We have investigated the effect of an enhanced biological productivity, caused by an artificial iron fertilization, in the southern ocean with a three-dimensional carbon cycle model, which is based on an ocean circulation obtained by an ocean general circulation model. The increase of the biological production did not result in a substantial increase of CO2 uptake by the ocean, as it has been proposed by Martin et al. (1990). We have found that - assuming that the basic structure of the particulate downward flux does not change when higher production occurs-the ocean circulation provides a higher return flux of remineralized products that compensates two thirds of the effect of the higher export production on the atmospheric pCO2. This finding is only slightly modified when assuming a much deeper penetration of particle flux due to the fertilization. Owing to the nonlinearity of the buffer factor the absolute value of CO2 reduction in the atmosphere is higher at high levels of pCO2. In view of the logarithmic dependency of the radiative forcing on CO2, the fertilization is more efficient at low CO2 levels. With our carbon cycle model we have, in essence, confirmed earlier three-dimensional model studies with a simplified geochemical cycling of the Princeton group (Sarmiento and Orr, 1991). ¿ American Geophysical Union 1993