Facility-scale inventory of dairy methane emissions in California: implications for mitigation

Abstract

Dairies emit roughly half of total methane (CH 4 ) emissions in California, generating CH 4 from both enteric fermentation by ruminant gut microbes and anaerobic decomposition of manure. Representation of these emission processes is essential for management and mitigation of CH 4 emissions and is typically done using standardized emission factors applied at large spatial scales (e.g., state level). However, CH 4 -emitting activities and management decisions vary across facilities, and current inventories do not have sufficiently high spatial resolution to capture changes at this scale. Here, we develop a spatially explicit database of dairies in California, with information from operating permits and California-specific reports detailing herd demographics and manure management at the facility scale. We calculated manure management and enteric fermentation CH 4 emissions using two previously published bottom-up approaches and a new farm-specific calculation developed in this work. We also estimate the effect of mitigation strategies – the use of mechanical separators and installation of anaerobic digesters – on CH 4 emissions. We predict that implementation of digesters at the 106 dairies that are existing or planned in California will reduce manure CH 4 emissions from those facilities by an average of 26 % and total state CH 4 emissions by 5 % (or ∼36.5 Gg CH 4 /yr). In addition to serving as a planning tool for mitigation, this database is useful as a prior for atmospheric observation-based emissions estimates, attribution of emissions to a specific facility, and validation of CH 4 emissions reductions from management changes. Raster files of the datasets and associated metadata are available from the Oak Ridge National Laboratory Distributed Active Archive Center for Biogeochemical Dynamics (ORNL DAAC; Marklein and Hopkins, 2020; https://doi.org/10.3334/ORNLDAAC/1814).