@article{iiasa13959,
          volume = {8},
          number = {2},
           title = {The Global Methane Budget: 2000-2012},
       publisher = {Copernicus Publications},
         journal = {Earth System Science Data Discussions},
             doi = {10.5194/essd-2016-25},
           pages = {697--751},
            year = {2016},
             url = {https://pure.iiasa.ac.at/id/eprint/13959/},
            issn = {1866-3591},
        abstract = {The global methane (CH4) budget is becoming an increasingly important component for managing realistic pathways to mitigate climate change. This relevance, due to a shorter atmospheric lifetime and a stronger warming potential than carbon dioxide, is challenged by the still unexplained changes of atmospheric CH4 over the past decade. Emissions and concentrations of CH4 are continuing to increase making CH4 the second most important human-induced greenhouse gas after carbon dioxide. Two major difficulties in reducing uncertainties come from the large variety of diffusive CH4 sources that overlap geographically, and from the destruction of CH4 by the very short-lived hydroxyl radical (OH). To address these difficulties, we have established a consortium of multi-disciplinary scientists under the umbrella of the Global Carbon Project to synthesize and stimulate research on the methane cycle, and producing regular ({\texttt{\char126}}biennial) updates of the global methane budget. This consortium includes atmospheric physicists and chemists, biogeochemists of surface and marine emissions, and socio-economists who study anthropogenic emissions. Following Kirschke et al. (2013), we propose here the first version of a living review paper that integrates results of top-down studies (T-D, exploiting atmospheric observations within an atmospheric inverse-modelling framework) and bottom-up models, inventories, and data-driven approaches (B-U, including process-based models for estimating land surface emissions and atmospheric chemistry, and inventories for anthropogenic emissions, data-driven extrapolations).

For the 2003-2012 decade, global methane emissions are estimated by T-D inversions at 558 Tg CH4 yr?1 (range [540-568]). About 60 \% of global emissions are anthropogenic (range [50-65 \%{{]}}). B-U approaches suggest larger global emissions (736 Tg CH4 yr?1 [596-884]) mostly because of larger natural emissions from individual sources such as inland waters, natural wetlands and geological sources. Considering the atmospheric constraints on the T-D budget, it is likely that some of the individual emissions reported by the B-U approaches are overestimated, leading to too large global emissions. Latitudinal data from T-D emissions indicate a predominance of tropical emissions ({\texttt{\char126}}64 \% of the global budget, {\ensuremath{<}} 30o N) as compared to mid ({\texttt{\char126}}32 \%, 30o N-60o N) and high northern latitudes ({\texttt{\char126}} 4 \%, 60o N-90o N). T-D inversions consistently infer lower emissions in China ({\texttt{\char126}}58 Tg CH4 yr?1 [51-72], ?14 \%) and higher emissions in Africa (86 Tg CH4 yr?1 [73-108], +19 \%) than B-U values used as prior estimates. Overall, uncertainties for anthropogenic emissions appear smaller than those from natural sources, and the uncertainties on source categories appear larger for T-D inversions than for B-U inventories and models.

The most important source of uncertainty on the methane budget is attributable to emissions from wetland and other inland waters. We show that the wetland extent could contribute for 30-40 \% on the estimated range for wetland emissions. Other priorities for improving the methane budget include: i) the development of process-based models for inland-water emissions, ii) the intensification of methane observations at local scale (flux measurements) to constrain B-U land surface models, and at regional scale (surface networks and satellites) to constrain T-D inversions, iii) improvements in the estimation of atmospheric loss by OH, and iv) improvements of the transport models integrated in T-D inversions. The data presented here can be downloaded from the Carbon Dioxide Information Analysis Center (doi:10.3334/CDIAC/Global\_Methane\_Budget\_2016).},
          author = {Saunois, M. and Bousquet, P. and Poulter, B. and Peregon, A. and Ciais, P. and Canadell, J. G. and Dlugokencky, E. J. and Etiope, G. and Bastviken, D. and Houweling, S. and Janssens-Maenhout, G. and Tubiello, F. N. and Castaldi, S. and Jackson, R. B. and Alexe, M. and Arora, V. K. and Beerling, D. J. and Bergamaschi, P. and Blake, D. R. and Brailsford, G. and Brovkin, V. and Bruhwiler, L. and Crevoisier, C. and Crill, P. and Curry, C. and Frankenberg, C. and Gedney, N. and H{\"o}glund-Isaksson, L. and Ishizawa, M. and Ito, A. and Joos, F. and Kim, H-S. and Kleinen, T. and Krummel, P. and Lamarque, J.-F. and Langenfelds, R. and Locatelli, R. and Machida, T. and Maksyutov, S. and McDonald, K. C. and Marshall, J. and Melton, J. R. and Morino, I. and O\&apos;Doherty, S. and Parmentier, F-J. W. and Patra, P. K. and Peng, C. and Peng, S. and Peters, G. P. and Pison, I. and Prigent, C. and Prinn, R. and Ramonet, M. and Riley, W. J. and Saito, M. and Schroeder, R. and Simpson, I. J. and Spahni, R. and Steele, P. and Takizawa, A. and Thornton, B. F. and Tian, H. and Tohjima, Y. and Viovy, N. and Voulgarakis, A. and van Weele, M. and van der Werf, G. and Weiss, R. and Wiedinmyer, C. and Wilton, D. and Wiltshire, A. and Worthy, D. and Wunch, D. B. and Xu, X. and Yoshida, Y. and Zhang, B. and Zhang, Z. and Zhu, Q.}
}