Appendix 7. Anthropogenic emissions of methane
Since the beginning of the industrial revolution, the concentration of methane in the atmosphere has increased from 722 to 1803 ppb (IPCC, 2013). In the 1980s, the growth rate slowed down, and almost ceased by the end of the 1990s. The fact that the atmospheric methane concentration almost stabilized, while global livestock populations increased see Appendix 8), has been misinterpreted as lack of correlation between these two variables: this idea was proposed in a report by the International Atomic Energy Agency (IAEA), in 2008. The explanation was that the emissions of methane, which remained relatively stable at around 550 million tonnes per year for nearly three decades, were basically offset by decay (IPCC, 2013). Hence, the atmospheric concentration of methane was stabilizing. Since 2007, the atmospheric concentration of methane has however continued to increase again (IPCC, 2013).
Natural sources, mainly various types of wetlands, accounted for 35-50% of total methane emissions during 2000-2009. The remaining portion (50-
65%) came from anthropogenic sources, of which enteric fermentation of ruminants accounted for about a quarter (IPCC, 2013). Enteric fermentation
is a process in which microorganisms in the rumen of ruminant animals break down cellulose and produce methane (Lassey, 2007).
Methane emissions from cattle vary with type and amount of feed: grass result in higher emissions than protein-rich feed-stuff, such as grain, because
grass contains more cellulose (Crutzen et al., 1986). The FAO has estimated that the global livestock sector accounts for 14.5% of anthropogenic greenhouse gas emission (Gerber et al., 2013). Of the total greenhouse gas emissions from the global livestock sector, methane from enteric fermentation
of ruminants account for 39% - of which cattle account for three-quarters (Gerber et al., 2013). That methane from enteric fermentation affects the
climate has been known for a long time (Johnson & Johnson, 1995; Moss et al., 2000). Lassey (2007) showed that the increasing concentration of methane in the atmosphere can largely be attributed to the world’s increasing livestock population. Methane emissions from enteric fermentation of cattle
are at least 15 times higher than methane emissions from the global population of wild ruminants (own estimate based on IPCC, 2013, pp. 507 and
Crutzen et al. 1986).
As a greenhouse gas, methane is 34 times more powerful than carbon dioxide, measured over 100 years, including climate–carbon feedbacks (IPCC, 2013; Table 8.7).