The challenge of reconciling bottom-up agricultural methane emissions inventories with top-down measurements

Citation

Desjardins, R.L., Worth, D.E., Pattey, E., VanderZaag, A., Srinivasan, R., Mauder, M., Worthy, D., Sweeney, C., Metzger, S. (2018). The challenge of reconciling bottom-up agricultural methane emissions inventories with top-down measurements. Agricultural and Forest Meteorology, [online] 248 48-59. http://dx.doi.org/10.1016/j.agrformet.2017.09.003

Plain language summary

Canada prepares an annual estimate of greenhouse gas emissions, known as an ‘inventory’ covering all sectors of the economy, including agriculture. The agricultural inventory of greenhouse gas emissions is based on specific activities, such as growing crops and raising livestock, which causes emissions of nitrous oxide and methane, two powerful greenhouse gases. The agricultural methane inventory is calculated using animal population, and per animal rates of emissions and are typically reported by province. In this study, we compare inventory data of agricultural methane emissions for a primarily dairy-based region in eastern Ontario, Canada, with direct measurements of methane emissions made from a specially equipped aircraft. Low altitude (170 to 270 metres above ground level) aircraft-based measurements of methane emissions were made on 7 days during the spring of 2011, totalling 50 measurements of 20 kilometers each in length. These measurements show that natural (e.g. wetlands) and human (e.g. waste treatment) sources of methane that are commonly found in an agricultural landscape complicate the comparison of inventory calculations with direct measurements. However, when non-agricultural sources of emissions are minimized, agricultural inventory estimates of methane emissions are equal to aircraft measurements of methane emissions. Using the aircraft measurements, we demonstrate an approach to separate the emissions from closely occurring sources of methane. This research gives us increased confidence that inventory estimates of methane emissions from livestock production are accurate.

Abstract

Agriculture is estimated to produce more than 40% of anthropogenic methane (CH4) emissions, contributing to global climate change. Bottom-up, IPCC based methodologies are typically used to estimate the agriculture sector's contribution, but these estimates are rarely verified beyond the farm gate, due to the challenge of separating interspersed sources. We present flux measurements of CH4 using eddy covariance (EC), relaxed eddy accumulation (REA) and wavelet covariance obtained using an aircraft-based measurement platform and compare these top-down estimates with bottom-up footprint adjusted inventory estimates of CH4 emissions for an agricultural region in eastern Ontario, Canada. Top-down CH4 fluxes agree well (mean ± 1 standard error: EC = 17 ± 4 mg CH4 m−2 d−1; REA = 19 ± 3 mg CH4 m−2 d−1, wavelet covariance = 16 ± 3 mg CH4 m−2 d−1), and are not statistically different, but significantly exceed bottom-up inventory estimates of CH4 emissions based on animal husbandry (8 ± 1 mg CH4 m−2 d−1). The discrepancy between top-down and bottom-up estimates was found to be related to both increasing fractional area of wetlands in the flux footprint, and increasing surface temperature. For the case when the wetland area in the flux footprint was less than 10% fractional coverage, the top-down and bottom-up estimates were within the measurement error. This result provides the first independent verification of agricultural methane emissions inventories at the regional scale. Wavelet analysis, which provides spatially resolved fluxes, was used to attempt to separate CH4 emissions from managed and unmanaged CH4 sources. Opportunities to minimize the challenges of verifying agricultural CH4 emissions inventories using aircraft flux measuring systems are discussed.

Publication date

2018-01-15