The European Directive 2008/101/EC, including aviation in the European Emissions Trading Scheme (ETS), was approved on November 18, 2008, defining the main features of the system to be entered into force starting 1st of January 2012. The aim of the rule is to keep CO2 emitted by all operators landing at and taking off from European airports at the level of a predetermined base year. This base year emission level is legally defined as the average of CO2 emissions on the 2004, 2005 and 2006 years. Then, it raises the problem of calculating emissions made in the past and verifying the level of accuracy of the result. The philosophy of emissions trading is based on a cap and trade mechanism to start the granting of free permits and the trade of the rest of amounts. Therefore, a correct quantitative base is of paramount importance for the performance of the system and has great economic repercussions. This paper discusses the different alternatives to model the emissions for the base year estimation and cover the existing data gaps while keeping the calculation within a reasonable level of accuracy. Some improvements with respect to existing methodologies are suggested in order to improve the process of working with a huge data base of several million flights, as EUROCONTROL uses for emission inventory. A methodology to cover data gap and calculate their magnitude is also proposed. The last part of the paper advises on ways of action in case of changes in the geographical scope of the problem either by increasing the EU members or by building agreements with other countries with similar emissions trading mechanism
