Abstract
Understanding and quantifying the global methane (<span classCombining double low line"inline-formula">CH4</span>) budget is important for assessing realistic pathways to mitigate climate change. Atmospheric emissions and concentrations of <span classCombining double low line"inline-formula">CH4</span> continue to increase, making <span classCombining double low line"inline-formula">CH4</span> the second most important human-influenced greenhouse gas in terms of climate forcing, after carbon dioxide (<span classCombining double low line"inline-formula">CO2</span>). The relative importance of <span classCombining double low line"inline-formula">CH4</span> compared to <span classCombining double low line"inline-formula">CO2</span> 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 <span classCombining double low line"inline-formula">CH4</span> sources and from the destruction of <span classCombining double low line"inline-formula">CH4</span> 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).
<span idCombining double low line"page1564"/>For the 2008-2017 decade, global methane emissions are estimated by atmospheric inversions (a top-down approach) to be 576 Tg <span classCombining double low line"inline-formula">CH4</span> yr<span classCombining double low line"inline-formula">-1</span> (range 550-594, corresponding to the minimum and maximum estimates of the model ensemble). Of this total, 359 Tg <span classCombining double low line"inline-formula">CH4</span> yr<span classCombining double low line"inline-formula">-1</span> or <span classCombining double low line"inline-formula">ĝ1/4</span> 60 % is attributed to anthropogenic sources, that is emissions caused by direct human activity (i.e. anthropogenic emissions; range 336-376 Tg <span classCombining double low line"inline-formula">CH4</span> yr<span classCombining double low line"inline-formula">-1</span> or 50 %-65 %). The mean annual total emission for the new decade (2008-2017) is 29 Tg <span classCombining double low line"inline-formula">CH4</span> yr<span classCombining double low line"inline-formula">-1</span> larger than our estimate for the previous decade (2000-2009), and 24 Tg <span classCombining double low line"inline-formula">CH4</span> yr<span classCombining double low line"inline-formula">-1</span> larger than the one reported in the previous budget for 2003-2012 (Saunois et al., 2016). Since 2012, global <span classCombining double low line"inline-formula">CH4</span> 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 <span classCombining double low line"inline-formula">CH4</span> yr<span classCombining double low line"inline-formula">-1</span>, 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 (<span classCombining double low line"inline-formula">ĝ1/4</span> 65 % of the global budget, <span classCombining double low line"inline-formula"><</span> 30<span classCombining double low line"inline-formula">ĝ </span> N) compared to mid-latitudes (<span classCombining double low line"inline-formula">ĝ1/4</span> 30 %, 30-60<span classCombining double low line"inline-formula">ĝ </span> N) and high northern latitudes (<span classCombining double low line"inline-formula">ĝ1/4</span> 4 %, 60-90<span classCombining double low line"inline-formula">ĝ </span> N). 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 <span classCombining double low line"inline-formula">CH4</span> yr<span classCombining double low line"inline-formula">-1</span> lower due to improved partition wetlands and other inland waters. Emissions from geological sources and wild animals are also found to be smaller by 7 Tg <span classCombining double low line"inline-formula">CH4</span> yr<span classCombining double low line"inline-formula">-1</span> by 8 Tg <span classCombining double low line"inline-formula">CH4</span> yr<span classCombining double low line"inline-formula">-1</span>, respectively. However, the overall discrepancy between bottom-up and top-down estimates has been reduced by only 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 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-<span classCombining double low line"inline-formula">CH4</span> measurements) and urban-scale 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 to improve source partitioning.
The data presented here can be downloaded from <a hrefCombining double low line"https://doi.org/10.18160/GCP-CH4-2019">https://doi.org/10.18160/GCP-CH4-2019</a> (Saunois et al., 2020) and from the Global Carbon Project.
| Original language | English (US) |
|---|---|
| Pages (from-to) | 1561-1623 |
| Number of pages | 63 |
| Journal | Earth System Science Data |
| Volume | 12 |
| Issue number | 3 |
| DOIs | |
| State | Published - Jul 15 2020 |
| Externally published | Yes |
Bibliographical note
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