What is our assessment?

Our analysis finds that Deploy Ocean Biomass Sinking could have high potential environmental risks, including unknown impacts on marine ecosystems. It is also unclear how effective or durable carbon storage in the deep sea is from this approach. There are likely better alternative uses for terrestrial biomass, and cultivating seaweed at climate-relevant scales is probably not feasible. Even if it were, seaweed would probably provide greater value through other applications. Therefore, Deploy Ocean Biomass Sinking is currently “Not Recommended” as a climate solution.

What is it?

Ocean biomass sinking relies on sinking terrestrial plant material and/or seaweed grown in the ocean to the deep sea or seafloor where it can be stored long-term. Cultivating and sinking seaweed removes carbon from the surface ocean, whereas sinking terrestrial biomass material can help reduce emissions that might otherwise occur if the material instead decomposed on land. While not a current practice, terrestrial biomass grown explicitly for sinking would also constitute a form of carbon removal. When biomass sinks naturally, most of it is degraded into CO₂ or other forms of carbon before reaching the deep sea. Deliberate sinking of biomass might avoid some of this degradation by expediting its delivery to the deep sea, depending on the method used. Once sunk, the biomass and any CO₂ or other forms of carbon produced from its degradation can be isolated from the atmosphere for decades to centuries due to the ocean’s slow circulation times at depth. Biomass sinking can be accomplished using active methods, like submersibles, or passive methods, like letting weighted bundles sink on their own. There has been a recent focus on sinking material in low-oxygen ocean basins (e.g., the Black Sea), which might help further minimize degradation, while improving the durability of sequestered carbon due to the long circulation time-scales typical of these regions.

Does it work?

Global estimates suggest that ~11% of carbon produced in natural seaweed ecosystems might be sequestered at depth, generally defined as below the mixed layer at around 1,000 m. However, very few studies have documented the export efficiency, or the fraction of carbon in surface waters that makes its way to the deep sea, of purposefully sunk terrestrial and seaweed biomass, as this practice is currently in the early stages of development and research. If biomass is quickly sunk, most carbon might make its way to the deep sea, while passive sinking techniques, if slower, could result in higher degradation rates. Sequestration also depends on the storage efficiency and durability of carbon once at depth. Some initial research suggests that biomass degradation may be slowed in low-oxygen basins, but this also remains poorly characterized in field studies. It is similarly unclear how durable the carbon stored below the mixed layer will be over climate-relevant timescales, both in the deep sea in general and in low-oxygen basins specifically.

Why are we excited?

The advantages of ocean biomass sinking include its potential ability to use land-based biomass that might otherwise be degraded in landfills or incinerated, both of which lead to CO₂ emissions. In some regions, seaweed cultivation could help reduce nutrient pollution, provide habitat for marine organisms, and locally buffer against ocean acidification. Estimates of potential climate impacts suggest that ocean biomass sinking using biomass from seaweed farms could theoretically exceed 0.1 Gt CO₂‑eq/yr. Still, those estimates remain highly speculative and require more research. Costs are poorly quantified, but some estimates suggest they could be low to moderately expensive compared to other marine carbon dioxide removal approaches, close to US$100/t CO₂.

Why are we concerned?

Ocean biomass sinking has many environmental and social risks that, though not currently fully understood, could make it unfeasible to deploy the technology at scale. Deep-sea and seafloor ecosystems are highly understudied, and it's unclear how new biomass might alter these unique environments. Potential impacts include increased acidification, nutrient pollution, and oxygen depletion of the deep sea, which could affect diverse marine life. Large-scale seaweed cultivation could reduce phytoplankton abundance, disrupt food webs, and deplete nutrients needed by other ecosystems. Cultivation in open ocean areas might relieve demand for coastal space, but they are often nutrient-poor, and adding nutrients raises serious concerns (see Deploy Ocean Fertilization). Terrestrial biomass sources could introduce contaminants into the ocean due to inadvertent inclusion of plastics or other pollutants in sunken biomass. This practice also comes with social risks. Some countries might disproportionately bear negative impacts wherever biomass is cultivated and/or sunk, as it could alter marine food webs and livelihoods. There could also be issues with public perception due to historical injustices around ocean dumping, potentially impeding future projects without meaningful community engagement and transparency. 

Moreover, there are numerous technical challenges relating to the effectiveness and durability of carbon sequestration. Biomass sources differ in how easily they break down, affecting how much carbon is stored at depth. Sunk biomass could also potentially release other greenhouse gases, such as methane and nitrous oxide. The location where biomass is disposed of also matters, impacting how much carbon reaches and stays at depth. However, all of these factors remain poorly constrained. Operational and technical challenges are also significant. To remove at least 0.1 Gt CO₂‑eq/yr using marine biomass, nearly 7 million ha of ocean – over 60% of the global coastline – could be needed for seaweed cultivation, which is impractical. Measurement and verification pose additional hurdles. In the case of seaweed cultivation, tracking carbon removal requires monitoring both CO₂ uptake at the ocean’s surface and export as well as storage at depth across large spatial and temporal scales. In addition, the opportunity cost of sinking terrestrial biomass is high due to competing land-based uses, as waste biomass and crop residues are finite resources. Growing new biomass explicitly for ocean sinking would introduce new risks, given that land is also a finite resource. Similarly, seaweed probably has higher value and carbon benefits as food, fertilizer, and other products.

What is our assessment?

Based on the difficulty of capturing low concentrations of CO₂ from the air and the associated technological, energy consumption, and financial challenges facing DAC, it is unlikely that this climate technology can be scaled up to remove globally meaningful amounts of CO₂. Furthermore, based on the current financial and policy support for DAC from fossil-fuel interests, there is a clear risk that the technology will be used to enable and perpetuate the production and use of fossil fuels, which is antithetical to solving the climate crisis. Therefore, we conclude that deployment of DAC is “Not Recommended” as a climate solution.

What is it? 

DAC is a suite of engineered technologies that remove CO₂ directly from the atmosphere, concentrate it, and then inject it underground for permanent storage. CO₂ is captured from the atmosphere by moving large volumes of air, usually with large fans, past a reactive material that selectively binds CO₂, either a solid sorbent (referred to as solid-DAC or S-DAC) or a liquid solvent (referred to as liquid-DAC or L-DAC). The captured CO₂ is recovered from the reactive material by applying heat, pressure, or chemical reactions, and collected and compressed for transportation and storage. The concentrated CO₂ is then injected deep underground into geological formations, such as saline aquifers or basalt formations, where it can be permanently stored. 

Does it work?

The technology and chemistry for the selective capture of CO₂ from air are effective, although the CO₂ capture efficiency varies with the reactive material and other factors. A variety of solid and liquid reactive materials have been developed, along with material-specific processes for recovering captured CO₂ and regenerating the sorbents. This process is very energy-intensive and, for liquid-DAC, water-intensive. To capture and recover 1 t CO₂, solid-DAC uses about 1,100 kWh, while liquid-DAC uses about 2,500 kWh and consumes as much as 7 t of water. Most of the energy for DAC (70–90%) is used to generate heat for recovery of the captured CO₂ and regeneration of the sorbent material. Liquid-DAC requires temperatures up to about 900 °C (1,652 °F), while solid-DAC requires temperatures of only about 100 °C (212 °F). Because the process is so energy intensive, DAC achieves net carbon removal – capturing and sequestering more CO₂ than it emits – only if it is powered by zero or low-carbon energy sources and/or uses waste heat. For example, recent reporting showed that the amount of CO₂ captured and stored by Climeworks, the largest commercial DAC company currently in operation, was insufficient to offset the facility’s operational GHG emissions. CO₂ captured by a DAC facility can also be used for other purposes, such as enhanced oil recovery or production of algae biofuels. However, life cycle analyses conducted by the National Energy Technology Laboratory show that these pathways do not result in net carbon removal due to the emissions from production and/or use of these other products. Therefore, in addition to its requirements for zero or low-carbon energy, DAC can only be an effective method for net carbon removal if the CO₂ it captures is permanently stored deep underground. With appropriate pre-injection site selection, geologic testing, and post-injection monitoring, underground storage of CO₂ is safe and effectively permanent.

Why are we excited about it?

Unlike some other carbon removal technologies and practices, a DAC facility has a relatively small footprint and can be located anywhere there is sufficient low-carbon energy and infrastructure and capacity to transport or store captured CO₂. In addition, the amount of CO₂ removed from the atmosphere can be directly measured by monitoring the flow and concentration of captured CO₂ at the point of storage. Compared to many other carbon removal approaches, this method provides a higher level of confidence in the amount of CO₂ being removed for investors and carbon credit purchasers. The geological sequestration of captured CO₂ has high permanence, effectively removing CO₂ from the atmosphere for thousands of years with a low risk of reversal. There are numerous research and pilot projects underway to improve CO₂ capture efficiency, reduce energy use, and reduce costs, which may improve the effectiveness and cost of this technology. 

Why are we concerned?

The concentration of CO₂ in the atmosphere is small, currently about 420 parts per million, or about 0.04%. This means that a DAC facility must process huge amounts of air – more than 1,600 t by one estimate – and consume more energy than a typical U.S. household uses in a month to capture 1 t CO₂. Scaled up to remove a globally meaningful amount of CO₂ (>0.1 Gt CO₂ /yr), DAC would consume more energy than the annual energy consumption of 10 million U.S. households. In addition, removing and storing CO₂ using DAC is very expensive, costing up to US$1,000/t CO₂ stored. This is more than twice the cost per t for all other commercially available carbon removal technologies and practices. 

For these reasons, the technical and financial feasibility of scaling DAC to remove globally meaningful amounts of CO₂ from the atmosphere is low. Despite these challenges, as of September 2025, more than 30 companies have sold more than 2.4 million t of future carbon removal credits. However, less than 1,300 t CO₂ has actually been removed so far – or only 0.05% of these promised credits. To put this in perspective, despite spending billions of dollars, DAC has removed about as much CO₂ as would be saved by keeping 250-300 cars off the road for a single year.

There is also an opportunity cost for DAC. Even if a DAC facility is powered by solar, wind, geothermal, or nuclear energy, that carbon-free energy could have been used to displace coal- and gas-powered electricity instead, reducing emissions by far more than a DAC facility can capture and store. Similarly, the large amounts of public and private sector funding going to DAC could be more cost-effective and carbon-effective if used for other, more effective actions to cut emissions or remove CO₂. There is also the risk that DAC will be used to delay or avoid emissions reduction actions or for greenwashing by fossil fuel companies and other emitters. Substantial amounts of the funding supporting the development of DAC are coming from fossil fuel companies, which have publicly stated that they view carbon capture as a strategy to extend society’s use of fossil fuels. Finally, unlike most other emissions reduction or carbon removal actions, DAC provides no obvious other benefits to nature or human well-being.

What is our assessment?

Based on our analysis, Deploy Artificial Upwelling is not ready for large-scale deployment, as it has not been tested or proven effective for carbon removal. Even if demonstrated to be effective, it comes with considerable feasibility and cost concerns, as well as potentially insurmountable and widespread environmental risks at scale, and is therefore “Not Recommended” as a potential climate solution.

What is it?

Artificial upwelling generally involves using pumps or other devices to bring deep, nutrient-rich ocean water to shallower depths, where it can increase phytoplankton growth via photosynthesis by reducing nutrient limitations. This can increase biological uptake of dissolved CO₂ from the ocean, allowing it to absorb additional CO₂ from the atmosphere. Carbon removed by phytoplankton can then be transported to the deep ocean, where it may be stored long-term. A variety of pumps and devices have been described (e.g., air-lift, gravity wave, air bubble, electrical, wave-based) for bringing deep ocean water to the surface. Additionally, some efforts are considering artificial downwelling, or the deliberate transport of surface water to depth, both with and without artificial upwelling, as a means of moving surface carbon to deep waters before it is degraded and potentially returned to the atmosphere as CO₂.  

Does it work?

The fundamental biology underlying artificial upwelling relies on well-understood principles for the natural upwelling of deep, nutrient-rich seawater to the surface ocean, which supports biological production and carbon removal. Some efforts have demonstrated the ability to bring deep ocean water to the surface. However, no proof-of-concept field trials have demonstrated net carbon removal, which depends not only on carbon uptake by phytoplankton and subsequent CO₂ exchange with the atmosphere in the surface ocean, but also on its long-term storage in the deep ocean; both aspects remain largely unexplored in the context of artificial upwelling. Model simulations have been conducted, most of which indicate that artificial upwelling is ineffective for large-scale carbon removal.     

Why are we excited?

Artificial upwelling may provide some environmental advantages. For example, deliberate upwelling of deep ocean water could lower the temperature of surface ocean water, benefiting some marine organisms. Artificial upwelling could also be coupled with aquaculture operations to improve nutrient availability in nutrient-poor regions.

Why are we concerned?

Artificial upwelling presents significant challenges in terms of effectiveness, feasibility, cost, and environmental risk. Large-scale deployment is estimated to potentially require millions to hundreds of millions of pumps. Even short-duration deployments have operationally failed. Moreover, for pumps that require external power, energy requirements could be substantial but remain unclear at this stage. Another current limitation of this technology is the engineering challenge of physically moving large volumes of seawater from depth. Costs remain highly uncertain but are expected to be high given these major operational needs. 

The effectiveness of artificial upwelling is also unclear. Upwelled deep water often contains high concentrations of dissolved inorganic carbon (and low oxygen concentrations), meaning that upwelling might actually result in more CO₂ being emitted into the atmosphere if upwelled waters release more carbon than is removed. Existing studies suggest that additional carbon removal in the surface ocean from artificial upwelling is unable to compensate for this release of deep water dissolved CO₂. Circulation and ocean mixing could further limit the durability of carbon removed, as some research suggests that more than 70% of carbon is returned to the surface ocean within 50 years. 

Finally, environmental effects are poorly constrained but potentially significant. This solution shares similar ecological risks as Deploy Ocean Fertilization, wherein manipulating nutrient availability to increase biological productivity can alter the function of marine ecosystems across large areas and in unclear ways. Artificial upwelling could also alter oxygen availability and exacerbate ocean acidification in some regions. By redistributing cold, dense water and altering the layering of ocean water, some models suggest that large-scale deployment might also increase ocean heat uptake and alter ocean circulation dynamics in ways that impact processes in the lower atmosphere, such as precipitation and temperature modulation. 

What is our assessment?

Based on the scientific uncertainties regarding its effectiveness and the potential serious environmental and social risks, we conclude that Deploy Ocean Fertilization is “Not Recommended” as a climate solution.

What is it?

Ocean fertilization involves adding nutrients, such as iron, to seawater to promote photosynthesis in the surface ocean. As phytoplankton draw in seawater CO₂ and convert it into biomass, the ocean can absorb more CO₂ from the atmosphere. Some of the carbon eventually sinks or is transported to the deep sea or seafloor, where it can be stored for decades or centuries. Most ocean fertilization efforts are focused on adding iron because it is a micronutrient already required in small amounts for photosynthesis and because iron limitation is common in many global ocean regions. The Southern Ocean, in particular, has been highlighted as a potential target due to its widespread iron limitation.

Does it work?

As a carbon removal technique, ocean fertilization requires that the nutrient addition enhances phytoplankton uptake of seawater CO₂ and subsequent absorption of additional CO₂ from the atmosphere, and that the carbon is transported and durably stored in the deep sea. Research since the 1990s has shown that ocean iron fertilization does lead to increased seawater CO₂ uptake due to enhanced photosynthesis. However, the ultimate fate and durability of that carbon are less well understood. To be sequestered, carbon must be transported below water depths where annual mixing occurs, often considered to be ~1,000 m, but research suggests that, on average, 66% of carbon at these depths can be re-exposed to the atmosphere in less than 40 years. Ocean fertilization might also increase production of GHGs, such as nitrous oxide and methane, which could impact the effectiveness of this practice, although these effects remain understudied. In places like the Southern Ocean, sunlight and changes in the availability of other nutrients, such as silicate, can also limit the effects of iron addition. Additionally, nutrients such as iron can have high loss rates, up to 75%, after dispersal into seawater due to conversion into forms inaccessible to phytoplankton, potentially further reducing the effectiveness of nutrient addition.

Why are we excited?

If ocean fertilization were broadly deployed and functioned as intended, its global climate impact could reach 0.1–1.0 Gt CO₂ /yr. Ocean fertilization is expected to increase surface water pH, which could help temporarily reduce ocean acidification locally. However, some studies suggest this benefit will come at the cost of increased acidification of deeper ocean regions. While costs remain highly uncertain, estimates of ocean fertilization costs range between US$80/t CO₂ and US$457/t CO₂, suggesting this practice might also be relatively inexpensive compared to other marine CO₂ removal practices.

Why are we concerned?

Ocean fertilization poses several technical challenges, along with significant environmental and social risks. Tracking the amount of carbon sequestered from ocean fertilization is difficult because carbon export efficiencies – the amount of carbon produced in surface waters that makes its way to the deep sea – can be low and highly variable in time and space. Addressing this will require both field studies and models capable of capturing global and multi-decadal changes in carbon cycling due to fertilization, given the long time scales and large spatial areas involved. Implementing ocean fertilization at globally meaningful carbon removal levels could raise additional feasibility concerns, given the potential difficulty of dispersing sufficiently large quantities of nutrients across vast areas and the need for fertilization to be done continuously to minimize carbon returning to the atmosphere. 

Beyond these technical challenges, ocean fertilization also poses several potentially severe environmental risks. Enhancing primary production could disrupt existing nutrient pools in the ocean, reducing the nutrients available for ecosystems far from dispersal sites. Another consequence of ocean fertilization is that increased organic carbon supply can enhance microbial processes that consume dissolved oxygen, potentially impairing respiration in marine organisms and leading to mortality. Other unintended consequences of nutrient fertilization include promoting harmful algal blooms that can release toxins that negatively impact a wide array of life, from shellfish to marine mammals to humans. Ocean fertilization also carries significant social risks because global-scale modification of marine ecosystems is likely to create inequities in environmental and economic impacts.

What is our assessment?

Based on our analysis, restoring large herbivores can provide climate benefits, but there is limited (and mixed) evidence on its carbon removal effectiveness across different ecosystem types. We will “Keep Watching” this potential climate solution.

What is it?

Restoring large (>45 kg) wild herbivores – such as bison, moose, and elephants – means reintroducing or increasing their populations on natural and degraded lands where they are absent. Large herbivores play an important role in carbon, water, and nutrient cycling and shape habitats for plants and animals. Herbivores alter the diversity and abundance of plant species, the habitat structure (such as forest vs. grassland), and plant productivity. These changes help mitigate climate change in three main ways. First, carbon storage can increase if plant productivity increases. Second, herbivores generally create more open landscapes that reflect more sunlight, which has a local cooling effect. Third, herbivores can reduce the risk of intense wildfires by creating more open ecosystems and reducing fuel loads. 

Does it work?

There is limited and mixed data on the solution's effectiveness. It is difficult to measure responses to a single change within a complex ecosystem and to account for differences across ecosystem types, pre-implementation levels of degradation, herbivore species, and herbivore density. Several examples illustrate these context-dependent nuances. Forest elephants prefer palatable tree species, favoring those with denser wood (more carbon), and disperse large fruits from larger, carbon-rich trees. In contrast, elephants in the savanna reduce carbon stocks by browsing branches and knocking over trees, creating a more open, grassy habitat that stores less carbon in vegetation. However, this may be offset if soil carbon builds up over time. In the boreal forest and tundra, caribou reduce tree and shrub cover (lowering carbon stocks), creating more open habitat. The open habitat reflects more sunlight than the forest (creating local cooling). Still, the exposed ground may warm enough to increase the rate of decay of organic matter in the soil (increasing CO₂ emissions). 

Where restoring herbivores increases carbon storage in the ecosystem, it complements or contributes to Protect Forests, Restore Forests, Protect Grasslands and Savannas, and Restore Grasslands and Savannas. Restoring large herbivores, such as manatees, in coastal and marine ecosystems could boost carbon storage, though there is even less evidence in marine ecosystems than in terrestrial ones. 

Why are we excited?

Restoring large herbivores is a key component of active or passive ecological restoration approaches to increase species diversity, restore natural processes, and aid species dispersal on natural and degraded lands. Where restoring herbivores increases carbon storage in the ecosystem, it complements the climate solutions for protecting and restoring ecosystems. Even when it has limited mitigation potential, the solution still has many biodiversity benefits. It helps protect and restore large herbivore species, ~60% of which are threatened with extinction. In addition, herbivory is an important ecological process in many ecosystems. In boreal and tundra regions, reintroducing caribou reduces the risk of intense wildfires in the forest and can limit the northward expansion of forests, which accelerates warming. Reintroducing bison to the Great Plains on grasslands previously grazed by cattle has little impact on the carbon storage. However, it increases plant species diversity, reduces methane emissions, and rebuilds cultural heritage for Indigenous people. This solution is becoming more common, particularly in Europe, but we are unable to estimate the potential global adoption and impact.

Why are we concerned?

The disadvantage of this solution for climate mitigation is that it can be expensive per metric ton of carbon, often has a limited impact, and, in many cases, can decrease carbon stocks. For example, reintroducing herbivores, such as elephants or deer, at high densities can reduce carbon stocks and limit further carbon sequestration. As a result, excluding or reducing herbivore abundance is a strategy in many habitat restorations. Changes in carbon storage, particularly in soils, are very difficult to measure and to attribute solely to the reintroduction of herbivores. 

What is our assessment?

Overall, ocean electrochemical approaches appear plausible, but their costs, environmental impacts, and effectiveness require further research. Furthermore, improving energy efficiency and increasing the use of low-carbon electricity supply will be critical to its viability as a large-scale global climate solution. Therefore, we will “Keep Watching” this solution.

What is it?

Ocean electrochemical systems pass an electrical current through seawater to drive chemical reactions that redistribute major ions, creating separate acidic and basic solutions that can be used for CO₂ removal in different ways. One common method uses an acidic solution to lower the pH of seawater. This shifts the chemical balance in the carbonate system toward dissolved CO₂, which then degasses from the water and can be captured for long-term storage in underground geologic reservoirs. Another variation uses the basic solution to increase the alkalinity of seawater, which shifts the carbonate system so that the ocean can absorb and remove more atmospheric CO₂, similar to Deploy Ocean Alkalinity Enhancement. Several other variations exist, including designs that use the acidic stream to weather alkaline rocks and the basic stream to precipitate carbon-containing minerals, such as carbonates. These systems can operate as closed systems, in which carbon removal is achieved with on-site infrastructure, as open systems, which rely on effluent discharge into the ocean to complete carbon removal, or as combination hybrid systems that combine both approaches.

Does it work?

The fundamental chemistry and physics underlying electrochemical methods for shifting chemical reactions and ionic balances in solutions are well understood and sound. These approaches are already used on a large scale in industrial applications, such as the chlor-alkali process for producing sodium hydroxide and chlorine. However, using electrochemistry to remove CO₂ from seawater remains in early development. Studies show that CO₂ can be removed using these methods, and some pilot projects are currently underway; however, the effectiveness and costs of approaches, particularly when energy requirements are factored in, are still uncertain. In addition, the effectiveness of ocean electrochemistry for CO₂ removal also depends on how the removed CO₂ is stored. For example, to have a beneficial climate impact, CO₂ degassed from acidified seawater must be permanently stored deep underground rather than used as a chemical precursor for the manufacture of other products or for enhanced oil recovery, both of which create additional GHG emissions.

Why are we excited?

Ocean electrochemical systems for CO₂ removal offer several potential advantages, including useful co-products, opportunities to leverage existing infrastructure, and possible environmental benefits. Some electrochemical variations generate beneficial co-products, such as hydrogen gas, which could help offset costs and displace external fossil fuel energy demand. Some designs can also be coupled with existing brine streams in desalination or industry to leverage both infrastructure and waste feedstocks. In other cases, seawater itself serves as an abundant natural feedstock for these electrochemical processes. In some variants, environmental benefits may include local reductions in ocean acidification, which may benefit some marine organisms. Additionally, monitoring and verification of electrochemical variations using closed-system approaches may be more straightforward compared to other marine carbon dioxide removal (mCDR) approaches that require tracking carbon across large regions and depths of the ocean. 

Why are we concerned?

Ocean electrochemical approaches for carbon removal present several challenges with respect to cost, effectiveness, and potential environmental impacts. Many designs are energy-intensive, require substantial infrastructure, and need large volumes of seawater. Their effectiveness and overall climate impact will depend on access to low-carbon electricity and accurate accounting of full life cycle energy use, since use of fossil-based electricity could offset much of the CO₂ removal benefits. Costs are unclear but are largely estimated to be well above US$500/t CO₂ for many variations. Additionally, some approaches, such as those that aim for carbonate mineral precipitation, do not inherently yield net CO₂ removal unless the alkalinity used for mineral formation is replenished, which can be costly. 

If the electrochemical approach relies on capturing CO₂ from seawater, geologic storage will be required to dispose of the degassed CO₂. Furthermore, some co-products of electrochemical approaches, such as acidic brines or chlorine gas, could exceed viable market demand at large scales and may require waste disposal or the development of technologies to reduce or eliminate their production. Finally, for open-system electrochemistry variants that rely on effluent interactions in the ocean to complete carbon removal, carbon accounting will likely need to extend far beyond the facility itself, making monitoring and verification of carbon removal similarly complex to other marine carbon dioxide removal approaches. 

Environmental impacts are not well understood but will depend on the specific electrochemical approach. Approaches that discharge chemically altered effluent into the ocean could impact marine organisms near discharge points, with ecosystem responses that remain highly uncertain. Large-scale pumping of seawater could affect marine organisms through entrainment and other intake impacts, due to the large volume of influent needed for these approaches at scale. When these approaches require alkaline rocks, they raise concerns about the potential effects of mining, similar to Deploy Ocean Alkalinity Enhancement and Enhanced Rock Weathering.

What is our assessment?

Based on our analysis, OAE could be a promising carbon removal technique, but it is not ready for large-scale deployment until the risks, costs, and effectiveness become clearer. We will “Keep Watching” this potential climate solution.

What is it?

OAE is the practice of adding alkalinity to seawater to increase the ocean’s ability to remove atmospheric CO₂. The addition of alkalinity through OAE mimics the natural process of weathering, or the physical and chemical breakdown of rocks. Rock weathering on land produces alkaline substances that eventually flow into the ocean through rivers and groundwater. This natural supply of alkalinity reduces ocean acidity, which affects the distribution of various carbon forms in the ocean. As alkalinity increases, CO₂ dissolved in seawater shifts toward more stable carbon forms, like bicarbonate and carbonate ions, that cannot exchange with air. This allows the ocean to remove more gaseous CO₂ from the atmosphere because the ocean and the atmosphere maintain a balance of CO₂ through gas movement at the sea surface. Most of the dissolved carbon in the ocean is bicarbonate and carbonate ions, which can persist in seawater for thousands of years. Under natural conditions, the ocean removes nearly 0.5 Gt of CO₂ annually. OAE generally relies on dissolving large amounts of ground-up rocks, either directly in the ocean or indirectly in water that is added to the ocean, to increase alkalinity and remove CO₂. This practice typically requires mining for alkaline rocks, though alkaline materials can also be sourced from waste by-products of other industries (e.g., steel slag, mine tailings) or commercially through human-made substances.

Does it work?

The science behind OAE is theoretically sound, and OAE is expected to result in durable storage over long time periods (>100 years). At scale, OAE could potentially remove over 1 Gt CO₂ /yr, but additional lab and field-based studies are needed to understand whether this approach is effective and safe. The majority of our understanding of OAE comes from models and laboratory experiments. However, when crushed minerals have been dispersed in field studies, the dissolution has not always occurred as expected. Several large-scale experimental trials are currently underway or planned, which will produce real-world data and inform monitoring and verification tactics needed to help refine and guide future implementation. These tests will also provide critical information on any ecological or community impacts. Various ways of implementing OAE are being developed, including ship-based dispersal, shoreline-based systems, and other approaches that leverage existing industrial waste streams or combine with other marine carbon dioxide removal (mCDR) techniques, such as electrochemical alkalinity generation.

Why are we excited?

OAE removes CO₂ from the atmosphere and stores it in the ocean as bicarbonate and carbonate ions, which are stable over long time periods. This means the CO₂ would be durably stored from the atmosphere for thousands of years. OAE could be scaled globally and can also mitigate local ocean acidification, a growing issue that threatens a range of marine ecosystems. Indeed, adding alkalinity to seawater has already been shown to mitigate ocean acidification in some coral reefs. Mitigating ocean acidification could also benefit fisheries and aquaculture, highlighting the potential for OAE to provide additional local benefits beyond carbon removal.

Why are we concerned?

Several technical, environmental, and social concerns surround OAE. The effectiveness could be limited by real-world conditions that either transport the alkaline materials away from the ocean’s surface before CO₂ can be absorbed or result in unexpected chemical reactions or biological uptake of the added alkalinity. Measuring and verifying the amount of CO₂ permanently stored using OAE is also challenging and will rely on a combination of field data and complex numerical models, which will require significant effort to collect and develop. Beyond these technical challenges, OAE poses potential environmental risks on land and in the ocean. On land, OAE could require an expansion of mining that rivals the cement industry, which could have negative environmental impacts on human and ecosystem health. In the ocean, increased alkalinity and the potential release of metals from the source rocks could negatively affect some marine life, but our understanding of the effects on individual species and food webs is limited. OAE could also interfere with existing ocean uses (e.g., fisheries, recreation) in some places and could have other unintended consequences as well. For instance, research suggests that OAE reduces natural alkalinity production in some ocean areas. In addition, OAE faces several social challenges. To be successful, mCDR approaches, like OAE, will require rapid, meaningful, and just community engagement. Public concerns about OAE have already led to a pilot project cancellation, highlighting the importance of public perception for OAE feasibility. It is also unclear if OAE can be scaled globally at reasonable costs, with current estimates highly variable but generally over US$100/t CO₂. Lastly, acquiring and dispersing sufficient alkaline materials could be challenging at scale, particularly because some materials are currently energy-intensive to source, transport, and/or produce.

What is our assessment?

Based on our analysis, enhanced rock weathering is a promising carbon removal technique, but it is not ready for large-scale deployment. We will “Keep Watching” this potential climate solution.

What is it?

Enhanced rock weathering is a practice that removes CO₂ from the atmosphere by accelerating the natural chemical and physical breakdown, or weathering, of rocks such as basalt, olivine, or limestone. This is typically achieved by crushing the rocks into dust or sand-sized particles to increase their surface area before applying them to croplands, beaches, or directly into the ocean, the latter of which is also a form of carbon removal known as ocean alkalinity enhancement. During weathering, the rock surface chemically reacts with atmospheric CO₂ that is dissolved in rain or ocean water. This reaction produces bicarbonate ions containing the carbon from the captured CO₂ and positively charged cations, such as magnesium or calcium, depending on the type of rock. For land-based enhanced rock weathering, the bicarbonate needs to be flushed out to the ocean, where it is stable and can be securely stored for thousands of years.  

Does it work?

The basic idea of enhanced rock weathering is scientifically and geologically sound. Its effectiveness in converting atmospheric CO₂ into bicarbonate has been demonstrated in laboratory and field trials for several rock types and application sites. There are currently numerous research and demonstration projects underway. More than a dozen companies are selling enhanced rock weathering-based carbon removal credits, with nearly 10,000 t CO₂ reported to have been removed as of early 2025.

Why are we excited?

Enhanced rock weathering has several features that improve the likelihood that it can be scaled up to remove and store globally meaningful amounts of atmospheric CO₂ (>0.1 Gt CO₂/yr). Since enhanced rock weathering utilizes a natural process – mineralization – it does not need to be combined with other technologies to capture CO₂ from the air or durably store it. Moreover, it does not require external energy for the carbon capture and storage process, although it does use energy and generate emissions from the mining, crushing, transport, and deployment of the crushed rock. Suitable rock types, such as basalt, which is widely used in construction, paving, and concrete, are common and often locally available. Globally, there are large areas of land and ocean surface on which enhanced rock weathering could be deployed, including on croplands where current agricultural practices often already include regular application of soil amendments. A recent study suggested that extensive deployment of enhanced rock weathering on U.S. agricultural lands could sequester 0.16–0.30 Gt CO₂/yr by 2050. Other studies have shown that the application of crushed rock to croplands for enhanced rock weathering can improve soil pH, provide essential soil nutrients, and improve crop yields.

Why are we concerned?

There are numerous challenges for enhanced rock weathering, as well as potential risks and adverse impacts from its large-scale deployment. Numerous studies on both land- and ocean-based enhanced rock weathering have shown that the amounts of atmospheric CO₂ converted into bicarbonate are highly variable, dependent on rock type, soil type, application rates, and other variables, and are therefore difficult to accurately predict and model. This makes measurement, reporting, and verification of the amount of CO₂ captured and stored, which is essential for the carbon market, reliant on extensive and expensive field measurements and customized models. There are also concerns about the harmful impacts of heavy metals, like nickel or chromium, that can be released during weathering, as well as other ecological impacts and environmental justice concerns, particularly for crushed rock deployed on beaches or in the ocean. Finally, costs for deployment and the purchase of enhanced rock weathering-based carbon credits are relatively high (>US$200–US$500/t CO₂ removed) and will likely remain high if verification continues to depend on large numbers of field measurements and carbon removal cannot be easily modeled. There is a general consensus in the scientific community that the current knowledge base is not sufficient to reliably or accurately quantify the CO₂ captured and stored by most land- or ocean-based enhanced rock weathering deployments.