The Global Methane Budget 2000–2017

Marielle Saunois, Ann R. Stavert, Ben Poulter, Philippe Bousquet, Joseph G. Canadell, Robert B. Jackson, Peter A. Raymond, Edward J. Dlugokencky, Sander Houweling, Prabir K. Patra, Philippe Ciais, Vivek K. Arora (+79 others)
2019 Earth System Dynamics Discussions  
<p><strong>Abstract.</strong> Understanding and quantifying the global methane (CH<sub>4</sub>) budget is important for assessing realistic pathways to mitigate climate change. Atmospheric emissions and concentrations of CH<sub>4</sub> are continuing to increase, making CH<sub>4</sub> the second most important human-influenced greenhouse gas in terms of climate forcing, after carbon dioxide (CO<sub>2</sub>). Assessing the relative importance of CH<sub>4</sub> in comparison to CO<sub>2</sub> is
more » ... omplicated by its shorter atmospheric lifetime, stronger warming potential, and atmospheric growth rate variations over the past decade, the causes of which are still debated. Two major difficulties in reducing uncertainties arise from the variety of geographically overlapping CH<sub>4</sub> sources and from the destruction of CH<sub>4</sub> by short-lived hydroxyl radicals (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 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).</p> <p> For the 2008&amp;ndash;2017 decade, global methane emissions are estimated by atmospheric inversions (top-down approach) to be 572&amp;thinsp;Tg&amp;thinsp;CH<sub>4</sub>&amp;thinsp;yr<sup>&amp;minus;1</sup> (range 538&amp;ndash;593, corresponding to the minimum and maximum estimates of the ensemble), of which 357&amp;thinsp;Tg&amp;thinsp;CH<sub>4</sub>&amp;thinsp;yr<sup>&amp;minus;1</sup> or ~&amp;thinsp;60&amp;thinsp;% are attributed to anthropogenic sources (range 50&amp;ndash;65&amp;thinsp;%). This total emission is 27&amp;thinsp;Tg&amp;thinsp;CH<sub>4</sub>&amp;thinsp;yr<sup>&amp;minus;1</sup> larger than the value estimated for the period 2000&amp;ndash;2009 and 24&amp;thinsp;Tg&amp;thinsp;CH<sub>4</sub>&amp;thinsp;yr<sup>&amp;minus;1</sup> larger than the one reported in the previous budget for the period 2003&amp;ndash;2012 (Saunois et al. 2016). Since 2012, global CH<sub>4</sub> emissions have been tracking the carbon intensive scenarios developed by the Intergovernmental Panel on Climate Change (Gidden et al., 2019). Bottom-up methods suggest larger global emissions (737&amp;thinsp;Tg&amp;thinsp;CH<sub>4</sub>&amp;thinsp;yr<sup>&amp;minus;1</sup>, range 583&amp;ndash;880) than top-down inversion methods, mostly because of larger estimated natural emissions from sources such as natural wetlands, other inland water systems, and geological sources. However the strength of the atmospheric constraints on the top-down budget, suggest that these bottom-up emissions are overestimated. The latitudinal distribution of atmospheric-based emissions indicates a predominance of tropical emissions (~&amp;thinsp;65&amp;thinsp;% of the global budget, <&amp;thinsp;30&amp;deg;&amp;thinsp;N) compared to mid (~&amp;thinsp;30&amp;thinsp;%, 30&amp;deg;&amp;thinsp;N&amp;ndash;60&amp;deg;&amp;thinsp;N) and high northern latitudes (~&amp;thinsp;4&amp;thinsp;%, 60&amp;deg;&amp;thinsp;N&amp;ndash;90&amp;deg;&amp;thinsp;N). Our analyses suggest that uncertainties associated with estimates of anthropogenic emissions are smaller than those of natural sources, with top-down inversions yielding larger uncertainties than bottom-up inventories and models. The most important source of uncertainty in the methane budget is attributable to natural emissions, especially those from wetlands and other inland waters. Some global source estimates are smaller compared to the previously published budgets (Saunois et al. 2016; Kirschke et al. 2013), particularly for vegetated wetland emissions that are lower by about 35&amp;thinsp;Tg&amp;thinsp;CH<sub>4</sub>&amp;thinsp;yr<sup>&amp;minus;1</sup> due to efforts to partition vegetated wetlands and inland waters. Emissions from geological sources are also found to be smaller by 7&amp;thinsp;Tg&amp;thinsp;CH<sub>4</sub>&amp;thinsp;yr<sup>&amp;minus;1</sup>, and wild animals by 8&amp;thinsp;Tg&amp;thinsp;CH<sub>4</sub>&amp;thinsp;yr<sup>&amp;minus;1</sup>. However the overall discrepancy between bottom-up and top-down estimates has been reduced by only 5&amp;thinsp;% compared to Saunois et al. (2016), due to a higher estimate of freshwater emissions resulting from recent research and the integration of emissions from estuaries. Priorities for improving the methane budget include: i) a global, high-resolution map of water-saturated soils and inundated areas emitting methane based on a robust classification of different types of emitting habitats; ii) further development of process-based models for inland-water emissions; iii) intensification of methane observations at local scales (e.g., FLUXNET-CH<sub>4</sub> measurements and urban monitoring to constrain bottom-up land surface models, and at regional scales (surface networks and satellites) to constrain atmospheric inversions; iv) improvements of transport models and the representation of photochemical sinks in top-down inversions, and v) development of a 3D variational inversion system using isotopic and/or co-emitted species such as ethane. </p> <p> The data presented here can be downloaded from ICOS (<a href="https://doi.org/10.18160/GCP-CH4-2019" target="_blank">https://doi.org/10.18160/GCP-CH4-2019</a>) and the Global Carbon Project.</p>
doi:10.5194/essd-2019-128 fatcat:fwprfquq65dipotvbhnv6txwu4