Dairy Methane Emissions in California's San Joaquin Valley Inferred With Ground‐Based Remote Sensing Observations in the Summer and Winter

Abstract

Global concentrations of atmospheric CH 4 have substantially increased since 1750 due to the expansion of human agriculture, waste management, and fossil fuel usage (Dlugokencky et al., 2011). CH 4 is a 28 times stronger greenhouse gas than carbon dioxide (CO 2 ) on a century timescale and increased atmospheric CH 4 accounts for 25% of anthropogenic climate forcing to date (Etminan et al., 2016). CH 4 has also contributed to increases in tropospheric ozone, an air pollutant and short-lived greenhouse gas (Fiore et al., 2008), and to stratospheric water vapor, which acts as a sink for stratospheric ozone (Myhre et al., 2013). Additionally, CH 4 has an atmospheric lifetime of 9.8 years, so its atmospheric abundance quickly responds to changes in its emissions (Saunois et al., 2016). Furthermore, there are proven and cost-effective ways to reduce emissions (UNEP, 2011). These characteristics make reducing CH 4 emissions an attractive policy option for mitigating the effects of climate change in the near and intermediate term.The US state of California is committed to reducing its CH 4 emissions to 40% of 2013 levels by 2030 as part of its climate change policy (CARB, 2017). The state has funded specific policies to reduce its emissions, including from its large dairy sector, which makes up 19% of the United States's milk production (CDFA, 2020). CH 4 emissions from the dairy sector are from two main sources: enteric fermentation emissions produced by dairy cows' digestion process and manure management emissions created by the anaerobic decomposition of dairy waste. Emissions from the dairy sector are estimated by the state as 880 Gg/yr, accounting for about half of California's total CH 4 emissions (CARB, 2019). The dairy industry is centered in the San Joaquin Valley (SJV), which contains 87% of the state's 1.7 million large dairy herd (CDFA, 2020). The SJV has also been identified as a large North American CH 4 emissions hotspot in many studies, highlighting the global importance of the region's dairy

Methane Emissions from Dairy Operations in California’s San Joaquin Valley Evaluated Using Airborne Flux Measurements

“…Despite the general agreement between our results, previously published summertime observational studies, 11,41 and inverse model predictions for May-June conditions, 8 considerable disagreement remains regarding the temperature dependence of dairy emissions. While we observe a substantial increase in emissions with temperature (Figure S17), in line with Arndt et al (2018) 9 and the Maasakkers et al (2016) 37 parametrization, both observational studies 11 and recent inverse models 6,12 have suggested little to no variability between wintertime and summertime emissions in this region. Higher wind speeds during cooler months have been suggested to account for the lack of seasonality in emissions.…”

“…While these inventories predict similar total emissions from dairy operations in California, 3 the spatial distributions of the published inventories vary considerably, especially in the southern SJV (Figure S18), with implications for their use as model priors in inversion-based analyses of methane emission sources. 8,12,43 Correlations between our measurements and the VISTA-CA dairy inventory (R = 0.51) are considerably stronger than those with the EPA 2016 livestock GHGI (R = 0.28) over the surveyed region (Figure 4). VISTA-CA uses detailed facilitylevel information (e.g., latitude−longitude) to spatially distribute dairy-related emissions in California, 3 and our measurements confirm that those efforts markedly improve the inventory's spatial representativeness relative to the EPA 2016 GHGI, which assigned emissions using general livestock probability maps based on the land type.…”

“…Higher wind speeds during cooler months have been suggested to account for the lack of seasonality in emissions. 11,12 During RECAP-CA, lower temperatures were not associated with considerably higher wind speeds (average of ∼3.3 m s −1 in the lowest temperature tercile and 2.8 m s −1 in the highest) and therefore, any effect of local temperature variability on methane fluxes was unlikely to be offset by simultaneous changes in local wind speeds. Assumptions regarding the seasonality/meteorological dependence of dairy methane emissions critically influence assessments of annual average inventory predictions using short-term measurements.…”

“…While the discussion above compares our measurements with annual average inventory estimates, observations suggest dairy methane emissions may vary on seasonal time scales. 9,15,37 Such a variability may be driven by fluctuations in local temperature and resulting impacts on microbial activity, 9,15 seasonal differences in wind-driven agitation of manure management lagoons, 11,12 or some combination of meteorological factors.…”

“…Estimates using inverse modeling techniques applied to both long-term , and short-term , measurements suggest that methane emissions from dairy livestock may be twice as large as the California Air Resources Board (CARB) predicts. However, recent short-term measurements at individual dairy facilities, surveys of multiple dairy facilities using mobile measurement techniques, , and short-term inverse modeling studies have supported the accuracy of CARB emission predictions.…”