Abstract
In moving-ship type CO2 ocean sequestration, liquid CO2 is discharged into a domain in a water column. Since the maximum CO2 concentration that is reached depends on the horizontal shape of the water column and the depths of release, it is very important to optimize these parameters for each injection site in order to minimize the biological impact. We conducted numerical experiments using an offline Oceanic General Circulation Model with a horizontal resolution of 0.1 degree × 0.1 degree. Experiments using a different horizontal site shape show that a site elongated in the meridional direction is effective to reduce the CO2 concentration. This is because CO2 has a tendency to be transported in a zonal direction. Optimization of the vertical distribution of CO2 injections is inherently determined by the balance of the following two factors; (1) dilution effect by eddy activity which decreases with depth, and the (2) predicted no effect concentration (PNEC), a criterion concentration causing no effect on biota, which increases with depth. Based on superposition of simulated CO2 concentration, we determined the optimized vertical distribution of CO2 injection which keeps the ratio of a simulated maximum CO2 concentration to PNEC constant.
Highlights
CO2 ocean sequestration has been proposed as an effective strategy to mitigate global warming [1]
In moving-ship type CO2 ocean sequestration, liquid CO2 is discharged into a domain in a water column
Since the maximum CO2 concentration that is reached depends on the horizontal shape of the water column and the depths of release, it is very important to optimize these parameters for each injection site in order to minimize the biological impact
Summary
CO2 ocean sequestration has been proposed as an effective strategy to mitigate global warming [1]. In the moving ship method [2, 3], liquid CO2 is discharged into the mid-depth ocean from a suspended conduit. CO2 is discharged within a site which is a domain in a water column within a few degrees of the horizontal. CO2-enriched seawater is advected by currents from the injection site and diffuses into the surrounding water [7,8,9]. If the CO2 concentration is decreased sufficiently by dilution, the biological impacts are expected to be small. A biological study proposed an index that identifies no effect on biota, which is called predicted no effect concentration (PNEC) [11]. The PNEC is defined as the highest CO2 concentration which causes no effects on the weakest species divided by assessment factors with various uncertainties taken into
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