The aim of this article is to prove the connection between CO2 emission and climate change, and to estimate the CO2 emission of the transport sector in Hungary. We have to clarify the emission of the transport sector in order to get information on externalities, which is a further step toward a sustainable society. The sustainable development is a development, where the pace of technical development, the satiation of increasing supply and the raw materials and resources of Earth are poised so that the rate of living and opportunities of the next generations need not be worse. One of the most emphasized goals of the transport policy of the European Union is sustainable mobility. For this reason transportation systems must be developed and standardized, the effectiveness of transportation services must be increased, while the environmental pollution must be decreased or prevented. Decoupling motorization from environmental pollution is the task for engineers. Decoupling the increase of economical activity from mobility is the task for economists.