Oceans act as a carbon sink when they absorb more carbon dioxide from the atmosphere than they release. Carbon is stored temporarily in seawater and biomass, or sequestered for longer periods in deep water and seabed sediments.
This is HL-only content in IB ESS subtopic 4.1 Water systems.
How The Mechanism Works
Atmospheric carbon dioxide dissolves into surface seawater through gas exchange. Absorption is generally greater where surface water is colder because cold water can dissolve more gas; currents can then transport this carbon-rich water into the deep ocean.
Dissolved carbon dioxide also reacts with water, forming carbonic acid and then bicarbonate and hydrogen ions:
This stores carbon in dissolved forms but increases hydrogen-ion concentration, causing ocean acidification.
| Carbon-sink mechanism | How carbon is stored |
|---|---|
| Solubility pump | Carbon dioxide dissolves in surface water and may be transferred to deep water by sinking and circulation. |
| Biological pump | Phytoplankton use dissolved carbon dioxide in photosynthesis, converting it into organic biomass. |
| Sedimentation | Dead organisms, faecal material and calcium-carbonate structures sink; some carbon becomes stored in seabed sediments. |
The Southern Ocean is an important real-world example because cold water and ocean circulation promote carbon uptake and deep-water transport.
A common misconception is that all absorbed carbon remains permanently locked away. In reality, oceans exchange carbon dioxide continuously with the atmosphere and can become a carbon source where release exceeds absorption. Warming may weaken uptake because warmer water holds less dissolved gas.
Ocean carbon uptake provides a climate-regulation ecosystem service, but acidification can damage marine organisms and fisheries, creating environmental-justice concerns for coastal communities dependent on seafood.
Exam Technique
For an explain question, give a linked sequence: atmospheric absorption, conversion or biological uptake, transport, and storage. Distinguish short-term dissolved storage from longer-term sequestration, and do not claim that ocean uptake has no environmental cost.
