From an empirical point of view, the liberalization of the internal energy market reduces carbon dioxide emissions, promoting a wider range of renewable energy sources. The aim of this paper is to examine the effect of the liberalization of the internal energy market on CO2 emissions, which was implemented in the European Union in 2011. The research data cover 27 countries of the European Union during the period 2004−2017 and was processed by estimating a two-way effects econometric model. The results suggest that the liberalization of the internal energy market is negatively related to CO2 emissions; the policy was effective in reducing CO2 emissions and, therefore, slowing down climate change. This result is significant at the level of the European Union, and in high-income countries since the year the policy was implemented, being different in the upper-middle-income countries, which begins to be effective after two years, which is due to the economic characteristics of the countries. The public policies to be implemented to reduce carbon dioxide emissions should focus on reducing the barriers imposed on foreign trade, which prevent efficient use of resources and providing financial and operating facilities to renewable energy providers in order to stimulate their production and consumption.