Mitigating the atmospheric CO₂ increase and ocean acidification by adding limestone powder to upwelling regions

The feasibility of enhancing the absorption of CO 2 from the atmosphere by adding calcium carbonate (CaCO 3 ) powder to the ocean and of partially reversing the acidification of the ocean and the decrease in calcite supersaturation resulting from the absorption of anthropogenic CO 2 is investigated. CaCO 3 could be added to the surface layer in regions where the depth of the boundary between supersaturated and unsaturated water is relatively shallow (250–500 m) and where the upwelling velocity is large (30–300 m a −1 ). The CaCO 3 would dissolve within a few 100 m depth below the saturation horizon, and the dissolution products would enter the mixed layer within a few years to decades, facilitating further absorption of CO 2 from the atmosphere. This absorption of CO 2 would largely offset the increase in mixed layer pH and carbonate supersaturation resulting from the upwelling of dissolved limestone powder. However, if done on a large scale, the reduction in atmospheric CO 2 due to absorption of CO 2 by the ocean would reduce the amount of CO 2 that needs to be absorbed by the mixed layer, thereby allowing a larger net increase in pH and in supersaturation in the regions receiving CaCO 3 . At the same time, the reduction in atmospheric p CO 2 would cause outgassing of CO 2 from ocean regions not subject to addition of CaCO 3 , thereby increasing the pH and supersaturation in these regions as well. Geographically optimal application of 4 billion t of CaCO 3 a −1 (0.48 Gt C a −1 ) could induce absorption of atmospheric CO 2 at a rate of 600 Mt CO 2 a −1 after 50 years, 900 Mt CO 2 a −1 after 100 years, and 1050 Mt CO 2 a −1 after 200 years.