The fertilization of the ocean with nutrients such as nitrate and/or phosphate is studied as one of the measures for solving the CO 2 problem in this paper. The desired amounts of nutrients were calculated based on the Redfield ratio of the phytoplankton in the ocean. Then, the dynamic decrease in the atmospheric CO 2 concentration by fertilization was estimated, using the unsteady state model developed by modifying the conventional two box model. The calculation results indicated that atmospheric CO 2 can be absorbed quickly, within a few years, by the fertilization. The estimation of energy requirements also indicated that this technique is feasible from the viewpoint of the energy balance.