High CO2 Retained Fraction in Ocean Sequestration Estimated by a High Resolution Model

In the CO2 ocean sequestration, CO2 retained fraction is one of the most important topics. It shows how long CO2 injected into the ocean is isolated from the atmosphere. The retained fraction of the injected CO2 calculated in previous studies are around 70 percent in average after 200 years (IPCC report) which are calculated by relatively coarse resolution models with the horizontal resolution larger than one degree. Since the time scale of deep ocean circulation is about 2000 years, 30 percent leakage into the atmosphere after 200 years seems to be too large. We considered that distribution of a point-source tracer as in the CO2 ocean sequestration depends on whether a model resolves mesoscale eddies. We estimated the CO2 retained fraction using a high resolution model with the horizontal resolution of 0.1 degree. The fine grid mesh explicitly represents effects of mesoscale eddies on CO2 transport and dilution. As expected, the injected CO2 is kept in a smaller ocean volume than that obtained from coarse resolution models. Only 0.4 percent of the injected CO2 is leaked into the atmosphere after 200 years, although about half of the injected CO2 is horizontally transported to the outside of the narrow model domain. We supposed that the difference in the retained fraction between high and coarse resolution models is caused by difference in horizontal CO2 transport. As shown in previous studies, CO2 leakage occurs mostly in around the equator and the Antarctic Ocean. If CO2 is kept in a small ocean volume, it takes a long time before attaining these regions. Difference in simulated western boundary current regions also may effect on the difference of the retained fraction. Now, we are extending the model domain to prevent the horizontal CO2 transport outside of the model domain. Results of the new model will be shown in the meeting.