Revised methane emissions factors and spatially distributed annual carbon fluxes for global livestock

Authors: Wolf, Julie; ASRAR, GHASSEM - Pacific Northwest National Laboratory; WEST, TRISTRAM - US Department Of Energy

Submitted to: Carbon Balance and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/16/2017
Publication Date: 9/29/2017
Citation: Wolf, J.E., Asrar, G.R., West, T.O. 2017. Revised methane emissions factors and spatially distributed annual carbon fluxes for global livestock. Carbon Balance and Management. https://doi.org/10.1186/s13021-017-0084-y.
DOI: https://doi.org/10.1186/s13021-017-0084-y

Interpretive Summary: Livestock are associated with emissions of methane gas to the atmosphere. Per-animal estimates are used to quantify how much methane is emitted by livestock; these quantities allow atmospheric scientists to better understand the global dynamics of all methane sources and sinks. The most recent per-animal estimates were published in 2006, and were based on information from the 1970s-1990s. This information is outdated, suggesting that per-animal estimates are also outdated and no longer valid. Large discrepancies between top-down (using measurements made by equipment on satellites, airplanes, or tall towers) and bottom-up (using per-animal estimates) estimates of livestock methane emissions in recent years highlight the problem. In this work we: i) created new per-animal estimates of methane emissions from livestock, based on recent information; ii) used these new per-animal estimates to estimate global livestock methane emissions in recent years, and, iii) created fine-scale maps of these emissions. This work will allow better reconciliation with top-down estimates made by atmospheric scientists.

Technical Abstract: Livestock play an important role in carbon cycling through consumption of biomass and emissions of methane. Recent research suggests that existing bottom-up inventories of livestock methane emissions in the U.S., such as those made using 2006 IPCC Tier 1 livestock emissions factors, are too low. This may be due to outdated information used to develop these emissions factors. In this study, we update information for cattle and swine by region, based on reported recent changes in animal body mass, feed quality and quantity, milk productivity, and management of animals and manure. We then use this updated information to calculate new livestock methane emissions factors for enteric fermentation in cattle, and for manure management in cattle and swine. Using the new emissions factors, we estimate global livestock methane emissions of 119.1 ± 18.2 Tg in 2011; this quantity is 11% greater than that obtained using the IPCC 2006 emissions factors, encompassing an 8.4% increase in enteric fermentation methane, a 36.7% increase in manure management methane, and notable variability among regions and sources. For example, revised manure management methane emissions for 2011 in the U.S. increased by 71.8%. For years through 2013, we present: i) annual livestock methane emissions, ii) complete annual livestock carbon budgets, including carbon dioxide emissions, and, iii) spatial distributions of livestock methane and other carbon fluxes, downscaled to 0.05x0.05 degree resolution. Our revised bottom-up estimates of global livestock methane emissions are nearly identical to recently reported top-down global estimates for recent years, and account for a significant part of the increase in atmospheric methane concentrations since 2007. Differences at regional and local scales may help distinguish livestock methane emissions from those of other sectors in future top-down studies. The revised estimates allow improved reconciliation of top-down and bottom-up estimates of methane emissions, will facilitate the development and evaluation of Earth system models, and provide consistent regional and global Tier 1 estimates for environmental assessments.