The decision to sequester CO2 in the deep ocean should ultimately be based not only upon what would happen to deep sea marine biota but also upon what would happen to surface organisms if nothing were done to limit atmospheric CO2. Thus such a decision should be based on a proper understanding of long-term chronic effects, from the global-scale perturbation in near-surface ocean CO2, in addition to acute effects, from large local increases in CO2 caused by purposeful sequestration. Here we focus on the long-term chronic effects of CO2 on shallow water benthic organisms that have calcium carbonate shells. With two duplicate 6 month manipulative experiments, we demonstrate that a 200 ppm increase in CO2 adversely affects the growth of both gastropods and sea urchins. Thus even moderate increases in atmospheric CO2 that could well be reached by the middle of this century will adversely affect shallow water marine benthic organisms. This provides another reason, beyond concerns for climate, to enhance efforts to limit increases in atmospheric CO2 to the lowest possible levels.