Livestock methane emission: From the individual grazing animal through national inventories to the global methane cycle

Methane is a potent greenhouse gas whose atmospheric abundance has grown 2.5-fold over three centuries, due in large part to agricultural expansion. The farming of ruminant livestock, which generate and emit methane during digestion (‘enteric fermentation’), is a leading contributor to this growth. This paper overviews the measurement or estimation of enteric methane emissions at a range of spatial scales. Measurement of individual animal emissions focuses particularly on grazing livestock for which the SF6 tracer technique is uniquely appropriate. Gaining insight into factors that influence methane production requires that feed intake and feed properties be determined, enabling the methane emitted to be expressed per unit of intake. The latter expression is commonly encapsulated in the ‘methane conversion factor’, Ym, an entity that enables small-scale methane emission estimates to be extrapolated to national and global enteric methane inventories. The principles of this extrapolation and sources of uncertainty are discussed, along with the significance of this global source within the global methane cycle. Micrometeorological and similar measurement techniques over intermediate spatial scales are also surveyed.