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Uncertainty of global warming potential for milk production on a New Zealand farm and implications for decision making

Basset-Mens C., Kelliher F.M., Ledgard S., Cox N.. 2009. International Journal of Life Cycle Assessment, 14 (7) : p. 630-638.

DOI: 10.1007/s11367-009-0108-2

Background, aim and scope As a food exporting nation, New Zealand recognises that the Global Warming Potential (GWP) impact of agriculture has become important to food customers. Food production policy and industry analysts make GWP decisions based on greenhouse gas inventory and life cycle assessment (LCA) results. For decision making, the level of confidence associated with information is important. However, treatment of uncertainty has been problematic in LCA, especially in agricultural systems. In this paper, the GWP of 1 kg of milk was used as a case study to test the feasibility of quantifying uncertainties by Monte Carlo simulation in an LCA applied to an agriculture product. The study also contributes to the development of good practice and has implications for the incorporation of uncertainties into decision making. Materials and methods We distinguished between three sources of variation. First, there is variability amongst basic units such as dairy cattle, soils and farm characteristics which may be quantified by the standard deviation (SD). Second, there is uncertainty about true population means, which is typically provided by a sample and can be measured by the standard error of the mean (SEM). Third, choices, such as the time horizon for computing GWP, can strongly affect the LCA outcomes. The first two sources were analysed by compiling input variable statistics and undertaking Monte Carlo numerical simulations. The third source of variation was quantified by sensitivity analysis. Results Up to the farm-gate stage, the mean GWP of 1 kg of milk (computed over 100 years) was 0.96 kg CO2-eq. The associated SD was 38% of the mean when using the SD of input variables (and called "variability") and 7% when using the SEM (and called "uncertainty"). The GWP was most sensitive to uncertainty of pasture dry matter intake by grazing cattle. The second and third key input variables were the cattle excreta nitrous oxide emission factor and the enteric fermentatio

Mots-clés : production laitière; changement climatique; nouvelle-zélande

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