The aim of this paper is to analyse the implications of the European Commission proposal of a mixed energy cum carbon tax to curb CO2 emissions from a global perspective. The paper deals with the effects of this proposal on emissions and welfare in both the EC and the rest of the world by concentrating on three main issues: i) the effectiveness of the proposed tax measures in terms of curbing EC and global CO2 emissions; ii) the implied costs for the EC and the other countries/regions of the world; and iii) the implications of the EC proposal for the world distribution of emissions and the competitiveness of the EC economy. In this connection, the relevance of the so-called "carbon leakages" -- i.e. the displacement of polluting activities from countries participating in an emission reduction agreement to countries not concerned by the agreement -- is examined. The paper provides quantitative answers to these issues using simulations with GREEN, the global dynamic applied general ...