Despite the growing number of in situ iron fertilization experiments, the efficiency of such fertilization to sequester atmospheric CO2 remains largely unknown. For the first time, a global ocean biogeochemical model has been evaluated against those experiments and then used to estimate the effect of a long-term and large-scale iron addition on atmospheric CO2. The model reproduces the observed timing and amplitude in chlorophyll, the shift in ecosystem composition, and the pCO2 drawdown; it also proves to be of utility in interpreting the observations. However, a full ocean fertilization during 100 years results in a 33 μatm decrease in atmospheric CO2, that is 2 to 3 times smaller than found previously.