Electrochemical Processes

Direct removal of CO₂ from seawater or increasing the pH of seawater by, and thus increasing seawater’s capacity for uptake of CO₂, by passing an electric current through the water to induce water splitting, or electrolysis.

Knowledge Base

Low-Medium Processes are based on well understood chemistry with a long history of commercial deployment. Yet to be adapted for CO₂ removal /ocean alkalinity enhancement beyond benchtop scale.

Efficacy

High Confidence
Monitoring within an enclosed engineered system, CO₂ stored either as increased alkalinity, solid carbonate, or aqueous CO₂ species.  Additionality possible with the utilization of by-products to reduce carbon intensity.

Durability

Medium-High
> 100 years; similar dynamics to ocean alkalinity enhancement

Scalability

Medium-High Potential C Removal >0.1-1.0 GtCO₂/yr (medium confidence)
Energy and water requirements may limit scale. For climate relevancy, the scale will be double to an order of magnitude greater than the current chlor-alkali industry

Environmental Risk

Medium-High (low confidence)
Impact on the ocean is possibly constrained to the point of effluent discharge. Poorly known possible ecosystem impacts similar to alkalinity enhancement.  Excess acid (or gases, particularly chlorine) will need to be treated and safely disposed. Provision of sufficient electrical power will likely have remote impacts

Social Considerations

Similar social considerations to ocean alkalinity enhancement and to any industrial site; Substantial electrical power demand may generate social impacts.

Co-Benefits

Medium-High (medium confidence)
Mitigation of Ocean Acidification; production of H2, Cl2, silica.

Cost of Scale-up

High
>$150/tCO₂ (medium confidence)
Gross current estimates $150 - $2500/tCO₂ removed. With further R&D, may be possible to reduce this to <$100/tCO₂.

Cost & Challenges of Carbon Accounting

Low-Medium

Cost of Environmental Monitoring

Medium (medium- high confidence)
All CDR will require monitoring for intended and unintended consequences both locally and downstream of CDR site; monitoring costs may be substantial fraction of overall costs during R&D and demonstration-scale field projects. 

Additional Resources Needed

Medium-High
High energy requirements (1 - 2.5 MWh/tCO₂ removed) and build out of industrial CDR

Ocean Alkalinity Enhancement

Enhancing surface water update of CO₂ from the atmosphere by altering seawater chemistry.  This can be accomplished by raising the alkalinity, or ph, of the seawater through various mechanisms such as enhanced mineral weathering and electrochemical or thermal reactions.

Knowledge Base

Low-Medium
Seawater CO₂ system and alkalinity thermodynamics well understood. Need for empirical data on alkalinity enhancement - currently, knowledge is based on modeling work. Uncertainty is high for CDR efficacy and possible impacts.

Efficacy

High Confidence
Need to conduct field deployments to assess CDR, alterations of ocean chemistry (carbon but also metals), how organic matter can impact aggregation, etc. 

Durability

Medium-High
> 100 years; processes for removing added alkalinity from seawater generally quite slow; durability not dependent simply on return time of waters with excess CO₂ to ocean surface

Scalability

Medium-High
Potential C Removal >0.1-1.0 GtCO₂/yr
(medium confidence)
Potential for sequestering >1 Gt CO₂/yr if applied globally. High uncertainty coming from potential aggregation and export to depth of added minerals and unintended chemical impacts of alkalinity addition

Environmental Risk

Medium (low confidence)
Possible toxic effect of nickel and other leachates of olivine on biota, bio-optical impacts, removal of particles by grazers, unknown responses to increased alkalinity on functional diversity and community composition. Effects also from expanded mining activities (on land) on local pollution, CO₂ emissions.

Social Considerations

Expansion of mining production, with public health and economic implications;
Potential perception of ‘dumping’ by the general public, leading to public acceptability and governance challenges. 

Co-Benefits

Medium (low confidence)
Mitigation of Ocean Acidification; positive impact on fisheries

Cost of Scale-up

Medium – High
>$100-150/tCO₂ (low-medium confidence)
Cost estimates range between $10’s and $160/tCO₂.
Need for expansion of mining, transportation and ocean transport fleet

Cost & Challenges of Carbon Accounting

Low-Medium
Accounting more difficult for addition of minerals and non-equilibrated addition of alkalinity, than equilibrated addition

Cost of Environmental Monitoring

Medium (medium- high confidence)
All CDR will require monitoring for intended and unintended consequences both locally and downstream of CDR site; monitoring costs may be substantial fraction of overall costs during R&D and demonstration-scale field projects. 

Additional Resources Needed

Medium-High
Adaptation and likely expansion of existing fleet for deployment. Infrastructure for storage at ports.

Ocean Nutrient Fertilization

Addition of nutrients (e.g., iron, phosphorus, or nitrogen) to the surface ocean to stimulate production of marine phytoplankton and, consequently enhance uptake of CO₂ through photosynthesis.

Knowledge Base

Medium-High
Considerable experience relative to any other ocean CDR approach with strong science on phytoplankton growth in response to iron / less on fate of carbon and unintended consequences.  Natural iron rich analogs provide valuable insight on larger temporal and spatial scales.

Efficacy

Medium-High Confidence
Biological carbon pump known to work and productivity enhancement evident.  Natural systems have higher rates of carbon sequestration in response to iron but low efficiencies seen thus far would limit effectiveness for CDR. 

Durability

Medium
10-100 years
Depends highly on location and biological carbon pump efficiencies, with some fraction of carbon flux recycled faster/shallower ocean depths but some reaching deep ocean with >100 year horizons for return of excess CO₂ to surface ocean

Scalability

Medium-High
Potential C Removal >0.1-1.0 GtCO₂/yr (medium confidence)
Large areas of ocean have HNLC conditions suitable to sequester >1 Gt CO₂/yr.  Co-limitation of macronutrients and ecological impacts at large scales are likely.  LNLC areas have not been explored to increase areas of possible deployment. (medium confidence based on 13 field experiments)

Environmental Risk

Medium (low to medium confidence)
Intended environmental impacts increase Net Primary Production and carbon sequestration due to changes in surface ocean biology.  If effective, there are deep ocean impacts and concern for undesirable geochemical and ecological consequences.  Impacts at scale uncertain.

Social Considerations

Potential conflicts with other uses of high seas and protections;
Downstream effects from displaced nutrients will need to be considered;
Legal uncertainties; Potential for public acceptability and governance challenges (i.e., perception of “dumping”)

Co-Benefits

Medium (low confidence)
Enhanced fisheries possible but not shown and difficult to attribute.  Seawater dimethyl sulfide increase seen in some field studies that could enhance climate cooling impacts. Surface ocean decrease in ocean acidity possible.

Cost of Scale-up

Low
<$50/tCO₂ (low-medium confidence)
Deployment of <$25/tCO₂ sequestered for deployment at scale are possible, but need to be demonstrated at scale

Cost & Challenges of Carbon Accounting

Medium
Challenges tracking additional local carbon sequestration and impacts on carbon fluxes outside of boundaries of CDR application (additionality)

Cost of Environmental Monitoring

Medium (medium- high confidence)
All CDR will require monitoring for intended and unintended consequences both locally and downstream of CDR site; monitoring costs may be substantial fraction of overall costs during R&D and demonstration-scale field projects. 

Additional Resources Needed

Low-Medium
Cost of material- iron is low and energy is sunlight.

Seaweed Cultivation

Large-scale seaweed farming can act as a CDR approach by removing CO₂ from the atmosphere through photosynthesis; the seaweed is then transported into the deep sea or into sediments where the carbon can be sequestered.

Knowledge Base

Medium-High
Science of macrophyte biology / ecology is mature, many mariculture facilities are in place globally. Less is known about the fate of macrophyte organic carbon and methods for transport to deep ocean or sediments

Efficacy

Medium Confidence
The growth and sequestration of seaweed crops should lead to net CDR.  Uncertainties about how much existing Net Primary Production & carbon export downstream would be reduced due to large-scale farming.

Durability

Medium- High
> 10-100 years; Dependent on whether the sequestered biomass is conveyed to appropriate sites (e.g., deep ocean with slow return time of waters to surface ocean)

Scalability

Medium
Potential C Removal >0.1 GtCO₂/yr and < 1.0 GtCO₂/yr
(medium confidence) Farms need  to be many million hectares which creates many logistic / cost issues; Uncertainties of nutrient availability & durability of sequestration, seasonality will limit sites, etc

Environmental Risk

Medium--High (low confidence)
Environmental impacts are potentially detrimental especially on local scales where seaweeds are farmed (i.e, nutrient removal due to farming will reduce Net Primary Production, carbon export & trophic transfers) and in the deep ocean where the biomass is sequestered (leading to increases in acidification, hypoxia, eutrophication & organic carbon inputs). The scale and nature of these impacts are highly uncertain.

Social Considerations

Possibility for jobs / livelihoods in seaweed cultivation;
Potential conflicts with other marine uses. Downstream effects from displaced nutrients will need to be considered. 

Co-Benefits

Medium-High (medium confidence)
Placing cultivation facilities near fish / shellfish aquaculture facilities could help alleviate environmental damages from these activities. Bio-fuels also possible

Cost of Scale-up

Medium 
~$100/tCO₂ (medium confidence)
Costs should be less than $100/tCO₂. No direct energy used to fix CO₂.

Cost & Challenges of Carbon Accounting

Low-Medium
The amount of harvested and sequestered carbon will be known. However, an accounting of the carbon cycle impacts of the displaced nutrients will be required (additionality).  

Cost of Environmental Monitoring

Medium (medium- high confidence)
All CDR will require monitoring for intended and unintended consequences both locally and downstream of CDR site; monitoring costs may be substantial fraction of overall costs during R&D and demonstration-scale field projects. 

Additional Resources Needed

Medium
Farms will require large amounts of ocean (many million hectares) to achieve CDR at scale.

Artificial Upwelling/Downwelling

Use of pipes and pumps to bring up deep, cold, nutrient-rich water to increase phytoplankton production in the surface waters. Similar to nutrient fertilization, this can enhance uptake of CO2 through photosynthesis. Artificial downwelling is the downward transport of surface water, which might be a means to carry carbon into the deep ocean.

Knowledge Base

Low-Medium 
Various technologies have been demonstrated for artificial upwelling, although primarily in coastal regimes for short duration. Uncertainty is high and confidence low for CDR efficacy due to upwelling of CO₂ which may counteract any stimulation of the biological carbon pump

Efficacy

Low Confidence
Upwelling of deep water also brings a source of CO₂ that can be exchanged with the atmosphere. Modeling studies generally predict that large-scale artificial upwelling would not be effective for CDR.

Durability

Low-Medium.
<10-100 years; 
As with ocean iron fertilization, dependent on the efficiency of the biological carbon pump to transport carbon to deep ocean 

Scalability

Medium
Potential C Removal >0.1 GtCO₂/yr and < 1.0 GtCO₂/yr (low confidence)
Could be coupled with aquaculture efforts. Would require pilot trials to test materials durability for open ocean and assess CDR potential. Current model predictions would require deployment of 10’s – 100’s of millions of pumps to enhance carbon sequestration. (low confidence that this large-scale deployment would lead to permanent and durable CDR)

Environmental Risk

Medium-High (low confidence)
Similar impacts to ocean iron fertilization but upwelling also affects the ocean’s density field and sea surface temperature and brings likely ecological shifts due to bringing colder, inorganic carbon and nutrient rich waters to surface

Social Considerations

Potential conflicts with other uses (shipping, Marine Protected Areas, fishing, recreation); Potential for public acceptability and governance challenges (i.e., perception of “dumping”)

Co-Benefits

Medium-High (low confidence)
May be used as a tool in coordination with localized enhancement of aquaculture and fisheries

Cost of Scale-up

Medium-High.
>$100-150/tCO₂ (low confidence)
Development of a robust monitoring program is the likely largest cost and would be of similar magnitude as ocean iron fertilization. Materials costs for pump assembly could be moderate for large-scale persistent deployments. Estimates for a km scale deployment are in the 10’s of million USD. 

Cost & Challenges of Carbon Accounting

High
Local and additionality monitoring needed for carbon accounting similar to ocean iron fertilization

Cost of Environmental Monitoring

Medium (medium- high confidence)
All CDR will require monitoring for intended and unintended consequences both locally and downstream of CDR site; monitoring costs may be substantial fraction of overall costs during R&D and demonstration-scale field projects. 

Additional Resources Needed

Medium-High
Materials, deployment, and potential recovery costs

Ecosystem Recovery

Recovery of the marine ecosystem can enhance the natural biological uptake of carbon dioxide through protection and restoration of coastal ecosystems, such as kelp forests and free-floating Sargassum, and also through the recovery of fishes, whales, and other animals in the oceans.

Knowledge Base

Low-Medium
There is abundant evidence that marine ecosystems can uptake large amounts of carbon and that anthropogenic impacts are widespread, but quantifying the collective impact of these changes and the CDR benefits of reversing them is complex and difficult.

Efficacy

Low-Medium Confidence 
Given the diversity of approaches and ecosystems, CDR efficacy is likely to vary considerably. Kelp forest restoration, marine protected areas, fisheries management, and restoring marine vertebrate carbon are promising tools. 

Durability

Medium
10-100 years;
The durability of ecosystem recovery ranges from biomass in macroalgae to deep-sea whale falls expected to last > 100 years

Scalability

Low-Medium
Potential C Removal <0.1-1.0 GtCO₂/yr (low-medium confidence)
Given the widespread degradation of much of the coastal ocean, there are plenty of opportunities to restore ecosystems and depleted species. Ecosystems and trophic interactions are complex and changing and research will be necessary to explore upper limits.

Environmental Risk

Low (medium-high confidence)
Environmental impacts would be generally viewed as positive. Restoration efforts are intended to provide measurable benefits to biodiversity across a diversity of marine ecosystems and taxa.

Social Considerations

Tradeoffs in marine uses to enhance  ecosystem protection and recovery;
Social and governance challenges may be less significant than with other approaches.

Co-Benefits

High (medium-high confidence)
Enhanced biodiversity conservation and the restoration of many ecological functions and ecosystem services damaged by human activities. Existence, spiritual, and other nonuse values. Potential to enhance marine stewardship and tourism.

Cost of Scale-up

Low
<$50/tCO₂ (medium confidence)
Varies but direct costs would largely be for management and opportunity costs for restricting uses of marine species and the environment. No direct energy used.

Cost & Challenges of Carbon Accounting

High
Monitoring net effect on carbon sequestration is challenging.

Additional Resources Needed

Low
Most recovery efforts will likely require few materials and little energy, though enforcement could be an issue. Active restoration of kelp and other ecosystems would require more resources. 

Knowledge Base

Efficacy
Confidence

Durability

10-100 years

<10-100 years; 

> 10-100 years;

10-100 years;

> 100 years

> 100 years

Scalability

Potential C Removal >0.1-1.0 GtCO₂/yr (medium confidence)
experiments)

Potential C Removal >0.1 GtCO₂/yr and < 1.0 GtCO₂/yr
(low confidence)

Potential C Removal >0.1 GtCO₂/yr and < 1.0 GtCO₂/yr
(medium confidence)

Potential C Removal <0.1-1.0 GtCO₂/yr
(low-medium confidence)

Potential C Removal >0.1-1.0 GtCO₂/yr
(medium confidence)

Potential C Removal >0.1-1.0 GtCO₂/yr
(medium confidence)

Environmental Risk

(low to medium confidence)

, 
(low confidence)

 
(low confidence)

(medium-high confidence)

(low confidence)
.

(low confidence)

Co-
Benefits
.

(low confidence)
 

(low confidence)

(medium confidence)

(medium-high confidence)

(low confidence)

(medium confidence)

Cost of Scale-up

<$50/tCO₂
(low-medium confidence)

.
>$100-150/tCO₂
(low confidence)

 
~$100/tCO₂
(medium confidence)

<$50/tCO₂
(medium confidence)

>$100-150/tCO₂
(low-medium confidence)

>$150/tCO₂
(medium confidence)
.

Cost & Challenges of Carbon Accounting

Additional Resources Needed