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Ocean storage
When CO2 is injected into the ocean a fracture om the injected gas may be temporarily isolated from the atmosphere. Ocean storage of CO2 is not a permanent solution and implicitly assumes that the emission of the captured CO2 will be less harmful by the time it reaches the atmosphere, either through improved technologies, other long-term mitigation measures or as part of natural processes.
The retention time depends on the depth of the CO2 injection, the method of injection and local factors such as currents and water temperature. Ocean storage has not yet been deployed or tested at a pilot scale.
Injection of large volumes of CO2 would produce a measurable change in the ocean chemistry. The full environmental consequences of this are not well understood, but may cause significant damage to ocean ecosystems.
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Storage mechanisms
Ocean storage of CO2 depends on the relatively slow exchange of the water in the upper ocean and in deep ocean. Most CO2 that is dissolved at great depths will take centuries to reach the surface where and exchange with air can take place. All CO2 injected this way is still a part of the oceanic CO2 cycle, however, and after a period of hundreds to thousands of years the atmospheric concentration of CO2 will be the same as if the CO2 had been emitted directly to air.
The behavior of the CO2 after being released depends on the injection depth and the temperature of the water. CO2 injection is most effective at depths below 3000 meters, where CO2 is a liquid with greater density than water. At this depth the CO2 will sink through the water column. At less than about 2500 meters the CO2 has lower density than water and will rise through the water column, but most of the CO2 will dissolve within the first few hundred meters of migration. At above about 500 meters the CO2 is a gas. If the gas is diffused to form small enough bubbles, all the CO2 may be dissolved before reaching the surface, but the shallow storage depth will make the effective retention time very short. CO2 dissolves slower in cold water.
Liquid CO2 deposited close to a depression on the ocean floor may form a lake. If relatively uninterrupted by currents this would slow the rate of dissolving, as only the CO2 at the surface of the lake is in direct contact with water, increasing retention time.
Environmental issues
Injecting a few GtCO2 would produce a measurable change in the ocean chemistry in the region of injection, while the injection og hundreds of GtCO2 would eventually change the chemistry of the entire ocean volume. When CO2 dissolves in water this causes the water to become more acidic. Acidification of the ocean will also occur if the CO2 is released directly to the atmosphere due to equilibration of CO2 at the surface level, but ny injecting large volumes of CO2 directly into the ocean, the change in Ph will be considerably higher. By contrast, if the captured CO2 was instead stored permanently in a geological formation, there would be no acidification at all.
Dissolving CO2 in water may also cause direct harm to marine organisms. Experiments have shown that elevated levels of CO2 may cause reduced rates in calcification, reproduction, growth, circulatory oxygen supplu and mobility, as well as increased mortality over time. The long term effects of CO2 storage on on ocean ecosystems have not been studied, and the effects of large scale CO2 injection on marine life is not well understood.
References
IPCC, 2005: IPCC Special Report on Carbon Dioxide Capture and Storage
