If carbon capture and storage is to be adopted as a CO2 mitigation strategy, it is important to understand the associated risks. The risk analysis consists of several elements such as leakage probability, assessing the strength of environmental perturbation, and quantifying the ecological, economic, and social impacts. Here, the environmental perturbation aspect is addressed by using a marine system model of the North West European Shelf seas to simulate the consequences of CO2 additions such as those that could arise from a failure of geological sequestration schemes. Little information exists to guide the choice of leak scenario and many assumptions are required; for consistency the assumptions err towards greater impact and what would be in likelihood extreme scenarios. The simulations indicate that only the largest leakage scenarios tested are capable of producing perturbations that are likely to have environmental consequences beyond the locality of a leak event. It is shown that, given the available evidence, the chemical perturbation of a sequestration leak, regionally integrated, is likely to be insignificant when compared with that from continued non-mitigated atmospheric CO2 emissions and the subsequent acidification of the marine system. The potential ecological impacts of a large environmental CO2 perturbation are reviewed, indicating that the biogeochemical functioning and biodiversity are sensitive. The key unknowns that must be addressed in future research are identified; namely, the fine scale dispersion of CO2 and the ability of ecological systems to recover from perturbation.