Description: Public fear for environmental and health impacts or potential leakage of CO2 from geological reservoirs is among the reasons why over the past decade CCS has not yet been deployed on a large enough scale so as to meaningfully contribute to mitigate climate change. Storage of CO2 under the seabed moves this climate mitigation option away from inhabited areas and could thereby take away some of the opposition towards this technology. Given that in the event of CO2 leakage for sub-seabed CCS the ocean would function as buffer for receiving this greenhouse gas, rather than the atmosphere, offshore CCS could particularly address concerns over the climatic impacts of CO2 seepage. In this paper we point out that recent geological studies confirm that leakage for individual offshore CCS operations may be highly unlikely from a technical point of view, if storage sites are well chosen, well managed and well monitored. But we argue that on a global long-term scale, for an ensemble of thousands or millions of storage sites, leakage of CO2 could take place in certain cases and/or countries for e.g. economic, institutional, legal or safety cultural reasons. We investigated what the impact could be in terms of temperature increase and ocean acidification if leakage would nevertheless occur, and addressed the question what the relative roles could be of on- and offshore CCS if mankind desires to divert the atmospheric damages resulting from climate change. For this purpose, we constructed a top-down energy-environment-economy model, with which we performed a probabilistic cost-benefit analysis of climate change mitigation with on- and offshore CCS as specific CO2 abatement options. One of our main conclusions is that even if there is non-zero leakage for CCS activity on a global scale, there is high probability that both onshore and offshore CCS could – on economic grounds – still account for anywhere between 20% and 80% of all future CO2 abatement efforts under a broad range of CCS cost assumptions.