Revisiting ocean carbon sequestration by direct injection: a global carbon budget perspective

Abstract

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. In additional parameter perturbation runs, we varied the default terrestrial photosynthesis CO2 fertilization parameterization by ±50 % in order to test the sensitivity of this uncertain carbon cycle feedback to the targeted atmospheric carbon reduction through direct CO2 injections. 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 16–30 % as a result of carbon cycle feedbacks and backfluxes in both land and ocean reservoirs. The target emissions reduction in the parameter perturbation simulations is about 0.2 and 2 % more at the end of the injection period and about 9 % less to 1 % more at the end of the simulations when compared to the unperturbed injection runs. An unexpected feature is the effect 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 the attribution of carbon fluxes and accounting for injected CO2 may be very challenging in the real climate system with its much larger internal variability.

The Effects of Carbon Dioxide Removal on the Carbon Cycle

“…However, it is not clear whether storage can be accessed fast enough or is appropriately located to meet the mitigation demands for fossil fuels with carbon capture and storage and CDR, so the use of more temporary storage sites has also been investigated. Reith et al [104] recently revisited the idea of using the deep ocean as a site to store carbon from DAC and quantified leakage rates and terrestrial and ocean carbon cycle responses. They found that their targeted atmospheric CO 2 reductions, which were equivalent to the amounts of CO 2 injected, were 16–30% lower on decadal to centennial timescales because of leakage and land and ocean carbon back fluxes that occurred in response to lowering atmospheric CO 2 .…”

Integrated Assessment of Carbon Dioxide Removal

“…Such open ocean deep convection in the Southern Ocean have been identified in many CMIP5 models (de Lavergne et al, ), the UVic model (Meissner et al, ; Reith et al, ) and also in the Kiel Climate Model, for which the cause could be linked to internal climate variability (Martin et al, ). An important model limitation in this respect is a coarse grid resolution, which for example prevents the correct representation of bottom water formation processes on the continental shelf and thus might favor such events (Bernardello et al, ).…”

“…CDR in UVic was simulated by injecting CO 2 at seven separate injections sites, which are located in individual grid boxes near the Bay of Biscay (42.3°N, 16.2°W), New York (36.9°N, 66.6°W), Rio de Janeiro (27.9°S, 37.8°W), San Francisco (31.5°N, 131.4°W), Tokyo (33.3°N, 142.2°E), Jakarta (11.7°S, 102.6°E), and Mumbai (13.5°N, 63°E) (Reith et al, , Figure ). Injections were simulated to be carried out at a 2900 m depth to minimize leakage and maximize retention time.…”

“…The simulated injection were based on the OCMIP carbon sequestration protocols and carried out in an idealized manner by adding CO 2 directly to the dissolved inorganic carbon (DIC) pool (Orr et al, ). Thus, we neglected any gravitational effects and assumed that the injected CO 2 instantaneously dissolved into seawater and was transported quickly away from the injection point and distributed homogenously over the entire model grid box with lateral dimensions of a few hundred kilometers and many tens of meters in the vertical direction (Reith et al, ).…”

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

“…(e.g., Matthews and Caldeira, 2008;Gillet et al, 2011) occurs in these simulations due to past emissions and climate cycle feedbacks. This committed warming is at some point overlaid by leakage of injected CO 2 out of the ocean (e.g., Orr, 2004;Reith et al, 2016), as well as by oceanic and terrestrial carbon cycle feedbacks that lead to a CO 2 increase in the atmosphere and respective additional warming (see section 3.1). To diagnose the contribution from leakage, we design a leakage-free sensitivity simulation (A1_ Comitw), in which CO 2 emissions are set to zero once the 1.5°C target is reached, and no CO 2 is 160 injected into the deep ocean.…”