fonds marins

The Oceanic Carbon Pump

The ocean is a climate regulator, mainly because it captures ⅓ of the carbon dioxide (CO2) emitted into the atmosphere each year. This phenomenon of “oceanic carbon pump” is due to a double process: a physical process and a biological process.

The physical process is related to the dissolution of carbon dioxide in water. At the surface between air and water, CO2 reacts with water molecules to form bicarbonate ions. These are dissolved and then carried away by cold, dense water – the dissolution of the gas is favoured in cold, high-latitude waters more than in warm, tropical waters. Through ocean currents, most of this dissolved carbon sinks with the cold, dense water to the sea floor. And it will remain buried there for hundreds of years!

courants oceaniques
Les océans du globe sont reliés par des courants profonds (lignes bleues) et des courants de surface (rouges). Le carbone de l'atmosphère pénètre dans les profondeurs océaniques dans les zones de formation d'eau profonde dans l'Atlantique Nord et au large de la péninsule Antarctique. Lorsque les courants profonds remontent vers la surface, ils peuvent libérer le dioxyde de carbone "fossile" stocké depuis des siècles. Carte de Robert Simmon, adaptée du GIEC 2001 et de Rahmstorf 2002 © Earth Observatory NASA

The biological process is linked to the phenomenon of photosynthesis. On the surface of the oceans and from water, sunlight and CO2, the plant plankton synthesises matter by releasing oxygen (O2). It therefore absorbs CO2 and transforms it into organic matter..

But some of this plankton dies and falls to the bottom of the sea, while the other part is ingested by marine organisms in the food chain, which in turn die and fall to the deep sea. Both the dissolved carbon and the carbon absorbed during photosynthesis are therefore stored in the deep sea. This is why our oceans are said to be “great natural carbon sinks”, essential to our human life.

schema pompe carbone
Schématisation du principe de fonctionnement de la pompe à carbone biologique dans les océans. Source : Plateforme Océan Climat,

As CO2 is the main greenhouse gas, the oceans are fully playing their role as moderators of global warming: it is estimated that they have already absorbed more than 25% of man-made CO2 emissions since 1950!

However, as the waters warm and global warming increases, the dissolution of CO2 becomes more difficult. Unabsorbed CO2 stagnates in the atmosphere, which only accelerates global warming. This is why oceanographic campaigns and research are so important today: it is essential to better understand the evolution and movements of the ocean in order to better understand tomorrow’s climate phenomena.

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