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300 400 500 600 700 800 900 1000 equilibrium pC02, Ma,m

Figure 6.3 Equilibrium partitioning of CO2 between the ocean and atmosphere. On the time scale of millennia, complete mixing of the oceans leads to a partitioning of cumulative CO2 emissions between the oceans and atmosphere with the bulk of emissions eventually residing in the oceans as dissolved inorganic carbon. The ocean partition depends nonlinearly on CO2 concentration according to carbonate chemical equilibrium (Box 6.1) and has limited sensitivity to changes in surface water temperature (shown by the grey area for a range of climate sensitivity of 1.5 to 4.5°C for CO2 doubling) (adapted from Kheshgi et al., 2005; Kheshgi, 2004a). ApH evaluated from pCO2 of 275 ppm. This calculation is relevant on the time scale of several centuries, and does not consider changes in ocean alkalinity that increase ocean CO2 uptake over several millennia (Archer et al., 1997).

300 400 500 600 700 800 900 1000 equilibrium pC02, Ma,m

Figure 6.3 Equilibrium partitioning of CO2 between the ocean and atmosphere. On the time scale of millennia, complete mixing of the oceans leads to a partitioning of cumulative CO2 emissions between the oceans and atmosphere with the bulk of emissions eventually residing in the oceans as dissolved inorganic carbon. The ocean partition depends nonlinearly on CO2 concentration according to carbonate chemical equilibrium (Box 6.1) and has limited sensitivity to changes in surface water temperature (shown by the grey area for a range of climate sensitivity of 1.5 to 4.5°C for CO2 doubling) (adapted from Kheshgi et al., 2005; Kheshgi, 2004a). ApH evaluated from pCO2 of 275 ppm. This calculation is relevant on the time scale of several centuries, and does not consider changes in ocean alkalinity that increase ocean CO2 uptake over several millennia (Archer et al., 1997).

There has been limited experience with handling CO2 in the deep sea that could form a basis for the development of ocean CO2 storage technologies. Before they could be deployed, such technologies would require further development and field testing. Associated with the limited level of development, estimates of the costs of ocean CO2 storage technologies are at a primitive state, however, the costs of the actual dispersal technologies are expected to be low in comparison to the costs of CO2 capture and transport to the deep sea (but still non-negligible; Section 6.9). Proximity to the deep sea is a factor, as the deep oceans are remote to many sources of CO2 (Section 6.4). Ocean storage would require CO2 transport by ship or deep-sea pipelines. Pipelines and drilling platforms, especially in oil and gas applications, are reaching ever-greater depths, yet not on the scale or to the depth relevant for ocean CO2 storage (Chapter 4). No insurmountable technical barrier to storage of CO2 in the oceans is apparent.

Putting CO2 directly into the deep ocean means that the chemical environment of the deep ocean would be altered immediately, and in concepts where release is from a point, change in ocean chemistry would be greater proximate to the release location. Given only rudimentary understanding of deep-sea ecosystems, only a limited and preliminary assessment of potential ecosystem effects can be given (Section 6.7).

Technologies exist to monitor deep-sea activities (Section 6.6). Practices for monitoring and verification of ocean storage would depend on which, as of yet undeveloped, ocean storage technology would potentially be deployed, and on environmental impacts to be avoided.

More carbon dioxide could be stored in the ocean with less of an effect on atmospheric CO2 and fewer adverse effects on the marine environment if the alkalinity of the ocean could be increased, perhaps by dissolving carbonate minerals in sea water. Proposals based on this concept are discussed primarily in Section 6.2.

For ocean storage of CO2, issues remain regarding environmental consequences, public acceptance, implications of existing laws, safeguards and practices that would need to be developed, and gaps in our understanding of ocean CO2 storage (Sections 6.7, 6.8, and 6.10).

6.1.2 Relevant background in physical and chemical oceanography

The oceans, atmosphere, and plants and soils are the primary components of the global carbon cycle and actively exchange carbon (Prentice et al., 2001). The oceans cover 71% of the Earth's surface with an average depth of 3,800 m and contain roughly 50 times the quantity of carbon currently contained in the atmosphere and roughly 20 times the quantity of carbon currently contained in plants and soils. The ocean contains so much CO2 because of its large volume and because CO2 dissolves in sea water to form various ionic species (Box 6.1).

The increase in atmospheric CO2 over the past few centuries has been driving CO2 from the atmosphere into the oceans. The oceans serve as an important sink of CO2 emitted to the atmosphere taking up on average about 7 GtCO2 yr-1 (2 GtC yr-1) over the 20 years from 1980 to 2000 with ocean uptake over the past 200 years estimated to be > 500 GtCO2 (135 GtC) (Prentice et al, 2001; Sabine et al., 2004). On average, the anthropogenic CO2 signal is detectable to about 1000 m depth; its near absence in the deep ocean is due to the slow exchange between ocean surface and deep -sea waters.

Ocean uptake of anthropogenic CO2 has led to a perturbation of the chemical environment primarily in ocean surface waters. Increasing ocean CO2 concentration leads to decreasing carbonate ion concentration and increasing hydrogen ion activity (Box 6.1). The increase in atmospheric CO2 from about 280 ppm in 1800 to 380 ppm in 2004 has caused an average decrease across the surface of the oceans of about 0.1 pH units (ApH ~ -0.1) from an initial average surface ocean pH of about 8.2. Further increase in atmospheric CO2 will result in a further change in the chemistry of ocean surface waters that will eventually reach the deep ocean (Figure 6.4). The anthropogenic perturbation of ocean chemistry is greatest in the upper ocean where biological activity is high.

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