Electrochemical carbon dioxide removal technologies utilize 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. These techniques are sometimes called “direct ocean capture” to draw comparisons with direct air capture and encompass both electrodialytic and electrolytic processes.

Electrodialytic approaches are already commercial technologies used in applications such as whey demineralization, organic acids recovery, and desalination, among others (Bazinet & Geoffroy, 2020).

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.) (Lakshmanan & Murugesan, 2014).

An electrolytic approach to precipitate calcium carbonate has recently been proposed (Callagon La Plante et al., 2021). 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 level of ~5 and is currently undergoing small-scale field testing.

• Scalability - Given the immense stocks of dissolved inorganic carbon in seawater (~38,000 Gt (Ciais et al., 2013)), 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 considered (Campione et al., 2018).
    • 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 seawater (Eisaman et al., 2018), and in the case of CO₂ stripping, the additional cost of safely storing or utilizing the captured CO₂ (Eisaman, 2020)
    • 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 capture (Callagon La Plante et al., 2021).
  • The collective market size for a 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 suggesting a range of 0.1-1.0 GtCO₂/yr with medium confidence, while the 2023 synthesis report from NOAA suggests 1-10 GtCO₂/yr. 

• 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., Callagon La Plante et al., 2021) with sequestered carbon are stable on timescales of hundreds to thousands of years. 

Electrochemical methods that 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 sources (Rau, 2008):
  • 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 addition (Hartmann et al., 2013).
  • 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 silicifiers (Bach et al., 2019).

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 (Renforth & Henderson, 2017).  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 carbonate (Callagon La Plante et al., 2021), the impacts of large quantities of new calcium carbonate on the 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 (Rau, 2008):
   • Electrolysis: hydrogen gas, chlorine gas, and oxygen gas, as well as hydrochloric acid
   • Electrodialysis: hydrochloric acid

• Electrochemical technologies, as applied to mCDR, are very new. Because most field experiments are in the planning phases or are underway currently, it is not yet possible to have a comprehensive characterization of benefits, risks, and scaling considerations of electrochemical approaches 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 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 guidance (Gattuso et al., 2015) as the community builds out standardized protocols and treatment levels for consistency and inter-comparability. See the 2023 Guide to Best Practices in Ocean Alkalinity Enhancement as one example (note that while not all electrochemistry is OAE, there is much in common and there are overlapping needs).
• 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). 
  • • 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)

• 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)

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 restoration (Bertram & Merk, 2020). (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 mCDR 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). See the mCDR Field Trial Database for information on current field experiments. 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 existing (Koornneed & Nieuwlaar, 2009; Hartmann et al., 2013), 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 mining (House et al., 2007)
• 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)
   • 0.1 – 0.6 Gt CO2/ year (NOAA 2023)

Theoretically, macroalgal cultivation and sequestration  could be scaled to between 0.1 - 1.0 Gt CO2/year (NASEM 2022, NOAA 2023), 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 environments (Bak et al., 2020; Lovatelli et al., 2010), 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) (Pan et al., 2016; Oschlies et al., 2009).
    • 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 annually (NASEM workshop comments from Carlos Duarte, 2021), 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) (Energy Futures Initiative, 2020; Capron et al., 2020; NASEM, 2022), but these cost estimates are preliminary and subject to change as macroalgae-based pathways advance in technological readiness.
    • Researchers at UC Irvine have developed a biophysical-economic model (G-MACMODS) to better understand the geophysical, biological, and economic sensitivities of macroalgae-based CDR pathways (DeAngelo et al., 2022). Also, see the affiliated site suitability tool.

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 years (if remineralized in the deep ocean) or for thousands-to-millions of years (if buried in marine sediments) (GESAMP 2019).
    • There are natural analogs 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 (Krause-Jensen & Duarte, 2016).
  • Relying on sedimentary burial below or adjacent to the farm (typically more applicable for farms not located over deep water) (Duarte et al., 2023)
  • Harvesting the macroalgae for:
    • Bioenergy (Hughes et al., 2013)
      • Combining combustion pathways with carbon capture and storage (thousands-to-millions of years if stored in a geologic reservoir) (Mereira & Pires, 2016)
      • Pyrolysis, resulting in biochar (hundreds to thousands of years) (Zhang et al., 2020; Roberts et al., 2015)
    • Production of long-lived bioproducts, such as bioplastics, that are capable of permanent (>100 years) sequestration (Albright & Fujita, 2023; Chia et al., 2020)

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% (Roque et al., 2019; Vijn et al., 2020). 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 supplements (Biris-Dorhoi et al., 2020). 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. (Koweek et al., 2016; Hirsh et al., 2020; Kapsenberg & Cyronak 2019; Fernández et al., 2019; Xiao et al., 2021)

• Nutrient remediation and metal uptake in eutrophied, polluted coastal waters (Neveux et al., 2018)
• Building macroalgae cultivation facilities near shellfish or fish aquaculture facilities may alleviate negative impacts from such activities (NASEM 2022) (e.g., deoxygenation, eutrophication)

• Macroalgae farms may attenuate wave energy (Mork, 1996)
• Creation of habitat with resulting nurseries for fish and other marine life (Smale et al., 2013)

• 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 halomethanes (Carpenter et al., 2009; Mehlmann et al., 2020) , 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 causes (Tegtmeier et al., 2015).
• 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 macroalgae (Pan et al., 2015).
• 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
• The remineralization of sunken seaweed may lead to oxygen depletion and acidification (Wu et al., 2023, Ocean Visions 2022)

• Impacts to biodiversity and ecosystem function from large-scale cultivation operations, including (Campbell et al., 2019):
  • 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 change (e.g., seaweed farming at Atlantic Sea Farms).
• Value-Added Products: High-value bioproducts (pigments, lipids, proteins, etc) can replace more carbon-intensive alternatives in feed, food, fuel, and other commodities (Albright & Fujita, 2023).

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
   • Protected sites include marine protected areas, world heritage sites, culturally significant areas, treaty-protected resources, ecologically or biologically significant marine areas, sensitive or vulnerable marine ecosystems, marine protected areas, and other effective area-based conservation measures.
2. Competition for Food: Given that macroalgae can be converted to nutrient dense food stuffs, there may also be social resistance to this method as it involves the willful destruction of viable food sources that could be used in furtherance of human food security (Stedt et al., 2022)
3. Financing: Financing any CDR approach brings with it the risk of creating inequity and decreasing social welfare (Cooley et al., 2022). 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 (Hurd et al., 2023; Burger et al., 2023)(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 technology (Bak et al., 2020).  (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) 
• A standard for mCDR is still needed. Evidence of a move towards a standard for seaweed can be seen in Verra's Seascape Carbon Initiative.

While interests and awareness amongst researchers and technologists is growing (i.e., Pessarrodona et al., 2023; DeAngelo et al., 2022; Troell et al., 2024), public awareness and support remain low. 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 impacts (van der Molen et al., 2018; Coastal Dynamics Laboratory)
• 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. (Visit the mCDR Field Trial Database for the latest information on current field experiments.) 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 (See partnership between Ocean Networks Canada and Running Tide)
• Develop a standardized list of biological indicators to measure during field trials to facilitate intercomparison between field trials (See Verra's Seascape Carbon Initiative.)
• 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 macroalgae CDR may be accelerated and strengthened by creating partnerships with key industries/sectors, including:  

• Integrated multi-trophic aquaculture (García et al., 2020): 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 need 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 permits.

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 pathways (Bertram & Merk, 2020)
• 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) (Meysman & Montserrat, 2017; Renforth & Henderson, 2017; Lenton et al., 2018; Köhler et al., 2013; Köhler et al., 2010).
2. Production of hydroxide minerals, including lime/slaked lime (Kheshgi, 1995) from thermal calcination and magnesium hydroxide from synthetic weathering of olivine (Scott et al., 2021) 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 alkalinity (Renforth et al., 2022)

In contrast to recent reports (Gagern et al., 2019; EFI, 2020; Rackley, 2020), we consider these four mineral-based 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 mCDR, 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 CDR (Beerling et al., 2020). 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 years (Bruno & Boyd, 2011), largely as a response to the hatchery failures ~2010 in the US Pacific Northwest and British Columbia from ocean acidification (Washington State Blue Ribbon Panel on Ocean Acidification, 2012). The Chalk-Ex experiment released crushed calcium carbonate in a ~1.5 km² patch in the Gulf of Maine in the early 2000s. However, this experiment's primary objective was to quantify particles' effects on upper ocean layer optical properties, not to achieve CDR. There are now a number of ongoing field trials, with more planned in the future. OAE's TRL is currently around a 6 (TRL, RMI The Applied Innovation Roadmap for CDR). See the mCDR Field Trial Database for information on current field trials.

1. Carbon Capture

   Methods of OAE that rely on depositing minerals into the ocean to promote additional CO2 uptake depend on how quickly the minerals dissolve relative to how quickly they sink (Fakhraee et al., 2023). 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. However, 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)
   • 1 – 15+ Gt CO2/ year (NOAA 2023)

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. The most current cost estimates range from $25-160/ton CO₂ (NOAA 2023).

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 (Renforth & Henderson, 2017). Currently, OAE is thought to have sequestration duration >20,000 years (NOAA 2023).

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) (Rau & Caldeira, 1999; Rau, 2011). 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 growth (Rau, 2014); 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) (Hughes, 2012), 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 ecosystems (Gattuso et al., 2018; Feng (冯玉铭) et al., 2016).

• In certain areas, calcium or silica additions could act as fertilizers to support plankton populations (Adhiya & Chisholm, 2001).

• OAE may pose ecotoxicological risks from the release of elevated concentrations of trace metals from some minerals during mineral dissolution (Burns & Corbett, 2020).
  • 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 webs (Bach et al., 2019).
  • Silicate rocks are likely to have higher metal concentrations than carbonate rocks, and thus may pose a greater ecotoxicological risk (Bach et al., 2019)
  • 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 vessels (Caserini et al., 2021)

• 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 wakes (Caserini et al., 2021).
• 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 chain (NASEM 2022).
• Dedicated ships/vessels to distribute minerals could cause environmental impacts such as additional noise pollution (Kaplan & Solomon, 2016), the potential transfer of nonindigenous species (Muirhead et al., 2015), and the addition of pollutants from shipping emissions (Dalsøren et al., 2013).

• Risks associated with the expansion of mining activities for OAE
  • Demographic changes include, but are not limited to, shifts in gender balance, increase in non-resident workforces, appropriation of land from local communities, social inequality, and impacts on Indigenous communities (NASEM 2022)
  • Negative health impacts include cancer, respiratory diseases, injuries, and prolonged exposure to chemical agents (Candeias et al., 2011)
  • Environmental implications include impacts to soil and air quality, impacts to ground and surface waters, and erosion.

• Creation of job opportunities from the OAE supply chain
• Amelioration of ocean acidification could increase food security and preserve cultural values (Nawaz et al., 2023)

• Field trials are beginning to get underway (See mCDR Field Trial Database) however characterization of benefits, risks, and scaling considerations remains a gap. An increase in controlled field trials 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 guidance (Gattuso et al., 2015)  as the OAE community builds out standardized protocols and treatments levels for consistency and intercomparability
  • Existing database(s) of ecotoxicological tests need to be reviewed (EPA, Environmental Protection Agency) 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 catalysts (Subhas et al., 2017) 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 testing⁵ (Taylor et al., 2021; Naturresursavdelningen, 2010)
• To systematically assess several of the knowledge gaps listed (whether and how OAE can be a safe, scalable, and permanent CDR method), Additional Ventures created the non-profit initiative Carbon to Sea. Carbon to Sea has funded several research and technology projects which aim to close knowledge gaps and push technology innovation forward.

• 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 impact (Beerling et al., 2020).
• 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 calcination (Renforth et al., 2013; Renforth & Henderson, 2017)(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' wakes (Caserini et al., 2021).

Many of the opportunities and challenges around building public awareness and support for mCDR 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) (Bertram & Merk, 2020) (Growing and Maintaining Public Support).
  • There is a great deal of hesitancy and resistance around any mCDR pathway that relies on adding materials to the ocean. 
  • Earlier ocean iron fertilization experiments (Schiermeier, 2009) 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).
• Clear communication strategies need to be developed to respond to the “geoengineering” narrative of OAE (Growing and Maintaining Public Support). 
• The public discussion around OAE (and mCDR generally) is growing and has been featured in popular news sources (Wired, MIT Technology Review).
• Funding has increased markedly in the past years, including funding from the US government.

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). While there are a number of OAE field trials underway in the US and abroad, more efforts are necessary. The following activities and products are still needed:

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 the uptake of atmospheric CO₂ resulting from ocean alkalinity enhancement will ultimately need to be developed to enable trading of carbon removal credits. See efforts by [C] Worthy, ARPA-E SEA-CO2 grantees, and Carbon to Sea grantees. Key first steps to support the development of these tools 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 existing (Koornneed & Nieuwlaar, 2009; Hartmann et al., 2013) 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 Principles).
• Industries generating alkaline byproducts for feedstock into OAE processes (steel, aluminum, cement, lime, and nickel production; coal and biomass combustion) (Renforth, 2019).
  • 7 billion tons of alkaline byproducts generated annually from manufacturing and combustion could decrease costs and potentially facilitate transitions to larger-scale OAE (Renforth, 2019). 
• 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 acidification (Gattuso et al., 2018). 
• 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 (Williamson et al., 2012). 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 (Beal et al., 2018). This can be accomplished on shore in cultivation ponds or photobioreactors, or afloat in photobioreactors either stationary or towed at sea (Zhai et al., 2020; Kim et al., 2016; Zhu et al., 2017).

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 (Li et al., 2022). 

• 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 (Hazlebeck & Rickman, 2019). Utilizing gravity-based systems is another way of increasing energy efficiency for harvesting (Muylaert et al., 2017). 
    • 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 (Morales et al., 2020). 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 (Morales et al., 2020). New techniques with biofilms are in development (Mantzorou & Ververidis, 2018).

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 (Siegel et al., 2021). 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 (Siegel et al., 2021). Findings also suggest the North Atlantic and North Pacific oceans have the greatest carbon storage potential, while subtropical oceans have the least (Nowicki et al., 2022). 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). Also note work being done to remediate harmful algal blooms and the potential applicability of findings on the efficacy of sinking algal biomass to mCDR research (see BlueGreen Water Technologies as an example).
  • 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 (Bates et al., 2019). 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 the 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 (Avnimelech et al., 1982; Chen et al., 2018).

• 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 (Bi & He, 2022; Lee et al., 2020; Wang et al., 2023).
• 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 (OpenAir, 2022).

Nutrient Fertilization

1. The depth of remineralization for carbon, iron, and other macronutrients (NASEM, 2022).
2. Factors affecting carbon sequestration that remain uncertain (NASEM, 2022):
   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 (Martin et al., 1990). 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 (Yoon et al., 2018), 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 (Martin et al., 1988; Morel & Price, 2003; ExOIS, 2022) and has led scientists to look to iron as a strategy to enhance the ocean’s biological carbon pump (Martin et al., 1990; Martin & Fitzwater, 1988). 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 (NASEM, 2022). 

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 (OpenAir, 2022).

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” (Carlisle et al., 2020; Cox et al., 2020). 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 (Gannon & Hulme, 2018; Abate, 2016). 
• Human health risks: Exposure to aerosolization of toxins from harmful algal blooms (HABs) have been known to cause respiratory symptoms in humans (Stewart et al., 2006). 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 (Cooley et al., 2022). 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 (Cooley et al., 2022). 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 (Daneshvar et al., 2022).
• Job creation: Economic value-addition is likely to also create jobs in new and existing industries in aquaculture and beyond (Mayo-Ramsay, 2010).
• 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: The 1972 Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter (London Convention) and the 1996 Protocol to the Convention (London Protocol) aim to limit ocean dumping. The London Protocol was intended to eventually replace the London Convention however not all countries party to the Convention have ratified the Protocol (for example, the United States). 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 outright (Rohr, 2019). 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 (an 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 that regulate marine dumping adopted a statement calling out the need for evaluation of four marine geoengineering techniques that have the 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 the 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 isn’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 incentives to researchers as well as the funds necessary to accelerate research. 
• Bias amongst approaches: How will the ability to monitor and verify the 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 (NASEM, 2022). 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 (Liu et al., 2022). 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 fertilization (Bach & Boyd, 2021; Weis et al., 2022). 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 (Singh & Ahluwalia, 2013). (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 systems (Huntley et al., 2015). 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. 

The term “blue carbon” has historically referred to three (vascularly) vegetated coastal ecosystems: mangrove forests, salt marshes, and seagrass beds (Lovelock & Duarte, 2019). These ecosystems are among the most productive in the world (NASEM, 2019) and sequester carbon in their underlying substrates (e.g., soil or sediment). Estimates of their global distribution reach ~36-185 million hectares, storing as much as 33 billion tons of carbon (Macreadie et al., 2021), however there remain large knowledge gaps around spatial extent and storage capacity, owing to the high variability across regions and ecosystems. The carbon in the substrate can be stored as organic or inorganic carbon and can remain sequestered for millennia (Macreadie et al., 2021; Duarte et al., 2013), positioning these ecosystems as important parts of the global carbon cycle. Situated between land and water, these ecosystems also play critical roles in human society and provide many services including coastal buffering from storms, surges and erosion control generally, nursery grounds for fish, culturally significant sites, and sources of income from fishing and tourism. Blue carbon ecosystems have gained attention as levers for climate change mitigation, including carbon sequestration (Collins et al., 2022). 

Some of this attention is related to the major uptick in interest and attention around carbon dioxide removal (CDR)⁷, which has again raised questions about the capacity of blue carbon ecosystems to sequester carbon and combat climate change (Collins et al., 2022; NASEM, 2022; Cross et al., 2023; NASEM, 2019). This discussion has also broadened the conversation around the definition of blue carbon, with the IPCC (2019) defining blue carbon as ‘all biologically driven carbon fluxes and storage in marine systems that are amenable to management’ (IPCC, 2019). This definition can and has been interpreted to encompass not only coastal vegetated ecosystems that store carbon in their structure and sediments, but also other marine life such as fish and mammals, as well as natural seaweed stands and seaweed farms - all systems that not only store and recycle carbon through their living tissues, but also respond to anthropogenic manipulation and management. Two notable examples of this broadened definition include the National Academies of Sciences, Engineering, and Medicine’s (NASEM) 2022 report on ocean carbon dioxide removal in which they include a chapter on “Recovery of Marine Ecosystems” for CDR (including benthic, pelagic, and offshore ocean systems along with mangrove forests, salt marshes, and seagrass beds) and the National Oceanographic and Atmospheric Administration’s (NOAA) 2023 Strategy for Carbon Dioxide Removal Research which includes coastal blue carbon (defined as mangrove forests, salt marshes, and seagrass beds) as well as ‘Marine Ecosystem Recovery’ (fishes and marine mammals). (Also note NOAA, 2021, and a recent call for a broader definition of 'blue carbon ecosystems' by James et al., 2024)).

Following these recent advances, this road map will consider carbon stored in natural seaweed stands and in marine animals (e.g., fishes and mammals) in the definition of blue carbon, alongside mangrove forests, salt marshes, and seagrass beds. Natural seaweed stands and marine animals have received substantially less attention as potential pathways for additional carbon dioxide sequestration and will require accelerated research to determine their potential. The nascent nature of these pathways makes them particularly well suited to be assessed in this road map.

Note: While the blue carbon definition above could be interpreted to include microalgae cultivation, we have excluded it from consideration in this road map because we have already released a road map for Microalgae Cultivation and Carbon Sequestration. In addition, we limit discussion of seaweed (macroalgae) farms to coastal farms which passively sequester carbon through biomass export to underlying sediments or offshore to deeper waters. We exclude discussion of seaweed farming coupled to active sequestration (such as sinking or harvesting to produce biochar) because this topic is covered in a different road map, Macroalgae Cultivation and Carbon Sequestration.

This road map focuses on assessing the potential for blue carbon to contribute to carbon dioxide removal (CDR), or the additional sequestration of carbon, and what is needed to achieve that potential. In many cases, this will look like the large-scale restoration of ecosystems or rebuilding of animal populations (and, in some cases, expansion of ecosystems or animal populations beyond past limits). Recent work by the Environmental Defense Fund and the High Level Panel for a Sustainable Ocean Economy suggests that even the most rigorous and optimistic future estimates of blue carbon restoration may yield small gains in additional carbon sequestration when compared with the scale of anthropogenic emissions and other possible CDR pathways (Fujita et al., 2022; Collins et al., 2022a; Collins et al., 2022b; Hoegh-Guldberg et al., 2023). One reason for this is the spatial limitation presented by the restoration of coastal vegetated ecosystems (i.e., mangrove forests, seagrass beds, and salt marshes). These ecosystems have spatial limits on their potential for restoration, and thus, their potential for carbon removal, however, other sources of blue carbon, such as animal biomass and seaweed farms, are far less limited by spatial constraints. Moreover, blue carbon restoration offers low or no-risk pathways, which sets it apart from other proposed CDR solutions. Not only do restoration efforts of blue carbon pose fewer risks to the environment and society than other proposed CDR solutions, but they also offer an enormous capacity for other benefits to both ecosystems and communities. In this way, carbon removal may in fact be best thought of as a “co-benefit” to blue carbon restoration and efforts (Williamson & Gattuso, 2022). Regardless, there is still critical research that would increase our understanding of the role blue carbon plays in the carbon cycle and inform responsible decision-making at the ocean-climate nexus.

In light of current and future climate disruption and risk, there is a growing need for solutions that will help to remove and durably store legacy greenhouse gas emissions. However, there remains a substantial set of unanswered questions around the capacity and efficacy of carbon storage in blue carbon systems. As scientists, practitioners, and policymakers continue investigating blue carbon as a climate solution, it is critical to be clear about what is known, and what remains unknown, and identify a pathway by which to answer the most important outlying questions. This road map lays out the current state of knowledge around carbon sequestration and storage in blue carbon systems, details the most pressing unanswered questions to understand the potential for blue carbon sequestration, and proposes ways in which to accelerate the development and testing of these potential solution pathways.

A Note on Restoration vs. Protection

Protection of existing blue carbon ecosystems and their embedded carbon avoid emissions generated as these systems degrade. This is complementary and distinct from the restoration of blue carbon systems, which is the regrowth of these ecosystems. This regrowth is where additional carbon can be sequestered. Because the focus of this road map is the potential for blue carbon systems to contribute to carbon dioxide removal, this map will only focus on restoration efforts, their state of progress, challenges, knowledge gaps, and opportunities to drive forward progress.

A Note on Units of Measurement

This road map differs from the other Ocean Visions’ Ocean CDR Road Maps in its use of ‘Gigaton CO2 equivalent’ (Gt CO2e) in order to account for the important role played by non-CO2 greenhouse gases (e.g., methane, CH4, and nitrous oxide, N2O) within blue carbon systems. Although petagrams (Pg) is a unit commonly used in blue carbon literature, we use the equivalent gigatons (or gigatonnes; Gt) in this road map.

Mangrove forests, salt marshes, and seagrass beds grow where land meets the sea and store carbon in their vegetation and underlying sediments/soils. The sediments/soils can store both organic and inorganic carbon. Some of this carbon originates from within the ecosystem itself (autochthonous carbon) while some originate from outside the ecosystem (allochthonous carbon). The exact proportion of autochthonous versus allochthonous carbon is difficult to determine, and differs by site, but is important when trying to account for all the carbon fluxes in a system. What’s more, carbon can move laterally and vertically in, out, and through these systems, creating additional complexity for carbon accounting.

Of these three ecosystems, in terms of current global extent, current carbon storage, and carbon sequestration potential, mangrove forests are the best understood followed by salt marshes. Seagrass beds are the least well understood both in spatial extent and ultimate carbon sequestration potential.

Restoration and its limitations: CDR pathways within coastal vegetated ecosystems are primarily concerned with restoration – the act of bringing an ecosystem back as nearly as possible to its original condition¹. However, the goal of restoration may be unattainable for several reasons, including irreversible land and hydrological (starved of freshwater input) change, conditions that do not permit rehabilitation, and the inherent spatial limitations for ecosystems that occupy specific niches (Adam, 2019). Another challenge is the host of known and unknown consequences of climate change which may impact the feasibility and efficacy of restoration efforts. Monitoring of restoration projects will also be critical both as important data points for studies and predictive science as well as for any involvement with carbon markets. There has been historical reluctance from both government agencies and private developers to fund long-term monitoring(Adam, 2019), however, this will be crucial for informed decision-making about where and how to implement restoration projects.

Net sequestration of CO2e and its storage durability are key metrics when considering restoration projects for CDR. Ideally, monitoring of restoration projects should feed back into adaptive management practices, where management techniques are adjusted through an iterative process in response to other variables such as climate change.

Mechanism for CDR

• Restoring hydrology: Restoration of hydrology, through reversing drainage or removing tidal restrictions may assist in the recovery and expansion of existing mangrove sites. This path may be met with more resistance (especially political and social) than other types of restoration efforts.
• Planting mangrove seedlings: This is one of the most common restoration techniques, however, large-scale planting efforts have often failed (Zimmer et al., 2022) (many of these shortcomings a result of projects that do not adequately consider environmental and socioeconomic conditions (Lovelock et al., 2022))  and smaller projects may not deliver carbon storage on climate-relevant scales (Lovelock et al., 2022).

• • Research is being done on the efficacy of automated mangrove planting via drones (AlRaisi et al., 2023; Ayub et al., 2023). This work is still in early stages but may provide lower cost options for planting efforts, especially in difficult to access locations.

• Restoration efforts should be de-risked in smaller projects before moving to larger projects. This will require investment in capacity-building in communities and institutions, and mechanisms to match restoration opportunities with prospective supporters and investors (Lovelock et al., 2022).

CDR Potential

Estimated Sequestration Potential: 0.028 – 0.172 Gt CO2e/year (Hoegh-Guldberg et al., 2023)

Sequestration Durability: Decades -- 1,000 years (Cross et al., 2023)  (Durability of sequestered carbon will depend on multiple variables and can vary greatly between individual ecosystems)

Challenges

• Accounting:

• • Estimating future sequestration potentials or future rates of carbon sequestration is more difficult than estimating their current global extent and storage.
  • While there is general scientific consensus that mangroves are net sinks of CO2 (Rosentreter et al., 2023), there is growing research that indicates production of CH4 and N2O nonuniformly across habitats. In order to have accurate calculations of greenhouse gas budgets, these emissions will need to be included in the accounting (Collins et al., 2022).
  • Current estimates of sequestration potential do not account for the effects of climate change on the ecosystem (e.g., warming, sea level rise, coastal squeeze, increased tropical storms).
  • Mangroves are inextricably connected to other marine ecosystems like seagrasses and coral reefs, complicating the definition of boundaries for estimating “additional” carbon sequestration and introducing other complications, such as figuring out how much carbon is autochthonous and how much is allochthonous. Additionally, many carbon markets currently require that all carbon be autochthonous to count toward credits or have poorly defined frameworks for whether to credit for allochthonous carbon that accumulates on site as a result of restoration or management actions.

• Restoration:

• • There are high costs associated with restoration efforts (Vanderklift et al., 2018).
  • Mangrove ecosystems are heterogeneous across geographies, and this must be accounted for when assessing individual habitat potential for carbon sequestration. (e.g., findings from one location/ecosystem may not be directly applicable to other locations/ecosystems
    • There is work being done to use unmanned aerial systems (UAS) to assess mangrove estuaries on the Pacific Coast of Costa Rica (Yaney-Keller et al., 2019) and in Malaysia (Otero et al., 2018), enabling monitoring of difficult-to-access locations. This type of innovation may be important for monitoring and verification of restored mangrove forests.

Key Knowledge Gaps

• Quantitative understanding of the impacts of climate change on mangrove ecosystems and how these may affect restoration efforts.
• Incomplete understanding of greenhouse gas production and release (e.g., CH4 and N2O) by and from mangroves and how this impacts net carbon removal.
• Uncertainty around the durability of carbon stored in mangroves and their sediments as well as carbon burial rates (Williamson & Gattuso, 2022).

First-Order Priorities

*Adapted from the Environmental Defense Fund report on Coastal Natural Climate Solutions (Collins et al., 2022)

• Improve predictions of the effects of climate change on mangrove forests*
  • Predictions needed include:
    • Effects of increased tropical storm intensity and frequency on the destruction and degradation of mangrove ecosystems
    • The timescales over which any previously sequestered carbon could be released to the atmosphere
  • Understand/map the potential spatial extent of mangrove forests given climate change impacts on existing habitats and poleward migration of mangroves with future warming.
• Improved, spatially explicit modeling approaches to predict greenhouse gas fluxes and net carbon sequestration*
  • Models should function across different mangrove ecosystems whose sediment dynamics cannot be resolved solely using imagery taken from space or aircraft.
  • The development of carbon crediting projects requires we know not only how much carbon a mangrove ecosystem contains in its above and belowground biomass, but also the magnitude of various greenhouse gas fluxes from underlying soils or sediments at present and in the future.
  • These models should account for future ecosystem state due to the interactive effects of climate change and predicted land-use changes, including the phenomenon of coastal squeeze, the prevention of expansion due to rural areas on one side and the ocean on the other.
• Create studies designed to better differentiate between carbon originating from within the system (autochthonous) and carbon originating from elsewhere (allochthonous)*
  • The ability to distinguish among carbon sources is particularly critical since methodologies developed under the voluntary carbon market standards require the deduction of allochthonous carbon from any claimed credit.
• Accelerate the development of technology and biotechnology that can help scale restoration efforts
  • Optimize drones and autonomous robots for increased scalability of mangrove planting.
  • Test biological engineering techniques to enhance carbon storage within mangrove forests. 

Mechanism for CDR

• Most restoration efforts include recovery of tidal exchange (e.g., restoring hydrology and tidal flow), managed realignment, and sediment level amendment (Billah et al., 2022). Efforts can also include removal of non-native vegetation and/or replanting of native vegetation.
• For an in-depth look at salt marsh restoration efforts to date, refer to Adam 2019 “Salt Marsh Restoration” (Adam, 2019)

CDR Potential

Estimated Sequestration Potential: 0.004 – 0.015 Gt CO2e/year (Hoegh-Guldberg et al., 2023)

Sequestration Durability: Decades -- 1,000 years (Cross et al., 2023)  (Durability of sequestered carbon will depend on multiple variables and can vary greatly between individual ecosystems)

Challenges

• Little potential for expansion of current habitats due to extensive land conversion.
• Gaining access to suitable land for restoration projects may be difficult and expensive.
• Determining baseline historical extent for setting restoration objectives.

Key Knowledge Gaps

• Need accurate predictions of carbon sequestration which requires integrated modeling of ecological impacts, land-use, and climate effects (made more challenging by high variability in carbon burial rates (Williamson & Gattuso, 2022)).
• Discerning between competing feedbacks associated with climate and sea level rise, which can have both positive and negative impacts on ecosystems.
• Measurement of greenhouse gases other than CO2 (e.g., CH4 and N2O) that are produced and their significance to net sequestration.
• What is the fate of carbon produced outside of marshes and trapped in them?
• Total global areal existing extent remains poorly mapped, especially at high latitudes.

First-Order Priorities

*Adapted from the Environmental Defense Fund report on Coastal Natural Climate Solutions (Collins et al., 2022)

• Conduct research to better understand and characterize the multiple carbon fluxes within salt marsh systems
  • Improve predictions of carbon sequestered by salt marshes through modeling*
  • Better knowledge of the relative contributions of carbon produced outside of (allochthonous) and within (autochthonous) marshes to salt marsh organic carbon pools, and the labilities of the carbon derived from these two sources*
• Investigate and characterize climate impacts, both positive and negative, on salt marsh systems*
  • Sea level rise, warming, and increased coastal storm frequency and severity all need further investigation.
• Measure non-CO2 greenhouse gases that are produced and compare with sequestration capabilities to better understand the net sequestration of salt marshes*
• Better global mapping of the current extent of salt marshes, particularly at high latitudes, as well as habitat-suitability mapping/analysis*

Mechanisms for CDR

• Unlike salt marshes and mangrove forests, seagrass beds remove carbon from seawater, creating a disequilibrium in the CO2 concentration between the seawater and atmosphere. The resulting flux from the atmosphere into the seawater replenishes the deficit in dissolved inorganic carbon due to seagrass photosynthesis. This replenishment may be complete (which maximizes carbon removal) or incomplete (which results in an inefficiency in carbon removal accounting).
• In theory, restoration (e.g., seed/seedling dispersal or planting) is the most likely mechanism for CDR, however, efforts to date are often unsuccessful (Macreadie et al., 2021). Restoration propositions are further complicated by the outstanding questions around net carbon removal (especially under different climate change scenarios) and the extent of suitable locations.
  • A 2023 study in Australia found that sediment carbon stocks show little recovery immediately after restoration and can take >5 years to return to expected carbon sequestration rates (Rahayu et al., 2023).  
• Restoration of hydrology, through reversing drainage or removing tidal restrictions, may assist in the recovery and expansion of existing seagrass beds (NASEM, 2019). This path may be met with more resistance (especially political and social) than other types of restoration efforts.

CDR Potential

Estimated Sequestration Potential: 0.007 – 0.025 Gt CO2e/year (NASEM, 2019)

Sequestration Durability: Decades -- 1,000 years (Cross et al., 2023)  (Durability of sequestered carbon will depend on multiple variables and can vary greatly between individual ecosystems)

Challenges

• A 2023 review of all available literature on eelgrass (the dominant species of seagrass along the United States west coast) restoration projects in California, Oregon, and Washington found that of the 82 restoration projects, only 6 were published in peer-reviewed journals. This highlights the lack of data availability that can make assessing successes, failures, and best practices difficult (Ward & Beheshti, 2023).
• Difficult to differentiate between autochthonous and allochthonous sources of carbon in sediments (Macreadie et al., 2021)
• There is substantial inorganic carbon in seagrasses which is derived from outside the seagrass bed (Saderne et al., 2019) which in turn has sparked debate over whether seagrasses are ultimately a carbon sink or source.
• Seagrass restoration efforts are often unsuccessful (Macreadie et al., 2021)
• High variability in carbon burial rates (Williamson & Gattuso, 2022)
• High cost associated with restoration efforts (Vanderklift et al., 2018)

Key Knowledge Gaps

• Robust measurements of air-water CO2 exchange above seagrass beds
• Global extent of seagrasses (including future extent under changing climate)
• Causes of failed restoration projects
• What are the sources and fates of carbonates in seagrass beds?

First-Order Priorities

*Adapted from the Environmental Defense Fund report on Coastal Natural Climate Solutions (Collins et al., 2022)

• Take additional measurements of air-sea gas exchange above seagrass beds*
  • This will lead to the development of more robust predictive models to better understand CDR efficacy.
• Map the global extent of seagrasses and their likely future extent*
  • This is likely to involve some combination of active remote sensing and field surveys as well as habitat suitability analyses.
  • Mapping of likely future extent will involve considerations of future climate change impacts.
• Continue investigations into the causes of restoration project failures in seagrass systems*
  • This effort will likely involve better monitoring, recording, and publication of data on restoration efforts.
• Improve the understanding of the sources and fate of carbonates in seagrass bed sediments*
  • This includes the contributions to CO2 production both within seagrass beds and upstream of these ecosystems.

Restoration of coastal vegetated ecosystems can generate many benefits for surrounding marine ecosystems. While we are calling these “co-benefits” here, it should be noted that some of these benefits are so significant (while carbon sequestration feasibility and efficacy remain unknown) that these may in fact be the main benefits of restoration. (Note that there are significantly more co-benefits in coastal vegetated ecosystems than any of the other Ocean Visions’ marine CDR Road Maps.) 

• Potential for ocean alkalinity enhancement (Fakhraee et al., 2023)
• Coastal protection against erosion (Barbier et al., 2011)
• Nutrient cycling (Mitra, 2019)
• Pollutant filtering
• Reducing biodiversity loss (NASEM, 2022)
• Restoring the role of marine organisms in the carbon cycle (NASEM, 2022)
• Protection against storm surge and inundation  (Blankespoor et al., 2017; Liu et al., 2013; Zhang et al., 2012)
• Providing habitat and nurseries for a variety of marine organisms (Whitfield, 2017)

Restoration of coastal vegetated ecosystems can generate many benefits for surrounding communities and economies. While we are calling these “co-benefits” here, it should be noted that some of these benefits are so significant (while carbon sequestration feasibility and efficacy remain unknown) that these may in fact be the main benefits of restoration. Many of the environmental benefits relate directly to social benefits, such as fisheries enhancement and coastal protection. (Note that there are significantly more co-benefits in coastal vegetated ecosystems than any of the other Ocean Visions’ marine CDR Road Maps)

• Coastal protection from storms, floods, and sea-level rise (Kiesel et al., 2020; Chow, 2017; Dasgupta et al., 2019) 
• Increased adaptive capacity for communities to handle climate change and natural disasters
• Job creation and economic stimulation from enhanced fisheries and tourism (of particular value to coastal communities that are dependent on marine-based livelihoods)
• Cultural / intrinsic / recreational value
• Education and research – the pursuit of restoring blue carbon for CDR will naturally add value to the existing body of research and can provide valuable educational experiences for the upcoming generation of scientists and conservationists.

Efforts around restoring natural ecosystems are often met with social acceptance and public support, especially when compared to other climate solutions seen as “tampering with nature”(Wolske et al., 2019). This may be in part due to the minimal risks (real and perceived) associated with restoring natural ecosystems. While some risks, outlined below, do exist, there may be a higher tolerance for such risks given the long list of potential benefits and co-benefits (both environmentally and socially).

• Potential for conflict with other industries (aquaculture, fishing, tourism, etc.); However, conflicts may be mitigated with participatory approaches to planning and implementation.
• Potential for inequity in benefits: Macreadie et al. (2022) point to concerns around the distribution of benefits from any given blue carbon project and whether benefits are evenly distributed or whether activities reinforce or add to social inequity (Macreadie et al., 2022). Similarly, there may be confusion around who “owns” the blue carbon and who has rights to control transactions of credits resulting from blue carbon projects. This may be particularly challenging in marine spaces where the movement of carbon must be considered.

• Develop accessible (economically and geographically) sensors and technologies to measure greenhouse gas exchanges and fluxes both temporally and spatially at high enough resolution to create predictive models for net carbon sequestration and durability.
• Create models that account for future impacts/effects of climate change and land-use change (e.g., coastal squeeze)
• Create new frameworks (e.g., seascape (Huxham et al., 2018) or watershed approaches) both for research and financial mechanisms that account for intersystem linkages and take a more holistic approach to measuring carbon associated with coastal vegetated ecosystems.
  • Currently, ecosystems are considered and studied individually which can cause issues when thinking about the movement of carbon between systems
• Investigate and characterize the historical or pre-industrial ecosystem locations and ranges (which in many cases are currently unknown) in order to establish baselines for restoration projects.
  • In some instances, these prior states may need to be derived from historical sources including more traditional data sets and non-traditional data sets like paintings (Duarte et al., 2020).

Current Landscape

• Verra: In 2020 Verra, a standard for certifying carbon credits, released its first blue carbon conservation methodology, an update to the VCS REDD+ Methodology Framework. Verra has also formed a blue carbon Working Group (2020) that explores opportunities for coastal wetland restoration and conservation as well as developed the Wetland Restoration and Conservation Requirements. Addressing blue carbon in the open ocean, Verra created an Ocean Carbon Working Group as well as the Seascape Carbon Initiative (co-led by Verra, Silvestrum Climate Associates, Blue Marine Foundation, the blue carbon Initiative, and Oceans 2050).
• Gold Standard: Gold Standard is a carbon offset standard for non-governmental emission reductions projects.  Gold Standard has an approved methodology for the certification of mangrove projects and are exploring other blue carbon opportunities.
• Plan Vivo:  Plan Vivo is a certification body that certifies projects against the Plan Vivo Standard, a framework for community and smallholder land-use and forestry projects. In 2014, Plan Vivo certified the world’s first blue carbon project, Mikoko Pamoja (Kenya), for mangrove conservation.
• Fair Carbon: Fair Carbon supports the development of blue Carbon projects by increasing the accessibility of information and transparency. This includes their Global Blue Carbon Projects map which allows access to all publicly registered blue carbon projects across multiple voluntary carbon market registries. They also have a number of other resources around carbon credits for blue carbon.
• Sustainable Development Mechanism (formerly the Clean Development Mechanism): This is a compliance market designed for shorter-term emissions credits.
• Nationally Determined Contributions (NDCs): These are the climate actions detailed and executed by countries participating in the Paris Agreement and serve as the primary mechanism for meeting the goals set out in the Paris Agreement. The restoration of blue carbon ecosystems may help mitigate and adapt to climate change, however, their integration into Nationally Determined Contributions remains underdeveloped (Dencer-Brown et al., 2022). A 2022 report from thirty-three experts highlights the need for “genuine participation by communities, inclusive project governance, integration of local work into national policies and practices, sustaining livelihoods and income (for example through the voluntary carbon market and/or national Payment for Ecosystem Services and other types of financial compensation systems) and simplification of carbon accounting and verification methodologies to lower barriers to entry (Dencer-Brown et al., 2022)” in order for blue carbon projects to be responsibly and successfully integrated into Nationally Determined Contributions.
• New efforts to improve the quality of carbon credits and the voluntary carbon market are being discussed and developed (See EDF, 2023). The integrity of each carbon credit will be critical to enable the carbon market to address climate change. New guidance has been released in 2023 to ensure carbon credits deliver on their claims. The Integrity Council for the Voluntary Carbon Market released its Core Carbon Principles that set a global threshold to define what a high-integrity carbon credit is.

Challenges

• Verification of the amount of carbon sequestered and its durability can be extremely challenging across coastal vegetated ecosystems but is necessary for effective (in terms of mitigating climate change) carbon markets.
• Ensuring fairness and equity in benefits distribution

Seaweeds use photosynthesis to convert dissolved carbon in the surrounding seawater into organic compounds and living tissue (Paine et al., 2021). As dissolved carbon is removed from the ocean, it is replaced by carbon dioxide from the atmosphere. It is through this process that seaweed can remove CO2 from the atmosphere. The ultimate fate of carbon sequestered in living seaweed is varied and includes becoming remineralized, or converted back, to CO2 via grazing, exuded as dissolved organic carbon, and being transported to the deep sea as particulate matter. The rate and proportion of carbon through each of these pathways remain uncertain (Macreadie et al., 2019), though a 2016 study from Krause-Jensen and Duarte sheds some light on possible export efficiencies (Krause-Jensen & Duarte, 2016). What’s more, the rate and proportion of carbon that goes through each of the pathways mentioned are not only uncertain but context-dependent. For an in-depth exploration of the readiness of seaweed as a blue carbon solution (both natural stands and farmed) see Fujita et al. 2023 (Fujita et al., 2023).

Note that this road map will exclusively look at the ability of natural seaweed stands and coastal seaweed farms to passively sequester CO2 (without active human intervention).  For an in-depth look at growing seaweed at large scale to be sequestered using active human intervention (such as deep sea sinking and production of algal biochar), please see the Macroalgae Cultivation and Carbon Sequestration Road Map.

Here, natural seaweed stands denotes seaweed naturally occurring in the ocean, also commonly referred to as kelp forests.

Mechanisms for CDR

• Reforestation of natural seaweed stands is the process of assisting in the recovery of seaweed stands that have been degraded, damaged, or destroyed, and is a type of restoration.
  • The reforestation of natural seaweed stands to increase carbon storage involves many context-specific factors that may complicate such efforts. Factors that will affect the growth and ultimate success of seaweed include climate change, disease, grazers, and predator interactions, etc. (Fujita et al., 2023). It is also highly species-dependent with some species of seaweed better suited than others for CDR. This complex assemblage of factors, unique across geographies, will require restoration efforts to be highly tailored.
  • A 2022 meta-analysis of seaweed restoration projects found a high success rate of restoration projects, however many of the reported projects were small in scale and these findings may not reflect failed restoration projects that were not reported (Eger et al., 2022).
    • The Nature Conservancy’s guidance on developing kelp restoration projects (Gleason et al., 2021)
    • One emerging method for restoring degraded seaweed stands is seeding small rocks with juvenile kelp, often termed ‘Green Gravel’. See work by the Green Gravel Action Group.
    • Another proposed technique actively removes urchins from seaweed stands to allow for the settlement of juvenile seaweed and subsequent regrowth. See Urchinomics for one approach.
• Increasing productivity of existing seaweed stands via genetic manipulation
  • There is current research on improving strain selection for enhanced carbon sequestration, productivity, or resiliency of seaweed. See ongoing research by Charles Yarish and lab at the University of Connecticut.

CDR Potential

Estimated Sequestration Potential: 0 - 0.018 Gt CO2e /year (Hoegh-Guldberg et al., 2023)

Sequestration Durability: Potentially hundreds of years, however, more studies are needed to understand how much seaweed becomes “refractory”, or resistant to degradation on the timescale of hundreds of years(Fujita et al., 2022).

Environmental Co-Benefits

Restoration of natural seaweed stands may generate many benefits for surrounding marine ecosystems. While we are calling these “co-benefits” here, it should be noted that some of these benefits may be so significant that these may in fact be the main benefits of restoration. (Note that there are significantly more co-benefits in blue carbon than any of the other marine CDR Road Maps.)

• Potential amelioration of ocean acidification (Mongin et al., 2016)
• Contribution to biodiversity (Theuerkauf et al., 2021)
• Contribution to habitat provisioning (Langton et al., 2019)
• Contribution to heavy metal and nutrient pollution removal (Zheng et al., 2019; Jiang et al., 2020)
• Contribution to fishery enhancement (Rimmer et al., 2021; Theuerkauf et al., 2021)
• Improved water clarity through the facilitation of settlement of fine sediments¹³
• Nutrient cycling (Jiang et al., 2020)
• Reducing biodiversity loss (NASEM, 2022)
• Restoring the role of marine organisms in the carbon cycle (NASEM, 2022)

Environmental Risks

In contrast to other proposed marine CDR pathways, there are not as many apparent environmental risks to reforesting natural seaweed stands for carbon dioxide removal. However, proper restoration techniques which take context-specific factors into consideration are critical to ensuring no or limited negative environmental impacts. As with any CDR pathway, incremental scale-up with careful monitoring and verification of impacts will be critical to mitigate risks. Possible environmental risks may include:

• Species entanglement
• Nutrient competition
• Smothering of adjacent sensitive ecosystems
• Negative effects of organic exudates on neighboring ecosystems
• If hard bottom substrates are needed (for seaweed to attach to) this could disturb soft bottom habitats. See examples of artificial reef creation to mitigate the loss of natural reef and restore seaweed.

Social Co-Benefits

Reforestation of natural seaweed stands may generate benefits for surrounding communities and economies. While we are calling these “co-benefits” here, it should be noted that some of these benefits may be so significant (while carbon sequestration feasibility and efficacy remain unknown) that these may in fact be the main benefits of restoration. Many of the environmental benefits relate directly to social benefits, such as fisheries enhancement and coastal protection. (Note that there are significantly more co-benefits in blue carbon than any of the other CDR Road Maps.)

• Increased adaptive capacity for communities to handle climate change and natural disasters via mechanisms such as wave attenuation for coastal protection (Denny, 2021), providing a refugium from acidified waters that benefits shellfish (Edworthy, et al., 2023), mitigation of dead zones by the production of dissolved oxygen¹⁹, or possibly providing shading from seaweed blades to create thermal refugia during marine heat waves²⁰
• Job creation and economic stimulation from enhanced fisheries and tourism (of particular value to coastal communities that are dependent on marine-based livelihoods)
• Cultural/intrinsic/recreational value
• Education and research – the pursuit of restoring blue carbon for CDR will naturally add value to the existing body of research and can provide valuable educational experiences for the upcoming generation of scientists and conservationists.

Social Risks

Efforts around restoring natural ecosystems are often met with social acceptance and public support, especially when compared to other climate solutions seen as “tampering with nature” (Wolske et al., 2019). This may be in part due to the minimal risks (real and perceived) associated with restoring natural ecosystems. While some risks, outlined below, do exist, there may be a higher tolerance for such risks given the long list of potential benefits and co-benefits (both environmentally and socially).

• Potential for inequity in benefits: Macreadie et al. (2022) points to concerns around the distribution of benefits from any given blue carbon project and whether benefits are evenly distributed or whether activities reinforce or add to social inequity (Macreadie et al., 2022). Similarly, there may be confusion around who “owns” the blue carbon and who has rights to control transactions of credits resulting from blue carbon projects. This may be particularly challenging in marine spaces where the movement of carbon must be taken into account.
• Potential for conflict with other uses such as commercial fisheries, shipping, marine renewable energy, and mining (NASEM, 2022) however, there may be synergies for some industries, like utilizing infrastructure from nearby renewable energy for monitoring.

Key Knowledge Gaps

Adapted from EDF 2022 Carbon Sequestered by Seaweed (Fujita et al., 2022)

• What is the best way to measure rates of carbon sequestration by natural seaweed stands?
• Which of the carbon fluxes between natural seaweed stands and their environment are well quantified, and which are not?
• How does carbon sequestered by natural seaweed stands compare with other ocean-based carbon sequestration pathways?
• Is the restoration of natural seaweed stands a durable and feasible enough pathway to help stabilize the climate?

*Note that all of these are highly dependent upon what species of seaweed is being considered.

First-Order Priorities

*Adapted from EDF 2022 Carbon Sequestered by Seaweed (Fujita et al., 2022)

• Conduct research to better understand and characterize carbon fluxes in natural seaweed stands
  • Determine the best way to measure carbon sequestration rates by natural seaweed stands*
  • Determine which of the carbon fluxes relevant to carbon sequestration by seaweeds are well quantified and which are not. This includes carbon fluxes from the atmosphere to ocean, ocean to seaweed, seaweed to microbial and other food webs, and seaweed to deep water*.
  • Characterize how the potential carbon sequestration by natural seaweed stands compares with other ocean-based carbon sequestration pathways*
  • Determine the durability of carbon sequestered via natural seaweed stands and if it is long enough to help stabilize the climate*
• Accelerate the innovation and testing of new technology that can aid in restoration projects
  • Conduct further field testing of techniques such as green gravel for afforestation of kelp forests.
  • Conduct more research on the efficacy of utilizing genetic manipulation and biotechnology to enhance seaweed carbon capture efficiencies.

Mechanism for CDR

Expansion of seaweed farming

• Possibly the most promising pathway to increase carbon sequestration by seaweed due to rapid advances in infrastructure, farm operations, and monitoring.
• Achieving additional, passive, carbon sequestration would require biomass to be left in the ocean long enough for fragmentation, transport, and burial to occur.

CDR Potential

Estimated Sequestration Potential: 0.00144 – 0.0079 Gt CO2e/year (Hoegh-Guldberg et al., 2023)

Sequestration Durability: Potentially hundreds of years, however, more studies are needed to understand how much seaweed becomes “refractory”, or resistant to degradation on the timescale of hundreds of years (Fujita et al., 2022).

Environmental Co-Benefits

The creation or expansion of seaweed farms may generate benefits for surrounding marine ecosystems, including but not limited to:  

• Potential amelioration of ocean acidification (Mongin et al., 2016)
• Contribution to biodiversity (Theuerkauf et al., 2021)
• Contribution to habitat provisioning (Langton et al., 2019)
• Contribution to heavy metal and nutrient pollution removal (Zheng et al., 2019; Jiang et al., 2020)
• Contribution to fishery enhancement (Rimmer et al., 2021; Theuerkauf et al., 2021)
• Improved water clarity through the facilitation of settlement of fine sediments (Jiang et al., 2020)
• Nutrient cycling (Cotas et al., 2023)
• Restoring the role of marine organisms in the carbon cycle (NASEM, 2022)

Environmental Risks

• Burning of fossil fuels associated with production activities (harvest, transport, processing)
• Production of CH4, N2O, and other potentially hazardous gases by the macroalgae
• Impacts to biodiversity and ecosystem function from farming operations may include: 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 seaweed farms due to competition for nutrients and light
• Changes in light and nutrient availability (including possible changes in ocean albedo due to canopy cover)
• Entanglement of marine megafauna
• Addition of noise pollution due to vessel traffic and machinery

Social Co-Benefits

Seaweed farming may generate many benefits for surrounding communities and economies.

• Basis for circular marine bio-economies, generating multiple benefits (Zhang & Thomsen, 2019) including integrated multi-trophic farming efforts to collocate multiple species (i.e., seaweed and shellfish)
• Education and research – the pursuit of restoring blue carbon for CDR will naturally add value to the existing body of research and can provide valuable educational experiences for the upcoming generation of scientists and conservationists.

Social Risks

While some risks, outlined below, do exist, there may be a higher tolerance for such risks given the long list of potential benefits and co-benefits (both environmentally and socially).

• Potential for conflict with other industries (fishing, tourism etc.) (NASEM, 2022)
• Potential for conflict with other uses such as commercial fisheries, shipping, marine renewable energy, and mining (NASEM, 2022) (However, note there are also possible synergies that may exist such as co-location of seaweed farms with compatible marine infrastructure types such as the moorings for wind energy, which could reduce costs for both industries.)

Challenges

• Scaling
  • Rough seas, light and nutrient availability, and the availability of space are all constraints.
• Measuring Carbon
  • Difficulties in measuring and verifying how much carbon is being sequestered passively by the presence of seaweed farms.

Key Knowledge Gaps

*Adapted from EDF 2022 Carbon Sequestered by Seaweed (Fujita et al., 2022)

• What is the best way to measure rates of carbon sequestration by seaweed farms?
• Which of the carbon fluxes between seaweed farms and their environment are well quantified and which are not?
• How does carbon sequestered by seaweed farms compare with other ocean-based carbon sequestration pathways?

First-Order Priorities

*Adapted from EDF 2022 Carbon Sequestered by Seaweed (Fujita et al., 2022)

• Investigate and characterize carbon fluxes in coastal seaweed farms
  • Determine the best ways to measure rates of carbon sequestered by seaweed farms*
  • Investigate which carbon flows relevant to quantifying carbon sequestration by seaweed farms are well documented and which remain uncertain*
  • Estimate total greenhouse gas emissions (including N2O and CH4) and carbon sequestration associated with seaweed farms (Duarte et al., 2023)*

Whales are thought to play a role in the carbon cycle both directly (living biomass and whale falls) and indirectly (the whale pump, the whale conveyor belt, and bioturbation), however the extent of this role remains uncertain(Meynecke et al., 2023). A 2022 report from The Environmental Defense Fund (Collins et al., 2022)  found there to be fewer than 10 scientific studies from which to draw any broad conclusions in this area and cautioned against the creation of a carbon credit system based on whale carbon sequestration. A 2020 study estimates that restoring baleen whale populations to pre-whaling abundances could sequester 0.032 Gt CO2/year, including carbon sequestered from fertilization via whale feces(Dufort et al., 2020). However, rebuilding whale populations is sure to elicit many other environmental and social benefits, explored further on in this road map.

Mechanisms for CDR

Direct Sequestration

• Living Biomass: Due to their size and longevity, whales can store large amounts of carbon for long periods of time in their bodies as biomass. While whaling indeed resulted in a loss of carbon stored in whales, there is not clear evidence to suggest that whaling was a net source of CO2 to the atmosphere (Heidi et al., 2023).
• Whale Falls: Whale falls, wherein whale carcasses sink to the ocean floor and their carbon-rich biomass is reallocated in various ways, is the “most scientifically comprehensible” of the pathways (direct vs indirect sequestration) (Collins et al., 2022) with a 2010 study estimating that restoration of large baleen whale populations to pre-whaling numbers could sequester 0.0006 Gt CO2e /year.

Indirect Sequestration

• Whale Pump: This is the term often used to describe the role that whales play in cycling nutrients through the ocean via their feces. The whale pump hypothesis sits on two assumptions that have not yet been verified through empirical observations (Collins et al., 2022). The first assumption is around the efficacy of surface water fertilization and subsequent primary production via whale feces. The second assumption concerns the fate of carbon export from primary production.
  • WhaleX Ocean Nourishment synthesizes limiting nutrients (nitrogen, phosphorous, iron, and silica) and disperses them across nutrient-poor regions of the ocean, simulating the dispersion of nutrients via whale feces.
• Whale Conveyor Belt: The whale conveyor belt is based on the idea that as whales migrate yearly, they move nutrients around the ocean, potentially increasing iron availability for phytoplankton growth (Roman et al., 2014).
• Bioturbation: Bioturbation is the process through which whale feeding behaviors stimulate the seabed and resuspend sediments (Meynecke et al., 2023). Conceptually, this is similar to the whale pump, but the pool of nutrients comes from seafloor sediments instead of whale feces (Nelson & Johnson, 1987; Alter et al., 2007).

Rebuilding Whale Populations: Increasing whale populations would likely amplify their role in the carbon cycle, including any sequestration.

• Increasing Krill Abundance:
  • A 2022 article by Savoca suggests krill as a limiting factor to whale population growth. Savoca points to studies that suggest a close relationship between whales and krill wherein whales provide necessary nutrients for krill to thrive. This is further complicated by competition between krill fisheries and whales in the Southern Ocean.

Reducing Ship Strike: A 2001 study found that in some areas, more than one-third of all fin whale and right whale strandings involve ship strike. This can especially impact small populations of whales such as the northern right whale in the western North Atlantic (Laist et al., 2001). Measures that require large, motorized vessels to slow their speeds in particularly vulnerable areas may be beneficial to whale populations.

CDR Potential

Estimated Sequestration Potential:  0.032 Gt CO2e /year, including carbon sequestered from fertilization(Dufort et al., 2021)

Sequestration Durability: 10 – 100 years (Cross et al, 2023)

Challenges

• Difficult to find and track outcomes from whale fertilization events (e.g., feces)
• Tracking the fate of carbon through the water column

Key Knowledge Gaps

Adapted from Pearson et al. 2022

• How effective are whales at creating and maintaining primary production hotspots that lead to enhanced carbon sequestration?
• How bioavailable are whale-derived nutrients?
• What is the carbon flux from cetaceans to the atmosphere?

First-Order Priorities

*Adapted from Pearson et al. 2022

• Investigate primary production hot spots from whales via determination of fecal nutrients to carbon recycling versus export*
• Conduct additional field and laboratory studies to ascertain the bioavailability of krill-derived iron in whale feces*
• Measure the carbon flux from cetaceans to the atmosphere via respiration to understand cetaceans’ net capacity for CO2 removal*

While fishes have not traditionally been considered as important parts of the ocean carbon cycle / biological pump (Sarmiento & Gruber, 2006), there are several recent studies, as highlighted in the 2022 Environmental Defense Fund report “Natural Climate Solutions in the Open Ocean”, that focus on the geochemical contributions of fishes to the global carbon cycle (Wilson et al., 2009; Boyd et al., 2019; Mariani et al., 2020; Bianchi et al., 2021; Saba et al., 2021). Many of the mechanisms by which fish are thought to contribute to carbon cycling are similar to whales (carbon stored in living biomass, bioturbation, carcasses falling to ocean floor, fertilization through feces) but also include contribution by intestinal precipitation of calcium carbonates (Wilson et al., 2009).

There is still uncertainty around the specifics of mesopelagic fish-mediated carbon export, however, the 2022 Environmental Defense Fund report suggests there is enough evidence of these species’ large contributions to global sequestration to warrant “the immediate pursuit of limitations or prohibitions on their harvest”.

Mechanism for CDR

CDR Potential

Estimated Sequestration Potential: Unclear, due to large knowledge gaps around biomass estimates of marine fish stocks, uncertainties in direct and indirect carbon export and overall carbon fluxes, few studies observing natural senescence / dead falls of fishes, and a poor understanding of the life cycle emissions of most fish species (Collins et al., 2022).

Sequestration Durability: Unknown.

Challenges

• More research is needed to understand carbon fluxes and life cycle emissions before developing strategies to increase carbon uptake and sequestration.

Key Knowledge Gaps

Adapted from EDF Natural Climate Solutions in the Open Ocean

• Lack of knowledge around the natural senescence or resulting deadfalls of fishes
• What is the contribution of fishes to inorganic carbon export via carbonate excretion?
• What is the total biomass of fishes, the fluxes in these communities, and conversion factors to go from biomass to units of carbon?
• How to discern between active carbon transport from mesopelagic fish versus from zooplankton?
• What is the composition of mesopelagic communities and what are the differences in roles played by small fishes versus other species such as cephalopods?

First-Order Priorities

*Adapted from EDF Natural Climate Solutions in the Open Ocean

• Conduct studies observing the natural senescence or resulting deadfalls of fishes*
• Conduct additional studies to understand the contribution of fishes to inorganic carbon export via carbonate excretion (Salter et al., 2019)*
• Create new models, observing technologies, and data to better constrain the total biomass of fishes, the fluxes in these communities, and conversion factors to go from biomass to units of carbon*
• Create new observational methods and models to better discern between active carbon transport from mesopelagic fish versus from zooplankton*
• Conduct studies to better understand the composition of mesopelagic communities to determine the difference in roles played by small fishes versus other species such as cephalopods*

• Nutrient cycling (Ratnarajah et al., 2014)
• Restoring the role of marine organisms in the carbon cycle (NASEM, 2022)

Restoration of marine animal populations may generate many benefits for surrounding communities and economies. While we are calling these “co-benefits” here, it should be noted that some of these benefits may be so significant (while carbon sequestration feasibility and efficacy remain unknown) that these may in fact be the main benefits of restoration. Many of the environmental benefits relate directly to social benefits, such as fisheries enhancement and coastal protection. (Note that there are significantly more co-benefits in “blue carbon” than any of the other CDR Road Maps.)

• Potential for economic stimulation and job creation from enhanced tourism (e.g., whale watching - of particular value to coastal communities that are dependent on marine-based livelihoods)
• Cultural / intrinsic / recreational value
• Education and research – the pursuit of restoring blue carbon for CDR will naturally add value to the existing body of research and can provide valuable educational experiences for the upcoming generation of scientists and conservationists.

Efforts around restoration efforts are often met with social acceptance and public support, especially when compared to other climate solutions seen as “tampering with nature” (Wolske et al., 2019). This may be in part due to the minimal risks (real and perceived) associated with restoration efforts. While some risks, outlined below, do exist, there may be a higher tolerance for such risks given the long list of potential benefits and co-benefits (both environmentally and socially).

• Potential for negative economic impacts to some industries such as oil and gas, fisheries, and mining (NASEM, 2022)
• Stricter fisheries management to increase species abundances can cause impacts on food security, displacement, and economic strain. (for example, Marine Protected Areas can cause varied impacts among communities and social groups and can be met with opposition) (Mascia et al., 2010).

Some restoration efforts (particularly those most studied such as mangroves and salt marshes) are generally feasible, local, and have high local governability(Gattuso et al., 2018) , however the scale and durability of these efforts will be critical to their success as effective CDR pathways. The restoration of marine habitats and species aligns with several broad-reaching international agreements and domestic laws (NASEM, 2022). Several international agreements and policies that recognize the value and importance of restoring marine life and ecosystems and that may be important when considering scaling up restoration efforts include:

• Parties to the United Nations Convention on the Law of the Sea (UNCLOS)
• The Convention on Biological Diversity (CBD)
• The United Nations Decade of Ecosystem Restoration (2021-2030)
• UNFCCC Paris Agreement
• Coastal ecosystems can be included in Nationally Determined Contributions (NDCs)
• Global Biodiversity Framework
• Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR)
• UN High Seas Treaty (BBNJ Treaty)
• The Habitats Directive (European Commission)

Examples of agreements aimed at protecting marine life

• The Endangered Species Act (United States)
• The Marine Mammal Protection Act (United States)
• The National Marine Sanctuaries Act (United States)
• Marine Protected Areas (Note that MPAs are designed to protect and conserve biodiversity, rather than carbon. For coastal systems, these will be located in national or state waters.)

Examples of US Blue Carbon Policy

• Executive Order 14008: Tackling the Climate Crist at Home and Abroad (2021): Places the climate crisis at the forefront of foreign policy and national security planning.
• America the Beautiful Initiative: Outlines inclusive and collaborative plan for locally led efforts to conserve, steward, and restore land and waters.
• Justice 40 Initiative: A national commitment to environmental justice which includes investments in the reduction of legacy pollution.  
• Ocean Climate Action Plan (OCAP): Includes ongoing and planned federal ocean-based climate mitigation and adaptation activities and recommends new or enhances ocean science and policy actions to tackle climate change. It also includes environmental justice considerations into ocean-based climate solutions.
• CREST Act of 2023: An expansion on the current research and development programs of the Department of Energy for capturing and storing carbon dioxide to include methods like CDR.
• Coastal Habitat Conservation Act of 2023: Authorizes the appropriation of $111.5 million over the 2024-2028 period for the US Fish and Wildlife Service to implement the Coastal Program.

• Prioritize leadership and partnership with local communities and recognize formal and informal tenure agreements.
• Networks of nations (e.g., International Partnership for Blue Carbon) can aid in building capacity to implement policies that support blue carbon projects.
• New IPCC inventory guidance is needed on emissions inventories for climate mitigation resulting from restoring seagrass, seaweed, and other blue carbon ecosystems (the current lack of guidance here also limits private sector investment)

• Property rights, coastal and offshore, are often unclear, not well defined, or not enforced, begging the question, who has jurisdiction over what. This is further complicated by the lateral movement of carbon in blue carbon ecosystems.
• Local governance of global commons is further complicated by local contexts, management structures, and benefit-sharing rules (Merk et al., 2022).
• Permitting may also pose an additional obstacle for some restoration projects.
• Regulations for coastal developments could impede coastal vegetated ecosystem restoration and limit the ability to launch larger restoration projects.

• Pacific Northwest Blue Carbon Working Group 
• Coastal Carbon Research Coordination Network 
• International Partnership for Blue Carbon 
• The Blue Carbon Initiative 
• The Blue Carbon National Working Group 
• Blue Carbon for Our Planet Act 
• The Blue Boat Initiative 
• Blue Carbon Network (Pew)
• Blue Carbon Lab (Deakin University) 
• Blue Carbon Canada (UN Decade Action) 
• Global Mangrove Watch 
• Blue Carbon Calculator 
• Blue Carbon Explorer (TNC) (Map)
• Coastal Carbon Atlas
• Blue Carbon Action Partnership
• Global Ocean Decade Programme for Blue Carbon (UN Decade Program)
• Blue Ventures (GEM etc)
• Blue Carbon Accelerator Fund 
• Sea4soCiety

• Delta Blue Carbon (Pakistan)
• Global Mangrove Alliance 
• Mangrove Action Project (US) 
• Blue Carbon Lab (Australia)
• Vida Manglar Carbon Project (Colombia)

• Ocean Alive (Portugal)
• MedGardens (Mallorca) 
• ResilienSEA (West Africa) 
• Ozfish (Australia)
• Project Seagrass (UK & Globally)
• For an example of a successful seagrass bed restoration, see Orth et al. 2020¹.

• San Francisco Estuary Invasive Spartina Project
• Napa River Salt Marsh Restoration Project

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 the uptake of any new technology; technologies associated with marine carbon dioxide removal (mCDR) are not an exception. To gain approval for field tests, which are a critical method of research, practitioners will need some level of public acceptance.

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 mCDR pathways are trapped in this latter cycle and hence remain critically underdeveloped. 

A few points of evidence include: 

• The relative paucity of information about mCDR pathways relative to terrestrial and technological pathways in CarbonPlan’s CDR database.
• The paucity of ocean-based applications submitted to Stripe’s first round of negative emissions purchases and Microsoft’s first negative emissions purchase.
• 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 Climate suggests a lack of scientific awareness that further slows broader public awareness.

Another cause of the low levels of development of mCDR 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 society (Lawford-Smith & Currie, 2017). 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.  This can reduce support even for research and development of mCDR. 

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 levels (IPCC 2018, IPCC 2022), 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 CDR pathways by the Academy largely ignored ocean-based pathways (with the exception of restoration of coastal aquatic vegetative habitats). We hope that the call for about $1.3 billion 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 generally (IPCC 2019). 

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 mCDR. However, for many people, the terrestrial impacts - increased wildfires, stronger hurricanes, heatwaves, and drought on land- are more visible and may make the climate crisis appear more dire on land than in the ocean.

Despite 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 mCDR pathways, many of which may have been born from controversies over ocean iron fertilization experiments in the 1990s and 2000s (Strong et al., 2015; Strong et al., 2009) or the ocean fertilization project by the Haida Salmon Restoration Corporation off Canada (Tollefson, 2017).

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 water (Mahli et al., 2020).

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 mCDR 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”) (Bertram & Merk, 2020).

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

Current governance-related challenges include:

• There are no specific regimes in the United States nor internationally governing mCDR (Webb 2024, Webb & Silverman-Roati 2023)
  • There have been recent efforts to regulate mCDR under three long-standing international agreements
    • The 1982 United Nations Convention on the Law of the Sea ("UNCLOS")
    • The 1972 Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter ("the London Convention") (see also Vivian & Del Savio 2024)
    • The 1996 Protocol to the London Convention ("the London Protocol") (see also Vivian & Del Savio 2024)
• In many countries, there are complex regulatory mazes to navigate to gain permits for in-water experimental trials, hindering the development of trials.  
• In the United States, this is particularly complicated by overlapping jurisdictions and authorities in the coastal zone (NASEM 2022).

These and other governance and regulatory uncertainties present real challenges and risks to those working to conduct research and development on mCDR pathways. Lack of such regimes both inhibits experimentation and the development of public confidence in mCDR experimentation. Governance structures and regimes with a specific focus on mCDR 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 mCDR pathways (i.e. different governance for macroalgal sequestration versus ocean liming) (Bellamy, 2018).

It is essential to swiftly raise awareness of and knowledge about mCDR across the key sectors and actors that are engaged in shaping climate and ocean policy. As various alternatives are evaluated, accelerating research into mCDR 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 the research and development of mCDR.  

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 of expanding support for accelerated research, development, and demonstration of mCDR pathways. 

The communications campaign should: 

• • • Identify all target audiences that will be part of creating an enabling environment for mCDR research, development, and design.
    • Research their current perceptions and understanding of mCDR. 
    • Develop appropriate content and information formats for these key audiences.
    • Identify and engage credible spokespeople (scientists, business leaders, and other opinion leaders) to help carry the messages.

Marine 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 mCDR approaches.
• Engage social scientists to design involvement processes and help address economic, social, and political challenges and opportunities around mCDR.
• Ensure broad dissemination of progress and results. 

• Engage relevant stakeholders in the development of a code (Hubert, 2020) to guide scientific experimentation and field trials in a manner that is transparent, participatory, rigorously monitored, and carefully controlled.
  • The Aspen Institute has been actively involved in developing a code of conduct for mCDR, aiming to provide guidance and principles for responsible deployment of CDR technologies, maximizing their potential benefits while minimizing risks and negative impacts on society and the environment. Key aspects of their work include stakeholder engagement, transparency and accountability, risk assessment and mitigation, governance and regulation, and capacity building and education.
  • The American Geophysical Union (AGU) has launched a plan to develop an Ethical Framework for Climate Intervention Research, Experimentation and Deployment. This will be a code of conduct to guide the ethics of climate intervention.
• 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 mCDR is considered in national programs for climate mitigation. There requires increasing governmental awareness and support for mCDR research, design, and development (RD&D) at national and international scales.

  • Increase outreach to relevant political jurisdictions to educate about mCDR opportunities and needs.
  • Seek to expand governmental involvement in mCDR through enabling legislation and policy. See recent work by the Sabin Center for Climate Change Law on model legislation for mCDR research.
  • Work with relevant jurisdictions to expand governmental support/funding for needed RD&D. See recent Marine Carbon Dioxide Removal Fast Track Action Committee (MCDR-FTAC) requests for input from all interested parties to inform the development of an implementation plan to advance a key recommendation of the Ocean Climate Action Plan (OCAP) regarding marine carbon dioxide removal (CDR) research.
  • Build a broad coalition/alliance to demonstrate the diversity of interests in support of mCDR testing.

• Sub-national, national, and international regimes to govern the development and testing of mCDR 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 mCDR 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 US (Webb et al., 2021)
      • Offshore carbon capture and storage in Canada (Webb & Gerrard, 2021)
      • The legal framework for sub-seabed carbon storage in the U.S and Canada (Webb & Gerrard 2017; Webb & Gerrard 2019)
      • Model federal legislation to advance safe and responsible marine carbon dioxide removal research in the United States (Webb & Silverman-Roati, 2023)

  Planned additional working papers will add geographic context and expand the suite of mCDR 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 mCDR (EFI, 2020)
    • 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 mCDR 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 mCDR 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 the 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 in mCDR-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.  
    • Carbon to Sea is bringing together scientists, engineers, field builders, and market shapers to systematically assess whether and how OAE can be a safe, scalable, and permanent CDR method, and to lay the groundwork for cost-effective and responsible deployment in the future.
    • Global ONCE is a research program endorsed in June 2022 under the UN Decade for Ocean Science for Sustainable Development (2022 – 2030) co-chaired by Prof. Nianzhi Jiao from Xiamen University, China, and Prof. Carol Robinson from the University of East Anglia, United Kingdom, and with 79 partners from 33 countries. The objective of Global ONCE is to provide data, knowledge, and best practices in the application of ONCE approaches towards achieving the global strategy of carbon neutrality by mid-century.
  • 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.  
    • Ocean Visions' Launchpad provides tailored expert support to innovators developing marine carbon dioxide removal strategies.

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

Without exception, all mCDR 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 mCDR pathways remain substantially underinvested given their potential to contribute to gigaton-scale, affordable CDR. 

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 mCDR 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 enhance public confidence in further exploration of mCDR pathways. While a handful of field trials and demonstration projects are getting off the ground (mCDR Field Trial Database) it will be critical to continue and increase efforts towards investment and awareness. 

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 the development and field testing of mCDR technologies, which in turn helps to “de-risk” private investment and finance of mCDR efforts. This public investment de-risking function is critical to creating the enabling conditions for building an industry and a market and has been done successfully in several fields, from healthcare to renewable energy.  So far, public investment in mCDR is lacking. 

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. The ARPA-E SEA-CO2 (Systems for the Enhanced Acquisition of Carbon Dioxide from the Ocean) initiative focuses on funding research and development projects that explore novel approaches to remove carbon dioxide from seawater, aiming to address climate change by harnessing the potential of the ocean as a carbon sink through innovative technologies and processes.

In the EU, the OceanNETs (NETs: Negative Emission Technologies) project is evaluating a wide range of mCDR 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 mCDR research, development, and demonstration (EFI, 2020).

Philanthropic investment is also important to build an enabling environment for increasing public awareness and support of mCDR pathways (Gagern, 2021). The ClimateWorks Foundation and the Grantham Environmental Trust are two of a small number of philanthropic organizations that include mCDR in their portfolios. 

The lack of well-developed markets for carbon removal and permanent storage also affects the flow of early-stage investment into mCDR pathways specifically. While terrestrial CDR has spurred the creation of numerous certifications and over two dozen standards-developing organizations, this proliferation has not consistently resulted in high-quality CDR implementations. There is much to learn from the successes and failures of the terrestrial carbon markets and apply them to the nascent mCDR markets (Arcusa & Sprenkle-Hyppolite, 2023).

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 the 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, Microalgae Cultivation and Carbon Sequestration, and Blue Carbon Restoration and Carbon Sequestration), many of the mCDR 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 hinder the development of more accurate business models for investors into mCDR pathways. 

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

Investment is needed not only in the development of mCDR technologies but also in the associated tools needed to support the field (examples of this include [C] Worthy and entities in the private sector). 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 the development and testing of mCDR 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 mCDR.
• Uncertainty and lack of expertise/knowledge around the jurisdiction and application of relevant international governance regimes to mCDR (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 the 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 mCDR pathways and inhibit private investment.

Regimes and protocols around monitoring, reporting, and verification for mCDR also need to be strengthened to clarify the roles and responsibilities of CDR providers (sellers), buyers, and regulators.

Investment in mCDR 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 mCDR can move forward such as the model legislation for mCDR research by the Sabin Center for Climate Change Law.  This must include things like clarified permitting processes, and clearly identified oversight authorities. Details on key elements of this priority can be found in the Engaging Key Audiences Road Map.

A growing carbon market will be ultimately necessary for the successful scaling of mCDR. Ensuring that mCDR 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 mCDR approach to provide assurances to negative emissions markets. See current work being done by [C]-Worthy on measurement, reporting, and verification (MRV). 
2. Develop the science needed to underpin standards for verifying carbon sequestration and permanence from various mCDR pathways.  For example, Oceans2050 is already developing a methodology for quantifying natural carbon sequestration underlying macroalgae farms (Duarte et al., 2022), but additional science and research on carbon fate and transport is needed for macroalgae cultivation and sinking. See recent framework to guide research on seaweed cultivation and sinking for carbon dioxide removal.
3. Lessons may be learned by looking to the more mature terrestrial carbon markets.

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 CDR (Stripe's first purchase, Stripe commits $8M, Shopify invests $5M) that have allowed for infusions of capital into entrepreneurial efforts needed to further test and develop these approaches. 

Co-products generated from mCDR (e.g., Albright & Fujita, 2023) pathways need viable routes to market to provide revenues that can offset the costs of mCDR 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 mCDR 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 mCDR pathways. (See Carbon to Sea as an example.)
• Cataloging outstanding science questions/gaps pertinent to mCDR 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 mCDR RD&D forward. This in turn requires a supportive policy environment. Supporters of mCDR must cultivate awareness and support among policymakers by: 

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

There are several emerging innovators and entrepreneurs working on mCDR, 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-Combinator, the Ocean Solutions Accelerator, Creative Destruction Lab, Larta Institute, or Indie Bio to increase their focus on mCDR 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 the inclusion of mCDR in prizes and competitions.
• Ensure that mCDR 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 mCDR, and from countries besides the US and the European Union, to gain a full picture of the global landscape around mCDR.
• Develop targeted outreach and materials for investor education to build their familiarity and comfort with CDR, and mCDR specifically (something akin to the Climateworks – Economist knowledge hub for mCDR pathways). Reach out to specific associations such as TONIIC.  
• Develop comparative analyses (e.g., cost-benefit) between mCDR and terrestrial-based or technology-based CDR to inform and reduce barriers to entry for CDR investors looking to support ocean-based pathways.