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). 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. Emissions from
geological 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 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-CH4 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
https://doi.org/10.18160/GCP-CH4-2019 (Saunois et al., 2020) and from the
Global Carbon Project.
HESS Opinions: The complementary merits of top-down and bottom-up modelling philosophies in hydrology
