Carbon atoms move continuously among the atmosphere, ocean, land, and living organisms. The ocean plays a dominant role in this global carbon cycle, absorbing roughly a quarter of the carbon dioxide emitted by human activities each year.

At the surface, CO2 dissolves into seawater and is converted into dissolved inorganic carbon. Phytoplankton incorporate some of that carbon into organic matter through photosynthesis; a fraction sinks into the deep ocean as particles or is transported by currents. Over longer timescales, the formation and dissolution of carbonate shells and the burial of organic matter in sediments remove carbon from the active cycle.

Changes in ocean circulation, temperature, and biology alter the strength of these fluxes. The uptake of anthropogenic carbon has already lowered surface pH and will continue to do so. Understanding the ocean’s carbon cycle is therefore central to predicting both climate change and the future chemistry of seawater.

Every year the ocean absorbs billions of tons of carbon that would otherwise remain in the atmosphere.