Evaluating Intergovernmental Panel on Climate Change Methods for Methane Emission Estimate From Lagoon-Based Swine Manure Management Systems

Abstract

To support national estimates of methane (CH₄) emissions from manure management systems, the Intergovernmental Panel on Climate Change (IPCC) proposed guidelines that include three tiers, of which Tier 1 and Tier 2 methodologies are the most used. The choice of each methodology is often driven by data availability. The application of the IPCC Tier 1 approach uses default emission factors and requires less data compared to IPCC Tier 2, which is considered a more advanced methodology. Nevertheless, no study has addressed the implications of the methodology choice on the estimation of CH₄ emissions from swine manure management systems. Subsequently, there is barely any quantitative information on the discrepancies to expect when using the IPCC Tier 1 or Tier 2 approach for estimating CH₄ emissions from significant sources such as open-air swine waste treatment lagoons. Hence, this study used the IPCC Tier 1 and Tier 2 methodologies to estimate CH₄ emissions from feeder-to-finish swine waste treatment lagoons across the North Carolina region. Precisely, updated IPCC guidelines were applied to farm, county, and state level data of feeder-to-finish swine farms to quantify CH₄ emissions from open-air anaerobic swine waste treatment lagoons. Results show that IPCC Tier 2's estimates of CH₄ emissions are higher, sustaining a discrepancy of 6.7% between the two IPCC Tiers’ methodologies. Quantitatively, the differences between the two IPCC tiers’ estimates totaled 7320 Mg CH₄/year (197640 Mg CO₂ equivalent/year) at the state level and were unequally distributed among counties and farms. Nevertheless, a comparative analysis highlighted a meaningful contrast between the IPCC Tiers estimation per swine head or unit animal weight and direct measurements of swine lagoons’ CH₄ emissions in North Carolina. The discrepancies are likely due in large part to sources of errors in the direct measurements, suggesting the need for advanced techniques for in situ CH₄ emission measurements. The study highlights the trade-off related to the emission estimation approaches, and the information reported could be useful to better understand CH₄ emission inventories.