Department of Environmental and Ocean Engineering

University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 133-5868, Japan

Abstract. In order to answer the question addressed in the title of this article, we have been elucidating the dilution process of CO₂ near releasing points in the deep ocean from the fluid-dynamical point of view, even though we know that it is very difficult to answer this tough question at present. A numerical simulation method of two-phase flow has been developed for analysing CO₂ droplet plume at the middle depth. The results indicate the entrainment of surrounding seawater, plume dynamics and the behaviour of CO₂-rich and, eventually, dense seawater. The near-future plans of this study include the computation of the impacts of dissolved CO₂ on marine lives by incorporating the so-called MIT mortality curve based on pH and exposure time. Even at that stage, we will be still far from the position to judge whether we can make the biological impacts of CO₂ negligible. However, we believe that we are progressing in the current interdisciplinary project and will come closer to the answer than we are now when we launch the next phase of the research in a couple of years time.

1. Introduction

As is well known, the ocean is chemically able to dissolve 1800Gt carbon, while the CO₂ originated from man-utilised fossil fuel in the world is about 6 GtC per year. When CO₂ in the air increases, the concentration of CO₂ in the surface ocean, the thickness of which is as much as 100 to 300m, becomes equilibrium to the air. However, most ocean, the average depth of which is about 4000m, is not affected because the deep ocean is completely separated from the surface water by the picnocline barrier. Therefore, it is natural to think of the artificial injection of CO₂ in the deep ocean. One of the uncertainties in such methods is the impact on the marine lives around the releasing point before CO₂ is diluted sufficiently. This is the reason why the title of this article is addressed.

Can the dilution of CO₂ wipe out the impact on the marine ecosystem around the releasing point? This is the question we find hard to answer. In fact, it is not a question, but rather philosophical

proposition. Before the current NEDO-RITE-KANSO project launched, it might have been able to say that the ocean scientists, the ocean biologists and the ocean engineers who were interested in the CO₂ ocean sequestration did research separately. Therefore, no one could possibly answer the question. The ocean engineers like us would only say “sorry, we do not know about the biological impacts” and the ocean biologists might say “it is, of course, impossible to say that there is no impacts”. However, this situation is gradually changing during the current project, one of the purposes of which is, I believe, to challenge this hard problem by a team of interdisciplinary research groups, although the answer is still difficult to obtain. At this stage, as shown in Fig. 1, it is fare to say that we, the team, have come to be on the same ground to discuss this issue. In this article, I would like to come close to the question addressed in the title of this article by trying to elucidate the dilution mechanism near CO₂-releasing points from the fluid-dynamical point of view, that will, I hope, invoke further interdisciplinary research inside/outside the project team.

Before proceeding down to the following sections, let me limit the scales of space and time of our discussion to local-ocean size and smaller. Figure 2 indicates the target scales of this article, which are the local-ocean and the droplet scales since we focus on the dilution process near the releasing points. The space size of the former is, say, several hundred meters and hours to days for time. In the latter, the scales are millimetres to centimetres for space and seconds to minutes for time.

(a) (b)

Fig. 8. (a) A presumable trajectory of a marine life superimposed on the contour map of CO₂

concentration in the case of the initial radius of droplets is 0.01m. (b) pH experiences

of the marine life, the trajectory of which is shown in (a), superimposed on the MIT

mortality curve^{4, 5}.