CHAPTER 2

Carbon Dioxide–Removal Pathways

CDR includes natural carbon sinks, which gradually draw down and store CO₂ (e.g., forests and seaweed), technological or engineered, and hybrid approaches that blend nature-based and engineered CDR pathways. Each CDR approach is associated with specific benefits and challenges.

This chapter is technical in nature and provides information and considerations on the various CDR pathways. It is organized into the following areas:

• CDR evaluation criteria
• Summary of CDR pathways considered
  • BiCRS
  • – DACS
  • Enhanced mineralization
  • Soil-based CDR
  • Forests (afforestation/reforestation)
  • Biochar
  • Wetland restoration
  • Coastal BCEs
  • Ocean-based CDR
• CDR pathways summary

For each pathway, it is helpful to understand the technical readiness level, potential for removal, permanence, and other criteria that airports can use to examine the potential fit of each approach. It is important to understand that the characteristics of each CDR pathway, as well as the volume of carbon removal, project location, and other factors all play key roles in determining which pathway is the best to pursue for any given airport. While the level of detail may not be applicable to all airports, the CDR evaluation criteria give the broader context of CDR and the scope of CDR pathways and provide additional details to be used with the technical tool, which more broadly summarizes many of these criteria into an easy-to-use and easy-to-communicate format.

2.1 CDR Evaluation Criteria

The terms outlined in Table 7 are the criteria used to evaluate the CDR pathways described in this chapter.

Table 7. CDR pathway evaluation standards.

2.2 CDR Pathways Considered

The following CDR pathways were considered. There are many sub-pathways to these and new technologies in development, so this section focuses on the primary pathways, not individual companies. There are technological pathways, nature-based pathways, and pathways that are hybrids of both.

2.2.1 BiCRS

Overview

BiCRS describes a range of processes that use plants and algae to remove atmospheric CO₂ and sequester it either underground or in long-lived products. BECCS is more established and aligns with CCS during energy production; BiCRS expands the scope of BECCS by using biomass for CO₂ capture and storage but is not a product of bioenergy production. The concept of BiCRS for energy production is illustrated in Figure 13.

State of Technology

BiCRS processes that produce energy and fuels (e.g., bioethanol, syngas, and electricity) are relatively advanced and well understood with TRL in the range of 8–9 (demonstration phase). In comparison, the processes that do not produce energy (e.g., engineered wood products, bioliquid injection, and biofiber entombment) are still underexplored with TRL in the range of 4–6 (development phase). Current biomass production accounts for about 7 percent of the total energy mix worldwide (Ritchie, Roser, and Rosado 2022), and this percentage is expected to increase with the shift away from fossil-based energy sources.

Required Resources

One of the most challenging elements of BiCRS is access to arable land (e.g., land that can be plowed and used to grow crops) because it would need a large area of land to grow sufficient biomass. It is estimated that about 500 square kilometers (km²) [0.05 mega hectares (Mha)] of arable land would be required to capture 1 Mt-CO₂/year (Negative Emissions Technologies and Reliable Sequestration: A Research Agenda 2019). The required land area for BiCRS is more than 70 times that required by a DAC process [National Academies of Sciences, Engineering, and Medicine (NASEM) 2019].

Volume and Costs

Thirteen commercial-scale facilities that use biomass to sequester carbon underground or in long-lived products are in operation today in the United States. A total estimation of approximately 2.5 Mt-CO₂/year of biogenic carbon is currently sequestered by these facilities with approximately 25 Mt-CO₂/year in planning and development. The largest BECCS plant in Illinois is able to capture 1 Mt-CO₂/yr from ethanol production and store the captured CO₂ underground [National Energy Technology Laboratory (NETL) 2017]. A previous BiCRS roadmap reports annual global biomass availability for biofuel production with 2.5 to 5.0 Gt-CO₂/year CCS can be achieved in BiCRS processes by 2050 with minimal environmental impact (Sandalow et al. 2021a). Higher estimates for biomass availability concerning land-use change and high biomass productivity imply an extended range of 2.5 to 15 Gt-CO₂/year carbon captured (IEA 2020) and stored if all this biomass were available for BiCRS processes. CO₂ capture costs from BECCS plants have been reported in the range of $15 to $85/t of CO₂, which makes the BiCRS process economically attractive considering recent increases in 45Q tax credits that provide $85/t of CO₂ captured and permanently stored. [The Inflation Reduction Act (IRA) 45Q tax credit provides financial incentive for implementing CCS projects. Depending on the project type, tax credits are available based on the amount (ton) of CO₂ captured and stored.]

Permanence

CO₂ captured in BiCRS processes can be stored underground or stored in long-lived materials for decades or centuries. The permanence of carbon storage in BiCRS projects depends largely on the end use of CO₂. For example, the largest BECCS plant in Illinois captures CO₂ from an ethanol production plant and stores CO₂ underground in Mount Simon Sandstone for over 1,000 years (National Energy Technology Laboratory 2017). When CO₂ is used for concrete production, it can be stored in concrete over 100 years (Caldas et al. 2021). The large volume and durability of concrete makes it attractive for CO₂ storage.

Monitoring and Verification Requirements

For large-scale BiCRS processes, land-use/land-cover (LULC) change requires careful monitoring for biomass production:

• Satellite-based remote sensing is the most effective approach to monitor LULC change.
• Satellite monitoring data can be used to track changes in vegetation cover and to forecast future productivity (Food and Agriculture Organization 2017).
• Projects like Climate TRACE can help translate the data into more accessible formats for policymaking and enforcement.

Co-Benefits

Among engineered CDR pathways, BiCRS offers dual benefits of substituting fossil fuels with a bio-based fuel and increasing the flow of atmospheric carbon into long-term storage. Some BiCRS processes offer additional value from energy and fuels production (e.g., bioethanol, electricity, and hydrogen) or conversion of biomass into biochar or construction materials (e.g., biofibers).

Potential Constraints

• Biomass conversion can be associated with increased impacts to ecosystems on land or in the oceans. For example, monocultures may be easier to maintain for fuel production, but the replacement of diverse forests with monocultures would impact biodiversity.
• Biomass conversion for carbon removal has the potential to compete with food production on arable land. This would increase food prices and affect food security, especially in already food-insecure areas.
• There is poor understanding of the life-cycle emissions for BiCRS projects.
• Incomplete accounting of the emissions associated with indirect land-use change could result in a net increase of CO₂ emissions.

Therefore, comprehensive environmental assessments and full LCAs are critically needed for all BiCRS projects. The expansion of BiCRS globally could increase to the risk of eco-colonialism through exporting biomass from developing to developed countries and disproportionally place the impacts to habitats and ecosystems in developing countries.

Additionality

BiCRS processes are likely to be additional as most biorefineries, such as corn ethanol plants, currently emit the produced CO₂ to the atmosphere, although the fermentation-derived CO₂ is nearly pure and represents a high capture and storage potential. However, to prove additionality, the economics of any applicable tax credits or policy incentives need to be considered. For example, ethanol plants with CCS have been discussed in the context of California’s Low Carbon Fuel Standard (LCFS). Any tonnes of CO₂ that received LCFS credits would be nonadditional because LCFS is a compliance program, and this would be a case of double-counting. However, tax credits under the federal IRA program would not preempt additionality because it is not a compliance program.

Research Gaps and Future Needs

For BiCRS processes that produce energy and fuels, future research needs include (1) optimizing biomass feedstock handling and pretreatment methods and (2) integration of energy and fuels production with downstream transportation. In comparison, BiCRS processes that do not produce energy are underexplored, and future research will be needed for these processes to investigate the CO₂ removal opportunities, including (1) modeling the durability and carbon stability in biochar; (2) optimization of process operations with deep bioliquid injection; and (3) better understanding of preferred feedstocks, treatment requirements, and techno-economics for biofiber production. In addition, while previous research attention has been paid to develop optimal biomass for energy production in BECCS processes, future research needs to focus on developing biomass that optimizes life-cycle carbon removal in BiCRS processes such as (1) genetic modification or plant breeding to increase carbon fixation in biomass and (2) system-level carbon flow assessment, including biomass production, harvest, treatment, and use.

2.2.2 DACS

Overview

DACS captures CO₂ emissions directly from ambient air (instead of from point sources, such as power plants or industrial facilities) via solvent, solid sorbent, or mineral processes. The captured CO₂ is then either compressed for geological sequestration or converted into aggregates

the United States using pipelines is around 68 Mt-CO₂ (Global CCS Institute 2015), indicating the high maturity of CO₂ transportation via pipelines, which can be used for air-captured CO₂ if the DAC facility is not placed nearby a storage site. There are only a few locations where on-site storage may be possible due to geologic storage capabilities.

The United States Environmental Protection Agency (U.S. EPA) has created a new well category specific for carbon sequestration, Class VI wells. To gain a permit to build a Class VI well, specific geologic criteria must be met. At this time, no Class VI well permits have been approved, but many are under review. The U.S. locations of CO₂ pipeline infrastructure and corn ethanol facilities are illustrated in Figure 15. The U.S. power purchase agreement (PPA) data for solar and wind electricity and installed capacity for the years 2016–2021 are shown in Table 8.

Geographic Considerations

There are several factors that need to be considered when evaluating DAC deployment. First, DAC facilities are attractive because they offer flexibility in siting and can colocate with renewable energy sources or CO₂ storage sites. Second, some DAC technologies require a specific range of wind speed, humidity, and temperature. For instance, under a humid environment, water can be co-captured with CO₂ in solid amine-based DAC (Erans et al. 2022), and thus introduce a competition with CO₂ capture that would reduce the capture efficiency of the technology. Also, air velocity can affect the air contactor pressure drop (Heidel et al. 2011), which directly impacts the required fan power of hydroxide-based DAC. Third, DACS requires water, especially for technologies that use an aqueous solvent. These require from 1 to 7 tons of water to capture one ton of CO₂ (Lebling et al. 2022; World Resources Institute 2021). Therefore, geographic considerations must be considered in parallel with choosing a specific DAC technology in which to invest.

Volume and Costs

The largest DAC plant in operation today is the Orca plant in Hellisheidi, Iceland, with a capture capacity of 4 kilotons CO₂/year (Climeworks, n.d-b) equivalent to merely 0.003 percent of the total aviation jet fuel CO₂ emissions in 2019. CO₂ capture costs by DAC technologies have been from $94/t-CO₂ to as high as $1,000/t-CO₂ (Keith et al. 2018). The wide range of cost indicates uncertainty currently.

Specific to amine-based DAC, the capital cost was found to be the most sensitive to the high levelized capture cost (McQueen et al. 2021). In addition, sorbent-related costs (e.g., lifetime, working capacity, cycle time) were found to influence the capture cost of solid amine-based DAC (McQueen et al. 2021). The cost of capture using hydroxide-based DAC, on the other hand, is mostly affected by the special contactor design and energy-intensive calcination step that is required for CO₂ regeneration (McQueen et al. 2021). DAC technologies are under rapid development and scale-up plans. Indeed, a 1 Mt-CO₂/year hydroxide-based DAC plant is expected to be in operation in the United States in 2024 (Carbon Engineering, n.d.), and a multimega-tonne-of-CO₂-per-year solid amine-based DAC plant is expected to be built by 2030 (Climeworks, n.d-b).

Permanence

Like BiCRS processes, the permanence of carbon storage in DAC projects depends largely on the end use of the captured CO₂. DAC can be coupled with geologic storage, with a permanence of over 1,000 years. DAC capture CO₂ utilized for concrete production has a potential permanence profile of 100 years. If the captured CO₂ is used in enhanced oil recovery, it can only be stored for 1 to 10 years (IPCC 2005).

Monitoring and Verification Requirements

Compared to many of the natural pathways for CO₂ removal, monitoring the CO₂ benefits from a DACS project is relatively straightforward:

• The CO₂ mass removed from the atmosphere can be directly measured based on material and energy consumptions at the facility level.
• Monitoring and verification technologies in place, such as two-dimensional or three-dimensional seismic technology for CO₂ geological storage and enhanced oil recovery applications, assess any leakage (Jenkins, Chadwick, and Hovorka 2015).

Co-Benefits

Some DAC pathways remove substantial amounts of water from the air, which is produced as freshwater vacuum distillate when CO₂ is recovered. This is particularly true for current solid-sorbent systems, which capture between one to two tons of water for every ton of CO₂ removed. This freshwater separation process may prove to be a very attractive co-benefit in water-scarce regions. In addition, CO₂-derived products from DAC technologies offer the potential for truly carbon-neutral or carbon-negative products, while CO₂ captured from fossil-fuel sources can only reduce CO₂ emissions.

Potential Constraints

There are a few main disadvantages of DAC technologies, especially for hydroxide-based and amine-based methods:

• High energy consumption, 5.25–9.5 gigajoules/t-CO₂ (Greenore Technology 2022; IEA 2022b)
• High total cost of capture, $94–1,000/t-CO₂ (Keith et al. 2018)
• The small scale at this time, ≤ 4kt-CO₂/year (IEA 2022b)

• Because of high energy consumption, energy-associated CO₂ emissions are an additional challenge, dependent on the energy source.
  • For a scenario where a DAC plant is fully powered via natural gas, the marginal energy-associated CO₂ emissions would be around 0.51–0.55 tonnes of emitted CO₂ per every tonne of captured CO₂.
  • Innovative process designs that capture the emitted CO₂ from natural gas (Keith et al. 2018) as well as utilization of renewable energy sources or waste heat (Beuttler, Charles, and Wurzbacher 2019) could improve the negativity of these plants.

DAC technologies offer flexibility in siting that can avoid occupying arable land and minimize environmental impacts. However, it is important to consider required equipment heights, a factor particularly important for airports to avoid potential impacts to airport operations.

• Both presented DAC technologies use modular equipment that do not exceed approximately 100 ft in height.
  • The FAA evaluates Part 77 surfaces surrounding a runway through 14 Code of Federal Regulations (CFR) Part 77 for air safety.
  • This is described in more detail in future chapters, but Part 77 surfaces would be a primary consideration relative to DAC siting.

Additionality

A DACS strategy must demonstrate additionality, meaning that the carbon-removal activity is a direct result of carbon finance and not required by regulations. If DAC facilities are coupled with geologic storage, any tons of CO₂ removed that do not receive incentives relative to a compliance market such as LCFS credits would likely be additional. However, if DAC facilities are coupled with enhanced oil recovery or CO₂ utilization processes, proof of additionality of a DACS strategy will be more challenging.

Research Gaps and Future Needs

DACS is still in development and improvements are underway. As previously mentioned, process optimization, integration, and design are hot topics in this field. Although progress is being made, additional research on optimizing process energy consumption and heat integration to improve energy efficiency is critical. Technical improvements are also needed, for example the integration of hydroxide-based air contactors with electrified CO₂ and solvent regeneration methods (e.g., electrodialysis and electrified calcination) and the utilization of carbonate/carbamate solutions as feedstocks to utilization methods. Future needs also include diligent siting to ensure DAC plants are built close to storage or utilization sites to reduce transportation costs and emissions. An additional key next step is to further research opportunities to integrate DAC with emerging and existing utilization technologies.

2.2.3 Enhanced Mineralization

Overview

Enhanced mineralization accelerates natural geologic processes around mineral reactions with CO₂ from the ambient air, leading to permanent carbon storage in carbonate form. There are two broad enhanced mineralization approaches: in situ (e.g., injecting CO₂ into rock formations underground) and ex situ (illustrated in Figure 16) or surficial (e.g., exposing crushed rocks on the Earth’s surface to CO₂-containing gases that usually involves extraction, transport, and grinding of minerals). Enhanced mineralization CDR includes nature-based processes, such as

spreading of silicate or carbonate rocks on agricultural land, and engineered processes, such as looping mineral reagents [calcium oxide (CaO) or magnesium oxide (MgO)] for CO₂ removal from air and injecting CO₂ captured from DAC facilities for in situ mineralization. Companies like Heirloom Carbon Technologies and CarbFix are working on process advancement and reducing the costs of carbon mineralization.

State of Technology

For in situ mineralization, there are two demonstrated projects: (1) the Wallula pilot project in Washington State, United States, and (2) the CarbFix Project in Iceland, with TRL in the range of 6–8 (demonstration phase) (McGrail et al. 2011; McGrail et al. 2017; Snæbjörnsdóttir et al. 2017). However, both projects currently use CO₂ captured from point sources like coal power plants and industrial facilities for carbon mitigation and cannot be counted as a CDR process. CarbFix has also partnered with a DAC company, Climeworks, and operates a small-scale pilot project for in situ mineralization since 2021, with a TRL range of 4–5 (development phase).

For ex situ mineralization, crushed rocks, such as mining wastes and industrial wastes, can provide large amounts of alkaline feedstock for CO₂ removal, with demonstration projects still underway (TRL in the range of 6–7). For example, the Mount Keith nickel mine in Australia uses mine tailings to remove 39,800 to 79,800 t-CO₂/year from air, offsetting 11–22 percent of the mine’s annual GHG emissions (Wilson et al. 2014). Also, there are pilot-scale demonstration projects (TRL 4–6) that loop reagent (CaO or MgO) through multiple cycles of CO₂ removal from air (Heirloom 2023; Erans et al. 2022).

Required Resources

The resources required for enhanced mineralization will vary depending on which approach is pursued. In situ processes need ample mineral resources (e.g., ultramafic rocks and basalt) or

Co-Benefits

There are two significant co-benefits to enhanced mineralization. Enhanced mineralization processes can generate valuable carbonate products such as synthetic aggregates for cement and concrete production. Mining and exposing additional surfaces (e.g., aggregate) to the atmosphere CO₂ will react with the rock surfaces and bond as a carbonate. Basalt is a particularly good example of a rock type that will positively react with atmospheric CO₂. Roadway shoulders, erosion control, and other processes that impact geologic structures are potential opportunities for enhanced mineralization to occur if the aggregate is compatible with the proposed use. An additional benefit of enhanced mineralization is that the processes are exothermic, meaning they do not require energy inputs and have low operating expenses.

Potential Constraints

Enhanced mineralization has several potential constraints, including the following:

• Slow reaction rates for carbon mineralization,
• Under exploration of the distribution of optimal mineral resources,
• The current scale and technical readiness, and
• Access to technical equipment and permits.

Additionality

Enhanced mineralization processes must demonstrate additionality, meaning that the carbon-removal activity is a direct result of carbon financing and not required by regulations. For in situ mineralization, it is a storage-only solution, and the only reason to implement such a project is to generate climate benefits. Thus, in situ mineralization is likely to be additional. For ex situ mineralization processes, if CO₂ is removed only to generate climate benefits without valuable products production, such processes are likely to be additional. However, proof of additionality for ex situ mineralization processes with valuable products production will be more challenging.

Research Gaps and Future Needs

Additional research needs before enhanced mineralization at scale can be deployed include an increase in the reaction rates for engineered processes for carbon mineralization and more assessments of surficial and subsurface mineral material distributions. Further, developing processes that can use carbonate materials produced via enhanced mineralization with comprehensive techno-economic and LCAs to gain a better understanding of relevant economic and environmental impacts will be necessary. Before scaling, researchers must investigate potential toxic and hazardous components that are present in mining and industrial wastes and through assessing injection-induced earthquakes. Future needs also include ensuring reliable, rapid, and cost-effective methods for monitoring the carbon mineralization rate and extent for in situ and ex situ processes. Finally, large-scale demonstrations for engineered mineralization processes with system integration, energy efficiency optimization, and cost reduction will be critical.

2.2.4 Soil-Based CDR

Overview

Soils have lost substantial amounts of carbon as natural systems were converted for agricultural production (Sanderman, Hengl, and Fiske 2017), but it is possible to restore some soil carbon by

estimates CDR potential of 0.4–8.6 Gt-CO₂e/year—the wide range in this estimate reflects persistent scientific uncertainty relating to the efficacy of practices that build soil carbon (IPCC 2019). Because soil carbon storage is temperature-dependent, this potential is likely to reduce by 14 percent by 2040 due to climate change without interventions (Padarian et al. 2022). The impacts of reduced or no-till practices depend on climate and soil characteristics, and gains in the upper soils can be negated by losses of carbon in deeper soil layers (Ogle et al. 2019; Cai et al. 2022).

It is estimated that integrating cover crops can help draw down 0.07–0.7 t-CO₂ eq/year/ha and reduced tillage can sequester an average of 1.2 t-CO₂ eq/year/ha, but the absolute sequestration rates and durability vary substantially (NASEM 2015a). Some studies report a positive impact of cover crops on soil carbon, while others show no effect, or even losses, depending on the amount of biomass produced by the cover crop (Abdalla et al. 2019; McClelland, Paustian, and Schipanski 2021). In addition, some practices can increase emissions of other GHGs,

Meeting rigorous additionality criteria is critical for offset-based programs because carbon credits are used to justify emissions elsewhere; however, for insetting programs, additionality criteria may be less important.

Research Gaps and Future Needs

Scaling soil-based carbon removal will require improving fundamental understanding of how soil carbon is stabilized in soils as well as the socioeconomic context for how soil-based solutions are deployed in the real world. The transition to using perennial and deeply rooted crops can have large soil carbon-sequestration benefits, but the development of crop varieties with these attributes is slow, and very few perennial commodity crops are commercialized today.

There is also a need for the development of rapid in-field and laboratory soil carbon assessment tools and a robust integration of on-the-ground measurements with remote sensing and modeling approaches. Soils store large quantities of inorganic carbon, but research is lacking on how to facilitate and accelerate soil carbonate formation—this is an important area for additional R&D.

2.2.5 Forests (Afforestation/Reforestation)

Overview

Earth’s forests already remove significant amounts of carbon, with some estimates indicating that they remove up to 25 percent of current global carbon emissions (almost 10 Gt-CO₂e/year between 2000–2007) (Friedlingstein et al. 2014). Forest capacity for carbon uptake can be enhanced when trees are replanted in an area where a forest has historically existed (reforestation) or when trees are planted where no trees have previously existed (afforestation) (Anderegg et al. 2020; Griscom et al. 2017). Afforestation and reforestation remove CO₂ from the atmosphere via photosynthesis, and new tree growth accumulates carbon in the form of biomass, with measurable carbon impacts over decades to hundreds of years depending on tree species and geographic region. Tree growth and subsequent carbon sequestration take decades to millennia (Zomer et al. 2017).

State of Technology

TRLs are generally not applied to land-use pathways in the same way as hybrid or technological pathways. To allow pathways to be more easily compared the research team has listed nature-based pathways, including afforestation/reforestation as high readiness level, because the mechanism and best practices associated with implementing these pathways are well understood. Challenges around implementing land-use measures are associated with ownership and land access, tracking and certification of additionality, policy, and financing, rather than the technical readiness level (e.g., deployment of a new technology, process, or equipment).

Required Resources

Afforestation in airport contexts would require access to land where trees can be planted and maintained until they are well established, including access to watering infrastructure to help with establishment. Given the height considerations for airports, planting trees may be limited to areas where they do not interfere with airport operations.

Geographic Considerations

CDR potential of afforestation and reforestation decreases with colder climates (the maximum potential of boreal forests in the United Kingdom is approximately 120 t-CO₂/ha but 5.7 times higher in Brazilian rainforests) (Chiquier et al. 2022). Furthermore, increasing forest cover in

Monitoring and Verification Requirements

The U.S. Forest Service monitors forest carbon pools through the Forest Inventory and Analysis (FIA) program using the following processes:

• They directly measure aboveground and belowground forest biomass, as well as dead wood, litter, and soil carbon across several permanent sites across the nation.
• The FIA surveys are repeated to track changes in forest carbon over time.
• The data from the FIA can be used for geospatial analyses and integrated with remote sensing to monitor forest changes over time.
  • Remotely sensed data must be validated using regionally calibrated methods and ground inventories (Harris et al. 2021).
  • If models are being used to estimate biomass, site-specific data must be used to parametrize models.
• In addition to estimating aboveground biomass, it is important to quantify any GHG fluxes associated with forestry activities and use those data within an LCA that includes any harvested products.

Co-Benefits

Reforestation and afforestation can bring many co-benefits in addition to potential climate mitigation, including providing habitat, enhancing soil fertility, controlling erosion, supporting local and indigenous livelihoods, and improving air and water quality.

Potential Constraints

Afforestation and reforestation are already being deployed globally. The changes in land and water resources relative to afforestation/reforestation need to be accounted for and tracked. Forests’ carbon-removal potentials vary across species and geographies, and for any given location, interdependencies among soil type, climate, and biological communities control carbon dynamics. Therefore, reforestation efforts must be optimized with species combinations that perform best for specific deployment contexts, especially considering the threats of fire, drought, and disease. As climate change increases the intensity or prevalence of these extreme events, this pathway’s long-term permanence is at risk.

Afforestation and reforestation will likely be challenging for airports due to the height constraint associated with trees near airports. In addition to height, wildlife hazards will also be problematic. Additional risks may include issues with monoculture or diverse mixes. Afforestation on a large scale could also compete with other land uses and threaten food security, water resources, and biodiversity (Di Sacco et al. 2021).

Additionality

Proving additionality (that the project would not have occurred without carbon finance) is a challenging but critical aspect of any carbon project, even in the context of reforestation and afforestation. Accounting for CDR associated with forest management change requires a comparison to a counterfactual scenario in which no management change took place, but counterfactual claims are difficult to verify because they cannot be directly observed. Establishing robust baselines, including via the use of historical remote-sensing data, and designated control plots can help forestry projects meet additionality criteria when counterfactual claims cannot be observed.

A complex set of socioeconomic factors govern forestry projects, from timber production to ecological conservation. In certain geographies, the requirement for additionality is easier to meet because there are no policies in place to require reforestation or afforestation activities,

Geographic Considerations

Wetland makes up between 2 and 6 percent of Earth’s terrestrial cover and includes saturated soil, small lakes, floodplains, and marshes (Schlesinger 1991). Site-specific factors, including restoration design; patterns in disturbance and succession; past land use and the effects of management practices, such as water level manipulations, all likely play a large role in the annual net carbon balance of wetland, and all these factors vary across geographies (Abbott, Elsey-Quirk, and DeLaune 2019).

Practically, wetland restoration may be an effective climate-mitigation strategy for some locations, but it will require a detailed understanding of past management and ecological context to design the appropriate restoration strategy and hydroperiod. There may be some opportunities for small-scale wetland restoration and improved wetland management at some airports (e.g., Oakland International Airport) (U.S. Fish and Wildlife Service, n.d.).

Volume and Cost

Long-term wetland carbon-sequestration rates range from 0.1 to 5 t-C/ha/year (Were et al. 2019), potentially at a relatively low cost of $10–$100/t-CO₂ (Griscom et al. 2017). The potential volume and cost depend on wetland type, land productivity, and public willingness to restore wetland relative to other land uses. For airports, targeting wetland restoration and management as areas where the relative benefits match or outweigh the costs may be advisable.

Co-Benefits

Wetlands improve water quality and prevent the loss of biodiversity as well as increase wildlife habitat. Buffer zones around streams and rivers sequester carbon; reforestation of buffer zones also reduces the degradation of banks, deposition of sediment, pollution from livestock, nutrient runoff, and risk of floods.

Permanence

Wetland-restoration permanence depends on the existence of minimizing the risk of reversal, the management of decomposition rates, and flooding regimes. Achieving a balance between carbon uptake (through waterlogging) and potentially increasing CH₄ emissions (under waterlogged conditions) may delay some of the climate benefits of restored wetland for decades or longer. One important consideration is designing the system to be resilient to sea-level rise in coastal environments to account for long-term impacts of climate change.

Monitoring and Verification

Wetlands are difficult to measure because they are a mosaic of water and vegetation, with distinct nutrient gradients, shifts in plant species, and dynamics in soil saturation and salinity (for estuaries) that can impact carbon cycling and GHG emissions (Diamond et al. 2021). As a result, wetlands require constant and persistent monitoring to maintain their integrity as a carbon-removal pathway.

Carbon sequestration in a wetland is typically determined using the following:

• Bulk density, soil C concentration, and accretion rate (Villa and Bernal 2018)
• Eddy covariance measurements, which can be combined with long-term, plot-level measurements of GHG emissions to map emissions and carbon-storage potentials for different wetland conditions and restoration strategies

Today, wetlands are relatively underrepresented in networks like AmeriFlux and the National Ecological Observatory Network, which are continental-scale observation facilities that collect long-term open-access ecological data to better understand how U.S. ecosystems are changing.

Potential Constraints

As wetland restoration becomes more widespread, restoration activities also face challenges:

• They need to be informed by a network of field experiments across a wider range of climates and geographies.
• They need to be coupled with continuous monitoring and modeling to understand how GHG dynamics vary among intact, disturbed, and restored wetland (see the following section).
• Legal considerations have limited the restoration of coastal wetland, including who owns the land or coastline and is ultimately responsible for any environmental impacts (Bell-James 2022).
• The increase in wildlife biodiversity in wetland habitats presents additional challenges to airport implementation.

Additionality

A wetland-restoration strategy must demonstrate additionality, meaning that the carbon-removal activity is a direct result of carbon finance and not required by regulations. This may prove difficult because wetland-restoration movements typically require the preservation of biodiversity, critical wildlife habitat, and watershed health. In instances where wetland restoration is a by-product of an existing activity, like preserving critical wildlife habitat, additionality is more challenging.

Research Gaps and Future Needs

Research indicates that restored wetland has high emissions of N₂O and CH₄ (Xue et al. 1999). For restored wetland to act as a valid carbon-removal pathway, these emissions must be monitored, understood, and reduced. The complexity of wetland ecosystems, their decomposition rates, and organic matter flows need to be better studied. More data need to be collected on wetland restoration in different climates to reduce overall experimental variability when testing specific wetland-restoration projects for viability as a carbon-removal pathway. The impacts of climate change need to be better understood to guide wetland-restoration activities, including which species to use in which geographies (Macreadie et al. 2019).

2.2.8 Coastal BCEs

Overview

Coastal BCEs refer to the CCS in coastal vegetation (e.g., marshlands, seagrasses, and mangroves). Although some consider kelp forests and warm-water coral reefs BCEs, this view is not the predominant consensus (Reynard et al. 2020). Oceans and coastal ecosystems are very effective in sequestering carbon; it has been reported that they have absorbed around 40 percent of anthropogenic carbon since the 1840s (Claes et al. 2022). Coastal blue carbon is absorbed by the vegetation and then stored in the sediment, soil, and biomass (Claes et al. 2022). The National Oceanic and Atmospheric Administration (NOAA) reports that “mangroves and salt marshes can remove carbon 10 times faster, and store three to five times more carbon per acre than tropical forests” (NOAA 2023). In addition to effectively absorbing and storing carbon, BCEs improve fishery biodiversity, ecosystem health, and water quality and help mitigate flood risk, particularly from sea-level rise (Reynard et al. 2020). Blue carbon has not always been a part of GHG inventories; in 2017 the United States became the first country to include it in its national inventory (NOAA 2023). There is a recent increase in blue-carbon projects,

impacts and location. McKinsey states that mangroves, salt marshes, and seagrass meadows are the most established types of BCE, but discrepancies regarding additionality, permanence, and leakage remain (Claes et al. 2022). For example, the science has yet to quantify to what extent underwater carbon capture through seaweed farming reduces atmospheric CO₂; there is a similar lack of data regarding the effect of reducing bottom trawling. Current research suggests that complex biogeochemical cycles in seawater and ocean currents influence net exchange of CO₂ with the atmosphere (Claes et al. 2022).

Required Resources

There are various resources required to create a successful BCE project. A coastal environment and ecosystem is necessary, especially one that is not exposed to harsh weather conditions that may be destructive. Figure 18 depicts the regionality of BCE potential, with some countries having a greater advantage by having more coasts than others. Although inland airports can fund BCE restoration projects, there may be more challenges with monitoring, creating potential issues with verification. Coastal blue carbon is likely only potentially applicable to a handful of coastal airports.

Geographic Considerations

A critical geographic consideration for coastal blue carbon is how close operations are to the project site. A CDR project that is close to the owner’s operations will be easier to monitor and verify. For example, an airport in the United States should avoid CDR projects that are not in North America but implement projects on land they own. In addition to proximity, avoiding highly exposed sites and areas that are already being impacted by the changing climate is critical. As climate change continues to manifest, coastal BCEs are becoming more vulnerable to extreme storms and warming waters. Loss of coral reefs can lead to increased wave height and intensity, which has the potential to significantly damage BCE vegetation (Macreadie et al. 2019). It is critical to ensure that any BCE projects are not particularly vulnerable to climate-change impacts, such as global sea-level rise (Macreadie et al. 2019). The space needed to implement these restoration projects must also be considered; Reynard et al. (2020) predicts that BCEs could cover 100 million–170 million hectares globally.

Volume and Cost

As previously mentioned, mangroves have the capacity to sequester greater amounts of carbon (two to five times greater) than mature tropical forests can. This equates to 6–8 Mg-CO₂e/ha (megatons of CO₂ equivalent per hectare) (The Blue Carbon Initiative 2019). Similarly, seagrasses absorb around 10 percent of the carbon stored in ocean sediment, approximately 27.4 teragram (Tg—one million metric tons) of carbon annually (The Blue Carbon Initiative 2019). In addition to the vegetation’s biomass storage, a large amount of carbon is held in the top meter of soil or sedimentation. The volume of carbon stored in the top meter of soil varies depending on the type of blue carbon, see Table 9.

Although various studies have different calculations, they all represent positive impacts and significant carbon-removal capacity. One study estimates that BCEs could potentially capture 0.5–2 percent of the estimated 10 Gt of carbon emitted annually, based on the current volume of projects and restoration potential (Reynard et al. 2020). According to a 2021 study, BCEs could potentially store more than 30,000 Tg of carbon over approximately 185 million hectares, which would avoid 304 (141–466) Tg of CO₂e annually (Macreadie et al. 2021). These more general references to BCE potential are useful, but it is critical to understand the potential of each type of BCE, as shown in Figure 19. Each type of BCE has individual carbon-removal potential and will react differently to restoration. It is evident that some BCEs will react more strongly to restoration efforts than others.

BCE projects can be challenging to finance and are a relatively costly nature-based CDR pathway. Due to the immaturity of this CDR pathway, there is no universal cost for a project but rather various conflicting approximations. Some studies estimate that mangrove restoration projects may cost approximately $9,000/ha (Claes et al. 2022). In contrast, another study estimates that mangrove restoration may cost $560/t-CO₂e for mangrove projects (Taillardat et al. 2020). Project locations will play a significant role in determining the cost of each project; there will be no global price for a mangrove restoration project (Taillardat et al. 2020). Estimates indicate a project restoring the 600,000 ha of mangroves lost between 1996 and 2016 would cost around $5.4 billion (Claes et al. 2022). In addition to the high cost of mangrove restoration, there is no consistent tool to finance BCE projects (Claes et al. 2022). Although the Blue Carbon Buyers Alliance is an available resource for BCE offset and credit buyers, it does not specifically advise on financing projects (Claes et al. 2022).

Additional research indicates that coastal ecosystem restoration “could result in 380 million tons of CO₂ sequestration and a return of $11.8 billion in carbon finance by 2040” (Morris 2023).

Despite the high up-front costs, if done properly, there is potential for a high return on investment in BCE projects.

Permanence

There is a need for more research on the permanence of BCE. Current studies indicate that there is relatively low permanence due to exposure to human activities. Many of the sites for BCE projects are vulnerable to aquaculture, agriculture, mangrove forest exploitation, pollution, and coastal development. The permanence of BCE will also depend on whether carbon is stored within the plant biomass or within the soil and sediments (Reynard et al. 2020). If the carbon is stored in the biomass, it may be stored short-term, ranging from seasons to decades, and if stored in sediments, it may be stored over extremely long periods of time, ranging from decades to millennia (Reynard et al. 2020). Although there are estimates regarding BCE permanence, more robust analysis is critical.

Monitoring and Verification

The process of monitoring and verifying a BCE is vital and challenging, with many obstacles to calculating the exact amount and duration of carbon stored, as previously mentioned:

• The carbon stored in the plant biomass has been studied more than the carbon found in the BCEs’ sediments and soils. The Blue Carbon Initiative (2019) published Methods for Assessing Carbon Stocks and Emissions Factors in Mangroves, Tidal Salt Marshes, and Seagrass Meadows, which aligns with IPCC GHG inventory standards.
• The VCS is the most prominent CDR verifier globally. VCS has verified approximately 1,800 CDR projects, removing upward of 865 million tons of GHGs (Friess et al. 2022). Specifically, VCS frameworks, such as VM0007, VM0033, and VM0024, which the VCS has put together to provide guidance on carbon-removal strategies, will provide valuable insights into the BCE verification process.
• An additional verification resource is the Taskforce on Scaling Voluntary Carbon Markets in 2021’s document, Core Carbon Principles (Claes et al. 2022). Although most BCE projects will be unable to meet the outlined standards, Core Carbon Principles contains ones that are relevant (Claes et al. 2022).

A main monitoring concern relates to the challenges and lack of knowledge on the permanence of the sequestered carbon. There are multiple BCE projects that have been successful, others in the verification process, and some in their design and feasibility phases. Freiss describes two successfully verified and monitored projects, one in Kenya and the other in Colombia (Friess et al. 2022). Both projects are on mangrove protection and restoration and successfully integrate the local community throughout the project. The former is considered the “world’s first mangrove avoided deforestation and restoration project” (Friess et al. 2022).

Co-Benefits

In addition to sequestering CO₂, BCEs may be a critical tool in adapting to the impacts of climate change. Many of these ecosystems play a significant role mitigating the impacts of storm surge and flood from sea-level rise (Reynard et al. 2020). Restoration and conservation efforts of BCEs will also improve biodiversity and fishery health (Reynard et al. 2020). Many of the ecosystems included in BCEs are home to key species that contribute to sustainable food webs and healthy oceans. Healthy fisheries and biodiversity provide added benefits, including ecotourism, and improve the livelihoods of local coastal communities (Friess et al. 2022). Financing the restoration and protection of BCEs for carbon removal will provide many additional ecosystem services and have the potential to boost local economies.

Potential Constraints

There are multiple constraints to implementing successful BCE projects:

• One key concern is the vulnerability of coastal ecosystems, of which approximately 340,000 to 980,000 ha are destroyed annually (The Blue Carbon Initiative 2019).
  • The loss removes potential CDR sites.
  • The loss releases stored CO₂ (The Blue Carbon Initiative 2019).
  • Climate change (e.g., water temperature, sea-level rise, pH) and human activities (e.g., coastal development, influx of N and pH) are the main contributors to BCE destruction and possible constraints (NASEM 2019).
• An additional constraint relates to water ownership.
  • BCE waters are “often governed by numerous subnational regulatory and administrative agencies,” creating an obstacle that other CDR pathways do not face (Claes et al. 2022).
  • Most nations will have “rights to the water within 200 nautical miles from their coasts” (Claes et al. 2022).
• Challenges around monitoring the projects and ensuring additionality pose additional constraints to this CDR pathway.

Additionality

One of the main challenges with BCE is calculating additionality. There is a high dependence on environmental policy and incentives to protect and leave these projects undisturbed (Claes et al. 2022). Claes discusses that each BCE type (e.g., seagrass, mangroves) will have different additionality inputs and can be impacted by different policies and anthropogenic actions (Claes et al. 2022). Determining credible baselines and historic sequestration for these ecosystems is challenging and creates a major obstacle to determine additionality (Claes et al. 2022).

Research Gaps

BCE is a relatively young CDR pathway, and more research on its CO₂ removal and storage capacity is needed. A particular area of focus relates to how a changing climate (e.g., warming waters and rising sea levels) will impact coastal ecosystems. Additional research is needed to improve the understanding of the permanence of CO₂ if there is sediment or soil erosion (NASEM 2019). Macreadie discusses the need to further study CO₂ storage depth in coastal ecosystems and the impacts distribution may have on the storage capacity (Macreadie et al. 2019). Ultimately, more modeling and insights into the sequestration, storage, permanence, additionality, and vulnerability to the changing climate is critical.

2.2.9 Ocean-Based CDR

Overview

Ocean-based CDR approaches remove dissolved CO₂ from the ocean and then store the carbon either in marine or geological reservoirs. While ocean-based CDR has limited applicability for airports, it is included for the few airports that are located near an ocean. Additional constraints for ocean-based CDR are detailed in the constraints section but include the complexity of the implementation and ownership structure for ocean-based CDR.

Engineered, or technological, ocean-based CDR approaches include the following:

• Nutrient fertilization: The addition of micronutrients, like iron, or macronutrients, like pH and N, to the surface ocean to increase photosynthesis.

• Artificial upwelling and downwelling: When the nutrient-rich water from deep ocean is pumped into the surface ocean to increase localized primary production of marine organisms that take up CO₂, or surface ocean water is transported downward to carry carbon into the deep ocean; upwelling is illustrated in Figure 20.
• Ocean alkalinity enhancement: The addition of pulverized alkaline substances, like silicate or carbonate minerals, over the open ocean, illustrated in Figure 20.
• Electrochemical approaches: Reactions at the cathode increase the alkalinity of the solution that can be discharged into the ocean.

State of Technology

• Nutrient fertilization process: The TRL is 2–5 (R&D phases) (NASEM 2022; Renforth 2019; Markels and Barber 2002), with strong scientific research supporting phytoplankton growth in response to iron. Natural iron-rich analogs provide valuable insight on larger temporal and spatial scales.
• Artificial upwelling and downwelling: The TRL is estimated to be 4–5 (development phase) as various technologies (Baumann et al. 2021; Fennel 2008; Handå et al. 2014) have been demonstrated for artificial upwelling primarily in coastal regimes for short duration (Baumann et al. 2021).
• Ocean alkalinity enhancement: The TRL is about 2–3 (research phase) as seawater CO₂ system and alkalinity thermodynamics are well understood, but current knowledge is based on modeling work, and empirical data on ocean alkalinity enhancement will be needed.
• Electrochemical approaches: The TRL is about 2–3 because these processes are based on well-understood chemistry, but this approach is yet to be adapted for CO₂ removal by ocean alkalinity enhancement on a large scale.

Required Resources

One of the key resources is ocean access that permits ocean-based carbon removal. Potential jurisdictional issues may be limiting. The required resources for this pathway will vary depending

on which type of removal is pursued. Access to resources, funding, technology, and permits will differ for ocean alkalinity enhancement, nutrient fertilization, artificial upwelling and downwelling, and electrochemical approaches. Airports pursuing this pathway will need a partnership to ensure a successful project and strong verification and monitoring entities since the ocean-based CDR sites may be challenging to access.

Volume and Costs

The total global volume of CO₂ that can be removed by the ocean iron fertilization approach has been estimated from a fraction of 1 Gt-C/year (3.7 Gt-CO₂/year) to up to 3–5 Gt-C/year (11.1–18.5 Gt-CO₂/year) (NASEM 2022). For artificial upwelling and downwelling processes, potential carbon-removal volume is estimated to be 0.1–1.0 Gt-CO₂/year. Current models predict that tens of millions to hundreds of millions of pumps will be needed to enhance the carbon sequestration, and pilot trials will be needed to test materials’ durability for open-ocean use and to assess carbon-removal potential. The global potential of CO₂ removal from ocean alkalinity enhancement is greatly related to the amount of global alkali discharge. For example, the global potential of CO₂ removal from quicklime discharge is estimated to be 1.5–3.3 Gt-CO₂/year (NASEM 2022). For an electrochemical approach, the scale will be double, to an order of magnitude greater than the current chlor-alkali industry to achieve potential CO₂ removal between 0.1–1.0 Gt-CO₂/year. Energy and water requirements may limit the scale for electrochemical approach.

The deployment costs for spreading nutrients in the ocean are relatively low, especially in the case of iron, where relatively small amounts of iron are needed. Previous cost estimates for ocean iron fertilization that include materials and delivery were as low as $2/t-C ($0.5/t-CO₂) (Primeau 2005; Markels and Barber 2002). With additional costs for monitoring and verification of carbon storage, lower than $25/t-CO₂ sequestered for deployment at scale is possible for ocean nutrients’ fertilization but will need to be demonstrated at scale. The costs for artificial upwelling and downwelling include materials for the pumps; deployment, development, and maintenance of an offshore monitoring and verification program; energy needed to power the pumps; and costs for the pumps to be removed at the end of their life cycle.

The estimated costs for CO₂ removal via artificial upwelling will be greater than $100–$150/t-CO₂. From the range of ocean alkalinity approaches, only ocean liming has received a techno-economic assessment. The cost of ocean liming is estimated to be around $120/t-CO₂ for oxy-fuel flash calcination of limestone, and this may be reduced to around $70/t-CO₂ using dolomite as the mineral feedstock (Renforth 2019). For an electrochemical approach, the use of sodium hydroxide (NaOH) from current chlor-alkali methods is estimated to be on the order of $500–$700/t-CO₂ removed (this does not include revenue that could potentially be generated from H₂ and Cl₂) or $450–$600/t-CO₂ removed (with revenue). The techno-economic assessment of removing CO₂ as a gas from seawater through bipolar membrane electrodialysis demonstrates CO₂ removal costs on the order of $373–$2,355/t-CO₂ (Eisaman et al. 2018).

Permanence

The time frame in which the ocean can store carbon is largely determined by depth and location for CO₂ sequestration and is also related to the mixing and circulation properties of the ocean. CO₂ storage time is generally greater than 200 years for depths below 500 m and more than 600 years for depths below 2,000 m (Primeau 2005). On the other hand, the shallow retention times are quite short, with less than 50 percent of the carbon retained more than 100 years if carbon is introduced above 200–500 m. For nutrient fertilization processes, the CO₂ storage time depends highly on location and biological carbon pump efficiencies. Some carbon will reach the deep ocean with greater than 100-year horizons. For artificial upwelling and downwelling process, some research concludes that they will have a shorter-term influence on atmospheric