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The Global Methane Budget 2000–2017

par Saunois, Marielle;Stavert, Ann;Poulter, Benjamin;Bousquet, Philippe PB;Canadell, Josep J.G.;Jackson, Robert B.;Raymond, Peter A.;Dlugokencky, Ed;Houweling, S.;Patra, Prabir P.K.;Ciais, Philippe;Arora, Vivek V.K.;Bastviken, David;Bergamaschi, Peter;Blake, Donald Ray;Brailsford, Gordon;Bruhwiler, L.;Carlson, Kimberly;Carrol, Mark;Castaldi, Simona;Chandra, Naveen;Crevoisier, Cyril;Crill, Patrick;Covey, Kristofer;Curry, Charles;Etiope, Giuseppe;Frankenberg, Christian;Gedney, Nicola;Hegglin, Michaela;Höglund-Isaksson, Lena;Hugelius, Gustaf;Ishizawa, Misa;Ito, Akihiko;Janssens-Maenhout, Greet;Jensen, Katherine M.;Joos, Fortunat;Kleinen, Thomas;Krummel, Paul;Langenfelds, Ray;Laruelle, Goulven Gildas ;Liu, Licheng;Machida, Toshinobu;Maksyutov, Shamil;McDonald, Kyle;McNorton, Joe;Miller, Paul A.;Melton, J.R.;Morino, Isamu;Müller, Jurek;Murguia-Flores, F.;Naik, Vaishali;Niwa, Yosuke;Noce, Sergio;O'Doherty, Simon;Parker, Robert J.;Peng, Changhui;Peng, Shushi;Peters, Glen P.;Prigent, Catherine;Prinn, Ronald;Ramonet, Michel;Regnier, Pierre ;Riley, William;Rosentreter, Judith;Segers, Arjo;Simpson, Isobel;Shi, Hao;Smith, Steven J.;Steele, Paul;Thornton, Brett F.;Tian, Hanqin;Tohjima, Yasunori;Tubiello, Francesco;Tsuruta, Aki;Viovy, Nicolas;Voulgarakis, Apostolos;Weber, Thomas S.;Van Weele, Michiel;van der Werf, Guido R.;Weiss, Ray F.;Worthy, Doug;Wunch, Debra;Yin, Yi;Yoshida, Yukio;Zhang, Wenxin;Zhang, Zhen;Zhao, Yuanhong;Zheng, Bo;Zhu, Qing;Zhu, Qiuan;Zhuang, Qianlai
Référence Earth System Science Data, 12, 3, page (1561-1623)
Publication Publié, 2020-07

Article révisé par les pairs

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Résumé :

Abstract. Understanding and quantifying the global methane (CH4) budgetis 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 interms of climate forcing, after carbon dioxide (CO2). The relativeimportance of CH4 compared to CO2 depends on its shorteratmospheric lifetime, stronger warming potential, and variations inatmospheric growth rate over the past decade, the causes of which are stilldebated. Two major challenges in reducing uncertainties in the atmosphericgrowth rate arise from the variety of geographically overlapping CH4sources and from the destruction of CH4 by short-lived hydroxylradicals (OH). To address these challenges, we have established aconsortium of multidisciplinary scientists under the umbrella of the GlobalCarbon Project to synthesize and stimulate new research aimed at improvingand regularly updating the global methane budget. Following Saunois et al. (2016), we present here the second version of the living review paperdedicated to the decadal methane budget, integrating results of top-downstudies (atmospheric observations within an atmospheric inverse-modellingframework) and bottom-up estimates (including process-based models forestimating land surface emissions and atmospheric chemistry, inventories ofanthropogenic emissions, and data-driven extrapolations). For the 2008–2017 decade, global methane emissions are estimated byatmospheric inversions (a top-down approach) to be 576 Tg CH4 yr−1 (range 550–594, corresponding to the minimum and maximumestimates of the model ensemble). Of this total, 359 Tg CH4 yr−1 or∼ 60 % is attributed to anthropogenic sources, that isemissions 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) is29 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 previousbudget for 2003–2012 (Saunois et al., 2016). Since 2012, global CH4emissions have been tracking the warmest scenarios assessed by theIntergovernmental Panel on Climate Change. Bottom-up methods suggest almost30 % larger global emissions (737 Tg CH4 yr−1, range 594–881)than top-down inversion methods. Indeed, bottom-up estimates for naturalsources such as natural wetlands, other inland water systems, and geologicalsources are higher than top-down estimates. The atmospheric constraints onthe top-down budget suggest that at least some of these bottom-up emissionsare overestimated. The latitudinal distribution of atmosphericobservation-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 methanebudget is attributable to natural emissions, especially those from wetlandsand other inland waters. Some of our global source estimates are smaller than those in previouslypublished budgets (Saunois et al., 2016; Kirschke et al., 2013). In particular wetland emissions are about 35 Tg CH4 yr−1 lower due toimproved partition wetlands and other inland waters. Emissions fromgeological 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 overalldiscrepancy between bottom-up and top-down estimates has been reduced byonly 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 methanebudget include (i) a global, high-resolution map of water-saturated soilsand inundated areas emitting methane based on a robust classification ofdifferent types of emitting habitats; (ii) further development ofprocess-based models for inland-water emissions; (iii) intensification ofmethane observations at local scales (e.g., FLUXNET-CH4 measurements)and urban-scale monitoring to constrain bottom-up land surface models, andat regional scales (surface networks and satellites) to constrainatmospheric inversions; (iv) improvements of transport models and therepresentation of photochemical sinks in top-down inversions; and (v) development of a 3D variational inversion system using isotopic and/orco-emitted species such as ethane to improve source partitioning. The data presented here can be downloaded fromhttps://doi.org/10.18160/GCP-CH4-2019 (Saunois et al., 2020) and from theGlobal Carbon Project.