The global methane budget 2000–2012

Abstract

Abstract. The global methane (CH 4 ) 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 CH 4 over the past decade. Emissions and concentrations of CH 4 are continuing to increase, making CH 4 the second most important human-induced greenhouse gas after carbon dioxide. Two major difficulties in reducing uncertainties come from the large variety of diffusive CH 4 sources that overlap geographically, and from the destruction of CH 4 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 (∼ 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 (exploiting atmospheric observations within an atmospheric inverse-modelling framework) and bottom-up models, inventories and data-driven approaches (including process-based models for estimating land surface emissions and atmospheric chemistry, and inventories for anthropogenic emissions, data-driven extrapolations). . Top-down inversions consistently infer lower emissions in China (∼ 58 Tg CH 4 yr −1 , range 51-72, −14 %) and higher emissions in Africa (86 Tg CH 4 yr −1 , range 73-108, +19 %) than bottom-up 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 top-down inversions than for bottom-up 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 30-40 % on the estimated range for wetland emissions. Other priorities for improving the methane budget include the following: (i) the development of process-based models for inland-water emissions, (ii) the intensification of methane observations at local scale (flux measurements) to constrain bottom-up land surface models, and at regional scale (surface networks and satellites) to constrain top-down inversions, (iii) improvements in the estimation of atmospheric loss by OH, and (iv) improvements of the transport models integrated in top-down inversions.

Effects of agricultural land use on fluvial carbon dioxide, methane and nitrous oxide concentrations in a large European river, the Meuse (Belgium)

“…Here, we show that the concentrations of dissolved CO 2 , CH 4 and N 2 O in rivers -and hence, their emissions, increased with fraction of agriculture on the catchment owing to a larger delivery of DIN and organic matter. Agriculture (livestock and rice cultivation) contributes globally to about 57% of total anthropogenic CH 4 emissions (Saunois et al, 2016). Here, we show that rivers can be an additional source of CH 4 related to agricultural practise (croplands and pastures) that has not been previously accounted, although already acknowledged for N 2 O (Yu et al, 2013).…”

Tropospheric Ozone Assessment Report: Assessment of global-scale model performance for global and regional ozone distributions, variability, and trends

“…Atmospheric methane concentrations have been increasing steadily since preindustrial times, and after remaining flat for about a decade in the mid-1990s to early 2000s, their growth has resumed since 2007 (Saunois et al, 2016). Increases in methane abundance lead to increases in global background tropospheric ozone concentrations (Prather et al, 2001;Lin et al, 2017).…”

Methane distribution and oxidation around the Lena Delta in summer 2013

“…Overall, about two-thirds of methane emissions are caused by human activities; the remaining third arises from natural sources (Kirschke et al, 2013). Methane sources and sinks also vary with latitude (Saunois et al, 2016); for example, methane sources at polar latitudes include wetlands, natural gas wells and pipelines, thawing permafrost, and methane hydrate associated with decaying offshore permafrost (Nisbet et al, 2014). The top-down and bottom-up estimates of methane from these various sources also show a divergence, so more data are needed, but the measurement network that focuses on methane concentrations and isotopes is rather…”