Oceans absorb a portion of atmospheric carbon dioxide as this gas naturally dissolves in water, through chemical reactions that form carbonic acid. This helps regulate the concentration of CO2 in the atmosphere.
When carbon dioxide (CO₂) floats above the ocean, some of the gas naturally dissolves in seawater, like sugar in cold tea. It is a simple chemical phenomenon where CO₂ moves from the atmosphere to the liquid because gases always seek to balance their concentrations between air and water. The more CO₂ there is in the air, the more the ocean absorbs to restore that balance. But be careful, this absorption increases the acidity of seawater, disrupting marine life. This dissolution is a spontaneous process, rapid at first and then slower as the ocean becomes saturated. The colder the water, the better CO₂ dissolves—this is why oceans near the poles capture this gas particularly well.
When CO₂ enters the oceans, it combines with seawater, forming a weak acid called carbonic acid. This acid quickly transforms into bicarbonate ions and carbonate ions, according to a chemical equilibrium that evolves with the conditions of the water. The majority of oceanic carbon thus exists in the form of bicarbonates, a small part as carbonates, and very little remains as pure dissolved CO₂. This equilibrium is super important because it determines the oceans' ability to absorb or release carbon dioxide. As more CO₂ enters the oceans, their water gradually becomes more acidic, which disrupts the natural carbonate balance, ultimately complicating the absorption of additional carbon.
Microscopic algae (phytoplankton) and other small marine organisms naturally capture a significant amount of CO₂ through photosynthesis. Using the sun's energy, they convert dissolved carbon dioxide in the water into oxygen and organic matter. This organic matter, when it sinks to the ocean floor after the death of these organisms, will trap carbon at the bottom for a very long time. This is known as the biological carbon pump: a super efficient mechanism to sustainably remove part of the CO₂ from the atmosphere and lock it away in the ocean depths. Not to mention corals, mollusks, and other marine creatures, which also use dissolved carbon to make their skeletons and shells out of calcium carbonate, further helping to absorb and store carbon underwater.
The wind and waves play a key role: by stirring the surface of the water, they facilitate gas exchanges between air and ocean. The CO₂ present in the air dissolves more easily in ocean waters. Then, ocean currents distribute all of this, transporting carbon to deeper waters where it can sometimes be stored for long periods, far from the atmosphere. This phenomenon of descending water masses to the depths (downwelling of cold waters) helps capture and isolate a portion of the carbon absorbed for hundreds, even thousands of years. It's somewhat like a giant and very effective pump: what is called the "physical pump" of carbon.
The efficiency with which the oceans pump carbon dioxide depends heavily on water temperature. The colder the water, the more easily it can dissolve CO₂: it's a bit like bubbles in soda; it works better at low temperatures. Wind and ocean currents also help a lot, as they constantly stir the water, allowing it to quickly absorb carbon at the surface. And let's not forget the health of plankton, which alters the oceans' ability to capture carbon through photosynthesis. In short, a cold, turbulent, and lively ocean will be much more effective at capturing CO₂ than warm, calm water.
When CO₂ dissolves in seawater, it produces carbonic acid, leading to a gradual acidification of the oceans, which poses a threat to coral reefs and many marine organisms that rely on calcium for their shells.
Phytoplankton, tiny organisms living at the surface of the oceans, play a crucial role in capturing carbon through photosynthesis, producing almost 50% of the oxygen we breathe on Earth.
Some regions of the ocean, known as 'biological carbon sinks', store carbon for extended periods through the sedimentation of marine organic matter, thereby limiting the rapid return of CO₂ to the atmosphere.
Deep ocean currents, such as thermohaline circulation, transport dissolved carbon to the depths of the oceans, thereby enabling its natural long-term storage for hundreds or even thousands of years.
No, the oceans' capacity to absorb CO₂ is limited. Chemical equilibrium and progressive saturation gradually limit absorption, thereby reducing the long-term effectiveness of this carbon sink.
Several factors are involved, including water temperature, salinity, ocean currents, weather phenomena (such as wind or storms), as well as the biological activities of marine organisms, all of which influence the oceans' absorption capacity.
Ocean acidification refers to the decrease in the pH of seawater due to the increase in dissolved carbon dioxide concentrations. This phenomenon affects marine biodiversity by disrupting ecosystems, harms the development of calcium structures in many species, and thus poses a significant threat to marine resources and associated food security.
Yes, by limiting CO₂ emissions, restoring coastal ecosystems such as mangroves and seagrasses, and promoting sustainable fishing practices, it is possible to reduce the pressure on the oceans and enhance their resilience to changes induced by acidification.
The absorption of carbon dioxide leads to ocean acidification, which can disrupt the development and growth of marine organisms, particularly those with shells or calcium carbonate skeletons such as corals, mollusks, or certain types of plankton.
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