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
Generated Summary
This study is a comprehensive review and synthesis of the global methane (CH4) budget, integrating both top-down (atmospheric observations and inversions) and bottom-up (process-based models, inventories, and data-driven approaches) methods to understand and quantify CH4 sources and sinks. The research analyzes data from a large ensemble of studies to assess the main features of the global methane budget and identify remaining uncertainties. The focus is on decadal budgets, with an update of the previous assessment for the period 2003–2012 to the more recent 2008–2017 decade. The study combines results from various sources, including atmospheric observations and inversions, process-based models, inventories of anthropogenic emissions, and biomass burning and atmospheric chemistry models. The aim is to improve the understanding and quantification of the global methane budget and address the challenges in reducing uncertainties.
Key Findings & Statistics
- Global methane emissions for the 2008–2017 decade are estimated by atmospheric inversions to be 576 Tg CH4 yr⁻¹ (range 550–594).
- Anthropogenic sources account for approximately 60% of the total, i.e., 359 Tg CH4 yr⁻¹ (range 336–376).
- The mean annual total emission for the new decade (2008–2017) is 29 Tg CH4 yr⁻¹ larger than the estimate for the previous decade (2000–2009) and 24 Tg CH4 yr⁻¹ larger than reported in the previous budget for 2003–2012.
- Bottom-up methods suggest almost 30% larger global emissions (737 Tg CH4 yr⁻¹, range 594–881) than top-down inversion methods.
- The range associated with the estimates (~10%–12%) is smaller than the range reported in Höglund-Isaksson et al. (2015) (~20%).
- The total anthropogenic emissions for the period 2008–2017 were 366 [349–393] Tg CH4 yr⁻¹.
- The estimated global methane emissions from agriculture and waste for the period 2008–2017 is 206 Tg CH4 yr-1 (range 191-223), which represents 56% of total anthropogenic emissions.
- Global mean emissions from fossil-fuel-related activities are estimated to be 128 [113–154] Tg CH4 yr⁻¹ for the 2008–2017 decade.
- Methane emissions from coal mining is estimated between 29 and 61 Tg CH4 yr⁻¹ for 2008-2017.
- For the period 2008–2017, biomass and biofuel burning generated methane emissions of 30 [26–40] Tg CH4 yr⁻¹.
- The range for natural wetland emissions is 102–182 Tg CH4 yr⁻¹.
- The average emission map from wetlands for 2008–2017 built from the 13 models is plotted in Fig. 3. The zones with the largest emissions are the Amazon basin, equatorial Africa and Asia, Canada, western Siberia, eastern India, and Bangladesh.
- A regional estimate for latitudes above 50° north (Wik et al., 2016b) estimated lake and pond methane emissions to be 16.5 Tg CH4 yr⁻¹ (compared to 13.4 Tg CH4 yr⁻¹ in Bastviken et al. (2011), above 54° N).
- Combining emissions from lakes and ponds from Bastviken et al. (2011) (71.6 Tg CH4 yr⁻¹) with the recent estimate of Deemer et al. (2016) for reservoirs and the streams and river estimates from Stanley et al. (2016) leads to total inland freshwater emissions of 117 Tg CH4 yr⁻¹.
- In this budget, the range of 5–10 Tg CH4 yr⁻¹ for marine geological emissions is reported, with a mean value of 7 Tg CH4 yr⁻¹.
- For top-down estimates, the CH4 chemical removal from the atmosphere is estimated at 518 Tg CH4 yr⁻¹ over the period 2008–2017, with an uncertainty of about ±5% (range 474–532 Tg CH4 yr⁻¹).
- The total estimated total CH4 emissions are 366 Tg CH4 yr⁻¹ (range 349–393) from bottom-up approaches and 576 Tg CH4 yr⁻¹ (range 550–594) from top-down approaches.
- The range of uncertainty for the global budget for 2000-2009 is [524–560] Tg CH4 yr-1.
- Inversions attribute about 60% of total emissions to anthropogenic activities (range of 55%–70%) and 40% to natural emissions.
- The bottom-up estimates rely on the global anthropogenic inventories, land surface models for wetland emissions, and published literature for other natural sources.
Other Important Findings
- The study’s findings are consistent with previous research, particularly in the global and latitudinal scales.
- In the 2008–2017 decade, the emissions from fossil fuels, agriculture and waste, and biomass burning are in good agreement with top-down approaches, but top-down approaches suggest less fossil fuel and more agriculture and waste emissions than bottom-up estimates.
- The main primary emission zones for wetlands are consistent between models.
- The total chemical loss for the 2000s reported here is 595 Tg CH4 yr¯¹ with an uncertainty of 22% (~130 Tg CH4 yr¯¹).
- Emissions from reservoirs are considered natural even though they are human-made.
- Emissions from wetlands, inland waters, and thawing permafrost are accounted for in natural emissions, even though they are influenced by climate change.
- The largest contribution to the range in the bottom-up budget is in the biomass burning estimates.
Limitations Noted in the Document
- The study acknowledges that uncertainties are reported as the range of available mean estimates rather than the uncertainty of individual estimates.
- The latitudinal distribution of emissions differs depending on the observations used (satellite or surface).
- The study does not fully account for all sources and sinks, especially from inland waters.
- The study’s ocean emission estimates do not include all brackish water emissions.
- The lack of high-resolution data and the complexity of atmospheric chemistry pose challenges in precisely measuring and modelling atmospheric CH4.
- The study relies on existing data and model outputs, which may have inherent biases.
- The use of different inverse systems can lead to variations in the estimated regional flux distribution.
- The study relies on the assumption of no trend in emissions and sinks for a global mean CH4 concentration.
Conclusion
The global methane budget for 2008–2017 reveals a persistent and challenging increase in atmospheric CH4 concentrations, underscoring the need for effective mitigation strategies. The study, using a combination of top-down and bottom-up approaches, confirms that both anthropogenic and natural sources contribute significantly to the global methane emissions. Notably, the study finds a good consistency in the partitioning of anthropogenic emissions between agriculture and waste, fossil fuels, and biomass burning. However, discrepancies remain between top-down and bottom-up estimates, especially for natural sources like freshwater systems, geological sources, and wetlands. The overestimation of the total methane emissions, especially from bottom-up approaches, leads to uncertainty in the methane budget and highlights the need for further refinement of the different approaches. The strong influence of climate change and human perturbation on the methane cycle is an important consideration, especially as there is a wide range of global and regional sources of methane. The study emphasizes the need for more detailed research on the emissions factors for inland waters. In particular, there should be more focus on reducing uncertainties in emission estimates for wetlands and freshwater systems and reducing double-counting issues. The study also highlights the importance of improving the capacity to understand and predict the methane budget through better integration of atmospheric observation and modelling, including improving the representation of chemical sinks and transport processes. As a key priority, the authors highlight that reducing uncertainties in emissions from wetlands and freshwater systems, along with decreasing double-counting issues, will be critical to better understand and mitigate the impact of methane emissions on climate change. Finally, the need for robust bias corrections on existing data and the inclusion of diverse and updated data sources are also highlighted as key to improving the accuracy and reliability of the methane budget. Overall, the study underscores the complexity of the methane cycle and the necessity of ongoing research efforts to reduce uncertainties and inform effective climate policies, with the GCP continuing to synthesize the methane budget on a regular basis.