A greenhouse gas is a gas that has the property of absorbing infrared radiation (heat energy) emitted from Earth’s surface and reradiating it back to the Earth’s surface, thus contributing to the greenhouse effect. The more greenhouse gases in our atmosphere the more our planet warms up. There are in fact a multitude of greenhouse gases among which carbon dioxide (C02), methane (CH4) and the water vapor play a major role. Greenhouse gases have a profound effect on the energy budget of the Earth system despite making up only a fraction of all atmospheric gases. A number of processes influence their concentrations. These processes are either derived from natural (tectonic activities, vegetation, soil, wetland and ocean) or Human activities (fossil-fuel combustion). If the natural causes have ruled Earth’s climate since its creation, the Human ones are so abrupt (emission started from the industrial revolution, late eighteen century) and important that they are already disturbing our climate. Indeed, previously stable since the tenth century, concentration in C02 increased by 40% in the last two centuries, due to the Human activity.
As CO2 has caused most of the warming and its influence is expected to continue, we will further discuss how oceans contribute to decreasing the CO2 atmospheric concentrations.
The ocean, a major carbon sink
A carbon sink is a natural or artificial reservoir that absorbs and stores carbon from the atmosphere through physical and biological mechanisms. Coal, oil and natural gas are all examples of carbon sinks.
As a result of very long processes, and under certain conditions, these sinks have been able to store carbon for millennia. On the other hand, the use of these resources, qualified as fossils, reinserts the carbon they contain in the atmosphere. Today, other carbon sinks come into play: soils that accumulate humus (such as peat bogs), some growing environments (such as forests in formation) and of course some biological and physical processes that occur in the marine environment.
These latter form the so-called “ocean carbon pump”. It consists of two compartments: on the one hand a biological pump, which transfers carbon from the surface to the seabed via the food chain (if not mineralized, it is then stored in the long term in sediment) and, on the other hand, the physical pump, which derives from the ocean circulation. At the poles, the denser water flows to the depths, causing that of the dissolved atmospheric carbon they contain. It is in high latitudes, in fact, that water is more easily loaded with CO2 because the dissolution of atmospheric CO2 is facilitated at low temperature (which hence the importance of polar regions in the carbon cycle).
When we talk about carbon storage, the notion of time is essential. The biological pump responds quickly to disturbances. It can therefore be unbalanced until carbon is emitted back into the atmosphere. The physical pump acts on another time scale. Disturbances affect it harder but for longer. Indeed, it has been estimated that it takes a thousand year for a water drop to complete ocean circulation. Along its journey that water could be enriched in carbon for multiple reasons (such as remineralization of organic matter or dissolution of calcareous sediment for example) and nothing suggests that when it resurfaced it will act as a carbon sink rather than a source for the atmosphere.
The biological pump is, therefore, the easiest to measure. It is mainly based on the planktonic ecosystem in offshore environments and on seagrasses, mangrove swamp, and corals on coastal environments. Everything that is organic matter, and which falls on the seabed participates in the biological pump and, when the conditions allow it, to the formation of oil. Particular organisms know at coccolithophorides (microscopic algae) participate particularly greatly to the biological pump. These latter build a calcareous capsid composed of carbon to defend themselves against predators. When they die, they generate a net vertical flow of carbon resulting from their preservation and reducing buoyancy. This carbon can then be stored in the sediment over long geological periods and chalk cliffs are nothing more than coccoliths-based rocks.
It is difficult to determine how much carbon is stored by the ocean carbon pump mechanisms, but it is estimated that the ocean concentrates 50 times more carbon than the atmosphere and they reduced the Human-derived C02 in the atmosphere by 30%. For some scientists, the open sea and its water column would be the largest carbon sink on the planet, but its large-scale future is still unknown, especially because of its acidification, this latter reducing C02 absorption (dissolution of corals, calcareous capsid of coccolitophorides etc.).