Three decades of global methane sources and sinks

Abstract

Methane is an important greenhouse gas, responsible for about 20% of the warming induced by long-lived greenhouse gases since pre-industrial times. By reacting with hydroxyl radicals, methane reduces the oxidizing capacity of the atmosphere and generates ozone in the troposphere. Although most sources and sinks of methane have been identified, their relative contributions to atmospheric methane levels are highly uncertain. As such, the factors responsible for the observed stabilization of atmospheric methane levels in the early 2000s, and the renewed rise after 2006, remain unclear. Here, we construct decadal budgets for methane sources and sinks between 1980 and 2010, using a combination of atmospheric measurements and results from chemical transport models, ecosystem models, climate chemistry models and inventories of anthropogenic emissions. The resultant budgets suggest that data-driven approaches and ecosystem models overestimate total natural emissions. We build three contrasting emission scenarios — which differ in fossil fuel and microbial emissions — to explain the decadal variability in atmospheric methane levels detected, here and in previous studies, since 1985. Although uncertainties in emission trends do not allow definitive conclusions to be drawn, we show that the observed stabilization of methane levels between 1999 and 2006 can potentially be explained by decreasing-to-stable fossil fuel emissions, combined with stable-to-increasing microbial emissions. We show that a rise in natural wetland emissions and fossil fuel emissions probably accounts for the renewed increase in global methane levels after 2006, although the relative contribution of these two sources remains uncertain

Soil Methanotrophy Model (MeMo v1.0): a process-based model to quantify global uptake of atmospheric methane by soil

“…The total global soil sink is similar in size to global emissions of CH 4 from rice paddies (Kirschke et al, 2013), and consequently, year-to-year changes in factors that impact rates of soil CH 4 oxidation may contribute to variability in the interannual growth rate of atmospheric CH 4 . Moreover, soil methanotrophy consumes up to 90 % of CH 4 produced via methanogenesis in persistently or periodically wet soil and thus factors that impact soil uptake of atmospheric CH 4 may reduce the capacity of soil methanotrophs to attenuate emission of soil-produced CH 4 (Oremland and Culbertson, 1992;Singh et al, 2010).…”

“…There is a large interannual variability and uncertainty in the accounting of the global CH 4 budget, particularly for processes that consume CH 4 (Kirschke et al, 2013). Our understanding of the main drivers of CH 4 uptake in soils and how those factors respond to climate change is incomplete.…”

“…Anthropogenic activities during the last 200 years have increased the concentration of CH 4 in the atmosphere from pre-industrial era levels of approximately 710 parts per billion (ppb) to the current mixing ratio of approximately 1800 ppb (Etheridge et al, 1998;Kirschke et al, 2013). The atmospheric lifetime of CH 4 is 9.1 ± 0.9 years (Prather et al, 2012) and most CH 4 is consumed in the troposphere via oxidation by OH radicals, which represents ∼ 90 % of the global CH 4 sink (Prather et al, 2012;Ciais et al, 2013).…”

The global methane budget 2000–2012

“…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). .…”

Fluxes and fate of dissolved methane released at the seafloor at the landward limit of the gas hydrate stability zone offshore western Svalbard

“…Microbially mediated oxidation of methane in bottom waters near seep sites means that seawater is typically undersaturated with respect to the atmosphere [Reeburgh, 2007]. The oceans are therefore considered to make a minor contribution to the global atmospheric methane budget [Kirschke et al, 2013], with inputs from surface seawater only occurring in localized regions of surface supersaturation, for example, where methane is transported directly to the sea surface in the gas phase. However, new sites of seafloor methane seepage continue to be discovered [e.g., R€ omer et al, 2014;Skarke et al, 2014], and recent studies suggest that sea to air methane fluxes at some locations may be far higher than previously thought [e.g., Shakhova et al, 2010a].…”