SUMMARY

Carbon Removal at Airports

Introduction

Many of the aviation industry’s leading organizations have committed to net-zero goals. “Net-zero” is typically defined as releasing no net greenhouse gas (GHG) emissions into the atmosphere after reduction and carbon-removal efforts. Many airports have been targeting emission reduction for years, and ACRP has been providing research findings that assist airports with identifying their GHG emissions, setting emission-reduction goals, targeting reduction efforts like renewable energy and energy efficiencies, and creating net-zero roadmaps. However, even with broadscale reductions, there will still be emissions in the atmosphere (residual emissions). Carbon dioxide (CO₂) removal (CDR)—removing CO₂ directly from the atmosphere—provides solutions for organizations, including airports, to reach net-zero emissions. (Note: Carbon removal and carbon dioxide removal are used interchangeably in the industry as well as in this report.)

The CDR industry is rapidly evolving. The technology is improving, new companies are established almost daily, and the current funding available provides many opportunities to reduce costs. The authors of this report anticipate that by the date of publication, some data will already be out of date because of the rapid development of CDR technology and pathways.

This guide aims to educate airports on the global carbon market and the current state of CDR pathways, including opportunities and challenges specific to the aviation industry. It also focuses on the available CDR pathways and their backgrounds, potential opportunities, and constraints for airports. Education for airports and users was a primary goal, and therefore, much of the content focuses on providing valuable insight into the CDR space. The guide is augmented by a technical tool to help better understand opportunities at specific airports and a communication tool to help airports connect with stakeholders and continue the process of education across executive, partner, and public lines.

All CDR pathways discussed in this report are presented for airport consideration, but not all pathways may be feasible or impactful. Ownership and operating structure, as well as monitoring, are potential key elements to successfully implementing CDR projects. Nature-based pathways may be easier to implement on airport property, but they are challenging to certify due to monitoring ambiguities. Technological or hybrid pathways may provide more carbon-removal potential at scale; however, they would likely need to be constructed in partnership with a developer and would also be more expensive for nascent technologies. Through the partnership, airports will be required to purchase carbon-removal offsets to gain credit for carbon removal toward their emission goals.

This executive summary serves to consolidate information from the guide into a short form focused on background, CDR pathways, and barriers and opportunities. Full details can be found within the guide.

Background

The Earth’s climate is changing because of a net increase in CO₂ and other GHGs. Temperatures are increasing; precipitation patterns are changing; and forest fires, droughts, and flooding are becoming more frequent and extreme. There is a need to rapidly reduce GHG emissions across all sectors, but even with a drastic decrease in emissions, there is a need to remove CO₂ from the atmosphere to limit global warming to less than 1.5 degrees Celsius (°C) [Intergovernmental Panel on Climate Change (IPCC 2022)]. Many organizations have set ambitious emissions-reduction and net-zero targets, which may require the use of CDR and carbon-removal offsets. Net zero is achieved by using carbon-removal strategies and cannot be achieved exclusively through the purchasing of traditional, non-carbon removal offsets (however, carbon neutrality can be attained by purchasing carbon offsets).

CDR is the process of removing CO₂ from the atmosphere. There are various CDR pathways, including nature-based (e.g., forests) and technological [e.g., direct air capture (DAC)]. CDR is different from carbon capture and storage, which captures the CO₂ emissions from the point source, like an industrial facility (see Chapter 1, Figure 5). Although CDR is relatively young, in fiscal year 2021 more than $80 million was allocated to CDR research and development, indicating rapid investment and growth in CDR technology (Riedl et al., n.d.).

Airports face a unique decarbonization challenge because most of their emissions are categorized as Scope 3, or emissions that are not under an airport’s direct control, such as emissions from tenant facilities and equipment. CDR may be an opportunity for airports to address hard-to-abate emissions, including aircraft and residual emissions (emissions that cannot be eliminated through reduction efforts). At large, the airport industry is vulnerable to a changing climate (e.g., forest fires and floods have the potential to negatively impact aviation operations), and CDR may be able to play a role in minimizing the long-term impacts on aviation by limiting global warming. The FAA and the Airports Council International (ACI) have set net-zero goals to achieve by 2050, aligning with the IPCC targets; these net-zero goals will require emission reductions and CO₂ removals.

Though the carbon-removal industry is nascent—and there are many fundamental and practical knowledge gaps—it is developing rapidly. This guide presents CDR options that may be scalable at airports, with caveats that the potential of these options is likely to change as the science and policy of CDR evolves.

Carbon-Removal Pathways

CDR pathways fall into two broad categories: technological and nature-based. The two key principles of CDR are removing CO₂ directly from the atmosphere and storing it durably. Storage, or sequestration, can occur in biological, geological, or ocean reservoirs or in long-lived products, such as concrete. Following are descriptions of each CDR pathway explored in the guide and general challenges, opportunities, and costs; these are summarized in Table 1 and illustrated in Chapter 1, Figure 8.

Table 1. CDR pathways summary.

CDR Pathway	Icon	Summary
BiCRS		Removing carbon from the atmosphere through biomass harvesting and storage, preventing it from being released in the atmosphere.
DACS		Removing CO2 from the atmosphere by engineered chemical reaction and then injecting it into a storage reservoir (typically geologic or used for long-lived products).
Enhanced mineralization		Employing accelerated weathering using reactive minerals to form a chemical bond with CO2 where it is mineralized, effectively sequestering it. This approach can be considered engineered or hybrid, depending on how the weathering is accelerated and where the rock is applied.
Soil-based		Using land-management practices that enhance soil carbon storage. Soil - based CDR capacity is relatively small per acre and varies substantially with geography and agricultural management practice.
Afforestation/reforestation		Planting trees or changing forest management to sequester atmospheric carbon in biomass and soils.
Biochar		Converting biomass to biochar via a pyrolysis process, which reduces biomass decomposition rates substantially, effectively sequestering biomass carbon for decades to hundreds of years.
Wetland restoration		Restoring wetland so that carbon can be sequestered by plants and trees that grow in wetland and stored in their biomass and surrounding soil.
Coastal BCEs		Using management practices that increase carbon stored in plants or sediments of mangroves, tidal marshes, seagrass beds, and other tidally influenced areas. Typically refers to coastal ecosystems rather than the open ocean.
Ocean-based		Supporting processes such as ocean fertilization, ocean alkalinity enhancement, and electrochemical approaches targeted at increasing absorption of CO2 into the ocean water.

Biomass Carbon Removal and Storage

• Plants and algae biomass remove CO₂ from the atmosphere; the CO₂ is then stored underground or in long-lived products after harvesting.
• This CDR pathway is higher in technical readiness level relative to other CDR engineered pathways.
• A potential constraint to implementation is access to arable land to grow enough biomass.
  • Seventy times more land is required for biomass carbon removal and storage (BiCRS) than for DAC [National Academies of Sciences, Engineering, and Medicine (NASEM) 2019].

• Costs are reported to range from $15 to $85 per ton of CO₂, making BiCRS more economically attractive relative to other engineered pathways.
• There is a need for greater understanding of the full life-cycle emissions associated with BiCRS, which varies in impact based on which biological process is chosen.

DAC and Storage

• CO₂ is captured from the atmosphere via a solvent, a solid sorbent, or a mineral process. It is then compressed and stored geologically or is converted into aggregates and other durable products.
• DAC and storage (DACS) has the potential to address emissions from hard-to-decarbonize sectors (e.g., aviation, infrastructure).
• Potential constraints include the energy intensity for DAC, water use, and the height of DAC projects, which is a challenge specific to airports.
• Costs are estimated to range from $94 to $1,000 per ton of CO₂ (Keith et al. 2018).
• The CO₂ removal permanence varies based on storage type:
  • Geological = approximately 1,000+ years, and
  • Concrete = approximately 100 years.

Enhanced Mineralization

• Enhanced mineralization speeds up the natural geologic processes of mineral reactions to form carbonates. These chemical reactions are exothermic, meaning they do not require energy inputs, and they can generate valuable materials like cement and concrete.
• The two approaches to enhanced mineralization are in situ (e.g., injecting CO₂ into rock formations underground) and ex situ or surficial (e.g., exposing crushed rocks on the Earth’s surface to CO₂-containing gases, which usually involves extraction, transport, and grinding of minerals).
• Potential constraints include access to resources (e.g., mining and industrial waste, processing equipment), scalability, and slow mineralization reactions.
• The costs range from $10 to $1,000 per ton of CO₂ removed.
• Permanence depends on the approach taken:
  • In situ is more than 1,000 years, and
  • Ex situ (e.g., concrete) is from 50 to 100 years.

Soil-Based

• Soil naturally sequesters carbon; the removal rates can be enhanced by increasing plant organic matter inputs (e.g., plants with deeper roots, and adding organic materials and cover crops) and by minimizing soil disturbance and erosion.
• There is higher carbon sequestration potential in warmer and wetter climates because of faster decomposition of organic matter. As a result, soil-based CDR is more likely to be effective in the southeastern United States and other humid climates (Zomer et al. 2017).
• Potential constraints include physical soil characteristics that limit sequestration rates, challenges in implementing practices that increase plant-based organic matter inputs, and the need for significant land area and new land-management practices.
• For airports, the most viable soil-based CDR approach may be ecosystem restoration and planting native grasses.
• The approximate cost of soil-based CDR is from $10 to $1,000 per acre or from $0 to $100 per ton of CO₂.

Forests (Afforestation/Reforestation)

• Afforestation is the process of planting trees in a new location, where there is no history of trees.

• Reforestation is the act of replanting trees where they used to grow.
• Afforestation and reforestation are nature-based CDR pathways. CO₂ is removed from the atmosphere by photosynthesis and new tree growth. As trees grow, carbon accumulates as biomass. The rate of sequestration varies depending on the tree species, soil, climate, biodiversity, age, and geographic region.
• Potential constraints for scaling afforestation and reforestation at airports include height, interactions with wildlife, and land availability.
• The approximate costs of afforestation and reforestation range from $10 to $100 per ton of CO₂ removed (NASEM 2015a).
• The southeastern and midwestern United States have the highest potential for reforestation.

Biochar

• Biochar is the product of burning biomass without the presence of oxygen (pyrolysis) and creating solid carbon. When carbon is in the form of biochar it decomposes much slower than in biomass form.
• A secondary benefit of biochar is that when mixed with soil, it can increase the soil’s carbon-sequestration potential.
• Building a biochar processing plant at an airport is not feasible, but adding biochar to the soil at airports could enhance carbon-removal capacity.
• Airports located near biochar facilities, such as those in the Pacific Northwest, will have the most opportunity to implement the biochar CDR pathway.
• The cost to produce biochar ranges from $30 to $120 per ton of CO₂ (Smith 2016).

Wetland Restoration

• Wetland restoration and preservation is another nature-based CDR pathway. Carbon stored in wetland has the potential to have a long residence time, but there is a significant risk of emitting methane during the restoration process.
• Like other pathways, sequestration rates will vary. Precipitation rates and moisture will impact wetland carbon removal and storage. It is reported that wetland restoration has the potential to remove from 0.1 to 5 tons of carbon per hectare annually (Were et al. 2019).
• The cost of long-term wetland carbon sequestration ranges from $10 to $100 per ton of CO₂ (Griscom et al. 2017).

Coastal Blue-Carbon Ecosystems

• Coastal blue-carbon ecosystems (BCEs) include coastal ecosystems with vegetation like marsh species, seagrass, and mangroves; kelp forests and warm-water coral reefs are not considered BCEs.
• It is reported that ocean and coastal ecosystems have sequestered approximately 40 percent of anthropogenic CO₂ since the late 19th century (Claes et al. 2022).
• Mangroves are a predominant BCE; they have many additional ecological benefits, like flood control and serving as a habitat for many critical species.
• 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 can” (NOAA 2023).
• Constraints include vulnerability to damage from extreme weather, the need to be near a coast to use this pathway, challenges with measuring the volume of carbon sequestered, and additionality. Additionality means that the project could not have occurred without revenue from the sale of the carbon credit.
• It is estimated that costs for mangrove restoration and carbon removal may cost approximately $560 per ton of CO₂, or approximately $9,000 per hectare (Claes et al. 2022; Taillardat et al. 2020).

Ocean-Based

• Ocean-based CDR removes dissolved CO₂ from the ocean and stores it in marine or geologic reservoirs.
• Strategies for ocean-based CDR include artificial upwelling, downwelling, and ocean alkalinity enhancement.
• Constraints for deploying ocean-based CDR at airports include location (since only a few airports are located near the ocean) and high capital costs.
• Because of the different implementation strategies, the costs for ocean-based CDR range from $70 to $2,355 per ton of CO₂ (Renforth 2019; Eisaman et al. 2018).

Carbon-Removal Roadmap

The CDR pathways referenced previously need to be considered within the context of a larger planning process to align with an organization’s broader decarbonization goals. Figure 4 (in Chapter 1) shows the process by which CDR pathways can be evaluated and implemented. Primary concerns include whether to implement a project on-site at an airport, work with a developer to implement a project on-site, or purchase offsets from a project.

Funding and Financing Opportunities

Bipartisan Infrastructure Law

The Bipartisan Infrastructure Law (BIL) was passed in 2021, making emissions-reduction funding accessible and a priority for the United States Department of Energy (U.S. DOE). As a part of the BIL, $3.5 billion was allocated to develop DAC project hubs. Provisions require the DOE to fund projects that help develop four regional DAC sites. The DOE will spend approximately $3 million on 12 feasibility studies, up to $12.5 million for eight design phase studies, and up to $50 million on two project developments (U.S. DOE 2022; FedConnect n.d.; Wu and Gibbs 2022). Within this program, there are various funds and prices allocated to specific aspects of DAC and carbon-removal technology.

Inflation Reduction Act

The Inflation Reduction Act (IRA) provides additional financial opportunities through the 45Q tax credit. Before the IRA, there were tax credits to incentivize the use of low-carbon technology, but the 45Q credit provides more significant financial incentives. The IRA will pay up to $180 per ton of CO₂ removed by DAC. These tax credits are available for up to 12 years once the equipment is installed and operational. Over time they will be re-evaluated and potentially adjusted to reflect inflation.

Introduction—Carbon Removal at Airports

The Industrial Revolution introduced an ever-growing dependence on the burning of fossil fuels. Since the 1800s, there has been a surge in land-use change (including agricultural uses) and fossil-fuel consumption, yielding massive amounts of CO₂ emissions being released into the atmosphere. These developments have negatively impacted the equilibrium of the climate and led to anthropogenic-forced (i.e., originating from humans) climate change.

Most carbon on the planet is stored in geologic formations, like rocks, soil, and sediments, while the remaining carbon is stored in the ocean, atmosphere, and living organisms.

In the past decade, ACRP has undertaken numerous research efforts to help airports understand, plan for, and reduce their GHG emissions, and enhance overall sustainability. Initiatives at airports across the country to date have been particularly focused on the reduction of GHG emissions [see other ACRP projects: ACRP Report 57: The Carbon Market: A Primer for Airports (Ritter, Bertelsen, and Haseman 2011); ACRP Research Report 220: Guidebook for Developing a Zero- or Low-Emissions Roadmap at Airports (Morrison et al. 2021); and ACRP Synthesis 100: Airport Greenhouse Gas Reduction Efforts: A Synthesis of Airport Practice (Barrett 2019)]. This research has focused on methodologies for GHG emissions reduction, energy efficiency improvement, renewable energy production, net-zero planning, and carbon-market primers; the associated reports, guides, and tools have provided a foundation for airports to make progress toward net zero.

A summary of aviation industry net-zero goals is provided in Table 2. Research indicates that net-zero 2050 goals cannot be reached by reduction and renewables alone (IPCC 2023b). More specifically, aviation goals will also require use of CDR. IATA and other aviation industry metrics show that up to 19 percent of the overall reductions will need to be accomplished via CDR or carbon offsets. However, the Science-Based Targets initiative (SBTi) requires organizations with validated targets through SBTi that no more than 10 percent of emissions are accounted for with carbon-removal technologies (Science-Based Targets 2022).

Airports are receiving greater pressure and attention as their market share of emissions becomes increasingly important to help meet industry, national, and international targets. As aviation activity continues to increase, other industries may increase their decarbonization efforts. In the event of this occurring, a large portion of global emissions will be attributed to the aviation industry. Although recent research, such as ACRP Report 57: The Carbon Market: A Primer for Airports (Ritter, Bertelsen, and Haseman 2011), discusses reduction measures and the potential for carbon markets and carbon offsets to address GHG emission goals, applicability of CDR strategies at airports has not been widely studied.

Table 2. Summary of aviation industry net-zero goals.

This report provides the context of CDR, the current state of carbon-removal methods (natural and technological pathways), current applicability to airports, as well as technical and communication tools to support airports in evaluating CDR as part of their net-zero plans. While GHG reductions are still the top priority to meet net-zero goals, CDR must be part of the solution to limit warming to internationally agreed-on levels. Figure 3 illustrates the general focus of ACRP Project 02-100, the research process, and the products of the report. The content includes a technical screening tool for airport use in assessing potential CDR deployment and a comprehensive guide that covers opportunities and constraints around implementing CDR at airports. Additionally, a communication toolkit is included to provide support in discussing CDR issues with airport stakeholders.

Carbon-Removal Guide and Appendices

The following sections of the guide provide global, national, and industrial context; education on carbon removal; and potential constraints for employing CDR measures at airports:

• Executive Summary
• Introduction
• Chapter 1: General background information on why CDR should be considered at airports, what is driving the need for CDR, the significance of GHGs in the aviation sector, and context on carbon markets and CDR certifications
• Chapter 2: Information on technological and nature-based CDR pathways to give the reader an understanding of the opportunities and challenges around various CDR pathways
• Chapter 3: General process to evaluate CDR and airport-specific considerations for CDR evaluation
• Chapter 4: Case studies

• Chapter 5: Implementation considerations: Stakeholder coordination and partnerships
• Chapter 6: Summary of potential funding and financing mechanisms
• Chapter 7: Tool information (technical tool and communication toolkit)
• Chapter 8: Future research needs

Because of the nature and complexity of the CDR space, this report encourages the reader to use supplemental information to gain a better understanding of the CDR space at large. This information is included in the following appendices:

Appendix A: Acronyms and Abbreviations, Glossary, and Other Important Terms

One of the primary challenges illustrated during the research process was that the language around CDR is complex and sometimes confusing. The glossary provides easy-to-reference terms, definitions, and acronyms to help users work through the guide.

Appendix B: Airport Frequently Asked Questions

Although the guide provides a more comprehensive view of CDR and applicability at airports, the Frequently Asked Questions section allows users to look for quick references on questions they might need answered readily. This section aims to provide a high-level understanding of CDR and is helpful for users who do not have time to review the whole guide.

Appendix C: Literature Review

This appendix contains a literature review, which is based in part on feedback from industry interviews completed as part of this research. The literature review focuses primarily on research that compiled CDR techniques, provided guidance for CDR applicability, and highlighted ACRP projects completed to date on GHG emissions-related sustainability planning.

Because CDR at airports is still in its infancy, the industry interviews included not only stakeholders within the aviation industry, but also private entities (such as Microsoft), leaders in the carbon landscape (Carbon 180, Carbon Direct), and other organizations (such as CDR developers) that provided important context for CDR. The literature review paired with the interviews helped to develop a baseline understanding of the current state of CDR and resulted in a gap analysis, which focused the content of this guide.

Appendix D: Carbon-Removal Pathways Technical Tool Overview

The technical tool, which can be found on the National Academies Press website (nap.nationalacademies.org) by searching for ACRP Research Report 270: Carbon Removal at Airports and looking under “Resources,” is intended to assist users in screening for potential carbon-removal opportunities at an airport. Users will enter requested information to receive recommendations for specific CDR techniques, as well as approximate costs and other considerations, to help evaluate the potential to apply CDR techniques at an airport. This tool can be used to evaluate any airport in the United States. While most inputs are optional, the only piece of information required to use the tool is the airport identifier. More information on use of the technical tool can be found in Chapter 7. Land-use cover types, assumptions, and instructions for use of the technical tool are included.

Appendix E: Carbon-Removal Communication Toolkit Overview

The communication toolkit, which is available on the National Academies Press website (nap.nationalacademies.org) by searching for ACRP Research Report 270: Carbon Removal at

• land-use cover, and other available geospatial information), this tool provides CDR recommendations only for public-use airports in the United States.

Key Takeaways

Because of the complexity of CDR, takeaways will differ between airports. Each airport’s unique location, resources, and financial structure will play a role in determining the best way to engage with CDR. There are three main pathways forward:

1. Own the project: It is unlikely that an airport will own a technologically based CDR project (such as DAC) at this time. Because of the nascency of CDR, technology needs to further develop and scale, making it easier to implement and operate. Land-use pathways are more likely to be developed by an airport that owns and manages the land. However, land-use pathway challenges are focused on the variable and sometime low carbon-removal potential and the ability to adequately certify, monitor, and take credit for the carbon removal. If the airport intends to use this to reach its net-zero goals, it might be challenging to certify carbon removal within existing certifications processes.
2. Partner with a developer: Partnership opportunities currently exist to use airport land for carbon-removal technologies. Challenges for partnering with a developer include the need for fair market value of the land use as well as lease terms for the ownership of removal credits. Research shows that even when partnering with a CDR project developer, airports may still have to purchase carbon-removal credits from them. This would likely look like developing a land lease agreement paired with a carbon-removal purchase option for the airport. Sequestration needs currently are driving the location of many of these carbon-removal developments, but sequestration sites will become more broadly available as CDR technologies and infrastructure develops.

3. Purchase carbon-removal offsets: Airports may consider purchasing carbon-removal offsets from the carbon market, but there are many important criteria that must be met. It is critical that carbon-removal offsets are verified and undergo consistent monitoring. Airports must understand all the nuances of the carbon-removal offsets being purchased to ensure that they are high quality and impactful. Separation of carbon-removal credits versus carbon-reduction measures continues to provide confusion for many people, so additional guidance to target carbon-removal offsets is needed.

Research Process

The intent of this project was to examine the current ACRP and other industry work on carbon reduction, assess the rapidly evolving CDR landscape, and then provide information for airports to help them assess how CDR could potentially fit within their net-zero goals. While much of the existing ACRP work has been completed on net-zero planning, the bulk of it has focused on reduction. The research process covered the period from 2022 to 2024. The research process involved four major elements:

• Literature review: The research team reviewed related ACRP projects to identify the gaps in existing research on carbon-reduction methods, carbon-capture methods, carbon offsets, and true carbon-removal methods to target the focus of the research for this project. The research team also reviewed academic research on CDR to identify estimates of CDR by pathway (nature-based and technological) relative to technical readiness, permanence, volume of CDR, cost, and other considerations. This research provided background on technological and nature-based CDR pathways and associated considerations for each and supported efforts for developing quantitative and qualitative inputs for the technical toolkit.
• Industry interviews: The research team interviewed organizations within the aviation industry, as well as organizations outside the industry, as CDR projects have not been widely implemented at airports. The purpose of these interviews was to gain perspective on approaches to CDR, including criteria to consider for and barriers to implementation of various CDR pathways.
• Regulatory context and evolving funding and financing: The research team evaluated funding and financing opportunities to provide context to airports on currently available funding for CDR projects as well as on constraints on the use of airport funds.
• Airport validation—Applicability of toolkits: The research team worked with several airports to validate the technical tool and the communication tool. Revisions to both were made based on this validation process.

Within the timeframe of this research, CDR technology, costs and opportunities and constraints were rapidly evolving. Therefore, it was important to conduct rolling interviews, literature reviews, and airport discussions throughout the process. While technology is expected to continue to advance, the guide and toolkits are intended to provide context for the process of evaluating and implementing CDR that should be applicable into the future, recognizing that the economics of CDR should also improve over time.