Abstract
Understanding and quantifying the global methane (CH4) budget is important for assessing realistic pathways to mitigate climate change. Emissions and atmospheric concentrations of CH4 continue to increase, maintaining CH4 as 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 for temperature change is related to its shorter atmospheric lifetime, stronger radiative effect, and acceleration in atmospheric growth rate over the past decade, the causes of which are still debated. Two major challenges in reducing uncertainties in the factors explaining the well-observed atmospheric growth rate arise from diverse, geographically overlapping CH4 sources and from the uncertain magnitude and temporal change in the destruction of CH4 by short-lived and highly variable hydroxyl radicals (OH). To address these challenges, we have established a consortium of multi-disciplinary scientists under the umbrella of the Global Carbon Project to improve, synthesise and update the global CH4 budget regularly and to stimulate new research on the methane cycle. Following Saunois et al. (2016, 2020), we present here the third version of the living review paper dedicated to the decadal CH4 budget, integrating results of top-down CH4 emission estimates (based on in-situ and greenhouse gas observing satellite (GOSAT) atmospheric observations and an ensemble of atmospheric inverse-model results) and bottom-up estimates (based on process-based models for estimating land-surface emissions and atmospheric chemistry, inventories of anthropogenic emissions, and data-driven extrapolations). We present a budget for the most recent 2010-2019 calendar decade (the latest period for which full datasets are available), for the previous decade of 2000-2009 and for the year 2020.
Generated Summary
This research presents an updated global methane (CH4) budget spanning the years 2000-2020, integrating data from top-down atmospheric observations and bottom-up models. The study employs a multi-faceted approach, using results from an ensemble of bottom-up models, including process-based models, inventories, and data-driven approaches, as well as top-down atmospheric inversions. The aim is to better understand the global CH4 cycle, identify uncertainties, and provide a benchmark for future CH4 studies. The core methodology involves analyzing the global and latitudinal distribution of CH4 emissions, focusing on the contribution of different sources, and assessing the discrepancies between bottom-up and top-down estimates. The study is an effort to reconcile top-down and bottom-up approaches to quantify the global methane cycle.
Key Findings & Statistics
- The total annual emissions inferred by the ensemble of 24 inversions is 575 Tg CH4 yr¯¹ [553-586] for the 2010-2019 decade, with the highest ensemble mean emission of 608 Tg CH4 yr¯¹ [581-627] for 2020.
- Global emissions for 2000-2009 (543 Tg CH4 yr¯¹) fall within the range in Saunois et al. (2016, 2020).
- The bottom-up estimates considered here differ substantially from the top-down results, with annual global emissions being about 15% larger at 669 Tg CH4 yr¯¹ [512-849] for 2010-2019.
- For the 2010-2019 decade, global CH4 emissions are estimated by atmospheric inversions (top-down) to be 575 Tg CH4 yr¯¹ (range 553-586).
- Of this amount, 369 Tg CH4 yr¯¹ or ~65% are attributed to direct anthropogenic sources.
- The combined wetland and inland freshwater emissions for 2010-2019 were estimated at 248 Tg CH4/year (159 to 369).
- Over the period 2010-2019, at the global scale, satellite-based inversions infer almost identical emissions to ground-based inversions.
- Total direct anthropogenic emissions were 358 [329-387] Tg CH4 yr-¹ for the decade 2010-2019.
- Agriculture and waste emissions amount to 144 Tg CH4 yr-¹, 40% of the direct anthropogenic emissions.
- Waste management was responsible for about 11% of total global direct anthropogenic CH4 emissions in 2000.
- Methane emissions from oil and natural gas systems vary greatly in different global inventories (67 to 80 Tg yr¯¹ in 2020).
- Coal mining show a reduced range of 37-44 Tg CH4 yr¯¹ for 2010-2019.
- Biomass and biofuel burning generated CH4 emissions of 28 [21-39] Tg CH4 yr¯¹ (Table 3), of which 30-50% is from biofuel burning.
- Total fossil fuel emissions from bottom-up approaches agrees well with the 13C-based estimate of Schwietzke et al. (2016) of 192 ± 32 Tg CH4 yr¯¹.
- Global emissions from agriculture and waste for the period 2010-2019 are estimated to be 211 [195-231] Tg CH4 yr¯¹.
- CH4 emissions from reservoirs were also considered as natural even though reservoirs are human-made.
- Geological submarine CH4 emissions are 12 Tg CH4 yr¯¹ (median) with a range of 6-20 Tg CH4 yr-¹.
Other Important Findings
- A dominant role of tropical emissions (~64%) compared to mid (~32%) and high (~4%) northern latitudes (above 60°N) emissions.
- Direct anthropogenic emissions were assessed to be statistically consistent between top-down (369 Tg CH4 yr-¹, range 350-391) and bottom-up approaches (358 Tg CH4 yr-¹, range 329-387).
- The range reported gives the minimum and maximum values among studies and does not reflect the individual full uncertainties.
- In the recent years 2020 and 2021, the highest growth rates of 15 ppb yr-¹ and 18 ppb yr-¹ were unprecedented since the 1980s.
- The study reports the total geological emission of 45 [18-63] Tg CH4 yr¯¹ with a breakdown between offshore emissions of 7 [5-10] Tg CH4 yr¯¹ and onshore emissions of 38 [13-53] Tg CH4 yr¯¹.
- The study also shows that the 2020 was a second highest year in terms of atmospheric CH4 growth rate (+15.2 ppb/yr) since systematic measurements began in the late 1980s.
- For the period 2010-2019, the most important source of uncertainty in the global CH4 budget is still attributable to natural emissions, especially those from wetlands and inland freshwaters.
- The total double-counting correction term of 23 Tg CH4 reduces the BU budget for combined wetlands and inland waters from 271 Tg CH4 yr¯¹ to 248 Tg CH4 yr¯¹ (see Fig. 4 and Table 3).
- The study identifies five major priorities for improving the CH4 budget.
- The largest discrepancies between the top-down and the bottom-up budgets are found in the mid-latitudes and boreal regions from the natural and indirect sources with bottom-up estimates twice as large as the top-down ones, especially in the inland freshwater category.
- The main remaining discrepancy between top-down and bottom-up budgets is found for the natural and indirect anthropogenic emission total (105 Tg).
Limitations Noted in the Document
- The uncertainties in the CH4 budget remain high because of the large range reported for emissions from freshwater systems.
- The uncertainty on the OH induced loss translates, in the top-down methods, into the minimum relative uncertainty associated with global CH4 emissions.
- The largest uncertainty in the global CH4 budget is still attributable to natural emissions, especially those from wetlands and inland freshwaters.
- The study has limited data representation from large lakes that are a large component of global lake surface area.
- For the 2010-2019 decade, top-down inversions infer “Other natural emissions” (Table 3) at 43 Tg CH4 yr¯¹ [40-46], whereas the sum of the individual bottom-up emissions is 63 Tg CH4 yr¯¹ [24-93], contributing to a 20 Tg discrepancy between bottom-up and top-down approaches.
- The study has a few limitations in top-down methods and their uncertainties.
- Uncertainties in the chemical loss of CH4 by OH, the predominant sink of atmospheric CH4.
- There is a lack of methane data representation from large lakes that are a large component of global lake surface area.
- The production of accurate bottom-up estimates is complicated by the fact that anthropogenic emissions result from leakage from fossil fuel production with large differences between countries depending on technologies and practices, the fact that many large leak events are sporadic, and the location of many emissions hotspots is not well known, and from uncertain emission factors used to summarise complex microbial processes in the agriculture and waste sectors.
Conclusion
The study provides a comprehensive overview of the global methane budget, offering key insights into its sources, sinks, and uncertainties. The consistency between the top-down and bottom-up approaches is a significant step towards defining a more accurate global methane emissions. The findings emphasize the importance of reducing emissions from anthropogenic sources, particularly those from the agriculture and waste sectors, while highlighting the need to address the uncertainties in natural emission sources, particularly wetlands and inland freshwaters. The research identified several key priorities for future research, including improving the quality of the data, and expanding the monitoring networks. These include integrating the spatial distribution of inland waters in atmospheric inversion models; refining the double-counting estimation; continued validation of the measurements; and development of diverse modeling approaches. The study underscores the complex interplay of factors influencing the CH4 cycle, highlighting the need for multidisciplinary research efforts. The research also reveals that the atmospheric growth rate of CH4 has increased to a level similar to that of the mid-1990s, indicating that emissions reductions may be more complex than anticipated. The budget also highlights the significance of the contribution from natural sources to the overall CH4 budget. The study suggests that a substantial reduction in CH4 emissions is an effective option to limit climate warming in the near-term. This work is a call to action for further research, improved measurements, and advanced models to improve our understanding of the global methane cycle and to support informed policy decisions.