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Ocean Uptake Potential for Carbon Dioxide Sequestration

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Ocean Uptake Potential for Carbon Dioxide Sequestration Geochemical Journal, Vol. 39, pp. 29 to 45, 2005 Ocean uptake potential for carbon dioxide sequestration MASAO SORAI* and TAKASHI OHSUMI** Research Institute of Innovative Technology for the Earth (RITE), 9-2 Kizugawadai, Kizu-cho, Soraku-gun, Kyoto 619-0292, Japan (Received June 26, 2003; Accepted May 7, 2004) For the assessment of the long-term consequences of the carbon dioxide ocean sequestration, the CO2 injection into the middle depth parts of the ocean was simulated using a geochemical box model of the global carbon cycle. The model consists of 19 reservoir boxes and includes all the essential processes in the global biogeochemical cycles, such as the ocean thermohaline circulation, the solubility pump, the biological pump, the alkalinity pump and the terrestrial ecosys- tem responses. The present study aims to reveal the effectiveness and consequences of the direct ocean CO2 sequestration P in relation to both lowering the atmospheric transient CO2 peak and reduction in future CO2 uptake potential of the ocean. We should note that the direct ocean injection of CO2 at the present time means the acceleration of the pH lowering in the middle ocean due to the eventual and inevitable increase of CO2 in the atmosphere, if the same amount of CO2 is added into the atmosphere-ocean system. The minimization of impact to the whole marine ecosystem might be attainable by the direct ocean CO2 sequestration through suppressing a decrease in the pH of the surface ocean rich in biota. The geochemical implication of the ocean sequestration is such that the maximum CO2 amount to invade into the ocean, i.e., the oceanic P CO2 uptake potential integrated with time until the end of fossil fuel era, is only dependent on the atmospheric CO2 value in the ultimate steady state, whether or not the CO2 is purposefully injected into the ocean; we gave the total potential P capacity of the ocean for the CO2 sequestration is about 1600 GtC in the case of atmospheric steady state value ( CO2 ) of 550 ppmv. P Keywords: ocean CO2 sequestration, global carbon cycle, CO2 uptake potential, box model, future atmospheric CO2 tralization effect provides a further oceanic potential to INTRODUCTION uptake excess CO2 (Nozaki, 1991). The detailed outline To mitigate the impact of the future atmospheric car- of the ocean CO2 sequestration has been presented both bon dioxide increase, the ocean CO2 sequestration has from technical and scientific aspects (Ohsumi, 1995). been proposed (Marchetti, 1977). The concept is built on The assessment of its long-term consequences requires the perspective that it would be hard to reduce drastically the comprehensive understandings on the global the future anthropogenic CO2 emissions without innova- biogeochemical cycle of the carbon. The modeling ap- tive technological developments. From a simple proach is effective in evaluation of each process govern- geochemical viewpoint, the ocean CO2 sequestration must ing the cycle. There have been so far several attempts to be one of the most reasonable options, because the ocean investigate the effects of the ocean CO2 sequestration and 2– originally contains both the carbonate ion CO3 and the its interaction with the biogeochemical processes. Hoffert – B(OH)4 ion enough to react with about 70% of the un- et al. (1979) showed clearly the “peak-shaving” effect of tapped fossil fuel carbon to form the bicarbonate ion the CO2 oceanic injection on the atmospheric CO2 con- – HCO3 and the boric acid B(OH)3 (Broecker, 2001). More- centration time profile in the future. In their work, some over, in the case of the CO2 sequestration to the bottom essential processes of the carbon cycle, such as the oce- ocean, the sedimentary calcium carbonate is expected to anic thermohaline circulation, the biological activities, – neutralize the injected CO2 to form HCO3 , and this neu- and the role of terrestrial ecosystem, were missing in the treatment. On the other hand, the EU-funded GOSAC (Global Ocean Storage of Anthropogenic Carbon) project, *Corresponding author (e-mail: [email protected]) which focused on improving the predictive capacity of *Presently at Mitsubishi Research Institute, Inc., 3-6, Otemachi 2- global-scale three-dimensional ocean carbon-cycle mod- Chome, Chiyoda-ku, Tokyo 100-8141, Japan. els, conducted a series of simulation runs where the an- **On leave from Central Research Institute of Electric Power Indus- thropogenic CO2 is injected into the several depth ranges try. of the selected points and revealed the relationship be- Copyright © 2005 by The Geochemical Society of Japan. tween the injection site and the subsequent holding ca- 29 Fig. 1. Global carbon cycle box model. The model consists of 19 reservoirs; the surface ocean is defined as 0–200 m depth, the middle part as 200–2,000 m depth, and the deep part as below 2,000 m. The fluxes between ocean boxes represent the thermohaline 6 3 –1 circulation and the numerical unit is sverdrups (1 Sv = 10 m sec ). Several key processes, such as the CO2 exchange between the atmosphere and the ocean, Fair-sea, the oceanic biological production, ONP, the terrestrial net primary production, TNP, the decomposition of the plant and soil, Fplant and FluxsoilT, are also shown by the arrows. pacity in the ocean. Although these simulation studies MODEL DESCRIPTION provided several useful implications on the ocean CO2 sequestration, the fundamental role of the ocean has not The three-dimensional box model consists of 19 boxes yet presented explicitly: that is, what is the amount of as shown in Fig. 1: atmosphere, the Arctic Ocean (sur- face and middle), the North and South Atlantic Oceans CO2 to remain in the ocean? The CO2 uptake potential in the ocean is the key to understanding the effectiveness of (surface, middle and deep for each), the Indian Ocean (surface, middle and deep), the Pacific Ocean (surface, the oceanic CO2 sequestration. To address these questions, the global analysis like the Hoffert’s study is useful, al- middle and deep) and the Antarctic Ocean (surface and though the model should be revised based on the latest middle), and the land (plant and soil). We defined the knowledge on the carbon cycle system in the surface of surface ocean as 0–200 m depth, the middle part as 200– the earth. 2,000 m depth, and the deep part as below 2,000 m. The In this study, we give an estimate of the potential ca- exceptions are the Arctic and the Antarctic oceans, where the middle part is set from 200 m depth to the seafloor. pacity of the ocean for the CO2 sequestration based on the geochemical box model of the global carbon cycle. The thermohaline circulations between the ocean boxes Our present work owed much to the Wigley’s work, in are also given in Fig. 1. The solubility pump, the biologi- which the modeling approach allowed to explain the past cal pump, and the alkalinity pump are considered as the mechanism of carbon cycling in the atmosphere-ocean changes in atmospheric CO2 concentration and to predict its future trends (Wigley, 1993). His successful method- system. The contribution of terrestrial ecosystem, which ology including the treatment of the terrestrial carbon includes both CO2 fertilization and global warming ef- cycle system was taken into account in our previous model fects, is also taken into account. (Sorai et al., 1997): it enabled us to analyze the response For an n-box system, the increase rate of carbon amount M in the box i is expressed as of the ocean CO2 sequestration qualitatively and straight- forwardly. Box model approach is efficient in analyzing the effects on the atmospheric CO concentration and the dM n n 2 QQ, 1 influences on the oceanic carbon concentrations by the dT =-ÂÂijÆ + jiÆ () j =11j = purposeful injection of CO2 into the ocean. jiπ jiπ 30 M. Sorai and T. Ohsumi where Q stands for the carbon flux from the box i to triple all the other fluxes in accordance with the mass iÆj the box j, and Q , the carbon flux from the box j to the balance of the system. As a result, the “apparent” mean jÆi box i. The first term on the right hand side of Eq. (1) is residence time was lowered to about 1,000 years in our the total carbon efflux from the box i, whereas the sec- model. This modification is justified also by the repre- ond term is the total carbon influx to the box i. In our sentation of DIC in the pre-industrial steady state: a good model, the change in carbon amount in each reservoir is agreement was obtained by using our tripled flow pattern given by Eq. (1): the variables related to the carbon as discussed later. amount include the atmospheric CO2 concentration For additional justification of our modification, we ( P ), the carbon amounts of the terrestrial plant and performed sensitivity analyses changing the magnitude CO2 soil, and the concentrations of the dissolved inorganic of oceanic circulation, which enabled us to realize the carbon (DIC) and the dissolved organic carbon (DOC), relationship between the oceanic potential and the circu- and the alkalinity in each ocean boxes. An inverse Euler lation (see Appendix II). In fact, it was found that such a method was used to avoid the solution instabilities difference of the oceanic circulation had a little influence (Walker, 1991). The detailed model description includ- on the oceanic potential. ing the balance equations for each variable had been re- ported elsewhere (Sorai et al., 1997). In this section, the Solubility pump key processes of our model are presented (also see Ap- The solubility pump is defined as the carbon exchange pendix I).
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