Climate change dramatically reshapes ocean conditions, as evidenced by ocean warming that extends from the surface to the subsurface across all ocean basins. Key climate impact drivers include rising sea-level, marine heatwaves and changing weather patterns. Marine heatwaves (MHWs) are becoming more frequent, intense, and long-lasting across European seas. They are only partially predictable and highly disruptive tomarine ecosystems, coastal communities, and ocean-based economies, requiring urgent integration into EU climate adaptation and marine policy frameworks.Scientifically, MHWs remain insufficiently understood and monitored, particularly when it comes to their subsurface manifestation and evolution.Strengthening scientific understanding, monitoring capacity, and early warning systems for MHWs will therefore be essential to support resilient marine management and informed climate policy across Europe. Read our policy recommendations in the document below. DisclaimerObsSea4Clim is funded by the European Union. Views and opinions expressed are, however, those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Executive Agency (REA). Neither the European Union nor the granting authority can be held responsible for them. About ObsSea4Climhttps://obssea4clim.eu/https://cordis.europa.eu/project/id/101136548