Earth system impacts of a realistic ocean alkalinization deployment scenario

Negative emission technologies (NETs) are an integral part of most climate change mitigation scenarios limiting global warming to 1.5 °C above preindustrial levels. Several different NETs have been proposed, including ocean alkalinization that has been considered as one method with high carbon removal potential. To date, most studies on 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. 

Here, we present a more realistic global deployment scenario for ocean alkalinization with Ca(OH)₂ dispersed at ocean surface in the exclusive economic zones of US, EU, and China, based on their respective production capacities. The dispersion scenario is based on current excess capacities in the lime and cement industries, 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 scenario with two different Earth System Models: EC-Earth and NorESM2-LM. In addition to this sophisticated scenario, we carry out an idealized scenario with a uniform addition of 0.5 Gt Ca(OH)₂ per year in the same coastal areas. 

The preliminary results show that the idealized 0.5 Gt Ca(OH)₂ flux decreased the atmospheric CO₂ concentration by 7 ppm in the first 15 years. The effects on ocean carbon uptake and surface ocean pH were strongly localized near the dispersion regions. The early version of the dispersion zone also included the Baltic Sea and the Mediterranean Sea, which led to significant increase in the alkalinity in these sea regions as the water exchange with the wider oceans are limited there. 

By providing a more realistic scenario for ocean alkalinization, we can give also more realistic assessment of climate effects and explore new research questions such as detectability of local changes in pH or carbon fluxes with slowly increasing deployment rates.