Abstract. In this study we look beyond the previously studied effects of oceanic CO2 injections on atmospheric and oceanic reservoirs, and also account for carbon cycle and climate feedbacks between the atmosphere and the terrestrial biosphere. Considering these additional feedbacks is important since backfluxes from the terrestrial biosphere to the atmosphere in response to reducing atmospheric CO2 can further offset the targeted reduction. To quantify these dynamics we use an Earth-system model of intermediate complexity to simulate direct injection of CO2 into the deep ocean as a means of emissions mitigation during a high CO2 emission scenario. In three sets of experiments with different injection depths, we simulate a 100-year injection period of a total of 70 GtC and follow global carbon cycle dynamics over another 900 years. Simulated seawater chemistry changes and marine carbon storage effectiveness are similar to previous studies. As expected, by the end of the injection period avoided emissions fall short of the targeted 70 GtC by 16 % to 30 % as a result of carbon cycle feedbacks and backfluxes in both land and ocean reservoirs. An unexpected feature are effects of the model's internal variability of deep-water formation in the Southern Ocean, which, in some model runs, causes additional oceanic carbon uptake after injection termination relative to a control run without injection and therefore with slightly different atmospheric CO2 and climate. These results of a model that has very low internal climate variability illustrate that attribution of carbon fluxes and accounting for injected CO2 may be very challenging in the real climate system with its much larger internal variability.

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.

Figure S1: Global mean surface air temperature, relative to preindustrial, of the A1_Comitw simulation (solid) and the A1_Comitw_sed run (dashed). The horizontal dashed black line denotes the 1.5°C climate target. Figure S2: Time-series of the default A2 simulation for (a) global mean surface air temperature, relative to preindustrial, and (b) northern hemisphere (NH) sea ice area.

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