4.1.8 (HL)—Oceans as carbon sink

CO₂ enters the ocean when atmospheric concentration exceeds surface-water concentration, but uptake is a changing flux, not a fixed saturated container.

Mixing and biology move carbon away from the surface and allow exchange; warming lowers gas solubility and changing chemistry can reduce the fraction absorbed. A sink can weaken while remaining a sink.

If emissions rise faster than ocean uptake, ocean carbon still increases even while the ocean absorbs less of each additional tonne.

Ocean uptake can weaken when warming lowers gas solubility or when the air–sea concentration gradient and carbonate chemistry change; a sink need not stop completely to lose capacity.

Sink does not mean unlimited uptake or zero outgassing; compare both directions and rates.