Ab initio Molecular dynamics simulations have been performed to study the dissolution of CO2 in water at high pH. The CO2 + OH- -> HCO3- forward and the HCO3- -> CO2 + OH- reverse paths have been simulated by employing the metadynamics technics. We have found that the hydration related and significantly entropic in origin, whereas the backward barrier is primarily enthalpic. The main motifs in the forward mechanism are the structural diffusion of the hydroxyl ion to the first hydration sphere of CO2, its desolvation, and the C-O bond formation in concert with the CO2 bending within the hydrate cavity. In the reverse reaction, the origin of the barrier is the rupture of the strong C-O(H) bond. The present findings support the notion that the free energy barrier of the bicarbonate formation is strongly solvation related but provide also additional mechanistic details at the molecular level.