropogenic emissions (including NGHGIs). For the other Annex I Parties in this study (USA and Russia), over 2015 to 2020, the mean of the four anthropogenic BU approaches reports 18.5 (13–27.9) Tg CH4 yr-1 for Russia and 29.1 (23.5- Tg CH4 yr-1 for the USA, against total TD mean estimates of 37 (30–43) Tg CH4 yr-1 and 43.4 (42–48) Tg CH4 yr-1, respectively. The averaged BU and TD natural emissions account for 16.2 Tg CH4 yr-1 for Russia and 14.6 Tg CH4 yr-1 for the USA, partly explaining the gap between the BU anthropogenic and total TD emissions. For the non-Annex I Parties, anthropogenic CH4 estimates from UNFCCC BURs show large differences with the other global inventory-based estimates and even more with atmospheric-based ones. This poses an important potential challenge to monitoring the progress of the global CH4 pledge and the Global Stocktake, not only from the availability of data but also its accuracy. By systematically comparing the BU with TD methods, this study provides recommendations for more robust comparisons of available data sources and hopes to steadily engage more Parties in using observational methods to complement their UNFCCC inventories, as well as considering their natural emissions. With anticipated improvements in atmospheric modeling and observations, as well as modeling of natural fluxes, future development needs to resolve knowledge gaps in both BU and TD approaches and to better quantify remaining uncertainty. Consequently, TD methods may emerge as a powerful tool for verifying emission inventories for CH4, and other GHGs and informing international climate policy. The referenced datasets related to figures are available at https://doi.org/10.5281/zenodo.10276087 (Petrescu et al., 2023b).