CO₂ Capture and Storage

6. Could CO₂ be stored in the deep ocean?

• 6.1 What are the methods of ocean storage?
• 6.2 What are possible environmental impacts and costs of ocean storage

6.1 What are the methods of ocean storage?

The source document for this Digest states:

Ocean storage methods

A potential CO₂ storage option is to inject captured CO₂ directly into the deep ocean (at depths greater than 1,000 m), where most of it would be isolated from the atmosphere for centuries. This can be achieved by transporting CO₂ via pipelines or ships to an ocean storage site, where it is injected into the water column of the ocean or at the sea floor. The dissolved and dispersed CO₂ would subsequently become part of the global carbon cycle. Figure TS.9 shows some of the main methods that could be employed. Ocean storage has not yet been deployed or demonstrated at a pilot scale, and is still in the research phase. However, there have been small- scale field experiments and 25 years of theoretical, laboratory and modelling studies of intentional ocean storage of CO₂.

Figure TS.9. Methods of ocean storage

Storage mechanisms and technology

Oceans cover over 70% of the earth’s surface and their average depth is 3,800 m. Because carbon dioxide is soluble in water, there are natural exchanges of CO₂ between the atmosphere and waters at the ocean surface that occur until equilibrium is reached. If the atmospheric concentration of CO₂ increases, the ocean gradually takes up additional CO₂. In this way, the oceans have taken up about 500 GtCO₂ (140GtC) of the total 1,300 GtCO₂ (350 GtC) of anthropogenic emissions released to the atmosphere over the past 200 years. As a result of the increased atmospheric CO₂ concentrations from human activities relative to pre-industrial levels, the oceans are currently taking up CO₂ at a rate of about 7 GtCO₂ yr-1 (2 GtC yr-1).

Most of this carbon dioxide now resides in the upper ocean and thus far has resulted in a decrease in pH of about 0.1 at the ocean surface because of the acidic nature of CO₂ in water. To date, however, there has been virtually no change in pH in the deep ocean. Models predict that over the next several centuries the oceans will eventually take up most of the CO₂ released to the atmosphere as CO₂ is dissolved at the ocean surface and subsequently mixed with deep ocean waters.

There is no practical physical limit to the amount of anthropogenic CO₂ that could be stored in the ocean. However, on a millennial time scale, the amount stored will depend on oceanic equilibration with the atmosphere. Stabilizing atmospheric CO₂ concentrations between 350 ppmv and 1000 ppmv would imply that between 2,000 and 12,000 GtCO₂ would eventually reside in the ocean if there is no intentional CO₂ injection. This range therefore represents the upper limit for the capacity of the ocean to store CO₂ through active injection. The capacity would also be affected by environmental factors, such as a maximum allowable pH change.

Analysis of ocean observations and models both indicate that injected CO₂ will be isolated from the atmosphere for at least several hundreds of years, and that the fraction retained tends to be higher with deeper injection (see Table TS.7). Ideas for increasing the fraction retained include forming solid CO₂ hydrates and/or liquid CO₂ lakes on the sea floor, and dissolving alkaline minerals such as limestone to neutralize the acidic CO₂. Dissolving mineral carbonates, if practical, could extend the storage time scale to roughly 10,000 years, while minimizing changes in ocean pH and CO₂ partial pressure. However, large amounts of limestone and energy for materials handling would be required for this approach (roughly the same order of magnitude as the amounts per tonne of CO₂ injected that are needed for mineral carbonation; see Section 7).

Table TS.7. Fraction of CO₂ retained for ocean storage

Source & ©: IPCC  Carbon Dioxide Capture and Storage: Technical Summary (2005)
6. Ocean storage, p. 37

6.2 What are possible environmental impacts and costs of ocean storage

The source document for this Digest states:

CO₂ can be injected into the deep ocean from oil platforms.
Source: Stephen Knowles

The injection of a few GtCO₂ would produce a measurable change in ocean chemistry in the region of injection, whereas the injection of hundreds of GtCO₂ would produce larger changes in the region of injection and eventually produce measurable changes over the entire ocean volume. Model simulations that assume a release from seven locations at 3,000 m depth and ocean storage providing 10% of the mitigation effort for stabilization at 550 ppmv CO₂ projected acidity changes (pH changes) of more than 0.4 over approximately 1% of the ocean volume. By comparison, in a 550 ppmv stabilization case without ocean storage, a pH change of more than 0.25 at the ocean surface was estimated due to equilibration with the elevated CO₂ concentrations in the atmosphere. In either case, a pH change of 0.2 to 0.4 is significantly greater than pre-industrial variations in ocean acidity. Over centuries, ocean mixing will result in the loss of isolation of injected CO₂. As more CO₂ reaches the ocean surface waters, releases into the atmosphere would occur gradually from large regions of the ocean. There are no known mechanisms for sudden or catastrophic release of injected CO₂ from the ocean into the atmosphere.

Experiments show that adding CO₂ can harm marine organisms. Effects of elevated CO₂ levels have mostly been studied on time scales up to several months in individual organisms that live near the ocean surface. Observed phenomena include reduced rates of calcification, reproduction, growth, circulatory oxygen supply and mobility, as well as increased mortality over time. In some organisms these effects are seen in response to small additions of CO₂. Immediate mortality is expected close to injection points or CO₂ lakes. The chronic effects of direct CO₂ injection into the ocean on ocean organisms or ecosystems over large ocean areas and long time scales have not yet been studied.

No controlled ecosystem experiments have been performed in the deep ocean, so only a preliminary assessment of potential ecosystem effects can be given. It is expected that ecosystem consequences will increase with increasing CO₂ concentrations and decreasing pH, but the nature of such consequences is currently not understood, and no environmental criteria have as yet been identified to avoid adverse effects. At present, it is also unclear how or whether species and ecosystems would adapt to the sustained chemical changes.

Costs of ocean storage

Although there is no experience with ocean storage, some attempts have been made to estimate the costs of CO₂ storage projects that release CO₂ on the sea floor or in the deep ocean. The costs of CO₂ capture and transport to the shoreline (e.g via pipelines) are not included in the cost of ocean storage. However, the costs of offshore pipelines or ships, plus any additional energy costs, are included in the ocean storage cost. The costs of ocean storage are summarized in Table TS.8. These numbers indicate that, for short distances, the fixed pipeline option would be cheaper. For larger distances, either the moving ship or the transport by ship to a platform with subsequent injection would be more attractive.

Table TS.8. Costs for ocean storage at depths deeper than 3,000 m

Legal aspects and public perception

The global and regional treaties on the law of the sea and marine environment, such as the OSPAR and the London Convention discussed earlier in Section 5 for geological storage sites, also affect ocean storage, as they concern the ‘maritime area’. Both Conventions distinguish between the storage method employed and the purpose of storage to determine the legal status of ocean storage of CO₂. As yet, however, no decision has been made about the legal status of intentional ocean storage.

The very small number of public perception studies that have looked at the ocean storage of CO₂ indicate that there is very little public awareness or knowledge of this subject. In the few studies conducted thus far, however, the public has expressed greater reservations about ocean storage than geological storage. These studies also indicate that the perception of ocean storage changed when more information was provided; in one study this led to increased acceptance of ocean storage, while in another study it led to less acceptance. The literature also notes that ‘significant opposition’ developed around a proposed CO₂ release experiment in the Pacific Ocean.

Source & ©: IPCC (WGI)  Intergovernmental Panel on Climate Change IPCC
Carbon Dioxide Capture and Storage: Technical Summary (2005)
6. Ocean storage, p. 38