What is our assessment?

Our analysis finds that Deploy Ocean Biomass Sinking could have high potential environmental and social 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 both seaweed and terrestrial biomass. 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 to remove carbon from the surface ocean. As biomass sinks, 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 natural 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 and provide habitat for marine organisms. Estimates of potential climate impacts suggest that ocean biomass sinking using biomass from seaweed farms could theoretically exceed 0.1 Gt CO₂ /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 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 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 /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. Similarly, seaweed probably has higher value and carbon benefits as food, fertilizer, and other products.

What is our assessment?

Based on the scientific uncertainties regarding its effectiveness and the potential serious environmental and social risks, we conclude that 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 to the deep sea or seafloor, where it can be stored for decades or centuries. Most ocean fertilization efforts are focused on 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 ~1000 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 greenhouse gases, 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 like 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 mitigate 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 range between US$80/t CO₂ and US$457/t CO₂, suggesting this practice might also be relatively inexpensive compared to other marine carbon dioxide 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, as 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 requirement that fertilization be done continuously to prevent the rapid return of sequestered carbon 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 the promotion of harmful algal blooms, which 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, as global-scale modification of marine ecosystems is likely to create inequities in environmental and economic impacts.

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 of 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 (i.e., >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 per 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 modelled. 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.