Ocean carbon removal rethink: local alkalinity projects beat global geoengineering dreams

Ocean alkalinity enhancement has long been framed as one of the most tantalizing ideas in the fight against climate change: sprinkle or dissolve alkaline minerals into the sea, and the ocean’s own chemistry will quietly pull gigatonnes of carbon dioxide out of the atmosphere. Now a major Perspective published in Nature Reviews Earth & Environment argues that this grand, top-down vision has been holding the field back. Led by Lennart T. Bach of the University of Tasmania’s Institute for Marine and Antarctic Studies, an international team of biogeochemists, modellers and social scientists contends that ocean alkalinity enhancement, or OAE, should be reimagined not as a planetary-scale geoengineering scheme but as a flexible tool for localized carbon dioxide management, deployed in diverse coastal settings and scaled up through many small, community-driven projects rather than one monolithic global program.

The underlying chemistry is elegant. Seawater’s capacity to store carbon dioxide is governed by its total alkalinity, essentially the charge balance of carbonate and bicarbonate ions dissolved in it. When alkaline minerals such as olivine, limestone, quicklime, hydrated lime, steel slag or magnesium hydroxide dissolve in seawater, they shift the carbonate equilibrium: carbon dioxide is converted into bicarbonate and carbonate ions, lowering the partial pressure of CO2 at the sea surface and drawing more of the gas in from the atmosphere. Because the added alkalinity is conservative, meaning it persists in the water until consumed by other reactions, the stored carbon can remain out of the atmosphere for tens of thousands of years. This durability is what makes OAE attractive compared with biological approaches such as tree planting, where stored carbon can be released by a single fire.

Yet the authors argue that the field’s obsession with gigatonne-scale potential has distorted research priorities and public expectations. Instead, they propose a portfolio approach: many different OAE pathways, each matched to the local environment where it makes sense. Spreading crushed olivine on sandy beaches might suit one coastline; discharging alkalinity from wastewater treatment plants or desalination facilities might suit another; electrochemical methods that split seawater or accelerate carbonate dissolution on ships could serve shipping lanes; and adding alkaline minerals to organic-rich sediments, as experiments in the Baltic Sea suggest, could tap into benthic weathering processes. The diversity of pathways, the team writes, favours implementation across heterogeneous coastal environments, with upscaling achieved through widespread bottom-up adoption rather than centralized rollout.

A central claim of the Perspective is that the intended increase in seawater bicarbonate and carbonate shows no tendency to be inherently harmful within plausible limits. This is a striking statement, because the ocean is already suffering from the opposite problem: acidification. By raising alkalinity, OAE would partially reverse the pH decline that threatens corals, shellfish and calcifying plankton. Indeed, historical precedent exists. Lime has been used in aquaculture ponds for decades to manage pH, control pests such as starfish, and improve conditions for farmed shellfish and abalone, providing a long, largely overlooked record of alkalinity addition to coastal waters.

But harmless bicarbonate is not the whole story. The authors emphasize that collateral perturbations tend to drive the real environmental risk. Dissolving certain minerals can cause transient pH and CO2 excursions in the immediate vicinity of deployment. Mineral powders add suspended solids that can smother or abrade organisms. Some alkaline materials, notably steel slag and olivine, release trace metals such as nickel, chromium and copper, and several recent experiments have shown that these can influence plankton communities in pathway-specific ways. Land-based life-cycle impacts matter too: mining, grinding, transporting and dispersing minerals consumes energy and generates emissions, and life-cycle assessments show that a poorly designed OAE operation could erode or even negate its own carbon benefit. Environmental assessment, the team argues, should therefore focus on these pathway-specific local effects rather than on the added carbonate chemistry itself.

One of the most technically intriguing sections of the paper concerns sediments. Because most practical OAE deployments will occur near heterogeneous coastal sediments, their effectiveness will depend heavily on what happens at the seafloor. Sediments are not inert. They host natural alkalinity production through carbonate dissolution, silicate weathering, sulfate reduction and pyrite burial. Added alkaline minerals can interact with these processes in complicated ways: secondary minerals may precipitate and consume the very alkalinity that was added, a phenomenon known as runaway carbonate precipitation that laboratory studies have shown can be triggered when aragonite supersaturation exceeds critical thresholds. Conversely, alkaline additions may stimulate natural benthic weathering, amplifying the carbon removal beyond what the added mineral alone would deliver. There is also an additionality problem: anthropogenic alkalinity can suppress natural alkalinity-generating processes, so the net removal is smaller than the gross addition. Recent work on anthropogenic seafloor disturbances, such as bottom trawling, has even revealed that alkalinity destruction in disturbed sediments generates hidden CO2 emissions, underscoring how sensitive the coastal alkalinity budget is.

Verifying that a local OAE project actually removed the carbon it claims is perhaps the hardest problem of all. The added alkalinity draws down atmospheric CO2 only gradually, as air-sea equilibration takes months to years and depends on wind, temperature and mixing. Meanwhile, the ocean’s background carbonate chemistry varies enormously with season, currents and biological activity. The authors call for integrated, site-specific observation programs and high-resolution models that can distinguish a project-scale carbon removal signal from this natural variability and quantify uncertainty transparently. Nested coastal models, such as one recently developed for Halifax Harbour, illustrate the kind of local simulation that will be needed. Emerging carbon-removal registries and the first OAE credits have already highlighted how contested verification methodologies remain, and the paper argues that accounting rules must be locally grounded and openly documented to support trust.

Governance is the third pillar of the rethink. The authors argue that global geoengineering frameworks, centred on the London Convention and Protocol and the United Nations Convention on the Law of the Sea, are poorly suited to regulating thousands of small coastal deployments. Instead, they envision locally grounded regulation, community co-design and transparent carbon accounting, with projects shaped by the governments, Indigenous communities, aquaculturists and coastal residents who will live alongside them. Social science research shows that public acceptance hinges on exactly this kind of local involvement; surveys and scenario studies across several countries indicate that communities evaluate marine carbon removal not only on environmental risk but on procedural fairness, economic benefit and who controls the technology. The authors also stress humility: research suggests OAE could contribute to climate mitigation, but major uncertainties persist, and rigorous assessment must continue as deployments grow.

The practical vision sketched in the Perspective is, notably, consistent with what is already happening in the real world. Early commercial deployments have been small, coastal and locally negotiated, from shoreline mineral dispersal trials to ship-based experiments in the North Atlantic, and buyers have purchased modest volumes of removal credits rather than betting on gigatonne promises. Whether this bottom-up mosaic can deliver meaningful climate impact remains an open question, and the authors are careful not to overpromise. But their message is clear: the future of ocean alkalinity enhancement will be written not in one sweeping global treaty or one planetary intervention, but in hundreds of carefully monitored, environmentally responsible, demonstrably carbon-negative projects, each designed for the particular stretch of coast where it operates. For a field that has oscillated between utopian hype and reflexive dismissal, that shift from planetary dream to local practice may be the most important recalibration yet.

Subject of Research: Ocean alkalinity enhancement as a localized marine carbon dioxide removal strategy

Article Title: Rethinking ocean alkalinity enhancement

Article References: Rethinking ocean alkalinity enhancement. (n.d.). https://doi.org/10.1038/s43017-026-00828-5

Image Credits: AI Generated

DOI: 10.1038/s43017-026-00828-5

Keywords: ocean alkalinity enhancement, carbon dioxide removal, marine biogeochemistry, coastal ecosystems, enhanced weathering, carbon verification, ocean acidification, sediment chemistry, climate governance, geoengineering, carbon markets, marine carbon dioxide removal