According to the generally accepted picture of CO2 dissolution in water, the formation of H2CO3 proceeds in a single step that involves the attack of a water oxygen on the CO2 carbon in concert with a proton transfer to a CO2 oxygen. In the present work, a series of ab initio molecular dynamics simulations have been carried out along with the metadynamics technique which reveals a stepwise mechanism: the reaction of a water molecule with CO2 yields HCO3- as an intermediate and a hydronium ion, whereas the protonation of the CO2 moiety occurs in a separate step representing a well-defined activation barrier toward the H2CO3 molecule. This alternative scenario was already taken into consideration decades ago, but subsequent experiments and calculations have given preference to the concerted mechanism. Employing extended periodic models of the CO2 water system that mimic the bulk aqueous environment, the present simulations yield the complete free energy profile of the stepwise mechanism and provide a detailed microscopic mechanism of the elementary steps. HCO3- formation is found to be the rate-determining step of the entire CO2 hydration process.