Meeting climate targets by direct CO<sub>2</sub> injections: What price would the ocean have to pay?

Abstract

We investigate the climate mitigation potential and collateral effects of direct injections of captured CO 2 into the deep ocean as a possible means to close the gap between an intermediate CO 2 emissions scenario and a specific temperature target, such as the 1.5°C target aimed for by the Paris Agreement. For that purpose, a suite of approaches for controlling the amount of direct CO 2 injections at 3000m water depth are implemented in an Earth System Model of intermediate complexi-10 ty.Following the representative concentration pathway RCP4.5, which is a medium mitigation CO 2 emissions scenario, cumulative CO 2 injections required to meet the 1.5°C climate goal are found to be 390 Gt C by the year 2100 and 1562 Gt C at the end of simulations, by the year 3020. The latter includes a cumulative leakage of 602 Gt C that needs to be re-injected in order to sustain the targeted global mean temperature. 15CaCO 3 sediment and weathering feedbacks reduce the required CO 2 injections that comply with the 1.5°C target by about 13 % in 2100 and by about 11 % at the end of the simulation.With respect to the injection-related impacts we find that average pH values in the surface ocean are increased by about 0.13 to 0.18 units, when compared to the control run. In the model, this results in significant increases in potential coral reef habitats, i.e., the volume of the global upper ocean (0 to 130m depth) with omega aragonite > 3.4 and ocean temperatures be-20 tween 21°C and 28°C, compared to the control run. The potential benefits in the upper ocean come at the expense of strongly acidified water masses at depth, with maximum pH reductions of about -2.37 units, relative to preindustrial, in the vicinity of the injection sites. Overall, this study demonstrates that massive amounts of CO 2 would need to be injected into the deep ocean in order to reach and maintain the 1.5°C climate target in a medium mitigation scenario on a millennium timescale, and that there is a trade-off between injection-related reductions in atmospheric CO 2 levels accompanied by reduced upper-25 ocean acidification and adverse effects on deep ocean chemistry, particularly near the injection sites.