Improved structure for uncertainty analysis in the Swedish Greenhouse Gas Emission Inventory

Uncertainty estimates in greenhouse gas emission inventories is an important element when prioritizing future improvements of the inventory accuracy. IPCC Good Practice Guidance recommends two methods for estimating the uncertainties, Tier 1 and Tier 2. Sweden has conducted uncertainty estimates for the inventories of submission 2004 and 2005 according to Tier 1. The emission inventory staff has however identified weaknesses in the background structure as well as lack of transparency in estimated uncertainties.

This study aims at improving the quality of the uncertainty estimates in the Swedish inventory by creating a more robust structure of input data and a clear documentation on methods and applied uncertainties. This facilitates easier replication and updating of results as well as internal and external peer reviews of uncertainty estimates. Furthermore, based on the results of the study, tables for uncertainty estimates of 1990 and 2004 for presentation in Sweden’s National Inventory 2006 have been produced. 

The uncertainty estimates for all source categories together with their rational are documented in Swedish in about thirty Expert Protocols. Most uncertainty estimates are based on expert judgements and IPCC recommendations. Very few measurement data have been available. 

The IPCC Good Practice Guidance Tier 1 method is used for calculating the uncertainty estimates for the base year 1990 and 2004, and the trend 1990-2004 for direct greenhouse gases, e.g. CO2, CH4, N2O and F-gases. The analysis is done for the sectors Energy, Industrial Processes, Solvent and Other Product Use, Agriculture and Waste. 

In order to make the analysis of uncertainties easier, the variance contribution is expressed for each source category for activity data, emission factors and emission data, respectively. 

In the underlying work, source categories have been specified on the level where independency is assumed to exist. When reporting the results in the NIR, however, uncertainties are as far as possible presented on the same aggregation level as the Key Source analysis. 

The results of the Tier 1 analysis show that the overall inventory uncertainty is estimated to be ±5.8%. The uncertainty in N2O from agricultural soils (CRF 4) alone accounts for about 67% of the total variance in the inventory. Other major uncertainty contributors are emissions of CH4 from solid waste (CRF 6A) and emissions of CO2 from chemicals (CRF 1A2c), accounting for 8.7% and 4.8% respectively of the total variance.