On the systematic underestimation of methane conversion factors in IPCC guidance

The Intergovernmental Panel on Climate Change (IPCC) provides guidance for calculating methane (CH4) emissions from liquid manure management. Tier 1 and Tier 2 procedures are the most straight forward methods, and are used more often by countries around the world. This paper evaluates and documents the systematic underestimation of the methane conversion factors (MCF) in the IPCC (2006) liquid manure management guidelines. Two main issues are documented in this paper that can lead to underestimation of MCF calculations. The first issue is the use of annual average temperature as an input to a non-linear function describing methane production. This was verified using the mathematical principle known as Jenson’s inequality which confirms that the MCF calculated based on annual average temperature is always an underestimate. In regions with large intra-annual temperature ranges, such as temperate climates, the underestimation can exceed 30%. A second issue is the lack of consideration for volatile solids retention time which can further impact calculations for methane emissions. Future updates to the IPCC methodology should therefore account for intra-annual temperature regime and retention time—not simply annual average temperature.