The ocean is storing much of Earth's excess heat. The resulting ocean warming contributes to problems such as rising sea level, ice-sheet melting, and coral bleaching. This Spotlight report covers these and other issues, including effects on coastal communities and difficulty predicting change in complex systems like global ocean currents. We also discuss opportunities, like restoring mangroves as barriers against storm surges and other "nature-based solutions." Policymakers could consider options such as research to improve data collection and simulation, or better intergovernmental coordination to address causes and effects of ocean warming.