Ocean Sequestration of CO2
In terms of capacity, the oceans are by far the largest potential location for storage of captured CO2. The oceans already contain some 40,000 GtC of carbon, mainly as stable carbonate ions, and have a virtually unlimited capacity to absorb even more.
Natural ocean uptake of CO2 is a slow process that works over millennia to balance atmospheric and oceanic concentrations of CO2. Anthropogenic emissions of carbon have upset this balance, and there is currently an estimated net flow of 2 GtC per year from the atmosphere to ocean surface waters, which are eventually transferred to the deeper ocean. Indeed, roughly 90% of present-day emissions will eventually end up in the ocean, but we know little about the effect on marine organisms and ecosystems (Chargin and Socolow 1997).
As with natural absorption, direct injection of CO2 increases the acidity of the ocean—but at a rate that may not give marine organisms time to adapt. By applying what they deem an “acceptable” increase in average ocean water acidity, scientists have estimated the storage capacity of the ocean at roughly 1,000 to 10,000 GtC. If 100% of global carbon emissions were captured and stored in the ocean, this would imply roughly 200 to 2,000 years of emissions storage.
The cost and technical feasibility for ocean storage depend on the transport distance and the depth of injection. Shorter transport distances favor pipeline injection, and the oil and gas industry have experience with underwater pipelines up to depths of 850 meters (Adams et al.1994). Pipeline transportation and storage would incur costs of about $10/tC to $50/tC, with a base case estimate of $20/tC. Although the ocean has a huge storage capacity, the environmental effects of ocean storage are more uncertain than for geologic storage.
Five methods for the direct injection of CO2 into the ocean:
- Dry ice released at the ocean surface from a ship (Nakashiki et al., 1991).
- Liquid CO2 injected at a depth of about 1000 m from a pipe towed by a moving ship and forming a rising droplet plume (Ozaki et al., 1995).
- Liquid CO2 injected at a depth of about 1000 m from a manifold lying on the ocean bottom and forming a rising droplet plume (Liro et al., 1992).
- A dense CO2 -seawater mixture created at a depth of between 500 and 1000 m forming a sinking bottom gravity current (Haugan and Drange, 1992).
- Liquid CO2 introduced to a sea floor depression forming a stable "deep lake" at a depth of about 4000 m (Ohsumi, 1995)
The relative merits of each scenario involve issues of sequestration efficiency, cost and technical feasibility, and environmental impact