With the increasing urgency to limit global temperature rise to below 2ºC by the end of this century, we cannot rely on decreasing emissions but will have to develop, test and upscale methods for active carbon dioxide removal from the atmosphere. The United Nations Ocean Decade aims to use and increase the ocean’s potential for CO2 uptake using natural or eco-engineering approaches. Those approaches include, but are not limited to, ocean negative carbon emissions (ONCE) that constitute land-ocean boundaries and open marine ecosystems via the integrations of diverse carbon pumps (biological carbon pump, microbial carbon pump, solubility pump and carbonate counter pump). A better understanding of the mechanisms underpinning these pumps would help guide eco-engineering approaches that hold promises for carbon sequestration in sea-farming fields, by ocean alkalinity enhancement, or through other innovative practices. To ensure transparency among the ocean research community, governmental officials and stakeholders, it is imperative to responsibly consider the potential outcomes from these eco-engineering designs. This will also call for a code of conduct that can be continuously updated to ensure prudent implementation of emerging ocean negative carbon emission technologies, aligning with the acceptability and feasibility standards of the research community and the public. Through this Union Symposium, we invite the European Geoscience Community to support, engage with, or supervise the global goal to facilitate ocean carbon negative emissions, striving for a sustainable ocean future beyond the UN Ocean Decade.