The Proceedings of the National Academy of Sciences published in Nov. 2013 a comprehensive and quantitative analysis of anthropogenic (i.e., human-made) methane emissions in the United States (Miller et al., 2013). The analysis used measurements of regional atmospheric methane concentrations to estimate and characterize total methane emissions for the U.S. One of the conclusions of the study, directly related to animal agriculture, was that “…emissions due to ruminants and manure are up to twice the magnitude of existing [i.e., US EPA] inventories,” This conclusion created a justifiable concern among animal scientists and deserves a closer look.
We contacted the authors and in a series of conversations concluded that their complex and important analysis had one significant drawback – the method used to differentiate methane emissions from “ruminants” (the term used by the authors also includes manure emissions and emissions from non-ruminant farm animals and wild ruminants) from other anthropogenic sources. In a nutshell, the study assumed that methane emissions over states with large animal industries, but fewer other sources of methane, are emissions from ruminants. There are apparently large uncertainties in this kind of approach. By the authors’ own words “The uncertainties on the sector-based [methane] budget estimates are large…” and as a result, “…the atmospheric methane…estimates by source sector [for example, ruminants vs. other methane sources] often have larger confidence intervals [i.e., low reliability of the prediction]”.
We evaluated the validity of the conclusions regarding methane emissions from ruminants by Miller et al. by utilizing a relatively simple “bottom-up” method based on current livestock inventories and enteric or manure methane emission factors (see Hristov et al., 2014). Our approach took into consideration the number of animals in each of several cattle categories (beef cows, dry and lactating dairy cows, beef and dairy heifers, steers and heifers on feed, bulls, and calves) and feed consumption and methane production rates per unit of dry matter intake for each animal category.