Enhanced Rates of Regional Warming and Ocean Acidification After Termination of Large-Scale Ocean Alkalinization

Termination effects of large-scale artificial ocean alkalinization (AOA) have received little attention because AOA was assumed to pose low environmental risk. With the Max Planck Institute Earth system model, we use emission-driven AOA simulations following the Representative Concentration Pathway 8.5 (RCP8.5). We find that after termination of AOA warming trends in regions of the Northern Hemisphere become similar to 50% higher than those in RCP8.5 with rates similar to those caused by termination of solar geoengineering over the following three decades after cessation (up to 0.15K/year). Rates of ocean acidification after termination of AOA outpace those in RCP8.5. In warm shallow regions where vulnerable coral reefs are located, decreasing trends in surface pH double (0.01 units/year) and the drop in the carbonate saturation state (Omega) becomes up to 1 order of magnitude larger (0.2 units/year). Thus, termination of AOA poses higher risks to biological systems sensitive to fast-paced environmental changes than previously thought. Plain Language Summary Climate engineering (CE) methods are intended to alleviate the environmental perturbations caused by climate change and ocean acidification. However, these methods can also lead to environmental issues. Among all the different CE techniques, the method of artificial ocean alkalinization (AOA) is commonly discussed. AOA involves the release of processed alkaline minerals into the ocean, which enhances the uptake of atmospheric carbon by the ocean while reducing the acidification of seawater. We study the impacts caused by the termination of AOA on environmental properties that are relevant for organisms and ecosystems because they are sensitive not only to the magnitude of environmental change but also to its pace. We analyze the rate at which the environment changes after termination of this method using an Earth system model that simulates the response of our climate to CE. We found that the abrupt termination of large-scale implementation of AOA leads to regional rates of surface warming and ocean acidification, which largely exceed the pace of change that the implementation of AOA was intended to alleviate. This enhanced rate of environmental change would restrict even more the already limited adaptive capacity of vulnerable organisms and ecosystems.