Any reservoir that absorbs more carbon than it releases over a given period functions as a carbon sink. The ocean is the largest active carbon sink on the planet, having taken up a substantial fraction of the carbon dioxide released by fossil-fuel burning and deforestation.

Two main processes drive oceanic uptake. The solubility pump dissolves CO2 into cold surface waters that later sink into the deep ocean. The biological pump transfers carbon fixed by phytoplankton into deeper layers via sinking particles and vertical migration. Coastal wetlands and seagrass beds act as additional, more localized sinks that bury carbon in sediments.

The capacity of these sinks is not unlimited and can change with warming, circulation shifts, and ecosystem alteration. Monitoring the ocean carbon sink requires a combination of surface measurements, interior observations, and models. Accurate quantification is essential for closing the global carbon budget and for evaluating climate-mitigation strategies.

• Reservoir that absorbs more carbon than it releases
• Ocean is the largest active sink
• Driven by solubility and biological pumps
• Capacity can change with climate and human impact