Abstract
Abstract. Dairies emit roughly half of total methane (CH4) emissions in California, generating CH4 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 CH4 emissions and is typically done using standardized emission factors applied at large spatial scales (e.g., state level). However, CH4-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 CH4 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 CH4 emissions. We predict that implementation of digesters at the 106 dairies that are existing or planned in California will reduce manure CH4 emissions from those facilities by an average of 26 % and total state CH4 emissions by 5 % (or ∼36.5 Gg CH4/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 CH4 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).
Highlights
Methane (CH4) is a greenhouse gas with a large influence on the rate of short-term warming due to its high global warming potential, roughly 85 times that of CO2 in a 20-year time frame (Dlugokencky et al, 2011)
Dairies provide a major opportunity for CH4 reduction, as roughly half of state-total CH4 emissions come from nearly equal contributions of enteric fermentation by ruminant gut microbes and anaerobic decomposition of dairy manure (Charrier, 2016)
The 2017 United States Department of Agriculture (USDA) Dairy Census reports the number of milk cows in California to be 1 750 329
Summary
Methane (CH4) is a greenhouse gas with a large influence on the rate of short-term warming due to its high global warming potential, roughly 85 times that of CO2 in a 20-year time frame (Dlugokencky et al, 2011). Climate mitigation policy in California targets a reduction in CH4 emissions by 40 % below 2013 inventory levels by 2030 (State of California, 2016). Dairies provide a major opportunity for CH4 reduction, as roughly half of state-total CH4 emissions come from nearly equal contributions of enteric fermentation by ruminant gut microbes and anaerobic decomposition of dairy manure (Charrier, 2016). Facility-level measurements of both the magnitude of total emissions and relative contributions of enteric fermentation versus manure management are only available for a few dairies in the state Marklein et al.: Facility-scale inventory of dairy methane emissions in California et al, 2018). Uncertainty in CH4 emissions from the dairy industry in California and globally makes it difficult to optimize mitigation actions at the spatial scales relevant to policy and to establish an emissions baseline against which mitigation efforts can be measured
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