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. 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 document presents the second version of a living review paper dedicated to the decadal methane budget, integrating results from top-down studies (atmospheric observations within an atmospheric inverse-modeling framework) and bottom-up estimates (including process-based models, inventories of anthropogenic emissions, and data-driven extrapolations). The study uses a multi-disciplinary approach, bringing together a consortium of 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. The focus is on decadal budgets and the update of the previous assessment made for the period 2003–2012 to the more recent 2008–2017 decade. The methodology involves gathering results of observations and models to better understand and quantify the main features of the budget, its uncertainties, and to make recommendations.
Key Findings & Statistics
- For the 2008-2017 decade, global methane emissions are estimated by atmospheric inversions to be 576 Tg CH4 yr¯¹ (range 550–594).
- Of this total, 359 Tg CH4 yr¯¹ or ~ 60% is attributed to anthropogenic sources.
- 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 the one 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.
- For the 2008–2017 decade, methane emissions from coal mining represent 33 % of total fossil-fuel-related emissions (42 Tg CH4 yr−1, range of 29–61).
- Global emissions from agriculture and waste for the period 2008–2017 are estimated to be 206 Tg CH4 yr-1 (range 191-223, Table 3), representing 56% of total anthropogenic emissions.
- Global methane emissions from rice paddies are estimated to be 30 [25-38] Tg CH4 yr¯¹ for the 2008–2017 decade.
- Global emission of methane from waste management is estimated in the range of 60–69 Tg CH4 yr−1 for the 2008–2017 period with a mean value of 65 Tg CH4 yr−1.
- During the period of 2008–2017, biomass and biofuel burning generated methane emissions of 30 [26–40] Tg CH4 yr−1, of which 30 %–50 % is from biofuel burning.
- The resulting global flux range for natural wetland emissions is 101–179 Tg CH4 yr−1 for the 2000–2017 period, with an average of 148 Tg CH4 yr−1.
- In this budget, we report a mean value of 159 Tg CH4 yr−1 from freshwater systems (lakes, ponds, reservoirs, streams, and rivers), with a range of 117–212 Tg CH4 yr−1.
- For the 2008–2017 decade, methane emissions from upstream and downstream oil and natural gas sectors are estimated to represent about 63% of total fossil CH4 emissions (80 Tg CH4 yr¯¹, range of 68–92 Tg CH4 yr¯¹, Table 3).
- The global emissions inferred by the ensemble of 22 inversions is 576 Tg CH4 yr−1 [550–594] for the 2008–2017 decade (Table 3), with the highest ensemble mean emission of 596 Tg CH4 yr−1 [572–614] for 2017.
- Total anthropogenic emissions for the period 2008–2017 were assessed to be statistically consistent between top-down (359 Tg CH4 yr−1, range 336–376) and bottom-up approaches (366 Tg CH4 yr−1, range 349–393).
- The latitudinal breakdown of emissions inferred from atmospheric inversions reveals a dominance of tropical emissions at 368 Tg CH4 yr−1 [337–399], representing 64 % of the global total (Table 5).
Other Important Findings
- The main drivers of uncertainty in the methane budget arise from the variety of geographically overlapping CH4 sources and from the destruction of CH4 by short-lived hydroxyl radicals (OH).
- Anthropogenic emissions are often reported by country, and the number of regions was chosen to be close to the widely used TransCom inter-comparison map but with subdivisions to separate the contribution from important countries or regions for the methane cycle.
- Methane is emitted by different processes (i.e., biogenic, thermogenic, or pyrogenic) and can be of anthropogenic or natural origin.
- The uncertainty in the chemical loss of methane by OH, the predominant sink of atmospheric methane, is estimated around 10% to 15%.
- The surface dry air mole fraction of atmospheric methane (CH4) reached 1857 ppb in 2018.
- The total number of livestock continues to grow steadily.
- The spatial distribution of methane emissions from fossil fuels is presented in Fig. 3 based on the mean gridded maps provided by CEDS, EDGARv4.3.2, and GAINS for the 2008–2017 decade.
- The largest emissions from rice cultivation are found in Asia, accounting for 30% to 50% of global emissions.
- The main primary emission zones are consistent between models.
- The most important source of uncertainty in the methane budget is attributable to natural emissions, especially those from wetlands and other inland waters.
Limitations Noted in the Document
- The range associated with the estimates is smaller than the range reported in Höglund-Isaksson et al. (2015) (~20%), perhaps because they analyzed data from a wider range of inventories and projections, plus this study was referenced to one year only (2005) rather than averaged over a decade.
- In most of the inverse systems, the atmospheric oxidant concentrations are prescribed with pre-optimized or scaled OH fields, and thus the atmospheric sink is not solved.
- The total methane emission derived from the sum of independent bottom-up estimates remains unconstrained.
- The chemical loss of methane in the stratosphere is subject to large uncertainties.
- The low number of measurements, the lack of clarity on ebullition fluxes, and the large degree of variance in measurements have precluded an accurate spatial representation of stream and river methane fluxes.
- Differences in the various models’ assumptions and parameters, we report here the median and range of Tian et al. (2016): 30 [11–49] Tg CH4 yr−1 for the periods 2000–2009 and 2008–2017.
- The range shows the minimum and maximum estimates but excludes the uncertainty from each single estimate, which is expected to be large.
- The perhaps overestimation likely results from errors related to up-scaling local measurements and double-counting of some natural sources.
Conclusion
The main challenge in mitigating climate change lies in accurately quantifying and understanding the global methane budget. This study, using a comprehensive approach of top-down and bottom-up methods, highlights the key components of the methane cycle, including the various sources, sinks, and their uncertainties. The study emphasizes the dominance of anthropogenic sources, particularly agriculture and fossil fuels, which contribute significantly to the overall emissions. The reliance on both top-down and bottom-up approaches, with their respective strengths and limitations, offers a robust framework for analyzing the methane budget. However, the disparities between the two approaches, especially in the estimates of natural emissions, indicate areas needing further refinement. The bottom-up methods, using process-based models, inventories, and data-driven approaches, often suggest larger total emissions, whereas the top-down methods, employing atmospheric observations, provide a valuable constraint on the global total source and sink. It is also highlighted the need for more studies integrating the different systems to improve the understanding of methane emissions. The study underscores that the path toward accurate and reliable estimates of the methane budget involves addressing several key areas. These include improving the partitioning of methane emissions from various sources, particularly wetlands and inland water systems. There’s also an urgent need for a more robust assessment of uncertainties across the various components of the budget. Such efforts will be crucial for refining the existing models and creating a more transparent and consistent knowledge base. Additionally, the study underscores that the regional and latitudinal variations in methane emissions should be thoroughly investigated. The regional distribution of inferred emissions differs depending on the observations used (satellite or surface), as differences in the ensemble explain the contrasting results. Therefore, further investigations into the causes of these differences and their implications are vital. Ultimately, the study’s findings offer vital information, reinforcing the need for a concerted, multi-faceted strategy to mitigate methane emissions. To accurately represent the global methane cycle and provide a reliable foundation for effective policy interventions, the study indicates that sustained and long-term monitoring of the methane cycle is needed.