Earth System Models generate projections of future Earth surface conditions, including climate, biogeochemical cycles and ecosystem functioning. These projections have traditionally been limited to the 21th century, while it is well known that the carbon cycle will be perturbed for millennia to come. Based on extended SSP scenarios up to the year 2500, this study presents projections for ocean carbon buffering, biological production and ecosystem functioning. Crucially, this study reveals that ocean carbon cycling (carbonate, carbon dioxide exchange and primary production) will be perturbed for centuries to come even if mitigation will be prioritized. Moreover, high emission scenarios are likely to lead to substantial declines in ocean buffer capacity and more severe and longer lasting ecosystem stress. Accordingly, emission choices at this moment are critical for ocean conditions in future centuries.