The research topic, assembled by editors Michael B. Fong of the National Institute of Standards and Technology, Maribel I. García-Ibáñez of the Spanish Institute of Oceanography, and Xinyu Li of the University of Washington, spans the full portfolio of ocean-based approaches. Biotic strategies enhance biological carbon fixation and export through nutrient fertilization, artificial upwelling, macroalgae aquaculture, and ecosystem restoration. Geochemical methods manipulate seawater chemistry directly, most prominently through ocean alkalinity enhancement, which raises the ocean’s capacity to absorb atmospheric CO2, and direct ocean carbon capture and storage, which strips dissolved inorganic carbon from seawater. Physical approaches such as artificial downwelling accelerate the sinking of carbon-rich water into the deep ocean.

What makes mCDR attractive beyond carbon accounting is its potential for co-benefits: enhanced fisheries and aquaculture, valuable biomass products, reuse of desalination byproducts, and even feedstocks for fuels and industrial chemicals. Yet the editors and contributing authors are blunt about the state of the field. Most methods remain at an early stage of development, with substantial gaps in carbon removal durability, additionality—the question of whether a project removes carbon beyond what nature would have absorbed anyway—monitoring, reporting, and verification, scalability, environmental impacts, and governance.

Four perspective articles frame the road ahead. Buesseler and colleagues, writing for the Exploring Ocean Iron Solutions group, lay out a coordinated five-year research roadmap to determine whether ocean iron fertilization can be safe, effective, and durable. Their framework centers on the concept of the centennial tonne: carbon that must remain isolated from the atmosphere for at least 100 years before it can count. They call for larger and longer field experiments, improved biogeochemical modeling, robust environmental monitoring and verification, and early engagement with coastal communities and regulators.

Hooper and colleagues examine the ecosystem risks of direct ocean carbon capture, synthesizing evidence from ocean acidification research, alkalinity enhancement studies, and natural analogues. Their conclusion is sobering: empirical knowledge about how marine organisms respond to the low-dissolved-inorganic-carbon, high-pH effluents that such facilities would discharge remains thin, and large-scale deployment is premature without a dedicated environmental impact evidence base. Martocello and colleagues assess macroalgae-based removal and find that high monitoring costs and the absence of validated regional circulation models are the key barriers to scalability and certification. They propose leveraging remote sensing and artificial intelligence for cost-effective monitoring while urging greater public investment in long-term field studies.

Ward and colleagues, in a separate perspective, tackle a foundational weakness: the ocean biogeochemical models that would underpin verification are inadequate for the task. Existing models struggle to resolve fine-scale processes and to represent the mechanisms that determine how long removed carbon stays out of the atmosphere. Their roadmap calls for refined parameterizations, modular technology-specific model components, and systematic testing across diverse ocean settings.

The original research contributions show how laboratory and field experiments are filling those gaps. Shaw and colleagues compared industrial magnesium hydroxide with material precipitated from seawater-derived brines for alkalinity enhancement, demonstrating that lower-crystallinity materials derived from seawater dissolve more rapidly and completely. The finding suggests that alkalinity enhancement performance can be tuned through material synthesis pathways, an engineering lever with real consequences for how much CO2 a given dose of alkalinity ultimately absorbs. Fucich and colleagues probed the role of microbial biology in hydroxide-based alkalinity enhancement using sterilized and unsterilized mesocosms. Biological activity accelerated the equilibration of CO2 in seawater but did not significantly change total carbon uptake, supporting the use of mesocosm experiments to set field-scale expectations and design verification strategies.

Monitoring emerged as perhaps the thorniest engineering challenge. Stewart and colleagues evaluated commercially available pH and carbon dioxide sensors for alkalinity enhancement verification and found that uncertainty in pH measurements dominates the uncertainty in estimated total alkalinity. Even dense sensor networks may underestimate surface pH changes because alkalinity plumes dilute rapidly after discharge. Savoie and colleagues confirmed the problem in the field: in a proof-of-concept trial of electrochemically generated alkalinity released through a coastal outfall, strong tidal mixing diluted the signal so quickly that near-field plume modeling was needed to guide sensor placement and interpret the observations. Ho and colleagues added a hydrodynamic twist, using a high-resolution regional ocean model of the San Francisco Bay Estuary to show that estuarine circulation can enhance atmospheric CO2 uptake by exporting alkalinity to the open ocean and enlarging the effective air-sea exchange area, making local currents a decisive factor in site selection.

Environmental risk and economics rounded out the collection. Ringham and colleagues, working with indigenous stakeholders from the Lower Elwha Klallam Tribe, exposed juvenile coho salmon to pulse conditions of electrochemically generated alkalinity representative of near-field mixing zones and observed no detectable adverse effects, a demonstration of how stakeholder-engaged laboratory science can inform risk assessment. Ward and colleagues performed a techno-economic analysis of a hypothetical large-scale ocean iron fertilization deployment, estimating levelized costs of carbon removal while finding large uncertainty driven primarily by poorly characterized oceanographic variables and the infrastructure demands of verification, such as research vessels. Their conclusion reframes uncertainty reduction itself as an engineering optimization: better science is a prerequisite for cost-effective monitoring.

Taken together, the eleven contributions chart both the promise and the distance remaining before marine carbon dioxide removal can be deployed responsibly. The editors emphasize that future progress will require improved modeling capabilities, tightly integrated observational and modeling frameworks for verification, rigorous ecological impact assessment, demonstrated economic feasibility, and transparent governance with genuine stakeholder engagement. The ocean’s carbon pump is powerful, but teaching humanity to steer it safely, measurably, and durably remains one of the defining engineering challenges of the coming decades.

Subject of Research: Marine carbon dioxide removal technologies including ocean alkalinity enhancement, direct ocean capture, iron fertilization, and macroalgae cultivation

Article Title: Editorial: Environmental engineering perspectives on ocean-based carbon dioxide removal

Article References: Fong, M. B., García-Ibáñez, M. I., & Li, X. (2026). Editorial: Environmental engineering perspectives on ocean-based carbon dioxide removal. Frontiers in Environmental Engineering, 5, Article 1908453. https://doi.org/10.3389/fenve.2026.1908453

Image Credits: AI Generated

DOI: 10.3389/fenve.2026.1908453

Keywords: marine carbon dioxide removal, ocean alkalinity enhancement, direct ocean carbon capture, ocean iron fertilization, macroalgae cultivation, monitoring reporting and verification, carbon storage durability, ocean biogeochemical modeling, environmental impact assessment, techno-economic analysis, coho salmon, San Francisco Bay Estuary

The core of the problem lies in a mismatch between two very different modeling worlds. Integrated assessment models, or IAMs, simulate the global economy, energy systems, agriculture, and land use, and they decide where and when carbon removal technologies should be deployed based on costs, resource demands, and policy assumptions. Earth system models, or ESMs, simulate the physics, chemistry, and biology of the atmosphere, ocean, and land surface in fully coupled detail. In the current state-of-the-art setup, used for the fifth and sixth phases of the Coupled Model Intercomparison Project and planned again for the seventh, the IAM simply hands the ESM a net emissions number in which the carbon removed by technologies like bioenergy with carbon capture and storage is folded in as a prescribed negative flux. The Earth system model never sees the bioenergy crops growing, never simulates the forests being planted, and never computes the ocean chemistry changes caused by dumping alkaline material into the sea. Everything the model does in response to that removal is therefore invisible.

Schwinger and colleagues argue that this prescribed approach must give way to what they call activity-driven simulation of carbon dioxide removal. Instead of prescribing a negative emission, the ESM explicitly represents the activity that draws down atmospheric CO2. For bioenergy with carbon capture and storage, that means growing the crops inside the model, harvesting the biomass, and routing a scenario-dependent fraction of the harvested carbon into geological storage. For ocean alkalinity enhancement, it means adding an alkaline agent such as slaked lime to the surface ocean and letting the model’s carbonate chemistry, circulation, and biology determine how much CO2 is actually sequestered, and how quickly. Only afforestation and reforestation have always been simulated this way, because the land-use patterns passed from IAMs to ESMs implicitly contain the forest transitions. Extending the same philosophy to other methods harnesses the full power of Earth system models: climate change effects on removal efficiency, from drought stress on bioenergy crops to ocean circulation changes, are computed rather than assumed.

But simulating the activity is only half the challenge. To measure what carbon removal actually achieves, scientists need a reference against which to compare, and this is where the study makes its most consequential conceptual contribution. The team defines two distinct metrics. The first is Process Carbon Removal, the gross amount of carbon a removal process takes up minus its own positive emissions, without any feedback effects. This is essentially what IAMs estimate, and it is the quantity relevant for carbon accounting, in the same way that emissions are priced per tonne regardless of how natural sinks respond. The second is Net Atmospheric Removal, the true difference in atmospheric carbon content between a world with the removal technology and a world without it, including every carbon-cycle and biogeophysical feedback the Earth system can muster. Crucially, the authors show that these two numbers can diverge substantially, because the planet’s carbon sinks respond to removal in ways that partially undo the benefit.

The reason for that divergence is the buffering nature of the carbon cycle. Today, terrestrial and oceanic sinks absorb more than half of every tonne of CO2 humanity emits. Models project that under declining emissions these sinks will take up less carbon, and under net-zero or net-negative emissions they can even flip into carbon sources, releasing CO2 back to the atmosphere as the reversed feedbacks operate in reverse. The practical consequence is stark: the net atmospheric reduction achieved by any carbon removal portfolio will always be less than the gross amount removed. How much less is a question the new framework is designed to answer quantitatively, scenario by scenario and model by model.

Constructing the necessary no-CDR baseline turns out to be anything but trivial. The authors distinguish two fundamentally different approaches. The first is an ex-post adjustment: take an existing IAM mitigation scenario and surgically remove the carbon removal, for instance by freezing the land-use transitions associated with afforestation or by omitting the alkalinity flux to the ocean. This preserves the original scenario’s geography and allows regional attribution of effects, but it produces a socio-economically inconsistent world, one where land earmarked for bioenergy simply sits idle rather than being reallocated to food production. The second is a counterfactual IAM scenario: re-run the economic model in a world where carbon removal was never an option, letting the energy system, land use, and emissions evolve consistently without it. This counterfactual world is socio-economically coherent but so different in its land-use and emissions geography that regional biogeophysical effects of the original removal deployment cannot be isolated. The two approaches are complementary, and the authors recommend that IAM teams begin producing counterfactual no-CDR baselines for selected scenarios, something that, to their knowledge, does not yet exist in the scenario literature.

To demonstrate the framework in action, the team turned to the Norwegian Earth System Model NorESM2-LM and a deep mitigation scenario generated by the REMIND-MAgPIE integrated assessment model, expanded to include ocean liming as a removal method. The scenario reaches an end-of-century carbon budget of 500 gigatonnes of CO2 from 2020, corresponding to 1.5 degrees Celsius of warming in 2100 after a substantial overshoot, with a peak cumulative budget exceeding 1000 gigatonnes around 2060 before massive upscaling of removals pulls it back down. In the Earth system model, ocean alkalinity enhancement was simulated activity-driven, with alkalinity deployed within exclusive economic zones excluding polar regions, while bioenergy removals remained prescribed. Three ensemble members were run for each of the required simulations.

The results are striking and, for anyone banking on ocean-based removal, sobering. For a deployment of 4.93 petamoles of alkalinity between 2050 and 2100, equivalent to 182.5 gigatonnes of calcium hydroxide, the model simulated a Process Carbon Removal of 28.2 petagrams of carbon by 2100. Carbon-cycle feedbacks from both land and ocean then clawed back 7.9 petagrams, roughly 28 percent, leaving a Net Atmospheric Removal of just 20.3 petagrams of carbon, equivalent to a 9.7 parts-per-million reduction in atmospheric CO2. The ocean and terrestrial biosphere contributed roughly equally to the feedback losses, at 4.8 and 3.1 petagrams respectively, though the authors caution that the land feedback in particular may vary greatly between models. Equally revealing was the comparison with the IAM’s own estimate: REMIND-MAgPIE, which assumes a constant efficiency of 0.57 moles of CO2 per mole of alkalinity and instantaneous uptake, overestimated the removal by about 7.4 petagrams, roughly 20 percent, partly because real ocean chemistry takes ten to fifteen years to fully equilibrate an alkalinity addition.

Perhaps the most humbling finding concerns the noise floor of the climate system itself. Even a tiny initial difference in carbon removal deployment acts as a perturbation that causes the scenario and baseline simulations to diverge chaotically, exactly as separate ensemble members with different initial conditions do. Within a few years of the alkalinity deployment beginning around 2040, the El Niño-Southern Oscillation states of the two simulations had become completely uncorrelated. The resulting internal variability in calculated removals and feedback fluxes remained comparable in magnitude to the global annual mean signal, and even decadal averages carried a noise level exceeding 0.25 petagrams of carbon per year. The implication is uncomfortable: detecting the small signals associated with carbon removal and its feedbacks in emission-driven simulations requires either sufficiently large initial-condition ensembles or averaging over long periods, and for some applications, such as quantifying regional biogeophysical effects, stylized modeling approaches may simply remain preferable.

The timing of this work could hardly be better. The upcoming seventh phase of the Coupled Model Intercomparison Project has adopted emission-driven scenario simulations as a priority, yet its ScenarioMIP protocol still foresees only prescribed carbon removal and no no-CDR baselines, limiting what those simulations can say about removal effectiveness. The authors argue that the next phase of the Carbon Dioxide Removal Model Intercomparison Project should adopt their framework, and they issue concrete recommendations to the modeling community: IAMs should provide gross positive and negative emissions separately for each removal method, deliver spatially explicit information on bioenergy with carbon capture in future land-use datasets, and flag the difference between passive forest regrowth and deliberate afforestation. As nations and companies commit billions to carbon removal portfolios whose real-world performance remains deeply uncertain, this framework offers something the field has lacked: a transparent, physically consistent way to separate what a removal technology promises from what the Earth system actually delivers.

Subject of Research: Earth system modeling of carbon dioxide removal efficiency and feedbacks in mitigation scenarios

Article Title: Assessing Earth system responses in mitigation scenarios with activity-driven simulation of carbon dioxide removal

Article References: Schwinger, J., Merfort, L., Bauer, N., Bernardello, R., Butenschön, M., Bourgeois, T., Gidden, M. J., Gupta, S., Lee, H., Mengis, N., Moustakis, Y., Muri, H., Nieradzik, L., Peano, D., Pongratz, J., Sauer, P., Tourigny, E., & Wårlind, D. (2026). Assessing Earth system responses in mitigation scenarios with activity-driven simulation of carbon dioxide removal. Earth System Dynamics, 17(5), 1341-1363. https://doi.org/10.5194/esd-17-1341-2026

Image Credits: AI Generated

DOI: 10.5194/esd-17-1341-2026

Keywords: carbon dioxide removal, Earth system models, integrated assessment models, ocean alkalinity enhancement, BECCS, afforestation, carbon cycle feedbacks, no-CDR baseline, CMIP7, net-zero emissions, NorESM2, climate mitigation scenarios

The new study, published in the journal Earth System Dynamics, is one of the first to examine how ocean alkalinity enhancement, or OAE, would reshape the seasonal rhythm of the carbon cycle rather than just the annual bottom line. Led by Chiara Ciscato of the Euro-Mediterranean Center on Climate Change in Bologna, with colleagues at the GEOMAR Helmholtz Centre for Ocean Research in Kiel, the work used a full Earth system model to simulate a continuous, decades-long dose of alkalinity along the European coastline under two very different futures: a low-emission pathway called SSP1-2.6 and a high-emission pathway called SSP3-7.0. The results reveal a carbon cycle that is stranger, and in some ways more productive, than anyone had mapped before.

OAE is built on a simple piece of chemistry. When alkaline substances such as quicklime or crushed olivine dissolve in seawater, they consume dissolved CO2 and convert it into bicarbonate and carbonate ions, stable forms of inorganic carbon that can persist in the ocean for millennia. This conversion lowers the partial pressure of CO2 at the ocean surface, opening a chemical gap between sea and air. The atmosphere rushes to fill it, and the ocean draws down more CO2. In theory, the approach mimics and accelerates the natural weathering of rocks that has regulated Earth’s climate over geological time, compressed into a human timescale.

In the simulations, the team injected alkalinity continuously into the surface layer of coastal grid cells ringing the European continent, excluding the Mediterranean and the Baltic. Starting in 2025 and ramping up over a decade, the addition reached the equivalent of one gigatonne of fast-reacting calcium hydroxide per year, held constant until 2100. That is an industrial-scale intervention, comparable in ambition to the largest proposed carbon removal schemes, and the model tracked its consequences through every season of the century’s final decades.

The most striking finding concerns summer. Under natural conditions, surface alkalinity in European waters peaks in winter, when deep mixing dredges alkalinity-rich water up from below, and bottoms out in summer, when a warm, stratified cap isolates the surface. But the model showed the opposite happening once alkalinity was added. Because summer stratification traps the added alkalinity in the top layer rather than letting it mix away, surface concentrations climbed highest in the warmest months. Along the coastline itself, the seasonal swing in alkalinity exploded from roughly 19 to 109 micromoles per kilogram under low emissions, and from 24 to 138 under high emissions. The seasonal cycle, in effect, had been turned upside down.

That reversal has cascading consequences. With alkalinity peaking in summer, the partial pressure of CO2 in surface waters falls most sharply precisely when it would naturally be highest, during the months of peak outgassing. Fast gas exchange between ocean and atmosphere then does the rest: the summer release of CO2 flips into summer uptake, and the region becomes a carbon sink all year round. In the broader European domain, the seasonal amplitude of ocean pCO2 shrank by 41 percent under the low-emission scenario, a damping of the natural cycle that the authors attribute to alkalinity addition working hardest against the season of greatest chemical resistance.

Yet the biggest carbon gains did not come in summer. The air-sea CO2 flux remained locked to the rhythm of the mixed layer, deepening in winter as storms and cooling stir the upper ocean and shoaling in summer as the water stratifies. Winter, with its cold water and vigorous mixing, is when CO2 dissolves most readily, and it is winter that saw the largest enhancement of uptake once alkalinity was added. In the European region, the seasonal amplitude of the CO2 flux grew by about 51 percent under low emissions and 52 percent under high emissions. Along the coastline, the effect was more dramatic still: the winter-to-summer swing of the flux nearly tripled under SSP1-2.6 and grew more than two and a half fold under SSP3-7.0.

Perhaps the most counterintuitive result is that the dirtier emissions scenario produced the bigger carbon sink. Under SSP3-7.0, atmospheric CO2 concentrations are far higher, which raises the partial pressure of CO2 in surface waters and weakens the ocean’s buffering capacity, its chemical resistance to absorbing more of the gas. The result is that each unit of added alkalinity pulls down more carbon. The study’s measure of seasonal carbon sequestration potential, the ratio of extra CO2 uptake to the alkalinity added, was consistently higher under high emissions, both along the coast and across the wider European region. A hotter, more carbon-polluted world, in other words, makes this particular removal technology more efficient, even as it makes the underlying climate problem worse.

The findings carry practical weight for the emerging industry of marine carbon removal. Because the excess CO2 uptake extended well beyond the strip of coastline where alkalinity was injected, with some of the additional flux appearing over the open North Atlantic, monitoring schemes confined to national waters or deployment sites could systematically underestimate how much carbon a project actually removes. The authors also flag a caution: summer surface alkalinity along the coast rose by more than 500 micromoles per kilogram in the simulations, levels that could push pH and aragonite saturation past thresholds harmful to local marine life. Choosing the right alkaline feedstock, and tailoring the rate and timing of addition to each site, may be essential to keeping the ecological risks in check.

The study is not without limits. The Earth system model, called FOCI, resolves the ocean at half a degree, which smooths over the fine coastal dynamics where alkalinity is actually added, and the model carries known biases in the North Atlantic, including a cold patch and a low-alkalinity shelf region near Norway. Riverine alkalinity inputs, which follow a strong seasonal cycle, were not represented. The authors argue that because their analysis rests on differences between paired simulations, such systematic errors largely cancel out, but they acknowledge that real-world deployment would demand finer tools. They also note that their continuous injection scheme is idealised; future projects are more likely to use pulsed additions, which earlier work suggests behave differently. Still, as coastal OAE moves from whiteboard to pilot project, the message of this study is clear: the seasons matter. Whoever deploys alkalinity in European waters will be rewriting not just the ocean’s carbon budget, but its calendar.

Subject of Research: Effects of coastal ocean alkalinity enhancement on the seasonal carbon cycle in European waters under low- and high-emission scenarios

Article Title: Impacts of simulated coastal ocean alkalinity enhancement on the seasonal carbon cycle in European waters under a low- and a high-emission scenario

Article References: Ciscato, C., Mehendale, N., Kemena, T. P., Avrutin, S., & Keller, D. P. (2026). Impacts of simulated coastal ocean alkalinity enhancement on the seasonal carbon cycle in European waters under a low- and a high-emission scenario. Earth System Dynamics, 17(5), 1513-1527. https://doi.org/10.5194/esd-17-1513-2026

Image Credits: AI Generated

DOI: 10.5194/esd-17-1513-2026

Keywords: ocean alkalinity enhancement, carbon dioxide removal, seasonal carbon cycle, European coastal waters, Earth system modelling, ocean pCO2, marine carbon sink, SSP1-2.6, SSP3-7.0, ocean stratification, air-sea CO2 flux, climate mitigation

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: Bach, L. T., Atamanchuk, D., Britton, D., Fuhr, M., Hylén, A., Nawaz, S., & Rohr, T. (2026). Rethinking ocean alkalinity enhancement. Nature Reviews Earth & Environment, 7(10), 725-742. 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

The scientific rationale behind the experiment is rooted in basic ocean chemistry. Since the beginning of industrialisation, atmospheric carbon dioxide concentrations have risen sharply, and a substantial fraction of that gas has dissolved into the sea. This influx has shifted seawater chemistry toward progressively lower pH, a process known as ocean acidification, which places particular stress on organisms that build calcareous structures such as mussels, corals and many planktonic species. At the same time, the added carbon dioxide is a principal driver of global warming. Increasing the alkalinity of seawater raises its buffering capacity, which counteracts acidification and allows the ocean to take up additional CO2 and convert it into more stable dissolved forms. In principle, the chemistry is straightforward. What remains uncertain is how marine ecosystems respond to different alkalinity sources, concentrations and modes of delivery, and where the ecological tolerance limits lie.

That uncertainty is precisely what the Taliarte experiment is designed to address. According to Emeritus Professor Ulf Riebesell, a marine biologist at GEOMAR and co-lead of the study, this year’s trial is about comparing a liquid source of alkalinity with ground concrete rubble and assessing how well both substances are tolerated by the marine environment. The work forms part of the international research project OceanAlkAlign, which aims to standardise measurement and assessment methods for Ocean Alkalinity Enhancement and thereby create a robust scientific foundation for future decisions about whether, where and how such techniques might ever be deployed. The choice of location is no accident: the Canary Islands Marine Research Institute PLOCAN, a long-standing research partner of GEOMAR, sits directly beside the Taliarte harbour, from which the mesocosms are deployed, filled and monitored throughout the entire duration of the experiment.

The tools of the trade are the KOSMOS mesocosms, short for Kiel Off-Shore Mesocosms for Future Ocean Simulations, which GEOMAR has been developing and deploying since 2006. These enclosures function like enormous test tubes suspended in the sea, each enclosing a natural slice of the coastal ecosystem, including planktonic food webs, microbial communities and the biogeochemical processes that connect them. Because the water inside each mesocosm is isolated from the surrounding ocean, researchers can manipulate conditions in individual units and compare outcomes against untreated controls with a precision that would be impossible in the open sea. Across 23 experiments to date, the KOSMOS platform has been used to investigate ocean acidification, warming, nutrient dynamics and potential countermeasures including artificial upwelling and various alkalinity enhancement approaches, making it one of the most experienced experimental infrastructures of its kind in the world.

The experimental design in Gran Canaria follows a comparative logic. Natural plankton communities will be observed over several weeks inside the twelve mesocosms. Some of the systems receive increasing quantities of finely ground concrete rubble, while others are treated with liquid sodium hydroxide as a reference representing pure dissolved alkalinity. A further set of enclosures remains untouched as controls. Throughout the seven-week run, the team will track changes in the seawater carbon dioxide system, including pH and total alkalinity, alongside the degree of CO2 uptake achieved. In parallel, an extensive suite of biological measurements will capture the composition and productivity of the phytoplankton, the responses of zooplankton, microbial process rates, and any indications of shifts rippling through the food web. Together, these data streams allow the researchers to evaluate both the effectiveness of each alkalinity source and any potential side effects along the chain from microbes to grazers.

The choice of concrete as a candidate material is driven by a striking waste-management reality. Concrete rubble is one of the largest waste streams on the planet, with an estimated five billion tonnes generated every year, and only a fraction of that material has so far been reused. Because concrete contains cement, which is inherently alkaline, finely ground demolition waste could in principle serve as a cheap and abundant source of alkalinity for the ocean. Model estimates are already exploring whether large quantities of carbon dioxide could be sequestered in this way over the long term, potentially turning a costly disposal problem into a component of climate mitigation. The appeal is obvious: if a material that already exists in staggering quantities can safely enhance the ocean’s carbon uptake, the economics and logistics of carbon dioxide removal could look very different from approaches that require manufacturing alkalinity from scratch.

Yet the researchers are emphatic that apparent availability does not equal environmental safety. A material that seems unproblematic on land can behave very differently at sea. Ground concrete introduces not only alkalinity but also solid particles, which can increase turbidity in the water column and potentially harm microorganisms, with consequences that could propagate through the food web. Associate Professor Kai Schulz of Southern Cross University in Australia, co-leader of the experiment, stresses that a waste product does not automatically become a sustainable solution simply because it is available. In his view, the field needs data showing under what conditions Ocean Alkalinity Enhancement could be ecologically acceptable and where its limits lie. That caution reflects a broader tension in the emerging carbon removal landscape, where the urgency of climate targets creates pressure to scale technologies faster than the ecological evidence base can comfortably support.

The wider context makes the stakes clear. According to many current scenarios, emission reductions alone will not be sufficient to meet the goals of the Paris Agreement, which has pushed methods for active carbon dioxide removal to the forefront of climate policy discussions. Among the portfolio of candidate approaches, Ocean Alkalinity Enhancement is regarded as an option with high potential, provided it can be implemented effectively and in an environmentally responsible manner. The ocean already absorbs a substantial share of humanity’s carbon emissions each year, and even modest enhancements to that capacity, applied across vast areas, could translate into gigatonne-scale removal. But the same vastness that makes the ocean attractive as a carbon sink also makes mistakes difficult to reverse, which is why controlled, ecosystem-level experiments like the one in Taliarte are considered essential prerequisites before any larger deployment could be responsibly considered.

The ultimate objective of the Gran Canaria study is to help define what the researchers describe as a safe operating space for alkalinity enhancement. The results are intended to inform threshold values: which dosages alter the water chemistry in the desired way, and at what point ecological effects become apparent? What differences emerge between solid particulate sources and dissolved alkalinity? And how can findings from laboratory studies, mesocosm experiments and open-ocean field trials be combined into a body of evidence robust enough to guide real-world decisions? Schulz distils the philosophy of the project into a simple maxim: understanding before scaling up. If Ocean Alkalinity Enhancement is ever to be discussed at a meaningful scale, he argues, it must rest solely on transparent data regarding both benefits and risks.

For now, the twelve floating enclosures in Taliarte harbour will continue their seven-week vigil, quietly recording how plankton communities, microbes and water chemistry respond to each dose of ground concrete or sodium hydroxide. Whatever the outcome, the experiment marks a rare moment when two enormous problems, the mounting mountains of construction demolition waste and the urgent need to remove carbon dioxide from the atmosphere, are tested against each other in the same seawater. If the rubble proves both effective and benign, it could open a genuinely circular pathway from demolition site to ocean carbon sink. If it reveals unexpected harms, the results will have done exactly what careful science is meant to do: draw a clear line around what is safe before anyone is tempted to cross it.

Subject of Research: Ocean alkalinity enhancement using ground concrete demolition waste tested in mesocosm experiments

Article Title: Construction waste for climate protection in the ocean?

Article References: Construction waste for climate protection in the ocean?. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: ocean alkalinity enhancement, carbon dioxide removal, concrete rubble, mesocosm experiment, ocean acidification, GEOMAR, KOSMOS, Gran Canaria, PLOCAN, OceanAlkAlign, marine ecosystems, climate mitigation

At the heart of the challenge is a delicate chemical balancing act. As atmospheric carbon dioxide dissolves into seawater, it forms carbonic acid, which lowers the ocean’s pH and erodes the carbonate chemistry that corals, shellfish and other calcifying organisms depend upon. Ocean carbon dioxide capture exploits a simple principle: if the ocean becomes more alkaline, it can absorb more CO2 from the air and lock it away safely as bicarbonate, one of the most stable and abundant forms of inorganic carbon on Earth. This process, known as ocean alkalinity enhancement, effectively turns the surface ocean into a vast, distributed carbon sink. But adding alkalinity to seawater is not trivial, and doing it in a way that is energy-efficient, scalable and safe has become one of the central engineering puzzles of the emerging carbon removal industry.

One of the most promising approaches relies on electrochemistry, specifically a technique called bipolar membrane electrodialysis. In this system, ion-selective membranes are stacked between two end electrodes, and the arrangement allows operators to split ordinary saltwater into its constituent acid and base. When brine containing sodium chloride flows through the stack, the process generates hydrochloric acid on one side and sodium hydroxide, a strong base, on the other. The elegance of the scheme lies in its product handling: if the hydrochloric acid is kept on land, where it can be stored, neutralized or put to industrial use, and only the sodium hydroxide along with the treated seawater is returned to the ocean, the receiving waters gain a net infusion of alkalinity. That added alkalinity shifts the carbonate equilibrium, allowing the ocean to draw down carbon dioxide from the atmosphere and convert it into dissolved bicarbonate that remains stable for millennia.

This electrochemical pathway has attracted serious commercial interest precisely because it produces what carbon markets call negative emissions, meaning carbon dioxide is durably removed from the atmosphere rather than merely avoided. Companies pursuing net-zero targets increasingly rely on such carbon removal credits to offset their residual emissions, and ocean alkalinity enhancement is widely regarded as one of the largest-capacity avenues available, given the sheer scale of the world’s oceans. Yet the technology has been held back by a deceptively mundane problem: what flows into the reactor matters just as much as what flows out.

Seawater, and many other brine streams that might feed these systems, contains divalent cations, most notably calcium and magnesium ions, alongside the sodium and chloride that the process is designed to split. When these divalent ions encounter the high-pH conditions created at the base-producing side of a bipolar membrane stack, they precipitate as solid calcium and magnesium compounds that coat the membranes in a crust known as scale. The consequences are cascading. Scaled membranes lose their ion-selective efficiency, forcing the system to push harder and consume more electricity to achieve the same output. Over time, the deposits shorten membrane lifetime, increasing maintenance frequency and replacement costs. The conventional answer has been water softening pretreatment, in which the incoming seawater is stripped of calcium and magnesium before it ever reaches the electrodialysis unit. While effective, such pretreatment adds significant capital and operating expense, undermining the economic case for the entire carbon removal operation and eroding the value proposition of the resulting carbon credits.

The Stony Brook researchers behind the new method have essentially designed around this bottleneck. Their technology enables electrochemical enhancement of ocean alkalinity without any pretreatment of the incoming seawater at all. By rethinking how the electrochemical system handles brine streams laden with divalent cations, the researchers have created a process that tolerates raw seawater directly, sidestepping the water softening infrastructure that previous designs demanded. According to the announcement from the Research Foundation for SUNY, the result is a system that produces negative emission carbon offsets of very high quality at a substantially reduced cost, simply because the most expensive upstream step has been engineered out of existence. The advantages claimed for the approach are straightforward and commercially significant: it is cheaper, more energy efficient, and requires no expensive water softening pretreatment.

The implications ripple across both the carbon markets and the broader climate technology landscape. Carbon offset buyers, including corporations striving to meet net-zero commitments, place a premium on removal credits that are durable, verifiable and cost-competitive. Ocean alkalinity enhancement already offers exceptional permanence, since bicarbonate dissolved in seawater represents one of the most long-lived forms of carbon storage known. By cutting costs at the front end, the Stony Brook method could help push the price of these high-quality ocean-based offsets toward levels that make them viable at gigatonne scale, a threshold that analysts consider essential if carbon removal is to play a meaningful role in stabilizing the global climate. Cheaper alkalinity generation also strengthens the second major application of the technology: mitigating ocean acidification directly, without necessarily framing it as a carbon market product at all.

Ocean acidification itself is a mounting crisis. Since the industrial revolution, the average surface ocean pH has dropped measurably as the sea has absorbed roughly a quarter to a third of anthropogenic CO2 emissions. The consequences have been documented across coral reefs, oyster hatcheries, pteropods and other calcifying organisms whose shells and skeletons become harder to build in increasingly acidic waters. A technology that adds alkalinity to coastal waters and open ocean regions simultaneously addresses this chemical stress and harvests the carbon removal benefit, a dual function that distinguishes ocean alkalinity enhancement from many other carbon removal approaches. In regions where fisheries, aquaculture and reef ecosystems are economically vital, localized deployment of alkalinity-enhancing electrochemical systems could offer both environmental resilience and revenue through carbon credit sales.

The scientific pedigree of the work reflects the growing maturity of the carbon removal field. Stony Brook University, a flagship research institution within the SUNY system, has become an active hub for climate and energy innovation, and the new method is protected under intellectual property identified in the licensing announcement. The Research Foundation for the State University of New York, the nation’s largest research foundation supporting the nation’s largest public university system, is actively seeking development partners, commercial partners and licensees to translate the laboratory advance into deployed systems. The foundation describes a broad portfolio of SUNY-led research spanning artificial intelligence for the public good, quantum technologies, next-generation semiconductors, biotechnology and medicine, and energy and climate solutions, with system-wide research expenditures approaching 1.5 billion dollars in fiscal year 2025.

For the carbon removal industry, the timing of the announcement is notable. As voluntary and compliance carbon markets mature, buyers are demanding higher fidelity in the offsets they purchase, favoring durable removal over avoidance-based credits. Electrochemical ocean alkalinity enhancement, with its well-understood chemistry and measurable inputs and outputs, is well positioned to meet rigorous monitoring, reporting and verification standards. The obstacle has always been cost, and the pretreatment bottleneck has been a substantial contributor. If the Stony Brook design performs as described at industrial scale, it could remove one of the largest cost wedges standing between laboratory demonstration and commercial deployment, accelerating a technology class that many climate analysts view as indispensable to achieving deep decarbonization.

The broader lesson of the advance may be as important as the technology itself. In the race to scale carbon removal, progress often comes not from exotic new chemistry but from eliminating the hidden costs and fragilities that make promising concepts uneconomic in practice. Water softening pretreatment is exactly the kind of unglamorous engineering detail that determines whether a climate technology remains a paper exercise or becomes infrastructure. By designing a bipolar membrane electrodialysis system that thrives on untreated seawater, the Stony Brook researchers have addressed one of those decisive details. The ocean, meanwhile, stands ready. Every unit of alkalinity added is an invitation for the sea to take up more carbon dioxide and, in doing so, to heal a little of the acidification that a warming world has inflicted upon it. What remains now is the work of scaling, partnering and commercializing, and the SUNY licensing announcement signals that the researchers and their institution intend to see this chemistry leave the laboratory and reach the sea.