Questioning the Accuracy of Greenhouse Gas Accounting from Agricultural Waste: A Case Study

The New Zealand Greenhouse Gas Inventory (the NZ Inventory) uses country-specific data to quantify CH4 emissions from anaerobic ponds treating dairy farm effluent (315 Gg CO2 equivalent [CO2-e] in 2009). In this study, we used literature data to: (i) evaluate the accuracy of the NZ Inventory's parameters used to quantify these CH4 emissions; and (ii) determine whether the NZ Inventory's scope is capturing the full spectrum of sources with bio-CH4 potential entering anaerobic ponds. The research indicated that the current NZ Inventory methodology is underestimating CH4 emissions from anaerobic ponds across New Zealand by 264 to 603 Gg CO2-e annually. Moreover, the NZ Inventory is currently not accounting for (i) manure from supplementary feed pads and stand-off pads (annual CH4 emissions = 207-330 Gg CO2-e); (ii) waste milk (153-280 GgCO(2)-e); and (iii) supplementary feed waste (90-216 Gg CO2-e). Annual CH4 emissions from anaerobic ponds on dairy farms across New Zealand are thus more likely to be 1029 to 1744 Gg CO2-e, indicating that the NZ Inventory is reporting as little as 18% of actual CH4 emissions produced by this sector. These additional wastes are not accounted for in the methodology prescribed by the Intergovernmental Panel on Climate Change for estimating CH4 emissions from dairy manure. Consequently, other significant dairying nations will also probably be underestimating their waste CH4 emissions. Our research highlights that, if governments attempt to include country-specific emission factors in their greenhouse gas inventories, these factors must be based on an assessment of the full spectrum of sources contributing to greenhouse gas emissions within any given sector.