Lethal by design? Guiding environmental assessments of ocean alkalinity enhancement toward realistic contextualization of the alkalinity perturbation

Ocean Alkalinity Enhancement (OAE) aims to mitigate climate change by increasing the chemical capacity of seawater to store anthropogenic CO2. OAE can be implemented through multiple pathways, each of which intentionally modifies marine carbonate chemistry through increases in total alkalinity (TA). Experimental research has only recently begun to assess how such TA perturbations (∆TA) affect ocean geochemical processes and ecosystems. Meaningful assessments need context on how ∆TA induced by different OAE pathways would evolve over time and in magnitude. Here, we use a dilution equation, a regional model, and a global model to explore how marine systems and life styles would experience ∆TA under realistic constraints. We find that a more extreme ∆TA of >1000 μmol kg-1, a perturbation commonly considered in OAE experiments, only occurs for minutes in a miniscule fraction of the OAE-perturbed seawater volume. In contrast, ∆TA between 1-100 μmol kg-1 is a ubiquitous perturbation range for OAE under real-world constraints, yet rarely in focus of environmental OAE assessments. These results suggest that there is a disconnect between real-world ∆TA that can plausibly be invoked by OAE and the experimental ∆TA range frequently used in the context of the environmental OAE assessment. While “unrealistic” ∆TA can provide crucial insights into response patterns to OAE, they can also cause overestimation of OAE effects, if the unrealistic ∆TA is not contextualized appropriately. Our results can be used to improve the contextualization of OAE studies, thereby making the interpretation of ∆TA effects on the environment more robust.

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