We analyze the evolution across time of CO2 emissions in the European Union (EU) using advanced econometric techniques in time series analysis. We estimate the time trends along with the orders of integration of series corresponding to global CO2 emissions in EU member states using both parametric and semiparametric methods. The results show that there is a significantly negative trend only in the case of the UK, this being also a country where the trend shows mean reversion. At the other extreme, Spain, Italy, Greece and Bulgaria are some of the countries where CO2 emissions show positive trends and orders of integration that are substantially above unity. Moreover, we examine the CO2 emissions of the EU as a whole, China and the US, finding some support for mean reversion only in the second case. Therefore, there is less urgent need for policy reforms in the U.K. and somehow China than in the rest of the EU or the US.