Electrochemical carbon dioxide removal technologies from seawater, sometimes called “direct ocean capture” to draw comparisons with direct air capture, encompass both Electrodialytic and Electrolytic techniques that capture and remove dissolved inorganic carbon from seawater (either as CO₂ gas or as calcium carbonate), and/or produce a CO₂-reactive chemical base, e.g. sodium hydroxide (NaOH), that can be distributed in the surface ocean to ultimately consume atmospheric CO₂ and convert it to long-lived, dissolved, alkaline bicarbonate.

Electrodialytic approaches are already commercial technologies used in applications such as whey demineralization, organic acids recovery, and desalination, among others.¹

The electrolytic production of H₂, O₂/Cl₂, hydroxide, and acid is a very mature technology that globally produces some 75 Mt of NaOH per year that is an essential reagent in a variety of important industrial processes (e.g., in food processing, soap production, pulp and paper, pharmaceuticals, etc.).²

An electrolytic approach to precipitate calcium carbonate has recently been proposed³. This approach electrochemically generates alkalinity in order to precipitate calcium carbonate, generating a potentially useful end product from the captured carbon. But, as described above, precipitation of calcium carbonate from seawater acidifies the seawater, reducing the ability of seawater to absorb CO₂ from air. This proposed pathway is at a technology readiness level⁴ of ~2, and thus needs more development before further consideration.

• Scalability - Given the immense stocks of dissolved inorganic carbon in seawater (~38,000 Gt¹), the theoretical scale of carbon capture from electrochemical methods is limitless for all practical intents and purposes. However, engineering, economic, political, and social considerations are likely to significantly reduce this upper bound. These include, but are not limited to:
• • Access to low or zero-carbon emissions energy and infrastructure (e.g., desalination plants, offshore wind) with the ability to pump large quantities of seawater into large-scale electrochemical reaction cells, perform electrodialysis or electrolysis, and capture outputs and byproducts. The exact energy and infrastructure needs are pathway-dependent. For instance, electrolysis requires more energy than does electrodialysis, but the relative value of beneficial products (e.g., H₂, O₂ in electrolysis, HCl in electrodialysis) produced by each process must also be considered².
    • In the cases where electrodialysis is used to produce HCl and strip CO₂ gas out of seawater or used to produce NaOH that precipitates calcium carbonate, the cost is high (>$350/ton CO₂) because of the cost of pumping seawater³, and in the case of CO₂ stripping, the additional cost of safely storing or utilizing the captured CO₂⁴
    • Costs can be lowered to ~$100/ton CO₂ if captured CO₂ is stored in the ocean as bicarbonate due to the elimination of seawater pumping costs⁵
    • Electrolytic pathways may offer CDR for $150-100/ton CO₂, depending upon whether revenues generated from co-products are used to offset gross costs of CO₂ capture⁶.
  • The collective market size for suite of co-products and services offered, including concentrated CO₂, H₂, O₂, HCl, control over critical seawater chemistry parameters like pH and total alkalinity, and CDR.

Ranges from recent synthesis reports vary with the 2021 NASEM report ⁷ suggesting a range of 0.1-1.0 Gt CO2/yr with medium confidence, while the 2020 synthesis report from the Energy Futures Initiative suggests a potential range of 1-5 Gt CO2/yr using the "Seawater carbon extraction pathway". ⁸

• Sequestration Permanence - Electrochemical methods that produce reactive alkaline minerals (e.g., NaOH) will result in additional CO₂ from the atmosphere being sequestered in the ocean as bicarbonate ions, which cannot exchange with the atmosphere. This is the same process that results in CO₂ sequestration for rock-based forms of ocean alkalinity enhancement. The residence time of bicarbonate ions in the ocean is ~10,000 years, suggesting that electrochemical alkalinity production will generate CO₂ sequestration with permanence of ~10,000 years.

Electrochemical methods that produce carbonate mineral products (e.g., ref⁹) with sequestered carbon are stable on timescales of hundreds-to-thousands of years. 

Electrochemical methods which capture and remove CO₂ gas from seawater have variable permanence depending upon the storage location for the captured CO₂. If the captured CO₂ is sequestered in geologic storage, permanence can be long (thousands-to-millions of years)

• Electrochemical CDR would likely provide localized reductions in ocean acidification, with expected benefit(s) to marine ecosystems.
• Carbon-negative byproducts from electrochemical CDR can be substituted for more C-intensive sources¹:
  • Electrolysis: hydrogen gas, chlorine gas, and oxygen gas, as well as hydrochloric acid
  • Electrodialysis: hydrochloric acid
• In comparison to rock-based forms of alkalinity enhancement, electrochemical methods may offer:
  • Reduced risk of toxicity from metals present in rock-based forms of alkalinity addition².
  • However, the purity of outputs from electrochemical CDR is determined partially by the purity of the input materials to the electrochemical processes, and purer input materials are more expensive.
  • Unlike ocean liming and coastal enhanced weathering, electrochemical alkalinity additions will not increase the silica concentration in the ocean and, therefore, may offer a reduced risk of disturbing phytoplankton community dynamics between calcifiers and silicifiers³.

1. Risks Shared by Electrolysis and Electrodialysis
   • The potential for changes in water column particle concentrations, turbidity, and optical properties if precipitation (inorganic mineral formation) of carbonates occurs due to local increases in alkalinity and pH.  
   • Mortality of marine life through seawater intake pumps and filters
   • Shifts in phytoplankton, invertebrate and vertebrate physiology, competition and/or mortality due to decreases in acidity as CO₂ is removed and/or alkalinity is added.¹ For example, do the preceding seawater chemistry changes provide an increased competitive advantage for calcifiers over and above the restoration of calcifiers/calcification from ongoing ocean acidification?
   • Risks about permanence and impacts of elevated concentrations of bicarbonate in the oceans.
2. Risks Specific to Electrolysis
   • Chlorine gas production and handling
3. Risks Specific to Electrodialysis
   • Production of large quantities of acid via electrodialysis that must be safely consumed or neutralized (e.g., via reaction with alkaline minerals).
   • For pathways that strip CO₂ gas from seawater and sequester the CO₂, risks of permanence of storage and potential for leakage
   • For pathways that remove dissolved inorganic carbon from seawater as calcium carbonate², impacts of large quantities of new calcium carbonate on diversity, abundance, and ecosystem function of marine environments

1. Localized reductions in ocean acidification, with expected benefit(s) to industries that rely on these localized improvements such as aquaculture and tourism
2. Creation of employment in industries producing carbon-negative byproducts from electrochemical CDR ²:
   • Electrolysis: hydrogen gas, chlorine gas, and oxygen gas, as well as hydrochloric acid
   • Electrodialysis: hydrochloric acid

• Electrochemical technologies, as applied to ocean-based CDR, are very new. Because of the lack of proof-of-concept field experiments, it has so far been impossible to characterize benefits, risks, and scaling consideration of OAE in real-world settings (i.e. not benchtop). Controlled field experiments across diverse ecosystems to determine marine chemistry and biology impacts and feedbacks are needed (Develop New Modeling Tools to Support Design and Evaluation, Accelerate Design and Permitting of Controlled Field Trials).
• It is challenging to verify additional CO2 uptake from the atmosphere as a result of electrochemical CDR given the ocean’s dynamic CO₂ flux “background state”. New methodologies are needed to observe additional sequestration from the atmosphere into the ocean (Develop New Modeling Tools to Support Design and Evaluation, Develop New In-Water Tools for Autonomous CDR Operations)
• Laboratory experiments are needed across a range of seawater chemistries expected because of expected electrochemical CDR variations in seawater total alkalinity and dissolved inorganic carbon¹ to characterize environmental impacts (Accelerate Design and Permitting of Controlled Field Trials).
  • Look to the ocean acidification community’s effort to develop standardized protocols for guidance² as the community builds out standardized protocols and treatments levels for consistency and inter-comparability
• Global, local, and regional predictions of physical, chemical, and biological outcomes and feedbacks of electrochemical CDR from high-resolution models (Develop New Modeling Tools to Support Design and Evaluation)
• Life cycle assessments to calculate net CDR benefits, taking into account all emissions associated with supply chains (Develop CDR Monitoring and Verification Protocols). 

• Current observational technologies (sensors, ROVs, AUVs, etc.) and modeling tools are not widespread and easily available to support field trials with the necessary spatial and temporal frequency of monitoring and sampling (Develop New In-Water Tools for Autonomous CDR Operations)
• In the case of surface seawater alkalinity addition from electrolysis or electrodialysis, cost-effective and safe methods need to be developed that optimize the distribution, dispersal and dilution of any strong chemical bases to avoid impacts of excessively alkaline (pH>9) waters on marine ecosystems. Dilution may require pumping large amounts of seawater, which need to be optimized for energy and cost (Develop New In-Water Tools for Autonomous CDR Operations).
• Offshore deployment of either electrodialysis or electrolysis would require further development to overcome challenges of working in the ocean (corrosion, biofouling, storms, physical and chemical stress on electrodes, catalysts, and membranes, etc.) (Develop New In-Water Tools for Autonomous CDR Operations). 
• Questions remain about how variations in seawater particulate and dissolved organic matter, temperature, and salinity affect electrochemical CDR efficiency (Develop New In-Water Tools for Autonomous CDR Operations). 
  • 1. Methods of handling chlorine gas produced as a byproduct of seawater electrolysis are needed (Develop New In-Water Tools for Autonomous CDR Operations, Improve Understanding of Markets for Co-Products). As an example, chlorine can be used to extract lipids from microalgae ¹ and the oil can then be used to produce high density polyethylene ² hull structures for offshore electrochemical CDR operations. As such, this chlorine management path may lead to a largely self replicating HDPE-based marine CDR infrastructure at the basic materials level. Self replicating offshore infrastructure is highly valuable for DOC operations and the USN is currently 3D printing thick walled HDPE submarines ³ yet offshore electrochemical and biomass CDR reactor hulls can simply be capped off extruded pipes of large diameter and lenght ⁴, no complex 3D production expense needed.
    2. Development of feedback control systems that integrate nearby observational data to determine optimal levels of electrochemical CDR (Develop New Modeling Tools to Support Design and Evaluation, Develop New In-Water Tools for Autonomous CDR Operations). 
       • Identifying and understanding how equipment, energy, and cost scale from benchtop experiments to small field experiments to globally-relevant deployments (Develop New Modeling Tools to Support Design and Evaluation, Develop New In-Water Tools for Autonomous CDR Operations, Develop CDR Monitoring and Verification Protocols, Accelerate RD&D Through New Partnerships)

• Current observational technologies (sensors, ROVs, AUVs, etc.) and modeling tools are not widespread and easily available to support field trials with the necessary spatial and temporal frequency of monitoring and sampling (Develop New In-Water Tools for Autonomous CDR Operations)
• In the case of surface seawater alkalinity addition from electrolysis or electrodialysis, cost-effective and safe methods need to be developed that optimize the distribution, dispersal and dilution of any strong chemical bases to avoid impacts of excessively alkaline (pH>9) waters on marine ecosystems. Dilution may require pumping large amounts of seawater, which need to be optimized for energy and cost (Develop New In-Water Tools for Autonomous CDR Operations).
• Offshore deployment of either electrodialysis or electrolysis would require further development to overcome challenges of working in the ocean (corrosion, biofouling, storms, physical and chemical stress on electrodes, catalysts, and membranes, etc.) (Develop New In-Water Tools for Autonomous CDR Operations). 
• Questions remain about how variations in seawater particulate and dissolved organic matter, temperature, and salinity affect electrochemical CDR efficiency (Develop New In-Water Tools for Autonomous CDR Operations). 
  • • Methods of handling chlorine gas produced as a byproduct of seawater electrolysis are needed (Develop New In-Water Tools for Autonomous CDR Operations, Improve Understanding of Markets for Co-Products)
    • Development of feedback control systems that integrate nearby observational data to determine optimal levels of electrochemical CDR (Develop New Modeling Tools to Support Design and Evaluation, Develop New In-Water Tools for Autonomous CDR Operations). 
    • Identifying and understanding how equipment, energy, and cost scale from benchtop experiments to small field experiments to globally-relevant deployments (Develop New Modeling Tools to Support Design and Evaluation, Develop New In-Water Tools for Autonomous CDR Operations, Develop CDR Monitoring and Verification Protocols, Accelerate RD&D Through New Partnerships)

Many of the opportunities and challenges around building public support are not specific to electrochemical CDR, but there are several points of interest specific to electrochemical CDR: 

• Electrochemical CDR faces challenges in terms of public perception regarding potential environmental risks not necessarily faced by more “nature-based” approaches, such as coastal blue carbon restoration¹ (Accelerate RD&D Through New Partnerships)

• Complexities of electrochemical CDR pathways inhibit understanding and evaluation by non-technical audiences
• There is a great deal of uncertainty and confusion around any ocean-based CDR pathway that relies on adding materials to the ocean. This point applies to both electrochemical alkalinity additions, which return a base to the seawater, and pathways that precipitate out calcium carbonate.
• Earlier ocean iron fertilization experiments² could offer opportunities to adopt best practices and avoid mistakes made when building public support for electrochemical CDR. 
• Electrochemical CDR has the advantage that it can be “turned off” more easily than the other approaches, enabling great control over its application

• Clearer communication strategies need to be developed to respond to the “engineering” narrative of electrochemical CDR.

High-resolution data-assimilative models that can support real-world testing of electrochemical CDR are required. These modeling tools must:

• Account for complex interactions in the immediate vicinity of the electrochemical CDR and downstream impacts
• Provide four-dimensional (space and time) estimates of biogeochemistry in zone of influence both in the presence and absence of OAE. The difference between these two simulations can be used to inform CDR estimates that account for background variability in the ocean. 
  • CDR estimates from electrochemical CDR must include estimates of the “opportunity cost” of electrochemical CDR - how did electrochemical CDR shift phytoplankton community composition, production, and export? 

To support the design of proof-of-concept field trials, these models should also: 

• Provide estimates of the size and scale of biogeochemical modification to the ecosystem from electrochemical CDR, allowing for informed placement of sensors to monitor the field trials
• Be capable of simulating passive tracers (e.g. SF₆) to inform whether and how these passive tracers may be useful in field trials (e.g., estimating rates of atmospheric CO₂ uptake)
• Inform a prioritized set of predictions to be tested during field trials

Field trials for the various electrochemical CDR technologies are urgently needed to test both carbon sequestration potential and environmental impacts (both positive and negative). A series of steps are needed to get to a series of controlled field trials.  They include:

Field trials for the various electrochemical CDR technologies are urgently needed to test both carbon sequestration potential and environmental impacts (both positive and negative). A series of steps are needed to get to a series of controlled field trials.  They include:

A new suite of durable, seagoing technologies are needed to support electrochemical CDR RD&D. Technology development needs include: 

Standardized methodologies from third parties to verify uptake of atmospheric CO₂ resulting from electrochemical CDR will ultimately need to be developed to enable trading of carbon removal credits. Key first steps to support development of these protocols include:

• Convening experts to review advances from modeling tools (Develop New Modeling Tools to Support Design and Evaluation) and controlled field trials (Accelerate Design and Permitting of Controlled Field Trials) to identify satisfied and outstanding data needs necessary to quantify additional CO₂ uptake as a direct result of electrochemical CDR. As advances in electrochemical CDR RD&D are made, the satisfied and outstanding data needs will need to be updated.
• Apply existing¹², or develop when necessary, life cycle analysis tools to calculate stored carbon after accounting for emissions from required materials, energy, transportation/dispersal, etc.
• Include aspects of sustained monitoring to verify CDR permanence over long time scales as CDR is scaled.

Research, development, and demonstration of electrochemical CDR may be accelerated and strengthened by creating partnerships with key industries/sectors, including: 

• Offshore renewable energy production, including wind and others, both as power sources and as integrated CDR platforms
• Coastal industries, including desalination and wastewater treatment facilities, which already have infrastructure for pumping/processing seawater or wastewater for CO₂ extraction or alkalinity addition.
• Marine research laboratories that already pump seawater and have expertise, technical equipment and infrastructure to support research and development
• Decommissioned or active offshore oil platforms, wells, and reservoirs as sites for CDR and CO₂ sequestration. 
• Finfish and shellfish aquaculture where the CDR-produced CO₂ and/or alkalinity can be used to optimize chemical conditions, including providing relief from ocean acidification. 

Developing and strengthening relationships with partner industries may also help promote public acceptance, as well as potentially offer faster routes to obtaining the necessary permitting.

Electrochemical CDR may generate a host of co-products. Assessing and mapping potential new markets to accommodate these co-products is an important step towards determining the feasibility of electrochemical CDR approaches at scale.

Potential co-products include:

• Hydrogen (H₂) gas – as a fuel, energy storage medium, and as a feedstock
• Hydrochloric Acid (HCl) – potentially to refine waste products from silicate mining¹
• Oxygen (O₂) gas
• Chlorine (Cl₂) gas – can be combusted with H₂ gas to form HCl

1. Carbon Capture - The ultimate CDR potential of large-scale macroalgal cultivation and sequestration is as yet difficult to determine. Recent consensus reports cite the following as possible carbon dioxide removal potential from macroalgae cultivation and sequestration:
   • 0.1 – 1.0 Gt CO2/year (NASEM 2022: A Research Strategy for Ocean-Based Carbon Dioxide Removal and Sequestration)
   • 0.1 – 0.6 Gt CO2/ year (NOAA 2022: Carbon Dioxide Removal Research)

Theoretically, macroalgal cultivation and sequestration  could be scaled to between one and greater than ten gigatons of CDR per year¹, but realized CDR may be much lower due to a number of factors, including:

• • Competition for space with existing ocean stakeholders – commercial shipping,  commercial fishing, indigenous rights, marine protected areas, non-seaweed aquaculture, etc. 
  • Challenges with expanding to offshore environments – engineering of moorings  and farm structures, challenges with operations and maintenance in distant environments²,³- and the likely associated cost increases (at least at first)  associated with offshore operations
  • Most open ocean environments require nutrient additions to sustain macroalgal yields. If these nutrients come from the deep ocean, they will be accompanied by high CO₂ water that can be released and will offset macroalgal carbon sequestration to some degree (which needs to be quantified).⁴,⁵
    • If macroalgal farms are located closer to the coastal zone, excess anthropogenic nitrogen inputs could support ~3.5 million km2 of macroalgal cultivation, corresponding with an estimated CDR range of 5- 10 Gt CDR annually⁶, alleviating competition for “natural” nutrient supplies with phytoplankton and alleviating the need for an upwelled supply of nutrients (and the associated risks of releasing CO₂ from upwelled deep ocean waters).
  • It is hypothesized that macroalgae-based pathways could supply CDR at moderate cost ($25-125 per ton of CO₂ removed)⁷,⁸,²², but these cost estimates are preliminary and subject to change as macroalgae-based pathways advance in technological readiness⁹.
    • Researchers at UC Irvine are developing a biophysical-economic model to better understand the geophysical, biological, and economic sensitivities of macroalgae-based CDR pathways¹⁰

2. Sequestration Permanence -For cultivated macroalgae to contribute to CDR, the carbon captured in the macroalgal tissue needs to be sequestered to prevent its decomposition and return to the atmosphere. Sequestration pathways include:

• • Intentionally sinking the macroalgae (pre- or post-processing) into the deep ocean (>1000 meters depth) where it can be sequestered from contact with the atmosphere for hundreds to a few thousand years¹¹ (if remineralized in the deep ocean) or for thousands-to-millions of years (if buried in marine sediments).¹²
    • There are natural analogues to this process already. In macroalgal systems, already ~50% of natural macroalgal production breaks off and becomes dissolved or particulate organic matter. Some of the particulate organic matter is transported to the deep ocean, where the carbon is sequestered.¹³
  • Relying on sedimentary burial below or adjacent to the farm (typically more applicable for farms not located over deep water).
    • Oceans2050 is working to determine the permanence of burial under coastal farms
  • Harvesting the macroalgae for:
    • Bioenergy¹⁴
      • Combining combustion pathways with carbon capture and storage¹⁵ (thousands-to-millions of years if stored in a geologic reservoir)
      • Pyrolysis, resulting in biochar¹⁶,¹⁷ (hundreds-to-thousands of years)
    • Production of long-lived bioproducts, such as bioplastics, that are capable of permanent (>100 years) sequestration^18}

3. Differentiating from Avoided Emissions - Macroalgae can provide a number of other services that either reduce or replace greenhouse gas emissions, but do not remove legacy carbon dioxide pollution from the atmosphere. One example of reducing emissions: red algal supplements added to cattle feed may play a key role in reducing enteric methane emissions from cattle by over 50%¹⁹,²⁰. Given methane’s potency as a greenhouse gas, such emissions reductions may play an outsized role in slowing planetary temperature increases through the reduction of greenhouse gases. Macroalgae biomass may also be converted into short-lived, high-value bioproducts, such as food and nutritional supplements²¹. This process may contribute to avoided emissions if it substitutes for carbon-intensive feedstocks, but it does not represent CDR because of the short timescale over which these products sequester carbon before it returns to the atmosphere. Although the focus of this roadmap is on macroalgae pathways for CDR, many of the obstacles faced, development needs, and near-term opportunities for advancement are relevant to these associated pathways for avoided greenhouse gas emissions.

• Localized buffering/reductions in ocean acidification due to CO₂ uptake. ¹,²,³,⁴,⁵

• Nutrient remediation and metal uptake in eutrophied, polluted coastal waters ⁶
• Building macroalgae cultivation facilities near shellfish or fish aquaculture facilities may alleviate negative impacts from such activities⁹ (e.g., deoxygenation, eutrophication)

• Macroalgae farms may attenuate wave energy ⁷
• Creation of habitat with resulting nurseries for fish and other marine life ⁸

• Carbon capture at sea may reduce the demand for terrestrial-based CDR alternatives that compete for land and/or freshwater

• Macroalgae are known to release bromoform and other halomethanes⁴,⁵, and thus, large-scale macroalgae cultivation seems likely to increase the release of these substances. This needs additional research as the natural marine sources of these gases are currently estimated to be responsible for around 9% of stratospheric ozone loss, including depletion due to anthropogenic causes⁶.
• Production of methane, nitrous oxide, and other potentially hazardous gases by the macroalgae
• The potential for CO₂ outgassing from pumping deep water to the surface  (artificial upwelling) to supply needed nutrients for the macroalgae³.
• Ecological and biogeochemical (especially acidification and hypoxia) impacts in the deep sea from sinking large quantities of macroalgae into the deep ocean
  • Such effects are likely to be location-specific

• Impacts to biodiversity and ecosystem function from large-scale cultivation operations, including¹,²:
  • Enhanced disease and parasite risk
  • Alteration of population genetics
  • Introduction of non-native species into new environments
• Enhancement in epiphytic calcifiers that could offset carbon sequestration through calcification-induced CO₂ release
• Reduced phytoplankton production in and around large macroalgae farms due to competition for nutrients and light
• Ecological and biogeochemical (especially acidification and hypoxia) impacts in the deep sea from sinking large quantities of macroalgae into the deep ocean
  • Such effects are likely to be location-specific

• Changes in light and nutrient availability (including possible changes in ocean albedo)
• In near-coastal zones, changes in circulation patterns due to drag by the seaweed could change residence time in nearshore environments
  • Changes in coastal residence time may affect the prevalence and intensity of harmful algal blooms (HABs)
• Entanglement of marine megafauna (e.g., whales)

• Job Creation: Seaweed farming could also present new job opportunities for fishers whose work is threatened by climate change⁴.
• Value-Added Products: High-value bioproducts (pigments, lipids, proteins, etc) can replace more carbon-intensive alternatives in feed, food, fuel, and other commodities.

1. Competition for Space: Competition for space with existing ocean stakeholders – commercial shipping, commercial fishing, indigenous rights, marine protected areas, non-seaweed aquaculture, etc.
   • Potential for entanglement of macroalgae cultivation equipment with shipping, commercial fishing gear, aquaculture farms etc, particularly if the macroalgae farm is free floating
2. Financing: Financing any CDR approach brings with it the risk of creating inequity and decreasing social welfare⁴. With macroalgae cultivation, this may be especially applicable to coastal dwelling communities who rely on the ocean for their livelihoods, food, and cultural meaning.

• Proof-of-concept field experiments have not been conducted in open ocean conditions to test growth rates, sequestration potential, and environmental impacts of macroalgal CDR pathways (Accelerate Design and Permitting of Controlled Field Trials)
• Siting analyses are needed to identify optimal nutrient, light, and wave conditions for growth; as well as potential conflicts with other marine industries (such as the NOAA Coastal Aquaculture Siting and Sustainability toolkit) (Develop New Modeling Tools to Support Design and Evaluation)
• A suite of tools and methodologies to estimate productivity, carbon capture, export, and sequestration, including direct and remote sensing approaches¹ needs to be built (Develop New Modeling Tools to Support Design and Evaluation, Measure the Scale and Impacts of CDR via Macroalgae Sinking, Develop New In-Water Tools for Autonomous CDR Operations). 
  • For sinking pathways, the challenges of following the carbon from source (farm) to deposition (seafloor) in energetically active environments (horizontal and vertical currents, turbulent areas, etc.) 
• We do not understand enough about the net CDR benefit from a life cycle perspective (Develop New Modeling Tools to Support Design and Evaluation, Accelerate Design and Permitting of Controlled Field Trials, Develop New In-Water Tools for Autonomous CDR Operations, Develop CDR Monitoring and Verification Protocols)
• Challenges exist verifying additional CO₂ uptake from the atmosphere to the ocean in a dynamic background as a result of macroalgae cultivation and sequestration CDR pathways (Develop New Modeling Tools to Support Design and Evaluation, Accelerate Design and Permitting of Controlled Field Trials, Develop New In-Water Tools for Autonomous CDR Operations)
• Inclusion of various macroalgal CDR into integrated assessment models to simulate and predict complex, global responses to macroalgal CDR. Examples include changes in CO₂ fluxes in other ecosystems via teleconnections (connections in Earth processes and non-continuous geographic regions, which are often caused by processes not immediately apparent from first principles), and to estimate permanence via the various macroalgal CDR pathways (Develop New Modeling Tools to Support Design and Evaluation)
• Increased knowledge base regarding the physiology (including heat tolerance) and genomics of a broader array of potential cultivars – beyond the ten most common - to understand their growth and carbon sequestration potential is needed (Develop New Modeling Tools to Support Design and Evaluation, Accelerate Design and Permitting of Controlled Field Trials) 
• Better understanding of how large-scale macroalgae cultivation affects the partitioning of carbon between particulate and dissolved phases, and its implications for CDR (Develop New Modeling Tools to Support Design and Evaluation, Accelerate Design and Permitting of Controlled Field Trials)

• Advances are needed in offshore mooring design, as well as viable, durable, and cost-effective farming systems for offshore cultivation technology¹  (Develop New In-Water Tools for Autonomous CDR Operations)
• Current observational technologies (sensors, ROVs, AUVs, etc.) and modeling tools are not widespread or easily available to support field trials with the necessary spatial and temporal frequency of monitoring and sampling (Develop New In-Water Tools for Autonomous CDR Operations)
• Technologies to accelerate macroalgal sinking are not well explored or understood (Accelerate Design and Permitting of Controlled Field Trials, Develop New In-Water Tools for Autonomous CDR Operations)
• Wave-powered devices (both for electrical power and upwelling) are in their infancy (Develop New In-Water Tools for Autonomous CDR Operations)
  • Upwelling systems must be designed and tested to access deeper water nutrients and keep them in the euphotic zone where they can support macroalgal growth (as opposed to immediately sinking out of the euphotic zone)
• Harvesting and processing technologies that minimize environmental impact and energy use are needed (Develop New In-Water Tools for Autonomous CDR Operations)
  • More efficient means of drying the harvested crop (for non-sinking pathways)
• Technologies to scale shore-based hatcheries to produce more juvenile kelp ready for outplanting are needed (Develop New In-Water Tools for Autonomous CDR Operations)

• Training, skills development, and technology transfers are needed globally to expand available workforce for macroalgal cultivation and sequestration (Accelerate RD&D Through New Partnerships, Broaden Funding Base for RD&D, Growing & Maintaining Public Support) 
• Stable, increased research and development funding to build capacity in this sector, such as has been seen with the ARPA-E MARINER program in the US (Broaden Funding Base for RD&D)

• Certification standards for macroalgal carbon sequestration are needed to support developing markets for macroalgae CO₂ sequestration (Develop CDR Monitoring and Verification Protocols) 
• Scalable business models and markets to support demand for the multitude of potential products that can be derived from macroalgae (nutritional supplements, high value food items, additives, biochar, bioenergy, etc.) are needed to support this industry (Accelerate RD&D Through New Partnerships, Broaden Funding Base for RD&D, Growing & Maintaining Public Support) 

Many of the obstacles and needs around building and maintaining public support are not specific to macroalgal cultivation and sequestration, but there are a few points of interest specific to macroalgal cultivation pathways (Growing & Maintaining Public Support):

• Industrial-scale farms could have a negative public connotation (e.g. corn fields/monoculture in the ocean)
• If genetically modified macroalgae were to be used to increase cultivation yields and sequestration potential, especially in the context of industrial-scale macroalgae farms, that might be perceived negatively by the public given public views on genetically modified organisms 
• Public perceptions of sinking macroalgal may differ from perceptions about conversion of cultivated macroalgae into high value bio products

High-resolution data-assimilative models are needed to support real-world testing of macroalgal CDR pathways. These modeling tools must:

• Account for complex interactions in the vicinity of the farm and downstream impacts¹²
• Provide four-dimensional (space and time) estimates of biogeochemistry in zone of influence both in the presence and absence of macroalgae cultivation. The difference between these two simulations can be used to inform CDR estimates that account for background variability in the ocean. 
  • CDR estimates from macroalgae must include estimates of the “opportunity cost” of macroalgal carbon sequestration (how much production and export would have occurred in natural phytoplankton communities in the absence of a macroalgal farm?) as well as any enhancements to marine ecosystem productivity due to the presence of a macroalgal farm (is phytoplankton production enhanced above background levels due to the presence of macroalgae farms?).

To support the design of proof-of-concept field trials, these models should also: 

• Provide estimates of the size and scale of biogeochemical modification to the ecosystem from macroalgae cultivation, allowing for informed placement of sensors to monitor the field trials.
• Be capable of simulating passive tracers (e.g. SF₆) to inform whether and how these passive tracers may be useful in field trials (e.g., estimating rates of atmospheric CO₂ uptake).
• Inform a prioritized set of predictions to be tested during field trials.

Proof-of-concept field trials are urgently needed to test both cultivation and sequestration technologies, as well as the full array of impacts. Field trials are needed to test predictions regarding: 

• Cultivation yields and their dependence on species, ocean basin, nutrient availability, and farm design among others
• Performance of deep-water moorings
• Impacts to pelagic marine ecosystems
• Performance of harvesting technologies
• Additional CO₂ uptake from the atmosphere into the macroalgae

• "Answering Critical Questions About Sinking Macroalgae for Carbon Dioxide Removal" (2022) ¹

Specific field experiments around the impacts of sinking seaweed in the deep ocean are urgently needed. Given the extensive amount of macroalgae cultivated and harvested in the coastal zone, we have more advanced knowledge of the scale impacts of natural accretion into sediments. We now need a global research effort around the fate and impacts of sinking seaweed in the deep ocean. To advance this agenda we need to convene scientists, engineers, seaweed farmers and more to: 

• Identify existing deep-sea observatories, facilities, and vehicles to accelerate research on the fate of macroalgal carbon intentionally sunk in the deep ocean
• Develop a standardized list of biological indicators to measure during field trials to facilitate intercomparison between field trials
• Collaborate with existing farms to conduct field trials. This may be especially important for offshore farms that may be more representative of the open ocean conditions necessary to scale cultivation to achieve globally relevant CDR.
• Evaluate the potential to conduct sinking trials with portions of the supply of floating Sargassum patches to evaluate CDR and environmental impacts of sinking. Globally, there are ~10 gigatons of carbon in these Sargassum patches¹, much of which is otherwise destined to end up on beaches, where it may become an environmental nuisance and will decompose, returning its carbon to the atmosphere. This may also serve as a way of generating public support for macroalgae CDR given the considerable social, economic, and environmental issues caused by Sargassum patches. 
• Review oceanographic data from past cruises, coastal observing systems, and buoy data to acquire needed physical, chemical, and biological information to assess site potential for field experiments. 
  • Data from pilot studies investigating the effects of kelp on local mitigation of ocean acidification (e.g., in the state of Washington, USA) may also provide useful information on macroalgae growth rates/carbon sequestration rates, as well as environmental co-benefits and risks.

Specific field experiments around the impacts of sinking seaweed in the deep ocean are urgently needed. Given the extensive amount of macroalgae cultivated and harvested in the coastal zone, we have more advanced knowledge of the scale impacts of natural accretion into sediments. We now need a global research effort around the fate and impacts of sinking seaweed in the deep ocean. To advance this agenda we need to convene scientists, engineers, seaweed farmers and more to: 

• Identify existing deep-sea observatories, facilities, and vehicles to accelerate research on the fate of macroalgal carbon intentionally sunk in the deep ocean
• Develop a standardized list of biological indicators to measure during field trials to facilitate intercomparison between field trials
• Collaborate with existing farms to conduct field trials. This may be especially important for offshore farms that may be more representative of the open ocean conditions necessary to scale cultivation to achieve globally relevant CDR.
• Evaluate the potential to conduct sinking trials with portions of the supply of floating Sargassum patches to evaluate CDR and environmental impacts of sinking. Globally, there are ~10 gigatons of carbon in these Sargassum patches¹, much of which is otherwise destined to end up on beaches, where it may become an environmental nuisance and will decompose, returning its carbon to the atmosphere. This may also serve as a way of generating public support for macroalgae CDR given the considerable social, economic, and environmental issues caused by Sargassum patches. 
• Review oceanographic data from past cruises, coastal observing systems, and buoy data to acquire needed physical, chemical, and biological information to assess site potential for field experiments. 
  • Data from pilot studies investigating the effects of kelp on local mitigation of ocean acidification (e.g., in the state of Washington, USA) may also provide useful information on macroalgae growth rates/carbon sequestration rates, as well as environmental co-benefits and risks.

Specific field experiments around the impacts of sinking seaweed in the deep ocean are urgently needed. Given the extensive amount of macroalgae cultivated and harvested in the coastal zone, we have more advanced knowledge of the scale impacts of natural accretion into sediments. We now need a global research effort around the fate and impacts of sinking seaweed in the deep ocean. To advance this agenda we need to convene scientists, engineers, seaweed farmers, and more to:

• Identify existing deep-sea observatories, facilities, and vehicles to accelerate research on the fate of macroalgal carbon intentionally sunk in the deep ocean
• Develop a standardized list of biological indicators to measure during field trials to facilitate intercomparison between field trials
• Collaborate with existing farms to conduct field trials. This may be especially important for offshore farms that may be more representative of the open ocean conditions necessary to scale cultivation to achieve globally relevant CDR.
• Evaluate the potential to conduct sinking trials with portions of the supply of floating Sargassum patches to evaluate CDR and environmental impacts of sinking. Globally, there are ~10 gigatons of carbon in these Sargassum patches¹, much of which is otherwise destined to end up on beaches, where it may become an environmental nuisance and will decompose, returning its carbon to the atmosphere. This may also serve as a way of generating public support for macroalgae CDR given the considerable social, economic, and environmental issues caused by Sargassum patches. 
• Review oceanographic data from past cruises, coastal observing systems, and buoy data to acquire needed physical, chemical, and biological information to assess site potential for field experiments. 
  • Data from pilot studies investigating the effects of kelp on local mitigation of ocean acidification (e.g., in the state of Washington, USA) may also provide useful information on macroalgae growth rates/carbon sequestration rates, as well as environmental co-benefits and risks.

A new suite of durable, seagoing technologies are needed to support macroalgae CDR RD&D. Technology development needs include: 

Standardized methodologies from third parties to verify uptake of atmospheric CO₂ resulting from macroalgae carbon sequestration will ultimately need to be developed to enable trading of carbon removal credits. Key first steps to support development of these protocols include:

Research, development, and demonstration of macrolgae CDR may be accelerated and strengthened by creating partnerships with key industries/sectors, including:  

• Integrated multi-trophic aquaculture¹: Integration with finfish and shellfish to leverage cost savings with operations and achieve permaculture-style benefits (macroalgae can take up waste products generated by fish; fish can feed on macroalgae, the increase in pH from the presence of algae can also be beneficial for shellfish whose growth is already threatened by ocean acidification).
• Offshore wind farms are often viewed as “dead space” for other marine spatial uses, but could provide power sources and platforms for macroalgal farms
• Microalgae companies to adopt best practices for shore-based nursery facilities, such as nutrient and light needs to optimize growth and facility design(s) that maximize growth potential and minimize cost.

Developing and strengthening relationships with partner industries may also help promote public support, as well as potentially offer faster routes to obtaining the necessary permitting.

Injection of significant funding is critical to move forward needed RD&D projects for CDR generally, for ocean-based CDR, and for macroalgae CDR specifically.  In addition to national and subnational governmental support that is vitally needed and has been outlined in the Expanding Finance and Investment road map, additional stakeholders that need to be engaged include: 

First-Order Priorities to build public support for ocean-based CDR pathways are found in the Public Support road map, but there are some specific elements that can be emphasized to cultivate public support around macroalgae-based CDR:  

These include: 

• The perceived advantage of nature-based approaches for macroalgae pathways¹
• Identification and quantification of the co-benefits associated with macroalgae cultivation
• Inclusion of coastal blue carbon ecosystems as part of a campaign to build broad public support for macroalgae. 

This document covers technologies designed to produce ocean alkalinity enhancement (OAE) from: 

1. Mining and distribution of natural rock-based alkaline minerals, including olivine and other silicate rocks as well as limestone and other carbonate minerals, either in open ocean (ocean liming) or coastal environments (coastal enhanced weathering^1891011.
2. Production of hydroxide minerals, including lime/slaked lime² from thermal calcination and magnesium hydroxide from synthetic weathering of olivine³ and distribution in the open ocean.
3. Accelerated weathering of limestone (AWL) in ex-situ reactors to sequester non-fossil, point source CO₂ emissions. Note that AWL requires a concentrated source of CO₂ in seawater because carbonate minerals are oversaturated in seawater at ambient CO₂ concentrations. 
4. Production and addition of hydrated carbonate minerals to seawater for increased alkalinity¹²

In contrast to recent reports⁴⁵⁶, we consider these three pathways of OAE together because of the common upstream and downstream considerations necessary to accelerate the development and testing of OAE pathways.

We consider all electrochemical-based technologies for ocean-based CDR, including alkalinity enhancement, in a separate road map due to their separate set of upstream and downstream considerations for electrochemical processes.

We also do not consider enhanced rock weathering in terrestrial ecosystems here despite its similarities with coastal enhanced weathering and its potential for gigaton-scale CDR⁷. Enhanced rock weathering in agricultural fields presents its own set of obstacles and development needs, many of which are distinct from those of the marine pathways considered in these road maps.

Addition of carbonate minerals to stabilize seawater carbonate chemistry has already been occurring in aquaculture facilities for ~10 years¹, largely as a response to the hatchery failures ~2010 in the US Pacific Northwest and British Columbia from ocean acidification². But for large scale OAE in open-ocean environments, current technological readiness is largely limited to laboratory experiments and/or modeling studies (~3-5 on a technological readiness scale³). However, there have been at least two field studies completed and 3 others in planning. 

• The Chalk-Ex experiment released crushed calcium carbonate in a ~1.5 km² patch in the Gulf of Maine, although the primary objective of this experiment was to quantify the effects of particles on upper ocean layer optical properties, not to achieve CDR⁴. 
• There has been at least one small-scale (~50m x 50m) OAE field trial (sodium hydroxide addition) on a coral atoll in the Great Barrier Reef in order to evaluate the ecosystem responses to ocean acidification mitigation⁵. 
• Vesta has plans for a demonstration of coastal enhanced weathering of olivine in Southampton, NY⁹, adding 500 cubic yards of olivine sand to the second phase of a beach nourishment effort. Vesta is currently working on another demonstration in Puerto Playa, Dominican Republic¹⁰⁶. 
• Researchers at the Universiteit Antwerpen and Delft University of Technology are planning a 1000 ton coastal enhanced weathering field experiment in the North Sea in 2024⁷.
• Mesocosm experiments have been conducted as part of the OceanNETs project in the European Union’s Horizon 2020 program⁸.

1. Carbon Capture

   Due to the vast quantities of alkaline rocks, and the capacity of the ocean to accommodate similarly vast quantities of bicarbonate ions, OAE has the theoretical potential to capture tens of gigatons of carbon dioxide annually¹. But given current technological readiness, this remains theoretical. Recent consensus reports cite the following as possible carbon dioxide removal potential from ocean alkalinity enhancement:

   • >0.1 – 1.0 Gt CO2/year (NASEM 2022: A Research Strategy for Ocean-Based Carbon Dioxide Removal and Sequestration)
   • 1 – 15+ Gt CO2/ year (NOAA 2022: Carbon Dioxide Removal Research)

Technical, economic, social, political, and governance factors may also decrease this theoretical CDR potential, although the degree to which they limit the CDR remains to be determined. Cost estimates currently vary between studies with estimated ranges of $72-159/ton CO₂ (variation due to variations in source rock, means of mining and production^{2}), $60-110/ton CO₂3, and $<25-125/ton CO₂4. More work is necessary to produce ground-truthed cost estimates.}

1. Sequestration Permanence

   OAE will result in additional CO₂ from the atmosphere being sequestered in the ocean as bicarbonate ions, which cannot exchange with the atmosphere. The residence time of bicarbonate ions in the ocean is ~10,000 years, suggesting that OAE sequestration would be stable for ~10,000 years. 

2. Accelerated Weathering of Limestone: Avoided Emissions and Hybrid Methods

   When AWL is used to sequester carbon dioxide resulting from the combustion of fossil fuels, it represents avoided emissions, not removal of atmospheric carbon dioxide (negative emissions)⁵⁶. However, there exist a number of hybrid possibilities to integrate AWL with various non-fossil sources of concentrated carbon dioxide to permanently sequester the carbon dioxide as bicarbonate ions in the ocean. These hybrid approaches include:

   • AWL coupled with direct air capture – captured CO₂ from a direct air capture (DAC) facility can be concentrated and reacted with limestone in the presence of seawater to trap the captured CO₂ as bicarbonate ions. 
     • The bicarbonate-rich effluent of DAC + AWL can also serve as a substrate to support algal growth⁷⁸; however newly-produced algal carbon must be sequestered from return to the atmosphere to generate negative emissions over relevant permanence timescales (> 100 years).
   • AWL coupled with bioenergy – CO₂ generated from the combustion of biomass (either terrestrial or marine-based) to generate energy can be concentrated and reacted with limestone in the presence of seawater to trap the CO₂ as bicarbonate ions. This process is analogous to bioenergy with carbon capture and storage (BECCS)⁹, except that the carbon is stored as bicarbonate ions in the ocean as opposed to being sequestered in geologic reservoirs. 

   We further consider AWL in these hybrid configurations that generate the removal of atmospheric carbon dioxide and sequestration as bicarbonate in the ocean.

• OAE would likely provide localized reductions in ocean acidification, with expected benefit(s) to marine ecosystems¹².

• In certain areas, calcium or silica additions could act as fertilizers to support plankton populations³.

• OAE may pose ecotoxicological risks from the release of elevated concentrations of trace metals from some minerals during mineral dissolution¹.
  • The toxicity will depend on the source rock, the concentration of source rock applied, and the seawater chemistry (which determines bioavailability). These trace metal additions could also pose risks to human health if accumulation occurs through food webs².
  • Silicate rocks are likely to have higher metal concentrations than carbonate rocks, and thus may pose a greater ecotoxicological risk²
  • In-situ applications of alkaline materials (e.g. coastal enhanced weathering, ocean liming) may pose greater ecotoxicological risks than ex-situ applications (e.g. reactors for accelerated weathering of limestone) because of the ability to capture and treat the effluent from ex-situ reactors before release into the ocean. However, options exist to mitigate these effects e.g., through rapid dilution in the wake of vessels³.

• The potential for changes in water column particle concentrations, turbidity, and optical properties from dispersing fine particulates
• The potential for changes in seafloor deposition of particles and their effects on smothering or burial, food webs interactions, light availability and more
• Large-scale mining of silicate or carbonate rocks may pose environmental risks, similar to those typically associated with existing mines including ground vibrations from blasting, noise pollution, decreased air and soil quality, etc.

• Ecological and geochemical impacts of discharging high alkalinity/high pH waters, including precipitation (inorganic mineral formation) of carbonates. However, options to mitigate these effects exist e.g., through rapid dilution in ship's wakes³.
• OAE may have the potential to shift phytoplankton community composition from carbonate-shell producing plankton (coccolithophores) to silica-shell producing plankton (diatoms) or vice-versa depending on the form of alkalinity addition.
• Potential for bioaccumulation and biomagnification in the food chain⁴
• Dedicated ships/vessels to distribute minerals could cause environmental impacts such as additional noise pollution⁵, the potential transfer of nonindigenous species ⁶, and the addition of pollutants from shipping emissions⁷.

• Risks associated with the expansion on mining activities for OAE
  • Demographic changes including, but not limited to, shifts in gender balance, increase in non-resident workforces, appropriation of land from local communities, social inequality, and impacts on Indigenous communities³
  • Negative health impacts including cancer, respiratory diseases, injuries, and prolonged exposure to chemical agents⁴
  • Environmental implications include impacts to soil and air quality, impacts to ground and surface waters, and erosion.

• Creation of job opportunities

• Because of the lack of proof-of-concept field experiments, it has so far been impossible to characterize benefits, risks, and scaling consideration of OAE in real-world settings (i.e. not benchtop). Controlled field experiments across diverse ecosystems to determine marine chemistry and biology impacts and feedbacks are needed (Develop New Modeling Tools to Support Design and Evaluation, Accelerate Design and Permitting of Controlled Field Trials).
• It is challenging to verify additional CO₂ uptake from the atmosphere as a result of OAE given the ocean’s dynamic CO₂ flux “background state”. New methodologies are needed to observe additional sequestration from the atmosphere into the ocean (Develop New Modeling Tools to Support Design and Evaluation, Develop New In-Water Tools for Autonomous CDR Operations).
• Laboratory experiments are needed across a range of seawater chemistries expected as a result of equilibrated OAE scenarios (e.g. high total alkalinity and high dissolved inorganic carbon¹), along with various alkaline materials and their associated major (e.g. magnesium, calcium) and minor (e.g. nickel, cadmium) elemental concentrations to characterize environmental impacts (Accelerate Design and Permitting of Controlled Field Trials).
  • Look to the ocean acidification community’s effort to develop standardized protocols for guidance² as the OAE community builds out standardized protocols and treatments levels for consistency and intercomparability
  • Existing database(s) of ecotoxicological tests need to be reviewed³ for possible OAE source materials (e.g. Ca(OH)₂) and co-occurring metals (e.g. Ni, Cd) to identify known ecotoxicological effect and lethality thresholds and current gaps in our understanding.
• Global, local, and regional predictions of physical, chemical, and biological outcomes and feedbacks of OAE from high-resolution models are needed (Develop New Modeling Tools to Support Design and Evaluation)
• Laboratory experiments are needed on silicate and carbonate mineral kinetics and dissolution catalysts⁴ to better understand mineral dissolution rates in marine environments and means to accelerate them (Accelerate Design and Permitting of Controlled Field Trials).
  • The coastal enhanced weathering facility jointly administered by Universiteit Antwerpen, the University of Gent, and VLIZ provides mesocosm and monitoring equipment to conduct mineral dissolution experiments in tank conditions closely approximating real-world beach and shallow water settings. 
• Life cycle assessments are necessary to calculate net CDR benefits, taking into account all emissions associated with supply chains (Develop CDR Monitoring and Verification Protocols). 
• There may be good value in summarizing best practices, lessons learned, and pitfalls from lime treatment of acid rain-affected lakes and watersheds to accelerate OAE development and testing⁵⁶⁷
• In order to address several of the knowledge gaps listed above, Additional Ventures is partnering with Ocean Visions and a consortium of philanthropic funders (2022) to identify and support the proposal that best articulates the highest priority questions facing the testing and development of OAE approaches.

• Current observational technologies (sensors, ROVs, AUVs, etc.) and modeling tools are not widespread and easily available to fully support field trials with the desired spatial and temporal frequency of monitoring and sampling needed (Develop New In-Water Tools for Autonomous CDR Operations).
• New technologies are needed to reduce the cost and environmental impacts of mining, grinding, and distribution of alkaline rocks with minimum environmental impact¹.
• Assessment of the potential to re-purpose existing supply chains for coal and cement to provide an easier ramp up to meet the production and distribution scales required (Accelerate RD&D Through New Partnerships). 
• Assessment of the potential to scale the cement and lime industries to meet the expected needs of calcination²³(Accelerate RD&D Through New Partnerships).
• Identification of sources of renewable energy at sufficient scale to power OAE (Develop New In-Water Tools for Autonomous CDR Operations) 
• For in-situ application with hydroxide minerals, methods to handle the heat generated from adding hydroxides to the ocean because this is an exothermic (heat-releasing) reaction (Develop New In-Water Tools for Autonomous CDR Operations).
• For point source OAE, cost-effective and safe methods need to be developed that optimize the distribution, dispersal and dilution of any strong chemical bases to avoid impacts of excessively alkaline (pH>9) waters on marine ecosystems. Dilution may require pumping large amounts of seawater, which need to be optimized for energy and cost (Develop New In-Water Tools for Autonomous CDR Operations). However, it may be possible to use existing large-volume discharges such as power station cooling water discharges that can be as high as 90 cubic meters per second for a 1600 MWe nuclear power station. In addition, another option would be to utilize the rapid dilution available in ships' wakes⁴.

• Development of a robust carbon market for OAE-derived CDR is especially important because most OAE pathways as described above do not produce co-products that could generate revenue and lower overall system costs.
  • However, synthetic production of magnesium hydroxide via accelerated weathering of olivine co-produces silica and metal oxides¹, both of which can be sold as co-products to reduce the overall system costs
• Strengthening mechanisms to protect intellectual property associated with various OAE pathways. 
• Development of third-party verification protocols is needed to support trading of carbon removal credits on carbon markets (Develop CDR Monitoring and Verification Protocols)

Many of the opportunities and challenges around building public awareness and support for ocean-based CDR are not specific to OAE, but there are several hurdles specific to OAE: 

• OAE faces challenges in terms of public perception of potential environmental risks that are not necessarily faced by what are considered more “nature-based” approaches (e.g., coastal blue carbon restoration)¹ (Growing and Maintaining Public Support).
  • There is a great deal of hesitancy and resistance around any ocean-based CDR pathway that relies on adding materials to the ocean. 
  • Earlier ocean iron fertilization experiments² could offer opportunities to adopt best practices and avoid mistakes made when building public support for OAE. 
• Negative perceptions exist about the large-scale mining that would likely be needed to generate sufficient rock supplies to support global-scale OAE (Growing and Maintaining Public Support).
• Clearer communication strategies need to be developed to respond to the “geoengineering” narrative of OAE (Growing and Maintaining Public Support). 

Advancing the development and testing of OAE will require governance structures that both enable the permitting of legitimate testing and development and ensure that the public interests are protected.  

• There is currently no clear international regime that governs research and development in OAE. The closest regimes would be the London Convention and the London Protocol, but currently they are not built to govern OAE scientific field trials in marine waters¹.
• Small-scale OAE field trials by “invited Parties and other Governments” with prior environmental impact assessments may be allowed under the United Nations Convention on Biological Diversity (CBD)  Section X/33(8)(w). The CBD, however, is not legally binding  and not all countries are party to the CBD (for example, the United States)

High-resolution data-assimilative models are needed to support real-world testing of OAE. These modeling tools must:

• Account for complex interactions in the immediate vicinity of the alkalinity release and downstream impacts
• Provide four-dimensional (space and time) estimates of biogeochemistry in zone of influence both in the presence and absence of OAE. The difference between these two simulations can be used to inform CDR estimates that account for background variability in the ocean. 
  • CDR estimates from OAE must include estimates of the “opportunity cost” of OAE - how did OAE shift phytoplankton community composition, production, and export? 

To support the design of proof-of-concept field trials, these models should also: 

• Provide estimates of the size and scale of biogeochemical modification to the ecosystem from OAE, allowing for informed placement of sensors to monitor the field trials
• Be capable of simulating passive tracers (e.g. SF₆) to inform whether and how these passive tracers may be useful in field trials (e.g., estimating rates of atmospheric CO₂ uptake)
• Inform a prioritized set of predictions to be tested during field trials

Field trials for the various OAE technologies are urgently needed to test both carbon sequestration potential and environmental impacts (both positive and negative). A series of activities and products are needed to get to a series of controlled field trials. They include:

A new suite of durable, seagoing technologies are needed to support OAE RD&D. Technology development needs include: 

Standardized methodologies from third parties to verify uptake of atmospheric CO₂ resulting from ocean alkalinity enhancement will ultimately need to be developed to enable trading of carbon removal credits. Key first steps to support development of these protocols include: 

• Convening experts to review advances from modeling tools (Develop New Modeling Tools to Support Design and Evaluation) and controlled field trials (Accelerate Design and Permitting of Controlled Field Trials) to identify satisfied and outstanding data needs necessary to quantify additional CO₂ uptake as a direct result of OAE. As advances in OAE RD&D are made, the satisfied and outstanding data needs will need to be updated.
• Apply existing¹² or develop when necessary, life cycle analysis tools to calculate stored carbon after accounting for emissions from required materials, energy, transportation/dispersal, etc.
• Include aspects of sustained monitoring to verify CDR permanence over long time scales as CDR is scaled.

Research, development, and demonstration of OAE may be accelerated and strengthened by creating partnerships with key industries/sectors, including:  

• Transoceanic shipping – cargo vessels often travel at less than full capacity, provide onboard power, and move from port to port providing frequent opportunities to acquire new alkaline material and offload byproducts from alkalinization processes. Developing partnerships with the shipping industry to help the industry fulfill its commitments to decarbonize shipping could be a stepping stone to accelerate ocean-based net negative emissions (e.g. Poseidon Principles¹).
• Industries generating alkaline byproducts for feedstock into OAE processes (steel, aluminum, cement, lime, and nickel production; coal and biomass combustion)².
  • 7 billion tons of alkaline byproducts generated annually from manufacturing and combustion could decrease costs and potentially facilitate transitions to larger-scale OAE³. 
• Finfish and shellfish aquaculture, as well as coral reefs, where the added alkalinity could be used to optimize chemical conditions, including providing relief from ocean acidification⁴. 
• Offshore renewable energy production, including wind and others, both as power sources and as integrated CDR platforms.
• Coastal industries, including desalination and wastewater treatment facilities, which already have infrastructure for pumping/processing seawater or wastewater for alkalinity addition.
• Marine research laboratories that already pump seawater and have expertise, technical equipment and infrastructure to support research and development.

Developing and strengthening relationships with partner industries may also help promote greater public support, as well as potentially offer faster routes to obtaining the necessary permitting.

First-Order Priorities to build public support for ocean-based CDR pathways are found in the Public Support road map Additionally, there are some specific opportunities to cultivate public engagement in and support around OAE including: 

• Developing targeted public outreach/advocacy campaigns to inform about OAE and its potentials with regards to ameliorating ocean acidification 
• Responding to the narrative of OAE as less “nature-based” than, for instance, coastal blue carbon restoration with the concept that OAE represents an acceleration of “natural” chemical weathering.

1. Open systems where the open ocean is directly manipulated to enhance biological production, capture atmospheric CO2, and export the captured carbon to the deep ocean. In open systems, CO2 fixation is facilitated by the addition of limiting macronutrients (e.g., phosphorus, nitrogen, silica) and/or micronutrients (e.g., iron) to the ocean’s surface to augment biological production ². Open system techniques accelerate natural processes already occurring in the ocean. Most approaches in the open ocean fall into the following two categories (some proposals that do not fit into these categories are also explored).
   1. Surface nutrient addition: the direct addition of nutrients (macro or micro) into ocean waters in situ to increase microalgal growth 
   2. Nutrient upwelling: artificial upwelling of nutrient-rich deep ocean waters to the surface to increase microalgal growth
2. Closed systems where inputs and growth conditions are controlled, and outputs (microalgae) are harvested within the confines of a pond or a photobioreactor. In closed systems, CO2 fixation is facilitated by the mixing of required inputs (sunlight, nutrients, CO2, water) and the introduction of microalgae culture with the intention of reproduction and continuous fixation in a contained system ³. This can be accomplished on shore in cultivation ponds or photobioreactors, or afloat in photobioreactors either stationary or towed at sea ⁴⁵⁶.

1. 1. Onshore: encompasses more established methods of microalgae cultivation, including photobioreactors, cultivation ponds, and hybrid onshore configurations. In these systems, all inputs are tightly controlled and regulated, and outputs must be directly managed through either storage or utilization of byproducts. While cultivation techniques are well-established and show high technological readiness, storage and utilization pathways remain underdeveloped and scale is a major consideration. Social and environmental risks for closed onshore systems are easier to monitor and mitigate due to the controlled nature of the system. 
   2. Offshore: includes floating photobioreactors (PBRs) that are incorporated into a floating platform which can be stationary or towed behind a ship. In these systems, cultivation occurs in the photobioreactor, inputs are regulated, and outputs can be actively managed or directed. While at sea, these photobioreactors operate much like their onshore counterparts to cultivate microalgae, however nutrients and energy are provided by the ocean water and wave action, respectively. After microalgae are cultivated, they can be sunk into the deep ocean for sequestration or hauled to shore to be used as biomass. This is an area that has garnered much attention in startup communities (see this Y Combinator request for startups), however, little is available about the technologies in the open-sourced or peer reviewed literature.

Regardless of approach or technology, there are two fundamental questions that make up the “CDR Potential” for a given pathway:

1) How much carbon can this approach sequester?  2) How long can it stay sequestered?

How much carbon can be sequestered?

• Carbon fixation rates and storage capacity of the microalgae species
  • The capacity of microalgae to fix carbon differs across species and selecting appropriate species for different approaches remains an area of study. Genetic modification of carbon fixation rates and carbon storage capacity may expand the CDR potential for microalgae ⁶. 

• Scalability of the approach (for example, offshore cultivation has fewer spatial imitations than onshore cultivation) 
  • The expense of acquiring, transporting, and delivering nutrients, as well as the bioavailability of those nutrients, and the resulting carbon-to-nutrient ratio in the microalgae
  • Additional considerations for closed systems include:
    • Harvesting Considerations: The energy necessary for harvesting algae from cultivation set-ups (e.g., raceway ponds) can be an impediment to scaling. Current estimates from the US Department of Energy cite energy usage as ~1.4 kWh/m³, and high capital costs, ~$152,000/million gallons per day. Automated harvesters, such as the Zobi Harvester, have the potential to cut time and costs significantly, using less than 0.14 kwh/m³ and a capital cost of less than $106,000 per million gallons per day of capacity, unlocking harvesting barriers ¹. Utilizing gravity-based systems is another way of increasing energy efficiency for harvesting ². 
    • Planktonic Cultivation vs Biofilms: Microalgae can be cultivated in two forms: planktonic or on biofilms. Planktonic microalgae are suspended in water and are highly diluted (less than 1%). Planktonic microalgae have been more extensively studied and can be cultivated in photobioreactors or ponds ³. Biofilms are aggregates of microorganisms attached to a surface and are much more concentrated than planktonic cultivation. The areal productivity for biofilms of microalgae is about two-fold the one for planktonic microalgae ⁴. New techniques with biofilms are in development ⁵.

How durable is the carbon sequestration? 

• Deep ocean: wherein microalgae, and their embedded carbon, cultivated via ocean fertilization or on land in photobioreactors or ponds are stored in the ocean for long-term storage. If purposely placed or injected into the water column at depth (>1,000 meters), many parts of the global ocean can offer 100 to 1,000 years-scale permanence. Sequestration durability is reduced to 10 to 100 years-scale for microalgae participating in the biological carbon pump due to high rates of remineralization in the upper ocean and during transit to the deep ocean as microalgae are grazed or decompose. There is extensive basin-scale variability in sequestration durability ⁷. Areas of the ocean best suited to long carbon storage are places where surface waters and sinking matter can reach depths greater than 1,000 meters. The Southern Ocean south of the biogeochemical divide separating the Antarctic from the sub-Antarctic would be one such suitable location as well as much of the North Pacific ⁸. Findings also suggest the North Atlantic and North Pacific oceans have the greatest carbon storage potential, while subtropical oceans have the least ⁹. Continued thorough measurements and models are needed to quantify the durability of sequestration for any given site, accounting for the fact that durability is likely best defined as a probability distribution given the various transit times of water masses.
  • A number of large-scale campaigns are underway to improve our understanding of deep ocean carbon storage. These include EXPORTS (USA), COMICS (UK), and ONCE (China).
  • In addition, there are a number of studies underway to investigate ways to accelerate deep ocean carbon storage, among them:

• • • Investigating Carbon Storage in Microalgae Polysaccharides: Scientists at the MARUM MPG Bridge Group Marine Glycobiology recently won the European Research Council’s Consolidator Grant to research carbon storage in marine algal polysaccharides and their role in the marine carbon cycle. 
    • Work from Mukul Sharma’s lab at Dartmouth College is investigating the use of clay minerals to enhance the removal of carbon from the euphotic zone and sequester it in the deep ocean ¹⁰. The principal idea behind this research is the formation of clay-gel hybrids when clay comes into contact with dissolved organic matter produced by microalgae and bacteria. These hybrids may facilitate conversion of dissolved organic matter to particulate organic matter and rapid export out of the mixed layer. While clay additions are very underexplored in open ocean environments, they have long been recognized to interact with, and flocculate, in the presence of microalgae ¹¹¹².

• Long-lived bioproducts: wherein carbon sequestered in microalgae is incorporated into products such as bioplastics for long-term storage.
• Biochar: Microalgal biomass can be processed via pyrolysis, gasification, or hydrothermal carbonization and turned into biochar. Biochar is a carbon-rich product that can be used in soils for agriculture (helping to prevent leaching of pesticides and nutrients) or as absorbents in air/water treatments for pollutant removal ¹³¹⁴¹⁵.
• Burial & land-based sequestration: Algal biomass that is sufficiently salty, dry, acidic, and rich in natural preservatives can be stored on land (buried) for long-term sequestration ¹⁶.

Nutrient Fertilization

1. The depth of remineralization for carbon, iron, and other macronutrients ⁹.
2. Factors affecting carbon sequestration that remain uncertain ¹⁰:
   1. Net primary productivity in the vicinity of nutrient fertilization and downstream (to address issues associated with “nutrient robbing”). 
   2. Efficiency of export of materials out of the euphotic zone, related to the attenuation of the particle flux 
      1. Ratio of iron to carbon (Fe:C) in exported materials and how that ratio changes with depth 
3. Ecosystem / trophic level interactions 
4. The effectiveness of particular methods of delivering nutrients 
5. Pulse (one-time) versus continuous (repeated) experiments will likely change carbon capture efficiency 
6. Field-trials have yet to observe full growth cycles of microalgae including the bloom demise

• Artificial iron fertilization: The first observations of a relationship between iron and atmospheric CO2 came from geological records that revealed a correlation between CO2 in ice cores and iron rich dust ¹¹. John Martin conducted pioneering work in the late 1980s to identify iron-limited regions of the ocean. The next decade (1993 – 2004) was spent executing studies in the Equatorial Pacific, Southern Ocean, Subarctic North Pacific, and Subarctic North Atlantic to explicitly examine the primary biological response of adding iron to surface waters. What resulted were thirteen studies in the open ocean accompanied by decades of analysis ¹², rendering artificial iron fertilization the most studied and well-understood approach in this road map. The application of iron to the ocean surface to investigate the relationship between iron limitation and phytoplankton growth suggests potentially high global capacity for iron fertilization to result in gigaton scale carbon dioxide sequestration ¹³¹⁴¹⁵ and has led scientists to look to iron as a strategy to enhance the ocean’s biological carbon pump ¹⁶¹⁷. Most of the open ocean experiments conducted to date were largely focused on the immediate biological response to added iron in the form of a microalgae bloom and not the geochemical response (in the form of carbon fluxes), thus there is limited knowledge on the efficacy of this technique for net carbon dioxide removal. However, there are a handful of groups around the globe currently investigating the fate of carbon from net primary production. In addition to the efficacy of such methods, biogeochemical and ecological impacts remain unknown. A 2022 whitepaper by ExOIS (Exploring Ocean Iron Solutions) lays out a number of questions that remain to be answered by science including: What controls the efficacy and durability of ocean iron fertilization as a CDR strategy? How can ocean iron fertilization efficacy be accurately and efficiently monitored and verified? What are the intended and unintended ecological consequences of ocean iron fertilization and how can they be mitigated? 

• • Engineered nanoparticles: Babakhani et al. (2021) propose the application of engineered nanoparticles in addition to iron fertilization in an attempt to enhance phytoplankton growth and possibly mitigate environmental risks posed by iron fertilization (e.g., nutrient robbing). Here, the goal is for the engineered nanoparticles to control the rate of nutrient release. Nanoparticles may be more expensive to deploy but may also be more effective at CDR. Focused field trials of engineered nanoparticles with artificial ocean fertilization are still needed to fully understand the impacts and efficacy of this technique. 

• • Triggering the formation of marine snow: The GEA275 Project (Croatia) is investigating if adding iron chelates to high-nitrate, low-chlorophyll regions of the ocean can induce the formation of marine snow (organic detritus) for carbon removal and storage in the deep sea. Unlike other forms of iron fertilization, this approach does not aim to stimulate phytoplankton growth but rather to accelerate the release of extracellular organic carbon by existing communities of phytoplankton and heterotrophic bacteria. 

• Macronutrient fertilization: Compared to iron fertilization, macronutrient fertilization has received less attention from the scientific community (but note Harrison 2017), with one clear disadvantage of this method being that larger quantities of nutrient material are needed per ton of carbon removal due to the higher cellular ratios of nitrogen and phosphorus to carbon (as compared to iron to carbon). However, ideal locations for macronutrient fertilization (low-nutrient, low-chlorophyll) may be more accessible than hard-to-reach locations ideal for iron fertilization, such as the Southern Ocean. In general, macronutrient fertilization presents general concerns around acquiring sufficient quantities of nutrients (some of which are nonrenewable, like phosphate), ecosystem impacts, and the fate of carbon once sequestered in phytoplankton. More work is needed to better understand macronutrient fertilization’s potential as a scalable CDR technique ¹⁸. 

Artificial Upwelling

Floating Photobioreactors

• Combining cultivation with wastewater treatment: Wastewater is a nutrient-rich medium that has been explored as a nutrient source for microalgae cultivation ²⁶. Wastewater is rich in nitrogen, phosphorous, heavy metals, etc., which are often costly and energy-intensive to remove through traditional methods ²⁷. Many species of microalgae can remove these compounds by accumulating and/or using them in their cells. One challenge in this approach is providing microalgae with enough CO2 for optimal growth.  There is potential for wastewater treatment to be paired with onshore cultivation systems such as raceway ponds or with offshore systems such as photobioreactors, however more research is needed to assess the feasibility of such set-ups. Most studies into coupling wastewater treatment with microalgae have looked at end products such as biofuels and not the permanent removal of carbon dioxide. As such, there is limited data on the actual carbon dioxide removal potential of this approach. While closed microalgae cultivation is widely considered to be an economically non-viable way to make fuels ²⁸²⁹, it may be viable for CDR purposes.  

• • Current Projects: Algae Systems (US) grows microalgae in bags of disinfected wastewater and CO2. The bags are deployed offshore in Mobile Bay, Alabama, and the combination of wave energy and sun create the necessary conditions for algae to grow. Currently, the algae are harvested and used for diesel and jet fuel, while the water is cleaned for re-use. The plant is currently at demo-scale and treats 40,000 gallons of wastewater per acre per day. 

Cultivation Ponds

• Current Projects: Brilliant Planet (London) began growing microalgae in 2013 on the shores of St Helena, South Africa and now have a thirty-thousand square meter production facility in the coastal desert of Morocco which has been operating for over five years. Their systems use seawater to cultivate microalgae in large ponds and are monitored with high-frequency satellites. Permanent carbon sequestration is achieved by burying resulting biomass in ultra-dry desert landfills and sealing it with a geomembrane liner ³²

Microalgae-based Bioenergy for Carbon Capture and Storage (MBECCS / ABECCS)

• Current Projects: The Johnson Lab at the Duke Nicholas School of the Environment was awarded a grant for a pilot demonstration of carbon capture/storage technology based on ABECCS.  The project uses a combined system that generates CO2 from forest products (bark/sawdust) and then uses that CO2 to grow algae for carbon sequestration in valuable bioproducts. Preliminary analysis of this process shows it to be carbon negative and economically viable. 

General Risks of Open Systems

Harmful Algal Blooms 

Land-Use

Summary Table of Potential Impacts

Social Risks 

• Lack of public support & social license: There are currently low levels of public support and/or acceptance of CDR broadly in the United States and beyond, particularly with activities viewed as “geoengineering” or “climate engineering” ³⁴. Other factors that may affect public support include perceptions of scale and perceptions of capture versus storage. These are areas that require more focused study to better understand. Early experiments are opportunities to adopt best practices and avoid mistakes when building public support and stakeholder engagement. 
• Risk to culturally significant landscapes: CDR interventions that alter marine landscapes may lead to disruptions (e.g., native species populations) to cultural, spiritual, or cosmological landscapes and other non-tangible uses or relationships to the ocean and its inhabitants. The introduction of any new technology that interacts with natural ecosystems is likely to always be contested due to the diversity of ways in which people view the relationship between humans and the non-human world ⁵⁶. 
• Human health risks: Exposure to aerosolization of toxins from harmful algal blooms (HABs) have been known to cause respiratory symptoms in humans ⁷. Although HABs have not been observed during artificial iron fertilization experiments, their occurrence is not impossible across many microalgae-based CDR approaches, including shore-based cultivation operations. 

• Irreversible consequences: Negative impacts (environmental or social) of an approach that are irreversible may impact future generations, raising concerns regarding intergenerational justice ⁸. This concern highlights the need for appropriate procedures so that any field trials that are approved and deployed do not have irreversible consequences. 

• Decreased equity and social welfare due to financing: The creation of financing systems like carbon markets for forest carbon sequestration caused tensions around land rights and had implications on human rights and equity for Indigenous and forest-dependent communities ⁹. These issues will likely vary greatly for closed systems on land, coastal closed systems, and open ocean/deep ocean commons. 

• Location considerations: Just as there are different governance implications for activities occurring in exclusive economic zones versus the high seas, these geographical differences may also carry importance for social impacts. Environmental or ecological impacts from activities taking place in near-shore waters may have more direct social impacts than activities taking place in the open ocean. 

• Food security: Microalgae-based interventions that risk disturbing food supply chains (e.g., fisheries) may threaten food security. This is especially relevant for communities that rely on the ocean for sustenance. 

• Impacts on livelihoods from tourism and fisheries: Activities that use or interfere with near-shore environments and ecology may impact industries like tourism and fisheries, thus impacting livelihoods. This is particularly important in areas that depend heavily on these industries economically and/or are socioeconomically vulnerable in general (e.g., the Global South).  

Social Co-Benefits 

• Economic incentives: The various potential utilization pathways of microalgae could create avenues for wealth generation and emissions-free economic growth – namely economic incentives within existing carbon markets, preventing costly environmental disasters resulting from uncontrolled climate change, and converting a damaging waste product (CO2) into a value-added product. ¹⁰
• Job creation: Economic value-addition is likely to also create jobs in new and existing industries in aquaculture and beyond ¹¹.
• Ownership and benefits accruing from uses of genetic resources: Many microalgae-based innovations bring with them a suite of potential biotech and genomic-based approaches. It will be important to understand when and how local benefits rights holders (e.g., coastal communities, Indigenous communities) can benefit from the use of marine-based genetic resources. This is an inherently complex subject that questions ownership of marine resources and is rich with justice and equity implications.

Existing Governance Structures that May be Applicable to Microalgae for CDR 

• United Nations Convention on the Law of the Sea (UNCLOS): Adopted/ratified by 167 countries and the European Union, UNCLOS defines countries’ rights and responsibilities in terms of use and management of offshore areas. Provisions that may be relevant to microalgae cultivation and/or sequestration in the open ocean include: 
  • Part XIII of UNCLOS which establishes rules for “marine scientific research” 
  • Part XII of UNCLOS which requires countries to “protect and preserve rare or fragile ecosystems as well as the habitat of depleted, threatened or endangered species and other forms of marine life” and includes a number of specific provisions on marine pollution from vessels, land-based sources, activities in the area, and dumping, among others. Without clear distinctions on what is or is not considered “pollution”, these provisions may be relevant for microalgae-based CDR interventions.
  • ​​Proelss (2022) investigates the United Nations Convention on the Law of the Sea and finds that although it covers ocean fertilization research stages, it does not apply to ocean fertilization for CDR, leaving the legality of ocean fertilization unclear under international law.  
• Convention on Biological Diversity (CBD): Adopted in 1992, the CBD aims to protect biological diversity. 195 countries and the European Union have ratified it; the United States has signed but not ratified it. In 2008 the Conference of the Parties of the Convention on Biological Diversity recommended that ocean fertilization activities not take place until there exists a sufficient scientific basis to justify it. The term “ocean fertilization” was not defined. In 2010 a decision was made to regulate “geoengineering activities” such that no geoengineering activities take place that may impact biodiversity until there is a scientific basis to justify such actions. An exception is made for small-scale scientific experiments in controlled settings (the definition of “controlled settings” may prevent research in the open ocean) after thorough assessment and justification, though key terms in the exceptions for scientific experimentation are not defined, resulting in significant uncertainty as to when it will apply. 
• London Protocol / London Convention: Adopted in 1972, this convention aimed to control all sources of pollution on the ocean, with particular focus on the dumping of waste or other materials in the ocean. There is a lack of clarity and general disagreement concerning what the future of policy and governance application of ocean iron fertilization may look like, particularly with the precedent of the London Protocol, which has banned nonscientific application outright¹. The London Protocol has not yet ruled on closed offshore systems and represents an opportunity to bring attention back to issues of ocean utilization for carbon dioxide removal. 
  • Amendment on Ocean CDR: This amendment, not yet entered into force, states: “Contracting Parties shall not allow the placement of matter into the sea from vessels, aircraft, platforms or other man-made structures at sea for marine geoengineering activities listed in annex 4 (amendment that regulates the placement of matter into the ocean for marine geoengineering), unless the listing provides that the activity or the subcategory of an activity may be authorized under a permit.”
  • Statement on Marine Geoengineering: In October of 2022, parties to the treaties which regulate marine dumping adopted a statement calling out the need for evaluation of four marine geoengineering techniques that have potential to mitigate the effects of climate change but could also threaten marine ecosystems. The four techniques given priority are: ocean alkalinity enhancement, macroalgae cultivation and sequestration (including artificial upwelling), marine cloud brightening, and microbubbles/reflective particles/materials. The London Protocol and London Convention Parties are conducting a technical and legal analysis to evaluate next steps and future regulation. (See the International Maritime Organization’s briefing) 
  • GESAMP (Group of Experts on the Scientific Aspects of Marine Environmental Protection) Working Group 41: Ocean Interventions for Climate Change (formerly Marine Geoengineering): In early 2022, GESAMP’s Working Group 41 formally advised the London Protocol Parties to consider marine geoengineering techniques to list in the new annex 4 of the protocol. These geoengineering techniques are those that may assist in mitigating or reversing the impacts of climate change. Included in this list are approaches that utilize microalgae to sequester carbon for long-term storage. (Timeline of Ocean Fertilization under the London Protocol)

Other Considerations

• Creation of new frameworks: In many cases it may be most efficient and effective to create new frameworks that put controls on research to ensure it occurs in a safe and responsible way that aren’t unnecessarily restrictive or hindering. 

• Modification of existing policy: Examine, and consider reforming, existing policies that have the potential to inhibit the research and development of this technology, e.g., the London Protocol. 
• Scale considerations: Pilot-scale experiments will likely have different associated risks than full-scale implementation. This may translate into policy and governance frameworks that consider the scale of manipulation or experimentation as a critical variable. 
• Government subsidies: Subsidies from the government could provide incentive to researchers as well as the funds necessary to accelerate research. 
• Bias amongst approaches: How will the ability to monitor and verify impacts and efficacy of a particular approach influence support from governance and policy? An approach-neutral stance (with financing, policy etc.) will help ensure that all approaches are given an opportunity to reach their full potential. 
• Resource competition for onshore cultivation: There is potential for competition for nutrients with traditional agriculture and other societal priorities. Although microalgae can be grown on marginal lands unsuitable for agriculture, land competition with human settlement, industrial processes, and other infrastructure may pose a challenge if microalgae production is scaled up significantly (Scott-Buechler, in preparation). 

• Using existing frameworks for onshore cultivation: Onshore cultivation of microalgae may look similar to established aquaculture or desalination plant policies/laws and thus have a more straightforward path for research and development. 

• Measurement, reporting, and verification (MRV) procedures will need to be developed following advances in research and development. Advances in research and development will inform what parameters need to be monitored and verified, and how to do so. MRV protocols should include measurements of key ecological parameters as well as carbon fluxes to ensure that microalgae CDR activities do not exceed accepted limits of environmental impact.

• Life cycle analyses will need to be conducted for any approach that uses microalgae for CDR to determine the net carbon removal potential after accounting for process and embedded emissions.

• Social co-benefits are not well understood or identified. While research into social impacts is growing, there remain large uncertainties around the range of potential impacts that may result from large-scale environmental interventions. Integrating social risks and co-benefits into CDR research agendas may help this area of knowledge grow alongside the technical research and development. 
• Environmental co-benefits have not been well studied or articulated across microalgae-based CDR methods ¹. Identifying and understanding the potential array of co-benefits may help offset other risks, both environmental and social, and aid in decisions around which methods are to be pursued further.
• Location and siting: Siting analyses are needed to identify optimal locations for microalgae CDR and address potential synergies and/or conflicts with other marine resource users (e.g., fisheries, renewable energy, etc.) (Map optimal locations for microalgae-based approaches and engage with interested and affected constituents)

Open Ocean Systems 

• Export and fate of carbon from net primary production in the upper ocean to understand sequestration efficacy and durability: A fundamental understanding of the export and fate of net primary production from the upper ocean is critical to any development, testing, or deployment of approaches that hope to accelerate this process. Currently a number of field campaigns are working on answering these questions, including EXPORTS (USA), COMICS (UK), and ONCE (China). In addition to these large campaigns focused on fundamental understanding of the biological carbon pump, these questions must be answered in the context of purposeful nutrient additions (Accelerate the design, permitting, and execution of the next generation of controlled field trials to answer questions specifically pertaining to carbon sequestration)
• Natural iron cycle / anthropogenic iron input: There is a lack of knowledge about the current state of iron cycling in the ocean system, particularly how much anthropogenic iron is already input into the system and what the spatially specific distribution of impacts are likely to be ¹. Research must prioritize understanding inputs and refining existing iron-cycle models. This will be critical work for ocean iron chemistry models to measure future changes in anthropogenic iron loading. Future research must also seek to better understand natural and pre-existing anthropogenic analogs that may be analyzed as simulations of future ocean iron fertilization²³. Analogs may include volcanic ash and wildfire ash deposits into ocean systems.

Closed Systems 

• Harvesting efficiency: In the cultivation stage, harvesting microalgae from closed systems contributes to 20–30% of the total cost of microalgal biomass production, so opportunities to increase efficiency present the greatest opportunity for cost reductions through innovation ⁴. (Optimize and/or efficiently automate all approaches for scalability and economic feasibility)

• Strain optimization: The algal industry is still relatively small, and primarily optimizes for lipid and/or protein content in microalgae strains. More research is thus needed to identify ideal species, strains, and specific growth conditions to maximize microalgal CO2 uptake; and to estimate associated costs. (Optimize and/or efficiently automate all approaches for scalability and economic feasibility)
• Fertilizer efficiency: High fertilizer needs are likely to be the most substantial constraint on closed onshore cultivation systems⁵. Research is needed into options to maximize fertilizer recycling and other strategies to lessen fertilizer needs. (Optimize and/or efficiently automate all approaches for scalability and economic feasibility)
• Overall CDR potential: There exists virtually no information on overall CDR potential of closed offshore systems (e.g., towed photobioreactors). (Accelerate the design, permitting, and execution of the next generation of controlled field trials to answer questions specifically pertaining to carbon sequestration)

• Mapping interested and affected constituencies: It will be important to identify the diverse stakeholders for microalgae CDR, including producers, potential buyers for carbon removal, potential buyers for other useful co-products, and communities where projects may be sited. Understanding perceptions, needs, concerns, and opportunities across multiple stakeholder groups is necessary to scale microalgae-based CDR.
• Industry transitions: Microalgae is already cultivated around the world today but is done so for non-CDR purposes—primarily feeds and fuels. Analysis of industry willingness and needs for transitioning from CO2 reuse to CO2 removal could ease the creation of onshore microalgae CDR pathways, but more research is necessary to determine if this is possible and under what conditions. 

• For open ocean systems, under various conditions

• • Determine the effectiveness (including bioavailability) of nutrient delivery at supporting new primary production
  • Determine the efficacy of export from the surface ocean into the deep ocean, and the associated estimates of carbon sequestration durability
  • Determine the resulting air-sea fluxes of carbon dioxide and other greenhouse gases to quantify the additionality of the nutrient fertilization
  • Characterize the environmental impacts, both intentional and unintentional, of any nutrient fertilization activity, including:
    • Changes to upper ocean biology, chemistry, and physics (especially for artificial upwelling)
    • Impacts to upper trophic levels and food web interactions
    • Impacts to existing marine industries and resource needs (e.g., fisheries)

• For closed systems
  • Demonstrate net negative carbon removal pathways in closed system cultivation
  • Determine sequestration efficacy and durability from various carbon storage possibilities, including land-based burial, storage in the deep ocean, and storage on the seafloor
  • Characterize the environmental impacts, both intentional and unintentional, of any cultivation and sequestration activity, including:
    • Land-based impacts from onshore cultivation construction and operation
    • Impacts from leakage or spill of onshore systems 
    • Impacts to other marine-based operations (e.g., fisheries)

• Perform siting analyses to identify optimal locations for microalgae CDR that consider the suite of technical, economic, social, and political factors necessary for deployment, as well as potential conflicts and co-benefits with other marine industries (e.g., fisheries, renewable energy, etc.)

• Develop tools and instruments to increase harvest efficiency for microalgae grown in contained systems 
• Determine and/or develop optimal microalgae strains for maximally efficient CO2 uptake for various environments and settings

• There has yet to be widespread adoption of a single code of conduct for CDR research, development, and deployment, although some organizations have proposed guidelines for the development of such a code (see American Geophysical Union and the Aspen Institute) or have an internal code of conduct to which they adhere (see Planetary).

• Clear frameworks that allow for responsible field trials of microalgae-based CDR to take place on land, in coastal waters, and in the open ocean are critical to accelerate progress. Without such frameworks there is risk of inappropriate experimental siting and delays in moving research out of the lab and into the real world. Climate interventions are time sensitive and responsible research, development, and deployment relies on comprehensive, cohesive, and effective government structures. 

Successful adoption of any new technology or innovation seldom relies on its function, engineering, or design alone. Some level of public awareness and acceptance is needed for uptake of any new technology; technologies associated with ocean-based carbon dioxide removal (CDR) are not an exception. 

Participants in the workshops and review cycles leading to the production of these road maps broadly agreed that ocean-based CDR technology pathways are currently at very low levels of public awareness and, accordingly, public support. These views are in broad agreement with the growing scholarly research on the subject.¹

Given these low levels of awareness and understanding, workshop participants agreed that a great deal more investment is needed to educate and inform key audiences on ocean-based CDR, and to connect directly to those key audiences that will be most critical to accelerate the research and development of ocean-based CDR.

For some technologies, public awareness and technological readiness can exist in a virtuous cycle, wherein successful development of said technology (e.g., smart phones) increases public attention and support which in turn further accelerates support for more development.  The converse is also true.  In a “vicious cycle”, low levels of development lead to low levels of awareness which in turn inhibits flow of resources even for research and development. 

Many ocean-based pathways are trapped in this latter cycle, and hence remain critically underdeveloped. 

A few points of evidence include: 

• The relative paucity of information about ocean-based CDR pathways relative to terrestrial and technological pathways in CarbonPlan’s CDR database.¹
• The paucity of ocean-based applications submitted to Stripe’s first round of negatives emissions purchases² and Microsoft’s first negative emissions purchase.³
• The lack of big environmental and climate NGOs that include ocean-based CDR, or any CDR for that matter, as priorities on their climate or ocean agendas.
• The minimal attention paid to ocean-based CDR pathways in the IPCC’s Special Report on the Ocean and Cryosphere in a Changing Climate⁴ suggests a lack of scientific awareness that further slows broader public awareness.

Another cause of the low levels of development of ocean-based CDR relates to what is referred to as the “moral hazard” of CDR itself.  

CDR, whether in the oceans or on land, has been dubbed by some as a “moral hazard” that will detract energy and attention away from the [more] important effort to reduce current and future greenhouse gas emissions across all sectors of society¹. The fear is that some actors will use CDR as a means to reduce their net emissions, while avoiding more difficult reductions in gross greenhouse gas emissions.  

Even though the international science community now recognizes that we need both CDR and overall emission reductions to stabilize planetary warming at 1.5ºC above pre-industrial levels²³, understanding and stakeholder recognition of the imperative for CDR remains low, and the “moral hazard” argument is partly responsible. 

Even when CDR is viewed as a critical tool, the role of the ocean lags far behind other pathways in public understanding and acceptance.  This despite the fact that the ocean covers ~71% of the surface area of the planet, and already plays a major planetary role in cycling atmospheric and terrestrial carbon and safely storing it in the deep sea.   

Despite the ocean’s potential for efficacious and scalable carbon removal, ocean-based pathways have not been investigated with the same level of interest and rigor as land-based (e.g., afforestation), technological (e.g., direct air capture), and hybrid (e.g., bioenergy with carbon capture and storage) approaches. For example, an earlier report on CDR¹ pathways by the Academy largely ignored ocean-based pathways (with the exception of restoration of coastal aquatic vegetative habitats)². We hope that the call for about $1.3 billion dollars over ten years in new prioritized research and development in the recently released (2022)  U.S. National Academy of Sciences publication of the research strategy for ocean-based CDR will help to increase the awareness of the ocean's potential for carbon removal.

The ongoing and future threats to marine ecosystems from both legacy and continuing greenhouse gas pollution are clear and urgent. They include sea level rise and unprecedented rates of warming and acidification that threaten the existence of critical ocean ecosystems, such as coral reefs and kelp forests, as well as severe alteration to marine biodiversity and ecosystem function generally¹. 

The harms posed by a warming, rising, and acidifying ocean are already clear and visible, especially for island and coastal countries. Polling among coastal residents of the United States done in 2022 also shows concern about the impact of climate change in the ocean and support for ocean-based carbon dioxide removal. However, for many people the terrestrial impacts - increased wildfires, stronger hurricanes, and heatwaves and drought on land- are more visible and may make the climate crisis appear more dire on land than in the ocean.

In spite of the clear impacts of and continuing risks posed by the build-up of carbon dioxide pollution in the air and water, there seems to be more fear about the risks of taking experimental action versus the risks of inaction.  This is particularly true for ocean pathways, with a reticence as some have stated to “use the ocean to fix the climate” (which overlooks the fact that they are inextricably intertwined). 

There are concerns about unknown and unbounded environmental risks from ocean-based CDR pathways, many of which may have been born from controversies over ocean iron fertilization experiments in the 1990s and 2000s²³ or the ocean fertilization project by the Haida Salmon Restoration Corporation off Canada⁴.

The precautionary principle has often been used to manage risk when a proposed activity has the potential for causing harm, and where extensive scientific knowledge and conclusive evidence to the contrary is lacking.  Implicit in the precautionary principle is that the burden of proof falls on the proponent of the action to show that potential harm can be avoided or minimized.  

Also implicit is that potential harm can be avoided by not taking the action. However, this assumes that the condition of the system will stay constant without the action.  This is certainly not the case in the oceans; there is overwhelming scientific evidence that the ocean is on a dangerous downward trajectory as a result of too much carbon in the air and water¹.  

If one accepts the foundational premise that “no action” on CDR is not a credible alternative, then any ocean-based CDR action has to be compared against other CDR actions that will equally solve for the problems in the ocean driven by high levels of greenhouse gases in the air and water.  It is not credible to compare ocean-based CDR to a “no-action” alternative:  inaction is not “safe”, and a “no-action alternative” is a fallacy as a choice to safeguard marine biodiversity and ecosystems. No action leads to continuing loss of ocean health and growing threats to communities and economies. 

Although ocean-based CDR technologies are nascent, there is already emerging evidence of public preference for pathways regarded as more “nature-based”, such as restoration of coastal blue carbon habitats, over “engineering-based” approaches, such as ocean alkalinity enhancement.  This appears to be due to concerns about the potential for environmental risk and unintended impacts with “engineering-based” approaches. 

This apparent bias for nature-based approaches creates a situation where the techniques with greatest CDR potential and permanence (“engineering-based”) face greater obstacles to public acceptance than those with reduced CDR potential and permanence (“nature-based”)¹.

Advancing the development and testing of ocean-based CDR approaches will require governance structures that both enable the permitting of legitimate testing and development and ensure that the public interests are protected.  

Current governance-related challenges include:

• There are no specific regimes in the US nor internationally governing ocean-based CDR¹.
• In many countries, there are complex regulatory mazes to navigate to gain permits for in-water experimental trials, hindering development of trials.  
• In the US this is particularly complicated by overlapping jurisdictions and authorities in the coastal zone².

These and other governance and regulatory uncertainties present real challenges and risks to those working to conduct research and development on ocean-based CDR pathways. Lack of such regimes both inhibits experimentation and the development of public confidence in ocean-based CDR experimentation. Governance structures and regimes with a specific focus on ocean-based CDR must be developed.

These governance structures must provide consistency; transparency; work to facilitate experimentation and demonstration; minimize negative environmental impacts; and ensure that field trials are controllable in size and scope. They may also require different skill sets appropriate to specific ocean CDR pathways (i.e. different governance for macroalgal sequestration versus ocean liming)³.  

It is essential to swiftly raise awareness of and knowledge about ocean-based CDR across the key sectors and actors that are engaged in shaping climate and ocean policy. As various alternatives are evaluated, ocean-based CDR needs a “seat at the table” and to receive due consideration. This requires a much broader array of voices articulating the case for consideration than we currently have. It is a priority to engage a broad and diverse range of credible individuals, organizations and entities to vocally support research and development of ocean-based CDR.  

The key first step is to design and launch a multi-dimensional, multi-year communications campaign to reach critical actors in this space. 

• Target actors include policymakers at multiple levels, climate and ocean specialists, marine sectors, scientists, investors, philanthropists, entrepreneurs and others.  The campaign should be designed to educate about the outsize and growing impacts of greenhouse gas pollution on the oceans, the role of CDR as one approach to slowing these impacts, and the potential role of ocean-based pathways for CDR.  All with the underlying objective to expand support for accelerated research, development and demonstration of ocean-based CDR pathways. 

The communications campaign should: 

• • • Identify all target audiences that will be part of creating an enabling environment for ocean-based CDR RD&D
    • Conduct research on their current perceptions and understanding of ocean-based CDR. 
    • Develop appropriate content and information formats for these key audiences.
    • Identify and engage credible spokespeople (scientists, business leaders, other opinion leaders) to help carry the messages¹.

Ocean-based CDR field trials will stand the best chance of success when they are developed with as many communities of interest participating as possible. All practitioners and researchers need to move field trials forward with transparency and broad stakeholder and interest group involvement, and with third-party scientific review and validation. It is particularly important to expand access for diverse communities of interest to participate in the development of this field.  

Researchers and practitioners need to: 

• Cultivate alliances with diverse stakeholders:
  • Communities subjected to ocean change, including fishers and tribal communities.
  • Indigenous and tribal communities who often hold legal rights to the sea and marine resources.
  • Work with partners already trusted by communities (e.g., SeaGrant in the US).
  • Work with local stakeholders to ensure that benefits of field testing (environmental and economic) are equitably shared, and that risks of field testing are not externalized on local communities.
• Ensure there are adequate resources as part of field trial designs to engage stakeholders.
• Identify and quantity co-benefits to the environment and people of ocean-based CDR approaches.
• Engage social scientists to design involvement processes and help address economic, social, and political challenges and opportunities around ocean-based CDR.
• Ensure broad dissemination of progress and results. 

• Engage relevant stakeholders in development of a code¹ to guide scientific experimentation and field trials in a manner that is transparent, participatory, rigorously monitored and carefully controlled.
  • The Aspen Institute, with support from the ClimateWorks Foundation,  published a set of key questions that should be considered by researches and practitions of ocean-based CDR.
• Develop assessment frameworks to guide scientific field trials under all relevant international conventions and national laws.

• Government actors at various levels responsible for climate action need to be specifically engaged to ensure that ocean-based CDR is considered in national programs for climate mitigation. There requires increasing governmental awareness and support for ocean-based CDR RD&D at national and international scales.

  • Increase outreach to relevant political jurisdictions to educate about ocean-based CDR opportunities and needs.
  • Seek to expand governmental involvement in ocean-based CDR through enabling legislation and policy.
  • Work with relevant jurisdictions to expand governmental support/funding for needed RD&D.
  • Build a broad coalition/alliance to demonstrate the diversity of interests in support of ocean-based CDR testing.

• Sub-national, national, and international regimes to govern development and testing of ocean-based CDR are absolutely critical for this field to progress. Key first order needs are:

  • Governance Reviews: Governance reviews are needed to identify and describe the existing legal frameworks for ocean-based CDR pathways in priority coastal countries/jurisdictions.
    • The Sabin Law Center at Columbia University has initiated such an effort with working papers to look at legal challenges and opportunities for:
      • Macroalgal cultivation and sequestration and ocean alkalinity enhancement in the US¹. 
      • Offshore carbon capture and storage in Canada²
      • The legal framework for sub-seabed carbon storage in the U.S and Canada ³⁴

  Planned additional working papers will add geographic context and expand the suite of ocean-based CDR pathways considered.

  • Identify Needed Improvements to Governance Regimes 
    • Review scans to assess gaps, needs, and opportunities.
    • Convene stakeholders at relevant scales to discuss and develop draft proposals for new or amended regimes, including developing specific legislative and executive directives that provide key agencies with scope to engage in ocean-based CDR.⁵
    • Engage with sub-national jurisdictions. Small-scale field testing under the jurisdiction of cities and/or states may help build data and evidence to support larger-scale testing in federal waters and under federal authority.
  • Review and Propose Needed Changes to International Regimes
    • Engage a global community of experts to review existing international governance regimes and laws for ocean-based CDR RD&D, and to make specific recommendations for improving the international regimes.  
      • Identify the specific processes and opportunities to more clearly embed governance and oversight of ocean-based CDR into international governance frameworks.
  • • Develop Tools to Help Researchers and Experimental Efforts
    • Permitting guides:  Develop specific tools and plans by country (and sub-national jurisdictions) to navigate the leasing, permitting, and legal considerations.
    • Convene relevant practitioners in working groups to identify and advocate for common tools and information needs that would benefit all practitioners of a given methodology (e.g. coastal enhanced weathering) in a specific jurisdiction. 
    • Co-develop such tools with the relevant permitting agencies to “bring regulators along for the journey”.
    • Identify and support development of the needed interagency and intergovernmental structures to facilitate review and compliance.

• It is essential to expand the field of researchers from across diverse disciplines to engage on ocean-based CDR-related research.  Many of the above activities to legitimize the field will help draw more scholarly attention but additional actions could also help, such as: 

  • Create prize competitions, awards, and funding opportunities to mobilize ocean scientists and engineers to work on critical needs identified throughout these road maps.  
  • Develop long-term strategies to engage more leading academic and research institutions against these challenges and opportunities to build the next generation of researchers and practitioners.  

One of the most critical gaps in further exploration, development, and testing of ocean-based CDR technologies is the very limited amount of financial resources being invested into the space.  

Without exception, all ocean-based CDR technologies examined in these road maps need substantial increases in public, philanthropic, and private investment to accelerate technology development, de-risk technologies, and develop monitoring, reporting, and verification protocols. The global community of experts who participated in the scoping workshops for these road maps broadly agreed that ocean-based CDR pathways remain substantially underinvested given their potential to contribute to gigaton-scale, affordable CDR. 

The lack of awareness of and support for ocean-based CDR amongst key audiences was clearly identified as one of the (if not the) primary obstacles to unlocking greater resources that could accelerate the development and testing of ocean-based CDR pathways. Readers may consult the road map “Growing & Maintaining Public Support” which details the specific obstacles, needs, and opportunities to advance public awareness and support. 

Participants in the expert workshops leading to the production of this road map broadly agreed that there exists a “vicious cycle” whereby the lack of investments into ocean-based CDR pathways has prevented proof-of-concept field trials from taking place. These field trials are in turn critical to answer key questions on environmental benefits and risks, as well as technical feasibility, which in turn could increase enhance public confidence in further exploration of ocean-based CDR pathways. 

Without this essential information, and/or without increasing public support, there is likely to be continued underinvestment into pilot projects. 

Public investment from governments must play a critical role in providing early-stage funding to development and field testing of ocean CDR technologies, which in turn helps to “de-risk” private investment and finance of ocean-based CDR efforts. This public investment de-risking function is critical to create the enabling conditions for building an industry and a market and has been done successfully in a number of fields, from healthcare to renewable energy.  So far, public investment in ocean- based CDR is lacking. 

A previous analysis by the Energy Futures Initiative and the Bipartisan Policy Center of US federal investments in ocean-based CDR RD&D found a total expenditure of only $44.3 million on marine pathways across 115 projects from 2005 to 2018¹. 

In the US, the US Department of Energy’s Advanced Research Projects Agency – Energy (ARPA-E) is the primary source of US public support through the $50 million MARINER program designed “to develop the tools to enable the United States to become a global leader in the production of marine biomass” (but for energy, not CDR) and through $2 million in recently announced awards to support research and development on direct ocean capture (electrochemical) technologies. 

In the EU, the OceanNETs (NETs: Negative Emission Technologies) project is evaluating a wide range of ocean-based CDR pathways for their theoretical, technological, economic, social, and political viability. OceanNETs is funded at € 7.19 million (~$8.75 million). 

While these government funding programs are steps in the right direction, they fall well short of the recently released call for a 10-year, $1.75 billion US federal investment in ocean-based CDR research, development, and demonstration². 

Philanthropic investment is also important to build an enabling environment for increasing public awareness and support of ocean-based CDR pathways³. The ClimateWorks Foundation⁴ and the Grantham Environmental Trust⁵ are two of a small number of philanthropic organizations which include ocean-based CDR in their portfolios. 

The lack of well-developed markets for carbon removal and permanent storage also affects the flow of early-stage investment into ocean-based CDR pathways specifically. 

There has recently been some progress to develop carbon removal markets (e.g. Nori, Puro Earth), and there are a growing number of corporate commitments to carbon removal, alongside several high-profile CDR ‘futures’ purchases from Stripe, Shopify, and Microsoft.

However, more work is needed to keep growing the markets for CDR and to better connect the growing demand for carbon removal to the potential suppliers of carbon removal. More clarity and certainty on the demand side (scale, specifications, pricing, etc.) will give clarity to potential early-stage investors and may increase capital flow into development of the technologies and enterprises that can ultimately deliver carbon removal at a competitive price.

As outlined in the technology-specific road maps (Electrochemical CDR, Macroalgae Cultivation & Carbon Sequestration, Ocean Alkalinity Enhancement), many of the ocean-based CDR pathways also generate co-products that can serve as important sources of revenue for carbon removal ventures. These include, but are not limited to: high value bioproducts, bioplastics, and bioenergy from macroalgae; hydrogen, chlorine, and oxygen gases, as well as hydrochloric acid from electrochemical processes; and valuable metals refined from ocean alkalinity enhancement pathways.  Unknowns about the current and projected sizes of the markets for these co-products hinders development of more accurate business models for investors into ocean-based CDR pathways. 

There was strong agreement in the expert workshops that a shortage of capacity, skills, and knowledge in the ocean CDR field amongst researchers, entrepreneurs, marine managers, and key stakeholders creates obstacles to the enhanced development and testing of ocean-based CDR pathways. More effort is needed to further identify the requisite knowledge and skills set for ocean-based CDR development and to work with relevant institutions to train and develop people with the capacity needed globally,  

Investment is needed not only in development of ocean-based CDR technologies but also in the associated tools needed to support the field. This includes a new suite of hardware and software products that can:

1. Provide physical, chemical, and biological information at sufficient spatial and temporal density necessary to make informed carbon sequestration estimates.
2. Provide remote verification of carbon sequestration and permanence.
3. Provide the autonomous or remote monitoring, maintenance, and troubleshooting necessary to support large-scale offshore operations.

Advancing the investment into development and testing of ocean-based CDR approaches will require governance structures that both enable the permitting of legitimate testing and development and ensure that the public interests are protected.  

Current governance-related gaps include:

• There are no specific regimes in the US nor internationally governing ocean-based CDR.
• Uncertainty and lack of expertise/knowledge around the jurisdiction and application of relevant international governance regimes to ocean-based CDR (e.g. London Convention/London Protocol, UN Convention on Biological Diversity).
• In many countries, there are complex regulatory mazes to navigate to gain permits for in-water experimental trials, hindering development of trials. This is particularly challenging in countries (e.g. the US) featuring overlapping jurisdictions and authorities in the coastal zone.

These and other governance and regulatory uncertainties present real challenges and risks to those working to conduct research and development on ocean-based CDR pathways and inhibit private investment.

Regimes and protocols around monitoring, reporting, and verification for ocean-based CDR also needs to be strengthened to clarify roles and responsibilities of CDR providers (sellers), buyers, and regulators.

Investment in ocean-based CDR will not happen at the scale needed without addressing the current situation of policy and regulatory uncertainty. It is critical to build enabling governance frameworks (a mixture of policy, regulatory structures, and public investment) that provide increasing clarity around how ocean-based CDR can move forward.  This must include things like clarified permitting processes, and clearly identified oversight authorities. Detail on key elements of this priority can be found in the Public Support road map.

A growing carbon market will be ultimately necessary for successful scaling of ocean-based CDR. Ensuring that ocean-based CDR pathways will be accepted into carbon markets will depend upon resolving some critical information challenges in the first instance, including:  

1. Scope and develop the measurement technologies, and verification tools to support creation and application of standards and protocols for each ocean CDR approach to provide assurances to negative emissions markets.  (link to “Develop CDR Monitoring and Verification Protocols” priority in each of the technical maps)

1. Develop the science needed to underpin standards for verifying carbon sequestration and permanence from various ocean-based CDR pathways.  For example, Oceans2050 is already developing a methodology for quantifying natural carbon sequestration underlying macroalgae farms¹, but additional science and research on carbon fate and transport is needed for macroalgae cultivation and sinking.

A growing carbon market will be ultimately necessary for successful scaling of ocean-based CDR. Ensuring that ocean-based CDR pathways will be accepted into carbon markets will depend upon resolving some critical information challenges in the first instance, including:  

1. Scope and develop the measurement technologies, and verification tools to support creation and application of standards and protocols for each ocean CDR approach to provide assurances to negative emissions markets.  (1,2,3)
2. Develop the science needed to underpin standards for verifying carbon sequestration and permanence from various ocean-based CDR pathways.  For example, Oceans2050 is already developing a methodology for quantifying natural carbon sequestration underlying macroalgae farms¹, but additional science and research on carbon fate and transport is needed for macroalgae cultivation and sinking.

Corporate buyers of carbon removal (e.g., Stripe, Microsoft) can play a key role in providing support for RD&D needed to make future carbon credit purchases possible.

• Scope creation of a “Carbon Removal Buyers Alliance” to pool demand and resources to help develop ocean-based pathways for carbon removal.  Such an Alliance could:
  • Provide early developmental and testing funding in exchange for future CDR options
  • Provide forward contracts for future purchase of CDR allowing entrepreneurs to make investments now
  • Develop and coordinate the specifications needed for the private removal market (as the Renewable Energy Buyers Alliance did for wind and solar).  This also would build on existing investments and/or purchase of CDR¹²³, that have allowed for infusions of capital into entrepreneurial efforts needed to further test and develop these approaches. 

Co-products generated from ocean-based CDR pathways need viable routes to market in order to provide revenues that can offset costs of ocean-based CDR deployment. Market analyses need to be performed that evaluate the potential for:

• Existing markets to accommodate additional supply of co-products (e.g. hydrogen gas produced from electrolysis) (Understand Market Potential for Co-Products)
• New markets to grow to match the supply of new co-products with buyers

Additional human and physical resources are needed to accelerate ocean-based CDR technology RD&D. Opportunities to accelerate RD&D include: 

• Building new public-private partnerships, such as between universities, national laboratories, and industry to address critical bottlenecks in the technological readiness of various ocean-based CDR pathways.
• Cataloging outstanding science questions/gaps pertinent to ocean-based CDR development and working with existing funders of basic and applied science to build funding programs around for the needed science.  One such option is encouraging relevant national science funding agencies (e.g., the US National Science Foundation), to open requests for proposals around key science needs.  

Much higher levels of public investment are needed to move ocean-based CDR RD&D forward. This in turn requires a supportive policy environment. Supporters of ocean-based CDR must cultivate awareness and support among policy makers by: 

• Developing targeted education and communication strategies for key policymakers at different levels of government to increase support for ocean-based CDR RD&D
• Engaging a diverse coalition of organizations to help educate and inform policy and appropriations for ocean-based CDR RD&D.

There are a number of emerging innovators and entrepreneurs working on ocean-based CDR, but they often lack access to the critical talent and expertise needed to move their innovations out of labs and into field trials and potential deployment. There are key actions that can help create a more supportive environment for these entrepreneurs.  

• Facilitate related technology transfer out of academia: These road maps can be used to work with specific universities to profile needed technology development and then to help connect academics with external actors.  To facilitate better tech transfer, a series of outreach materials and webinars to familiarize ocean scientists and engineers with technology transfer offices at key universities would be useful. 
• Build partnerships with leading accelerators, such as Y-Combinator¹, the Ocean Solutions Accelerator, Creative Destruction Lab, Larta Institute, or Indie Bio to increase their focus on ocean-based CDR ventures and to access their expertise for building these ventures.  
• Work with prizes and competitions, such as those hosted by The Ocean Exchange to ensure inclusion of ocean-based CDR in prizes and competitions.
• Ensure that ocean-based CDR research and development is well represented in the myriad “ocean clusters” springing up (e.g., Alaska Ocean Cluster) to support a global Blue Economy 

• Develop a publicly available database where people can track investments and projects. Ensure that this includes tracking of public investment in ocean-based CDR, and from countries besides the US and the European Union, to gain a full picture of the global landscape around ocean-based CDR.
• Develop targeted outreach and materials for investor education to build their familiarity and comfort with CDR, and ocean-based CDR specifically (something akin to the Climateworks – Economist knowledge hub for ocean-based CDR pathways). Reach out to specific associations such as TONIIC.  
• Develop comparative analyses (e.g., cost-benefit) between ocean-based CDR and terrestrial-based or technology-based CDR to inform and reduce barriers to entry for CDR investors looking to support ocean-based pathways.