CO2 Removal Potential of Two Ocean-based NETs in Earth System Models in a Realistic Deployment Scenario

Negative emission technologies (NETs) are an integral part of most climate change mitigation scenarios limiting global warming to 1.5 °C above pre-industrial levels. Several different NETs have been proposed, including ocean alkalinization and direct CO2 removal which have been considered as methods with high carbon removal potential. In ocean alkalinization partial pressure of CO2 sea surface is reduced by spreading alkaline material and in direct removal of CO2 it is extracted from sea water and transported to permanent reservoir. To date, most studies on ocean-based NETs with Earth System Models have been based on idealized scenarios where atmospheric carbon is either simply removed by prescribed amount or some NET is deployed at magnitudes that would be extremely challenging to reach if any economic, technical, or political constraints were considered. In this work, we present Earth System Model simulations using a more realistic global deployment scenario for ocean alkalinization with CaO dispersed at ocean surface in the exclusive economic zones of US, Europe, and China. The dispersion scenario is based on current excess capacities in the lime and cement industries in these three regions, and high-end projections on how they could evolve until 2100. We use the high-overshoot SSP5-3.4-OS as the socioeconomic background scenario. We simulate the deployment scenarios with several Earth System Models. We will show results from simulations with alkalinity enhancement deployment initiated in 2030 and 2040. Furthermore, we compare these results with simulations of direct removal of CO2. Here, the direct removal is calculated from the added alkalinity using approximation for CO2 uptake factor using the relation between alkalinity and dissolved inorganic carbon. The results show that the CO2 is being removed from the atmosphere to oceans after the alkalinity deployment. Compared to the control simulation the global CO2 concentration is reduced by about 7 ppm in the deployment scenario starting in 2030 and about 4 ppm in the deployment scenario starting 2040 by end of the century. For real life deployment the efficacy and detectability of the alkalinity enhancement is a major concern. We will show that the temperature change in the earlier deployment scenario (higher removal potential) cannot be distinguished from the annual variability illustrating the problem in detectability. Furthermore, the simulations show the deployment must be constrained in regions with low oceanic transport to inhibit the precipitation of CaCO3 to retain the CO2 removal potential. Using a more realistic scenario for ocean alkalinization we can give a more realistic assessment of its climate effects and explore new research questions such as detectability of local changes in pH or carbon fluxes with slowly increasing deployment rates. In the realistic deployment scenario, ocean alkalinization decreases the CO2 concentration but does not produce a large signal in the temperature. Therefore, this method can be seen as having potential but its role in removing carbon from the atmosphere is limited, according to these scenarios. Furthermore, the wider effects on the Earth system still require more analysis.