Carbon dioxide (CO2) cycle
Carbon dioxide (CO2) forms 1.6 percent of the total amount of dissolved gasses in seawater. The annual exchange of carbon dioxide between the oceans and the atmosphere is gigantic, estimated between 100 gigatons per year (a gigaton is 1000 billion kilo). There is a total of around 40,000 gigatons of carbon dioxide in the oceans. Every seven years, all of the carbon dioxide in the atmosphere (700 Gigaton) is exchanged with ocean surface water. Carbon dioxide is a life necessity for all ecosystems on earth: no plant life would be possible without CO2. The 'skeleton' for proteins, fats and sugars consists of carbons. However, carbon dioxide is also the most predominant of the greenhouse gasses. Scientists predict that the seas and oceans will turn acidic if they absorb too much CO2, with major consequences for sea life.
Absorbtion of CO2 in the North Sea
Seawater can absorb carbon dioxide in two ways. The first manner is by a physical pump: nature is always attempting to maintain an equilibrium. Because the concentration of CO2 in the air is greater than in seawater, there is a tendency for it to be absorbed into the water surface to balance out. During the winter, the surface water in cold polar seas cools off and becomes heavier (cold water is heavier than warm water). The heavier water sinks to the deep sea, carrying along the absorbed CO2.
The second manner is by fixing carbon dioxide in algea, under influence of sunlight (photosynthesis). This mechanism is important in the North Sea. Algae grow in surface water and form their cell material by absorbing CO2 out of the seawater. This makes room for more carbon dioxide to be absorbed from the air. The algae are consumed into the entire food web. This makes the water flowing out of the North Sea much richer in organisms (and carbon-holding waste products) than the inflowing blue ocean water.
Studies performed by the NIOZ have shown that the storage capacity of the North Sea is almost three times as high as the average world seas. Integration over the entire North Sea brings an annual net absorbtion of carbon dioxide of at least 8.5 million tons carbon per year.
Artificial storage in the North Sea
In order to decrease the amount of CO2 emission, studies are being made to store CO2 in depleted natural gas fields. Since 2005, Gaz de France has been experimenting with storing CO2 in gas fields in the sea bottom. Per year, 20,000 tons of gas are injected into a reservoir located in a porous sand layer 3800 meters deep. The K12B gas field is 80 kilometers off the coast of Den Helder. It is not likely that the gas will escape because there is a stable 800-meter thick layer of salt lying on top. A side effect is that more natural gas can probably be exploited in these fields, because the CO2 pushes away the gas. In the future, transportation pipes that are no longer in use in the North Sea could be used to pump CO2 to empty fields.
In 2008, the branche organization for oil and gas exploitation NOGEPA reported that there is a limit to the storage capacity: only 35% of the gas fields in the North Sea are suitable for storing CO2.
Source: de Vleet, Ecomare
