evidence: One proposal for the mitigation of ongoing global warming is the sequestration of carbon dioxide extracted at combustion sites or directly from the air1,2. Such sequestration could help avoid a large rise in atmospheric CO2 concentration from unchecked use of fossil fuels, and hence extreme warming in the near future3,4. However, it is not clear how effective different types of sequestration and associated leakage are in the long term, and what their consequences might be. Here I present projections over 100,000 years for five scenarios of carbon sequestration and leakage with an Earth system model5. Most of the investigated scenarios result in a large, delayed warming in the atmosphere as well as oxygen depletion, acidification and elevated CO2 concentrations in the ocean. Specifically, deep-ocean carbon storage leads to extreme acidification and CO2 concentrations in the deep ocean, together with a return to the adverse conditions of a business-as-usual projection with no sequestration over several thousand years. Geological storage may be more effective in delaying the return to the conditions of a business-as-usual projection, especially for storage in offshore sediments. [...] House, K. Z., Schrag, D. P., Harvey, C. F. & Lackner, K. S. Permanent carbon dioxide storage in deep-sea sediments. Proc. Natl Acad. Sci103, 12291–12295 (2006). [...] Archer, D. & Brovkin, V. The millennial atmospheric lifetime of anthropogenic CO2 . Clim. Change90, 283–297 (2008).