New estimates of the storage permanence and ocean co-benefits of enhanced rock weathering

Abstract

Avoiding many of the most severe consequences of anthropogenic climate change in the coming century will very likely require the development of "negative emissions technologies"-practices that lead to net carbon dioxide removal (CDR) from Earth's atmosphere. However, feedbacks within the carbon cycle place intrinsic limits on the long-term impact of CDR on atmospheric CO₂ that are likely to vary across CDR technologies in ways that are poorly constrained. Here, we use an ensemble of Earth system models to provide new insights into the efficiency of CDR through enhanced rock weathering (ERW) by explicitly quantifying long-term storage of carbon in the ocean during ERW relative to an equivalent modulated emissions scenario. We find that although the backflux of CO₂ to the atmosphere in the face of CDR is in all cases significant and time-varying, even for direct removal and underground storage, the leakage of initially captured carbon associated with ERW is well below that currently assumed. In addition, net alkalinity addition to the surface ocean from ERW leads to significant increases in seawater carbonate mineral saturation state relative to an equivalent emissions trajectory, a co-benefit for calcifying marine organisms. These results suggest that potential carbon leakage from the oceans during ERW is a small component of the overall ERW life cycle and that it can be rigorously quantified and incorporated into technoeconomic assessments of ERW at scale.

Keywords: carbon cycle; climate change; negative emissions technology.

Fig. 1.

Historical and future trajectories for temperature (left) and fossil fuel emissions (right) from an Earth system model of intermediate complexity (EMIC) compared to results from the CMIP5. Solid lines at left show cGENIE temperature trajectories for each RCP scenario, while shaded regions show corresponding ranges for CMIP5 models (22). Box plots (ensemble mean, ± 1 SD, and minimum to maximum range) show CMIP5 results for globally averaged temperature between 2080 and 2099 (22), while filled circles show cGENIE results. Solid lines and error envelopes at right show inverted fossil fuel CO₂ emissions for each RCP scenario from the cGENIE ensemble compared with ensemble mean trajectories from CMIP5 and a series of IAMs (23).

Fig. 2.

Carbon backflux through 2100 across a range of emission scenarios, shown as a time-integrated percentage relative to CDR deployment level. Curves and error envelopes show the ensemble median and uncertainty on baseline (modulated emissions) response and carbon leakage during ESW after correcting to the baseline response. Values for all scenarios are shown for a CDR deployment level of 10 GtCO₂ y⁻¹, though the relative magnitude of leakage is only weakly sensitive to CDR deployment level (see Supplementary Information).

Fig. 3.

Ocean carbon cycle response to CDR in an EMIC. Shown at left are sea-air CO₂ fluxes (top), depth-integrated inventory of dissolved inorganic carbon ([DIC]_int; middle), and zonally averaged DIC concentrations (bottom) for the baseline (modulated emissions) case and the ESW scenario. Shown at right are anomaly plots of sea-air flux (top), depth-integrated DIC inventory (middle), and zonally averaged DIC (bottom) between the ESW scenario and the equivalent modulated emissions case. Results are shown for year 2070 of an RCP4.5 emissions trajectory and a continuous CDR deployment level of 10 GtCO₂ y⁻¹ starting in 2030 (see Supplementary Information). Note that DIC concentration/anomaly results (D–I) are shown excluding the uppermost grid cell (80 m).

Fig. 4.

The response of surface ocean aragonite saturation state (Ω_arg) to CDR in an EMIC. Shown at left are surface Ω_arg values for the modulated emission (base) and ESW scenarios for RCP4.5 (top), RCP6.0 (middle), and RCP8.5 (bottom). Shown at right are anomaly plots of Ω_arg between the ESW scenario and the equivalent modulated emissions case. Results are shown for year 2070 of a given emissions trajectory and a continuous CDR deployment level of 10 GtCO₂ y⁻¹ starting in 2030.