Capacity and fractions retained
The physical capacity for storage of CO2 in the ocean is large relative to fossil-fuel resources. The degree to which this capacity will be utilized may be based on factors such as cost, equilibrium pCO2, and environmental consequences.
Storage capacity for CO2 in the ocean can be defined relative to an atmospheric CO2 stabilization concentration. For example, roughly 2,300 to 10,700 GtCO2 (above the natural pre-industrial background) would be added to the ocean in equilibrium with atmospheric CO2 stabilization concentrations, ranging from 350 ppmv to 1000 ppmv, regardless of whether the CO2 is initially released to the ocean or the atmosphere (Table 6.1, Figure 6.3; Kheshgi et al., 2005; Sorai and Ohsumi, 2005). The capacity of the ocean for CO2 storage could be increased with the addition of alkalinity to the ocean (e.g., dissolved limestone).
Effectiveness of ocean CO2 storage has been reported in a variety of ways. These different ways of reporting result in very different numerical values (Box 6.3).
Over several centuries, CO2 released to the deep ocean would be transported to the ocean surface and interact with the atmosphere. The CO2-enriched water would then exchange CO2 with the atmosphere as it approaches chemical equilibrium. In this chemical equilibrium, most of the injected CO2 remains in the ocean even though it is no longer isolated from the atmosphere (Table 6.1; Figure 6.3). CO2 that has interacted with the atmosphere is considered to be part of the natural carbon cycle, much in the way that CO2 released directly to the atmosphere is considered to be part of the natural carbon cycle. Such CO2 cannot be considered to be isolated from the atmosphere in a way that can be attributable to an ocean storage project.
Loss of isolation of injected CO2 does not mean loss of all of the injected CO2 to the atmosphere. In chemical equilibrium with an atmosphere containing 280 ppm CO2, about 85% of any carbon injected would remain the ocean. If atmospheric CO2 partial pressures were to approach 1000 ppm, about 66% of the injected CO2 would remain in the ocean after equilibration with the atmosphere (Table 6.1). Thus, roughly 1/5 to 1/3 of the CO2 injected into the ocean will eventually reside in the atmosphere, with this airborne fraction depending on the long-term atmosphere-ocean CO2 equilibrium (Kheshgi, 1995, 2004b). The airborne fraction is the appropriate measure to quantify the effect of ocean storage on atmospheric composition.
6.3.3 Estimation of fraction retained from ocean observations
Observations of radiocarbon, CFCs, and other tracers indicate the degree of isolation of the deep sea from the atmosphere (Prentice et al., 2001). Radiocarbon is absorbed by the oceans from the atmosphere and is transported to the deep-sea, undergoing radioactive decay as it ages. Radiocarbon age (Figure 6.16) is not a perfect indicator of time since a water
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