Greenhouse Gas Emission Accounting. Preliminary study as input to a joint Int. IPCC Expert Meeting / CKO-CCB Workshop on Comparison of Top-down versus Bottom-up Emission Estimates.

Bottom-up data for carbon dioxide, methane and nitrous oxide from the official national inventories (National Communications) were compared with data from EDGAR (Emission Database for Global Atmospheric Research) and top-down emission estimates, based on the results of dispersion and climate models using measured concentrations of greenhouse gases in the atmosphere. The aims of this preliminary study were to investigate the possibilities of comparing different types of emission inventories, to develop a methodology for this comparison and to use the results in an analysis to identify areas for improving the IPCC methodology. The main conclusion points to the value of an international program to review and evaluate national inventories of greenhouse gases. Both bottom-up and top-down emission data were found to improve the scientific understanding of the global and regional budgets. Data exchange promotes consensus on the data among scientists and policy-makers. For CO2 , it is not possible at the moment to estimate fossil fuel emissions derived from atmospheric measurements and global carbon dioxide budget calculations with more accuracy than for emissions based on bottom-up emission inventories. For CH4, a global or even a zonal comparison of bottom-up emission inventories with top-down results of transport models is possible; furthermore, uncertainty for specific sources may have been reduced. Uncertainties for N2O emissions are so large that we may expect both top-down and bottom-up emission estimates to benefit from results of a careful comparison of these emission estimates. Comparing national inventories with EDGAR data has identified areas for future improvement in the IPCC Guidelines