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, < 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.

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Methane (CH 4 ) has a large contribution to the global radiative budget and is responsible for about 0.5°C of present global warming over the period 1850 -1900(IPCC, 2021. Methane has a relatively short perturbation lifetime (12.4 years (Balcombe et al., 2018)) and high global warming potential (28-36 times that of CO 2 over a 100year period (IPCC, 2021)). As such, a decline in CH 4 emissions will rapidly reduce global CH 4 concentrations and mitigate the impact of climate change at decadal time scales (United Nations Environment Programme & Climate & Clean Air Coalition, 2021). However, any efforts to target CH 4 emissions reductions require a thorough understanding of the dominant CH 4 sources and sinks and their temporal and regional distribution and trends.Methane is produced in three ways-pyrogenically, thermogenically, or biogenically-from both anthropogenic and natural processes. Pyrogenic sources of CH 4 include biofuel combustion (e.g., wood burning for heating and cooking) and biomass burning (e.g., wildfires and peat fires). All pyrogenic sources produce CH 4 from the incomplete combustion of organic matter. Thermogenic CH 4 is produced from the breakdown of organic matter buried deep within the Earth's crust at high pressure and temperature. Although geological CH 4 is released naturally into the atmosphere through gas seeps, most is released through activities related to the exploration, mining, and transport of fossil fuels (Hmiel et al., 2020;Janssens-Maenhout et al., 2019;Petrenko et al., 2017). The majority of biogenic CH 4 is produced in anaerobic environments by the microbial mediated breakdown of organic matter. These environments include wetlands, inland waters, marine sediments, ruminants such as cattle, rice paddies, manure management and wastewater and landfill systems. Small quantities of CH4 are also produced from the aerobic bacterial metabolization of methylated compounds (e.g., Florez-Leiva et al., 2013) and even photochemically (Li et al., 2020). Counter-balancing these CH 4 sources are three chemically driven atmospheric sinks of CH 4 . The first two reactions with tropospheric OH radicals and tropospheric atomic chlorine account for ~88% (476 -677 Tg CH 4 yr −1 ) and ~2% (1-35 Tg CH 4 yr −1 ) of the total sink, respectively, with a third stratospheric sink (e.g., reaction with O('D), Cl and OH in the stratosphere) accounting for a further ~5% (12-37 Tg CH 4 yr −1 ) (Saunois et al., 2020). However, due to their highly reactive nature, the key reactants are inherently difficult to quantify, driving a significant level of uncertainty in the spatial and temporal distribution of atmospheric sink estimates (Zhao et al., 2019). Many fundamental aspects of the spatial distribution of OH are currently unresolved, for example, estimates of the interhemispheric gradient can vary from 0.85 to 1.4 (NH/SH) depending on the methodology