Abstract Carbon capture and storage looks to be the most promising option to stem the rise in atmospheric CO 2 concentration and its consequences including global warming. Consideration of the various options available suggests that geological storage will probably dominate future carbon-storage efforts. The safest geological storage host is the formation of carbonate minerals. Carbonate mineral formation is favoured by the presence of reactive divalent-metal silicate minerals, in particular basalts and ultra-mafic rocks. In the absence of such rocks the most stable form of injected CO 2 is likely to be via solubility trapping, which involves its dissolution into formation waters. The quantity of CO 2 that can be stored via solubility depends on the composition of the formation waters and is greatly enhanced by increasing the pH and the formation of aqueous carbonate complexes.