The Ocean as a carbon sink: until when?
To mark World Ocean Day and in connection with the United Nations Ocean Conference (UNOC), the Office for Climate Education (OCE) presents a series of four articles to explore the theme “Ocean and Climate” in the classroom.
After analyzing the regulatory role of the ocean and its ability to store heat, this third article focuses on another essential function of the ocean in the climate system: its role as a carbon sink. This valuable mechanism helps mitigate climate change but also leads to harmful effects on marine ecosystems.
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The Ocean: A Natural Carbon Sink
The ocean acts as a carbon sink, capable of capturing atmospheric CO₂ through the interaction of two processes—one physical and the other biological. On the one hand, atmospheric CO₂ naturally dissolves into the ocean, and this dissolution is enhanced at low temperatures. Since cold water is denser, it sinks, carrying the dissolved CO₂ with it—this is known as the physical carbon pump. On the other hand, phytoplankton absorb CO₂ through photosynthesis—this is called the biological pump. Thanks to these two processes, the ocean sequesters about 30% of the CO₂ emitted by human activity—approximately 38 million tons of CO₂ per day. That’s 16 times more than all terrestrial soils and plants combined, and nearly 60 times more than the atmosphere.
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Ocean Acidification
The flip side of this mechanism is that CO₂ absorption by seawater causes the progressive acidification of the ocean. When CO₂ dissolves in seawater, it forms carbonic acid, which then undergoes chemical reactions that release various ions.
These reactions increase the concentration of hydrogen ions, which cause acidification, and decrease the concentration of carbonate ions. Carbonate ions are essential for the formation of mollusk shells and coral skeletons, which are made of calcium carbonate. In more acidic conditions, corals, shellfish, and mollusks struggle to grow and survive.
Since the Industrial Revolution, the ocean’s average pH has dropped from 8.15 to about 8.05. While this 0.1 unit drop may seem small, it actually represents a 30% increase in acidity. Even though the pH is still above 7, this change significantly disrupts the biological balance of marine environments.
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