The Global Methane Budget 2000–2017

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).

Generated Summary

This research presents the second iteration of a living review paper focused on the global methane budget from 2000 to 2017. The study utilizes a combination of top-down (atmospheric observations within an atmospheric inverse-modeling framework) and bottom-up (process-based models, inventories, and data-driven extrapolations) approaches. The goal is to assess and quantify the main features of the global methane budget, addressing uncertainties and recommending future research directions. The study integrates results from a large ensemble of bottom-up approaches, including process-based models for estimating land surface emissions, atmospheric chemistry, and data-driven extrapolations, alongside top-down approaches that encompass methane atmospheric observing networks and atmospheric inversions.

Key Findings & Statistics

• Global methane emissions for the 2008-2017 decade: 576 Tg CH4 yr¯¹ (range 550–594 Tg CH4 yr¯¹) estimated by atmospheric inversions.
• For the 2008-2017 decade, total anthropogenic emissions were 366 Tg CH4 yr¯¹ (range 349–393 Tg CH4 yr¯¹).
• 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.
• 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).
• Total anthropogenic emissions are estimated by atmospheric inversions (a top-down approach) 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, that is emissions caused by direct human activity (i.e. anthropogenic emissions; range 336–376 Tg CH4 yr®¹ or 50 %-65%).
• 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).
• 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).
• 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, global methane emissions are estimated by atmospheric inversions (a top-down approach) to be 576 Tg CH4 yr¯¹ (range 550–594).
• Total anthropogenic emissions for the 2008-2017 decade were 366 [349–393] Tg CH4 yr-1.
• Mean emissions from fossil-fuel-related activities: 128 [113-154] Tg CH4 yr¹ for the 2008–2017 decade.
• 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).
• Methane emissions from oil and natural gas systems: 72 to 97Tgyr-1 in 2017.
• Global emissions from agriculture and waste for the period 2008–2017 are estimated to be 206 Tg CH4 yr-1 (range 191-223).
• Global methane emissions from rice paddies are estimated to be 30 [25-38] Tg CH4 yr¯¹ for the 2008–2017 decade.
• Global methane emissions from waste management is estimated in the range of 60–69 Tg CH4 yr 1 for the 2008–2017 period.
• 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.
• 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).
• For wetlands in 2008–2017, the top-down and bottom-up estimates of 181 Tg CH4 yr-1 (range 159–200) and 149 Tg CH4 yr-1 (range 102–182), respectively, are statistically consistent.
• For the 2008-2017 decade, the average ensemble emissions were 149 Tg CH4 yr¯¹ with a range of 102–182.
• 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 period 2008–2017, a total global geological emission of 45 [18–63] Tg CH4 yr-1.
• In this study, we report a decadal value of 9 Tg CH4 yr-1 (range [3-15] Tg CH4 yr¯¹).
• 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).
• 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).
• 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).
• The global mean CH4 concentration was fitted with a function that describes the approach to a first-order steady state (ss index): [CH4](t) = [CH4]ss – ([CH4]ss – [CH4]0)e-t/t; solving for the lifetime, t, gives 9.3 years.
• From 1999 to 2006, the annual increase in atmospheric CH4 was remarkably small at 0.6±0.1 ppb yr¯¹.
• Since 2006, the atmospheric growth rate has recovered to a level similar to that of the mid-1990s (~5 ppb yr¯¹), or even to that of the 1980s for 2014 and 2015 (> 10 ppb yr¯¹).
• We report here a climatological range for the tropospheric loss of methane by OH oxidation of 553 [476–677] Tg CH4 yr 1.
• We report here a climatological range of 12–37 Tg CH4 yr-1 associated with a mean value of 31 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 2008-2017, the top-down and bottom-up estimates of 181 Tg CH4 yr-1 (range 159–200) and 149 Tg CH4 yr-1 (range 102–182), respectively, are statistically consistent.
• The range associated with our estimates (~10%-12%) 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, as done here.
• For wetlands in 2008–2017, the top-down and bottom-up estimates of 181 Tg CH4 yr-1 (range 159–200) and 149 Tg CH4 yr-1 (range 102–182), respectively, are statistically consistent.
• Based on the ensemble of databases detailed above, total anthropogenic emissions were 366 [349–393]Tg CH4 yr-1 for the decade 2008-2017.
• Global methane emissions for 2000–2009 (547 Tg CH4 yr¯¹) are consistent with Saunois et al. (2016) and the range for global emissions; 524–560 Tg CH4 yr¯¹ is in line with the range in Saunois et al. (2016) (535–569), although the ensemble of inverse systems contributing to this budget is different than for Saunois et al. (2016).
• At the global scale, the total emissions inferred by the en- semble of 22 inversions is 576Tg CH4 yr−1 [550–594] for the 2008-2017 decade.
• The values reported for the upper-level anthropogenic categories (agriculture and waste, fossil fuels, and biomass burning & biofuels) are therefore consistent with the sum of their subcategories, although there might be small percentage differences between the reported total anthropogenic emissions and the sum of the three upper-level categories.
• In 2017, almost 40% (IEA, 2019b) of the world’s electricity was still produced from coal.
• 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).

Other Important Findings

• 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.
• Methane is emitted by different processes (i.e., biogenic, thermogenic, or pyrogenic) and can be of anthropogenic or natural origin.
• The main bottom-up global inventory datasets covering anthropogenic emissions from all sectors are from the United States Environmental Protection Agency (USEPA, 2012), the Greenhouse gas and Air pollutant Interactions and Synergies (GAINS) model developed by the International Institute for Applied Systems Analysis (IIASA) (Gomez Sanabria et al., 2018; Höglund-Isaksson, 2012, 2017), and the Emissions Database for Global Atmospheric Research (EDGARv3.2.2; Janssens-Maenhout et al., 2019).
• In order to report emissions for the period 2000–2017, some of the datasets were extended and interpolated. The USEPA dataset was linearly interpolated to provide yearly values. The FAO-CH4 dataset, ending in 2016, was extrapolated to 2017 using a linear fit based on 2014–2016 data. EDGARv4.3.2 was extrapolated to 2017 using the extended FAO-CH4 emissions for enteric fermentation, manure management, and rice cultivation and using the BP statistical review of fossil fuel production and consumption (BP Statistical Review of World Energy, 2019) for emissions from the coal, oil, and gas sectors.
• Changes in other chemical compounds (such as nitrogen oxides, NOx, or carbon monoxide, CO) also influence the forcing of atmospheric CH4 through changes to its atmospheric life- time.
• In this study, we use large-scale biomass burning (forest, savannah, grassland, and peat fires) from five biomass burning inventories (described below) and the biofuel burning contribution from anthropogenic emission inventories (EDGARv4.3.2, CEDS, GAINS, and USEPA).
• 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), with a lower uncertainty range than for coal emissions for most countries.
• Global emissions from agriculture and waste for the period 2008–2017 are estimated to be 206 Tg CH4 yr-1 (range 191-223.
• Atmospheric inversions rely on regional constraints from atmospheric sampling networks, which are relatively dense for northern mid-latitudes, with a number of high-precision and high-accuracy surface stations, but are sparser at tropical latitudes and in the Southern Hemisphere.
• 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.
• Methane hydrates, ice-like cages of trapped methane found in continental shelves and slopes and below sub-sea and land permafrost, can be of either biogenic or thermogenic origin.
• 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).
• Global emissions from agriculture and waste for the period 2008–2017 are estimated to be 206 Tg CH4 yr-1 (range 191-223).
• 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).
• Global emissions from agriculture and waste for the period 2008–2017 are estimated to be 206 Tg CH4 yr-1 (range 191-223).
• The main primary emission zones are consistent between models: Scandinavia, continental Europe, eastern Siberia, central United States, and tropical Africa.
• The top-down and bottom-up approaches have been highlighted as well as discrepancies between the two budgets.
• 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).
• The range associated with our estimates (~10%-12%) 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, as done here.

Limitations Noted in the Document

• The uncertainty in the chemical loss of methane by OH, the predominant sink of atmospheric methane, is estimated around 10% (Prather et al., 2012) to 15% (from bottom-up approaches in Saunois et al., 2016).
• The atmospheric constraints on the top-down budget suggest that at least some of these bottom-up emissions are overestimated.
• The fact that anthropogenic emissions result from unintentional leakage from fossil fuel production or agriculture further complicates production of accurate bottom-up emission estimates.
• The bottom-up estimates rely on global anthropogenic inventories, land surface models for wetland emissions, and published literature for other natural sources.
• For the top-down estimates, we use atmospheric inversions covering 2000–2017. The simulations run until mid-2018, but the last year of reported inversion results is 2017.
• The bottom-up models and inventories are presented for all source processes and for the five main categories defined above globally. Top-down inversions are reported globally and only for the five main emission categories.
• In summary, bottom-up models and inventories are presented for all source processes and for the five main categories defined above globally. Top-down inversions are reported globally and only for the five main emission categories.
• The main 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).
• 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).
• In the tables, we present mean values and ranges for the two decades 2000–2009 and 2008–2017, together with results for the most recent available year (2017).
• The CH4 emission estimates are provided with up to three digits, for consistency across all budget flux components and to ensure the accuracy of aggregated fluxes.
• Given the values of the uncertainties in the methane budget, we encourage the reader to consider not more than two digits as significant.
• The total (inner and outer) estuarine emission flux, which is based on only about 80 systems is thus in the range 4–5 Tg CH4 yr-1.

Conclusion

Methane emissions from natural and anthropogenic sources continue to be a major area of scientific interest, with this study offering the second version of a living review paper focused on the global methane budget. The study employs both top-down and bottom-up approaches to assess emissions, focusing on decadal budgets and updates from previous assessments. The main findings include significant emissions from both anthropogenic and natural sources, with a substantial portion of emissions attributed to anthropogenic activities. The study also highlights the limitations of the top-down and bottom-up approaches, including uncertainties and potential double-counting issues, particularly in the estimates of natural sources such as wetlands and inland water systems. The study emphasizes the need for further research to reconcile discrepancies between top-down and bottom-up estimates and to refine understanding of methane sources. The study highlights how important it is to distinguish between the different approaches to consistently compare natural and anthropogenic total emissions. While top-down methods offer constraints, they are limited by the prior emissions used, particularly in regions with sparse observations. Similarly, bottom-up estimates rely on global inventories, land surface models, and other sources, which can lead to uncertainties. The research also emphasizes that the contribution from human activities is statistically consistent with top-down and bottom-up approaches. The study underscores the need for continued monitoring of methane trends, with a focus on reducing uncertainties and improving the understanding of the various sources and sinks of methane. The study’s goal is to create a more accurate forecast of future climate, and for all of these reasons, the methane budget remains an area of active research and is important to mitigate global warming potential in the climate.

DOI

10.5194/essd-12-1561-2020