Analysis of uncertainties in the estimates of nitrous oxide and methane emissions in the UK’s greenhouse gas inventory for agriculture

The UK’s greenhouse gas inventory for agriculture uses a model based on the IPCC Tier 1 and Tier 2 26 methods to estimate the emissions of methane and nitrous oxide from agriculture. The inventory 27 calculations are disaggregated at country level (England, Wales, Scotland and Northern Ireland). 28 Before now, no detailed assessment of the uncertainties in the estimates of emissions had been 29 done. We used Monte Carlo simulation to do such an analysis. We collated information on the 30 uncertainties of each of the model inputs. The uncertainties propagate through the model and result 31 in uncertainties in the estimated emissions. Using a sensitivity analysis, we found that in England and 32 Scotland the uncertainty in the emission factor for emissions from N inputs (EF1) affected 33 uncertainty the most, but that in Wales and Northern Ireland, the emission factor for N leaching and 34 runoff (EF5) had greater influence. We showed that if the uncertainty in any one of these emission 35 factors is reduced by 50%, the uncertainty in emissions of nitrous oxide reduces by 10%. The 36 uncertainty in the estimate for the emissions of methane emission factors for enteric fermentation 37 in cows and sheep most affected the uncertainty in methane emissions. When inventories are 38 disaggregated (as that for the UK is) correlation between separate instances of each emission factor 39 will affect the uncertainty in emissions. As more countries move towards inventory models with 40 disaggregation, it is important that the IPCC give firm guidance on this topic.