Introduction
The problem is exacerbated by other human activities: Deforestation, or intensive logging, also raises CO2 concentrations in the atmosphere. Trees have the capacity to capture carbon. Due to deforestation, these carbon stores are released into the atmosphere when the wood is burned, or through the decomposition of dead trees. Intensive farming is another source of greenhouse gases, two of which are even more powerful than CO2: The extensive use of fertilisers, rice crops, and cattle farming are all sources of methane and nitrous oxide. Adding to this are man-made greenhouse gases such as fluorinated gases (CFCs or HCFCs), which are used in air conditioning. Despite their minimal atmospheric concentrations, HCFCs (Hydrochlorofluorocarbons) play an important role because their potential to enhance the greenhouse effect is two thousand times stronger than that of CO2. There is no natural equivalent to these artificial gases, and their heat trapping capacity is extremely high. They represent additional greenhouse gases which nature cannot support.
Fig.1 The Ocean naturally absorbs the CO2 from the air.
Image cover: phytoplankton seen from the sky. Crédit: NASA / GSFC / MODIS Rapid Response.
Physicochemical Mechanism
The physicochemical process behind the absorption and storage of a large portion of man-made carbon dioxide in the ocean is also called the ‘carbon sink‘ or ‘solubility pump‘.
One particular characteristic of CO2 is that it is more soluble in colder waters. As a result, atmospheric CO2 is dissolved better, faster, and in larger quantities in the glacial waters around the Earth’s poles.
The colder the water, the larger the amount of carbon that is absorbed. In the higher latitudes, the powerful marine currents plunge into the ocean’s depths, taking the CO2 with them. This is for example the case close to the polar ice sheet in the North Atlantic, an area which represents one of the largest carbon sinks on Earth.
The cold currents carry most of the absorbed carbon all the way to the end of their journey.
Some of it mixes with other parts of the ocean, and a small fraction remains locked in the ocean floor and forms the so-called carbon storage.
Biological Mechanism
The biological mechanism is based on processes triggered by microscopic algae and Cyanobacteria suspended in the water, called phytoplankton (vegetal plankton).
Just like green plants growing on soil, phytoplankton absorbs a considerable amount of CO2. Under the influence of sunlight, it synthesises carbon to create organic matter and gives off double oxygen molecules (O2). This process is known as ‘photosynthesis‘. The oxygen released by these organisms in the form of O2 is redistributed between the ocean and the atmosphere.
To illustrate the process of photosynthesis, we can draw a comparison between the absorption of CO2 by phytoplankton and the consumption of a whole olive by a human being whose body will only absorb the olive’s flesh while the stone is rejected.
During photosynthesis, phytoplankton captures CO2 (the olive containing the stone), rejects the oxygen in the form of O2 (the olive’s stone) and only retains the carbon, or C (the olive’s flesh). The surplus oxygen in the form of O2 is released into both the ocean and the atmosphere.
The ocean therefore supplies the atmosphere with oxygen in the same way as the Earth’s forests do. It is thanks to the phytoplankton that our atmosphere has developed the oxygen levels necessary for the evolution of the world as we know it.
The ocean is thus the main lung of the planet: It ‘inhales’ part of the atmospheric CO2 and ‘exhales’ the oxygen back into the air that we need to breathe.
Vegetal plankton (phytoplankton) is at the bottom of the aquatic food chain. The carbon is distributed throughout the entire marine biodiversity, from microscopic organisms to the largest species such as whales.
When these organisms die, a small part of the carbon sinks all the way to the ocean floor, where it is deposited and turns into sediment, a form of carbon storage.
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