University of Tasmania researchers recast ocean alkalinity enhancement as a local carbon tool

The strongest number in the paper favours the global vision. "In theory, the ocean has more than enough 'space' to safely sequester all human emissions," said Tyler Rohr, an IMAS researcher and co-author ^[10]. "We would only need to increase the mean alkalinity of the ocean by about 2% to store all of the approximately 2,660 billion tonnes of CO2 released since the Industrial Revolution," he said ^[11]. Then he named the catch: "The problem is that it's not obvious how we could evenly distribute that much alkalinity across the global ocean and ensure it is in contact with the atmosphere long enough to absorb its full potential of CO2." ^[18]

That 2% describes an ocean where the alkalinity has already spread out evenly. Real deployments start in one place. "In practice, highly concentrated alkalinity must be delivered to the ocean from some specific point sources and obtained from material that may contain contaminants," Rohr said ^[12]. The early global work measured the end state and skipped the first step of any project: the outfall or shoreline where concentrated material enters the water ^[13]. Rohr said current modeling tools "are not yet able to quantify those trade-offs" between ways of sourcing and spreading alkalinity ^[14]. "To really explore the possibilities for OAE, we need a more advanced modeling toolkit," he said, adding that it should not ignore chemical feedbacks and environmental impacts that may matter at the local scale ^[15].

The team's doubts are about delivery. "The chemistry is scientifically understood and predictable, and the available evidence suggests storing carbon as bicarbonate would not harm marine life," co-author Damon Britton said ^[8]. He noted that rivers already carry natural alkalinity into the sea and have regulated the climate throughout Earth's history ^[16]. The open question is what the side effects do at a given site. "Understanding how these and other perturbations from OAE interact with local marine life is critical to understanding where small-scale OAE can be safely implemented and whether the environmental risks outweigh the benefits," Britton said ^[17].

Lead author Lennart Bach, a biogeochemical oceanographer at the University of Tasmania's Institute for Marine and Antarctic Studies, put the proposal this way: "We argue that integrating small-scale OAE into diverse local contexts could deliver the achievable carbon mitigation goals defined by individual governments and communities." ^[3] His wastewater case is phrased as a possibility. "For example, a city council might realize that its emissions from domestic wastewater could be mitigated through alkalinity from silicates," he said ^[4]. The phys.org account gives no tonnages, costs or field results for that example or for the wind farm one, and it describes no trial. The thing this doesn't tell you is whether one city's wastewater emissions could be matched by an amount of silicate that a local stretch of coast can take without the turbidity and metals Britton describes.

I think the local framing is the easier claim to defend, because it asks for evidence at the scale where the unknowns are. It also raises the bar. Bach said upscaling could be achieved across these local areas, "but only if those pathways are environmentally responsible, demonstrably carbon negative and earn public trust." ^[6]