Bottle incubations were conducted to examine how exposure to seawater containing 8000 ppm carbon dioxide (CO2; pH 6.95) influenced the growth and reproduction of the keystone copepod Calanus finmarchicus. The chosen concentration Of CO2 is expected to occur over 100s of cubic kilometres of seawater as a result of marine CO2 storage/disposal, and is also representative of the predicted ‘worst-case’ atmospheric CO2 scenario in the year 2300. Growth (egg production and biomass loss) in adult female copepods was not affected by the simulated ocean acidification. In contrast, a maximum of only 4% of the eggs successfully yielded nauplii after 72 h in the experimental treatment. Our results demonstrate that environmental risk assessments for marine CO2 storage/disposal must look beyond adult mortality as an endpoint. Furthermore, if CO2 is to be disposed of in the deep sea, the location and timing of such activities must take into consideration the overwintering populations of C. finmarchicus.