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Suggested Citation: "Appendix A: Acronyms and Abbreviations, Glossary, and Other Important Terms." National Academies of Sciences, Engineering, and Medicine. 2024. Carbon Removal at Airports. Washington, DC: The National Academies Press. doi: 10.17226/28458.

APPENDIX A

Acronyms and Abbreviations, Glossary, and Other Important Terms

Acronyms and Abbreviations

AC	alternating current
ACA	Airport Carbon Accreditation
ACI	Airports Council International
ADOs	FAA Regional Airports Division and District Offices
BCE	blue-carbon ecosystem
BECCS	bioenergy with carbon capture and storage
BiCRS	biomass carbon removal and storage
BIL	Bipartisan Infrastructure Law
CaO	calcium oxide
CCM	compliance carbon market
CCS	carbon capture and storage
CCU	carbon capture and utilization
CDR	carbon dioxide removal
CFR	Code of Federal Regulations
CH₄	methane
Cl₂	chlorine gas
CO₂	carbon dioxide
CO₂e	carbon dioxide equivalent
COP	Conference of the Parties
CORSIA	Carbon Offsetting and Reduction Scheme for International Aviation
CSU	Colorado State University
DAC	direct air capture
DACS	direct air capture and storage
DEN	Denver International Airport
DOE	Department of Energy
EA	environmental assessment
EPA	Environmental Protection Agency
EPIC	Energy Program for Innovation Clusters
ESG	environmental, social, and governance
ETS	Emissions Trading System
FECM	Office of Fossil Energy and Carbon Management
FIA	Forest Inventory and Analysis
GHG	greenhouse gas
Gt	gigaton
H₂	hydrogen
HCl	hydrogen chloride
IATA	International Air Transport Association

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Suggested Citation: "Appendix A: Acronyms and Abbreviations, Glossary, and Other Important Terms." National Academies of Sciences, Engineering, and Medicine. 2024. Carbon Removal at Airports. Washington, DC: The National Academies Press. doi: 10.17226/28458.

ICAO	International Civil Aviation Organization
IEA	International Energy Agency
IND	Indianapolis International Airport
IPCC	Intergovernmental Panel on Climate Change
IRA	Inflation Reduction Act
km²	square kilometers
kWh	kilowatt-hour
LCA	life-cycle analysis
LCFS	Low Carbon Fuel Standard
LULC	land use/land cover
m	meter
Mg	megagram
MgO	magnesium oxide
Mha	mega hectares
MRV	monitoring, reporting, and verification
Mt	metric ton
MW	megawatt
N	nitrogen
N₂O	nitrous oxide
NaOH	sodium hydroxide
NASEM	National Academies of Sciences, Engineering, and Medicine
NDCs	nationally determined contributions
NEPA	National Environmental Policy Act
NETs	negative-emissions technologies
NETL	National Energy Technology Laboratory
NOAA	National Oceanic and Atmospheric Administration
NPIAS	National Plan of Integrated Airport Systems
NRCS	Natural Resources Conservation Service
NREL	National Renewable Energy Laboratory
OTT	Office of Technology Transitions
pH	phosphorus
PPA	power purchase agreement
R&D	research and development
RKS	Southwest Wyoming Regional Airport
RPZ	runway protection zone
SAF	sustainable aviation fuel
SAN	San Diego International Airport
Tg	teragram
TRL	technical readiness level
UNFCCC	United Nations Framework Convention on Climate Change
USD	U.S. dollar
USDA	United States Department of Agriculture
VCM	voluntary carbon market
VCS	Verified Carbon Standard

Glossary

This glossary was compiled by the authors of this report from a variety of sources, including the Carbon Dioxide Removal Primer, the IPCC, the National Academy of Sciences, the United Nations Environment Programme, the United Nations Framework Convention on Climate Change (UNFCCC), and the World Resources Institute.

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Suggested Citation: "Appendix A: Acronyms and Abbreviations, Glossary, and Other Important Terms." National Academies of Sciences, Engineering, and Medicine. 2024. Carbon Removal at Airports. Washington, DC: The National Academies Press. doi: 10.17226/28458.

A

Additionality	Additionality is a critical aspect of determining a pathway, or project’s added benefit. Additionality refers to the amount of carbon sequestered due to a CDR project, against a business-as-usual baseline. This category plays a significant role in analyzing nature-based pathways. A question to ask is, does this project create an additional climate benefit, or would the same volume of carbon be sequestered without CDR intervention?
Afforestation	Planting new forests on lands that historically have not contained forests.
Aggregates	Inert granular materials, such as sand, gravel, or crushed stone, that are essential ingredients in concrete.
Agroforestry	The practice of integrating trees and shrubs with agriculture crops.
Alkalinity	Measures how much acid can be added to water without changing its level of phosphorus (pH).
Alkalinity enhancement	“Ocean alkalinity enhancement is an approach to carbon removal that involves adding alkaline substances to seawater to accelerate the ocean’s natural carbon sink” (Ocean Visions 2022).
Anthropogenic emissions	Emissions of greenhouse gases (GHGs), precursors of GHGs, and aerosols caused by human activities. These activities include the burning of fossil fuels, deforestation, land use and land-use changes, livestock production, fertilization, waste management, and industrial processes.
Arable land	Any land that can be used to grow crops and can be plowed or tilled.

B

Bio-optical impacts	Impacts of ocean bio-optical properties like light absorption, scattering coefficients, and chlorophyll-specific coefficients of phytoplankton.
Bioenergy	A type of renewable energy that is made from biomass or organic matter. Some examples of sources for bioenergy include corn, wood, algae, and soy.
Bioenergy with carbon capture and storage (BECCS)	Use of biomass as an energy source and the capture and permanent storage of CO₂ produced during the conversion of biomass to energy.
Biological carbon pump	This is the natural process in which inorganic carbon becomes organic through the food chain. The biological carbon pump typically refers to an ocean setting in which phytoplankton is consumed by an organism, which is then consumed by a larger organism. It will also occur through the process of downwelling.
Biomass	The feed that goes into making bioenergy. Biomass is made from deceased organisms, typically plant matter.
Biota	The specific organisms (plant and animal) that live in a specific area during a particular time.

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Suggested Citation: "Appendix A: Acronyms and Abbreviations, Glossary, and Other Important Terms." National Academies of Sciences, Engineering, and Medicine. 2024. Carbon Removal at Airports. Washington, DC: The National Academies Press. doi: 10.17226/28458.

Bipartisan Infrastructure Law (BIL)	A federal investment focusing on improving American transportation infrastructure. Some areas of focus include equity, job creation, and climate change.
Blue carbon	Carbon that is captured and stored in the ocean and coastal systems.

C

Calcination	Heating of solids to a high temperature to remove other substances.
Carbon credit	A transferable permit, or credit, that an entity can purchase or sell that is equivalent to one metric ton of CO₂. A carbon credit allows an entity to emit CO₂ or CO₂e and then purchase a credit, neutralizing the emissions.
Carbon cycle	The carbon cycle is the natural flux of carbon from sinks or reservoirs into the atmosphere.
Carbon dioxide removal (CDR)	Anthropogenic activities removing CO₂ from the atmosphere and durably storing it in geological, terrestrial, or ocean reservoirs, or in products. It includes existing and potential anthropogenic enhancement of biological or geochemical sinks and DACS but excludes natural CO₂ uptake not directly caused by human activities.
Carbon capture and storage (CCS)	A process in which a relatively pure stream of CO₂ from industrial and energy-related sources is separated (captured), conditioned, compressed, and transported to a storage location for long-term isolation from the atmosphere.
Carbon capture and utilization (CCU)	A process in which CO₂ is captured and then used to produce a new product. If the CO₂ is stored in a product for a climate-relevant time horizon, this is referred to as CO₂ capture, utilization and storage (CCUS). Only then and only combined with CO₂ recently removed from the atmosphere can CCUS lead to CDR. CCU is sometimes referred to as CO₂ capture and use.
Carbon markets	A term for a carbon trading system through which countries may buy or sell units of GHG emissions to meet their national limits on emissions. The term comes from the fact that CO₂ is the predominant GHG, and other gases are measured in units called CO₂ equivalents.
Carbon offset	In climate policy, a unit of CO₂e emissions that is reduced, avoided, or sequestered to compensate for emissions occurring elsewhere.
Carbon offset (from ACA)	A carbon offset—or simply offset—represents one metric ton of carbon dioxide equivalent (Mt-CO₂e). Offsets are created, held, and transferred as digital credits in an electronic registry. When an offset is purchased, it is canceled, retired, or redeemed in the registry to compensate for the emission of one t-CO₂e. Consequently, it is no longer traded on a registry.
Carbon pool	A location where carbon is stored. There are many carbon pools, such as the trunks of trees, the bottom of the ocean, soil, fallen leaves, and many more. Carbon pools do not store carbon indefinitely but can release carbon.

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Suggested Citation: "Appendix A: Acronyms and Abbreviations, Glossary, and Other Important Terms." National Academies of Sciences, Engineering, and Medicine. 2024. Carbon Removal at Airports. Washington, DC: The National Academies Press. doi: 10.17226/28458.

Carbon-removal pathways	These are the different strategies and ways that atmospheric carbon can be removed and stored. Each pathway has different characteristics and will fall under a category of a nature-based solution or a technological solution.
Carbon sequestration	The process of storing carbon in a carbon pool.
Carbon sink	Anything that will absorb, capture, and store atmospheric carbon. Carbon sinks refer to the active inflow or uptake of carbon.
Cathode	A cathode is the electrode of an electrochemical cell where reduction reactions take place.
Charette	A meeting with all relevant stakeholders.
Chemical weathering	This is the process of changing the chemical makeup of soil or rock. In some scenarios, water can cause a chemical change in rock or soil.
Chlor-alkali	Electrolytic decomposition of sodium chloride to produce chlorine and sodium hydroxide.
Climate positive	Activity going beyond net-zero carbon emissions in which more carbon dioxide is being removed than generated.
Co-benefits	The positive effects that a policy or measure aimed at one objective might have on other objectives, thereby increasing the total benefits for society or the environment. Co-benefits are often subject to uncertainty and depend on local circumstances and implementation practices, among other factors.
Compliance carbon market (CCM)/emissions trading system/cap-and-trade market	The CCM is also referred to as the Emissions Trading System or the Cap-and-Trade Market. In CCMs, there are regulatory emissions requirements with which organizations must comply or else they will be financially penalized. Entities may purchase carbon credits and offsets on the CCM; pricing is based on government and regulatory policy. The CCM governing body will set a maximum amount of emissions that one organization can emit, if it overemits, they must buy offsets or credits from the market. In contrast, an entity can sell carbon credits or offsets if it does not overemit, which creates a financial incentive to reduce emissions. Today the three leading CCMs are the European Union’s ETS, the California Global Warming Solutions Act, and the Chinese national emissions trading scheme (CarbonCredits.com 2022).
Cradle-to-grave	A cradle-to-grave assessment looks at the full life cycle of a product or process. This type of analysis provides insight into each phase of a product’s usage from manufacturing to end of life.

D

Decarbonization	The process by which countries, individuals, or other entities aim to achieve zero fossil carbon existence.
Dimethyl sulfide	Predominant volatile sulfur species found in surface seawater with the formula (CH₃)₂S.
Direct emissions	Emissions that are controlled by the reporting entity. An example of a direct emission is electricity usage.

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Suggested Citation: "Appendix A: Acronyms and Abbreviations, Glossary, and Other Important Terms." National Academies of Sciences, Engineering, and Medicine. 2024. Carbon Removal at Airports. Washington, DC: The National Academies Press. doi: 10.17226/28458.

Downwelling	Downwelling is when surface water, which has a higher concentration of oxygen, moves into deeper waters. This process brings oxygen to deeper water, ultimately reducing the productivity.
Durability	See “Permanence.”

E

Effluent	Wastewater that is discharged into surface water (e.g., rivers, oceans, lakes).
Electrochemical	The study of the relationship between chemicals and electricity.
Electrochemical sorbent	CO₂ capture sorbent that can be regenerated through electro-swing process.
Electrolysis	“The process of using electricity to split water into hydrogen and oxygen. This reaction takes place in a unit called an electrolyzer” (Office of Energy Efficiency & Renewable Energy n.d-a).
Embodied carbon	“Embodied carbon refers to the GHG emissions arising from the manufacturing, transportation, installation, maintenance, and disposal of building materials. Embodied carbon is a significant percentage of global emissions and requires urgent action to address it” (Himes 2020).
Enhanced mineralization	“Enhanced mineralization, also known as enhanced weathering or accelerated weathering, accelerates the natural processes by which various minerals absorb carbon dioxide (CO₂) from the atmosphere” (American University 2020).

F

Fossil fuels

Fuels that are naturally occurring and contain decomposed organic material. Fossil fuels contain carbon and hydrogen, which are burned to generate energy. Examples include coal, natural gas, and oil.

G

Global-warming potential (GWP)

Different GHGs have different GWPs, or different warming capacities. GWPs “measure how much energy the emissions of 1 ton of a gas will absorb over a given period of time, typically a 100-year time horizon, relative to the emissions of 1 ton of carbon dioxide (CO₂)” (U.S. EPA 2015). GHGs with larger GWPs will warm the atmosphere more significantly than CO₂. Some gases last in the atmosphere longer than others, while some absorb more heat energy. Methane does not stay in the atmosphere as long as CO₂ does, but it may have more negative impacts as it takes up more heat.

Greenhouse gases (GHGs)

The atmospheric gases (both natural and anthropogenic) responsible for causing global warming and climatic change. The major GHGs are carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O). Less prevalent, but very powerful, GHGs include hydro-fluorocarbons, perfluorocarbons, and sulfur hexafluoride (SF₆).

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Suggested Citation: "Appendix A: Acronyms and Abbreviations, Glossary, and Other Important Terms." National Academies of Sciences, Engineering, and Medicine. 2024. Carbon Removal at Airports. Washington, DC: The National Academies Press. doi: 10.17226/28458.

Greenhouse gas effect	The phenomena in which GHGs are trapped close to the Earth’s surface and warm the atmosphere.
Greenhouse gas inventory	“EPA develops an annual report, called the Inventory of U.S. Greenhouse Gas Emissions and Sinks (Inventory), that tracks U.S. greenhouse gas emissions and sinks by source, economic sector, and greenhouse gas going back to 1990” (U.S. EPA 2023).
Greenhouse gas removal	Withdrawal of a GHG or a precursor, or both, from the atmosphere by a carbon sink.

H

Hard-to-avoid emissions	“Emissions that will be either unacceptable to avoid from a social justice perspective or extremely physically difficult to eliminate within the given timeframe” (Wilcox et al. 2021).
Historic emissions	Emissions that were released in the past, specifically since the Industrial Revolution.
Hydroxide-based DAC	A DAC process that uses aqueous hydroxides (e.g., KOH, NaOH) as CO₂ capture media.

I

Indirect emissions	These are emissions that are sourced from an organization’s operations but are emitted by another entity in the value chain. Scopes 2 and 3 emissions are considered indirect.
Influent	Something that flows in, typically used when describing water flow and movement. Influent has recently been used to describe the input of carbon.
Insetting	Insetting projects are interventions along a company’s value chain that are designed to generate GHG emissions reductions and carbon storage and, at the same time, create positive impacts for communities, landscapes, and ecosystems.
Intergovernmental Panel on Climate Change (IPCC)	A part of the United Nations that focuses on gathering and analyzing data and information on scientific research as it relates to climate change. The IPCC puts out robust reports on climate change.
International Energy Agency (IEA)	Established in 1974, the IEA is an intergovernmental organization that researches and advises the global community on all things related to energy. The IEA also advises on international energy policy to the global sector.
Ion-exchange resins	CO₂ capture sorbent that contains fixed quaternary ammonium cations and exchangeable carbonate ions.

K

Kyoto Protocol

“The Kyoto Protocol (1997) operationalizes the United Nations Framework Convention on Climate Change by committing industrialized countries and economies in transition to limit and reduce greenhouse gases (GHG) emissions in accordance with agreed individual targets” (UNFCCC n.d.).

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Suggested Citation: "Appendix A: Acronyms and Abbreviations, Glossary, and Other Important Terms." National Academies of Sciences, Engineering, and Medicine. 2024. Carbon Removal at Airports. Washington, DC: The National Academies Press. doi: 10.17226/28458.

L

Leachates	A liquid that collects contaminants, especially solid waste, as it flows through surface and subsurface areas.
Leakage	Leakage refers to an unintended increase in GHG emissions or the shifting of emissions from one place to another due to a carbon-credit project because of shifting demand from a protected place to an unprotected one (Klein 2020).
Legacy emissions	Emissions that are either physically extremely difficult to eliminate within a certain time frame (e.g., because of dependence on a particular infrastructure with a long lead time for carbon-free substitution or because avoidance would require a technology that relies on a scarce resource) or that would be unacceptable to avoid from a social justice perspective (e.g., if mitigation would deprive people of the means to satisfy their basic needs, like food security).
Life-cycle assessment (LCA)	Compilation and evaluation of the inputs, outputs, and the potential environmental impacts of a product or service throughout its life cycle.
Liquid hydroxide solvent	CO₂ capture solvent that consists of aqueous hydroxides (e.g., KOH, NaOH).
Liquid monoethanolamine (MEA sorbents)	CO₂ capture sorbent that consists of aqueous monoethanolamine with the formula C₂H₇NO.

M

Measurement	“Processes of data collection over time, providing basic datasets, including associated accuracy and precision, for the range of relevant variables. Possible data sources are field measurements, field observations, detection through remote sensing and interviews” (FAO, UNEP, and UNDP 2010).
Mitigation	In the context of climate change, mitigation relates to a human intervention to reduce the sources or enhance the sinks of GHGs. Examples include using fossil fuels more efficiently for industrial processes or electricity generation, switching to solar energy or wind power, improving the insulation of buildings, and expanding forests and other sinks to remove greater amounts of CO₂ from the atmosphere.
Monitoring, reporting, and verification (MRV)	The quantitative accounting of any carbon-removal project that ensures accountability of the amount of carbon removed.

N

Nationally determined contribution (NDC)

NDCs are submissions from countries describing the national actions they intend to take to reach the Paris Climate Agreement’s long-term temperature goal of limiting warming to well below 2°C. Once a country has ratified the Paris Climate Agreement, its intended NDC is automatically converted to its NDC, unless that country chooses additional updates.

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Suggested Citation: "Appendix A: Acronyms and Abbreviations, Glossary, and Other Important Terms." National Academies of Sciences, Engineering, and Medicine. 2024. Carbon Removal at Airports. Washington, DC: The National Academies Press. doi: 10.17226/28458.

Negative emissions	Removal of GHGs from the atmosphere by deliberate human activities, i.e., in addition to the removal that would occur via natural carbon cycle processes.
Negative emissions technologies	Technologies that remove and store CO₂ from the atmosphere, making an entity’s emissions negative.
Net-negative emissions	A situation of net-negative emissions is achieved when, as result of human activities, more GHGs are removed from the atmosphere than are emitted into it.
Net-zero CO₂ emissions	Net-zero CO₂ emissions are achieved when anthropogenic CO₂ emissions are balanced globally by anthropogenic CO₂ removals over a specified period. Net-zero CO₂ emissions are also referred to as carbon neutrality.
Net-zero emissions	Net-zero emissions are achieved when emissions of GHGs caused by human activity are balanced by removing an equivalent amount of GHGs through purchased offsets or carbon-removal solutions over a specified period. It is sometimes used interchangeably with the term “carbon neutral.”

O

Ocean acidification	Ocean acidification is the decrease of the ocean’s pH, making it more acidic over time. This is attributed to the ocean’s uptake of atmospheric CO₂. Ocean acidification has many negative impacts on the entire oceanic ecosystem, including reducing the stability of shells made of carbonate (U.S. EPA 2016).
Ocean alkalinization	A process in which alkaline substances (e.g., lime and olivine) are added to the ocean, increasing its capacity to work as a carbon sink. This is one nature-based carbon-removal strategy. This process is also referred to as ocean liming.
Ocean liming	See “Ocean alkalinization.”
Oxy-fuel flash calcination	A limestone production process that uses highly concentrated oxygen instead of air for fuel combustion.

P

Paris Climate Agreement	An international treaty focusing on all aspects of climate change and holding its global signatories accountable to their climate commitments. The Paris Climate Agreement is also called the Paris Climate Accords, or the Paris Accord, and went into effect in the autumn of 2016.
Peat	Peat is a highly organic material that is often found in swamps, bogs, and other wetland. Peat can be used to generate energy, particularly for heat, but unfortunately emits a lot of carbon. In some regions of the world, peatlands are burned and are a major source of carbon pollution.
Permanence (or durability)	The duration for which CO₂ can be stored in a stable and safe manner.

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Suggested Citation: "Appendix A: Acronyms and Abbreviations, Glossary, and Other Important Terms." National Academies of Sciences, Engineering, and Medicine. 2024. Carbon Removal at Airports. Washington, DC: The National Academies Press. doi: 10.17226/28458.

Photosynthesis	The process by which plants convert sunlight, water, and CO₂ into chemical energy. Plants convert these inputs into sugars, and oxygen is an output.
Phytoplankton	Phytoplankton are self-feeding organisms that are at the very bottom of the food chain but play an essential role in aquatic food webs and the overall health of the body of water ecosystems.
Power purchase agreement (PPA)	An arrangement in which a third-party developer installs, owns, and operates an energy system on a customer’s property. The customer then purchases the system’s electric output for a predetermined period.
Pyrolysis	The heating and eventual thermal conversion of biomass in an environment without oxygen.

R

Reforestation	Planting of forests on lands that have previously contained forests but that have been converted to some other use.
Regenerative agriculture	A way to farm that allows the soil to remain productive over time by using techniques that improve and protect biodiversity and soil health. Regenerative agriculture can sequester carbon if done properly.
Renewable energy	Energy that is produced by a natural source that is replenished faster than it is consumed. Examples include solar, wind, and geothermal energy.
Reporting	Reporting is an ongoing process that assesses an organization’s climate impact and discloses its climate reduction targets and initiatives. There are many frameworks and standards for reporting, such as the Global Reporting Initiative, the Carbon Disclosure Project, the Task Force on Climate Related Financial Disclosures, and many more.
Residual emissions	Emissions for which abatement remains uneconomical or technically infeasible under the assumptions of a specific model and mitigation scenario (Buck et al. 2023).

S

Sink	A reservoir (natural or human, in soil, ocean, and plants) where a GHG, an aerosol, or a precursor of a GHG is stored.
Solid amine sorbent	A sorbent used for capturing CO₂ that consists of a porous support onto which an amine is attached or immobilized.
Solvent	Solvents are chemicals that absorb carbon dioxide and are the basis for many carbon-capture technologies.
Sustainable aviation fuel (SAF)	“A biofuel used to power aircraft that has similar properties to conventional jet fuel but with a smaller carbon footprint. Depending on the feedstock and technologies used to produce it, SAF can reduce life-cycle GHG emissions dramatically compared to conventional jet fuel” (Office of Energy Efficiency & Renewable Energy n.d-b).

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Suggested Citation: "Appendix A: Acronyms and Abbreviations, Glossary, and Other Important Terms." National Academies of Sciences, Engineering, and Medicine. 2024. Carbon Removal at Airports. Washington, DC: The National Academies Press. doi: 10.17226/28458.

T

Technical readiness level (TRL)	A way to understand how mature a technology is and if it is ready to be deployed at scale.
Thermodynamics	The scientific relationship between heat, temperature, and work and how they relate to energy, entropy, and radiation. Thermodynamics falls into the category of physics.

U

Upwelling

Upwelling is when deep, colder, more nutrient-dense water moves up to the surface. Surface water winds often move the warmer water away, creating space for this cooler water to rise. In regions where upwelling occurs, there is usually more oceanic activity and productivity.

V

Verification	“Carbon verification is when a third-party verification organization inspects a carbon-offset program and ensures it meets its standards. If the program meets the standards, the carbon-offset credits are verified, making them safe for consumers to purchase and offset their own carbon footprint” (Terrapass 2022).
Voluntary carbon market	The market that allows carbon emitters to purchase offsets or credits to achieve net-zero emissions targets that are not required by regulations.

Other Important Terms Related to CDR

Carbon credits vs. carbon offsetting	Carbon credits and offsets are related but cannot be used interchangeably. Carbon offsets are more relevant to CDR as they refer to removing GHGs from the atmosphere. On the other hand, carbon credits refer to reducing the amount of GHGs emitted into the atmosphere.
Carbon cycle	The carbon cycle is the natural flux of carbon from sinks or reservoirs into the atmosphere. The Earth is in a closed system, therefore the amount of carbon in the system does not change, just the location of it does. Carbon is often referred to as the “backbone of life”; it is found in living organisms, rocks and sediments, peat swamps, the ocean, and the atmosphere. These locations are referred to as carbon sinks or reservoirs because they store carbon for periods of time before being released into the atmosphere. Carbon release is caused by living organisms dying, volcanic eruptions, forest fires, and of course the burning of fossil fuels. Human impacts, such as burning peat swamps, slashing and burning forests, and using fossil fuels have altered the balance of the natural cycle by removing too much carbon from the sinks and reservoirs and releasing it into the atmosphere, causing climate change.
Carbon market	A carbon market is a platform for different entities to trade carbon credits and offsets. Carbon markets are critical in incentivizing carbon reductions through costs by putting a price on emissions.

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Suggested Citation: "Appendix A: Acronyms and Abbreviations, Glossary, and Other Important Terms." National Academies of Sciences, Engineering, and Medicine. 2024. Carbon Removal at Airports. Washington, DC: The National Academies Press. doi: 10.17226/28458.

	Organizations, individuals, and countries that are emitting CO₂ can offset their footprint by participating in the carbon markets. Today, there are two different carbon markets: the compliance market and the voluntary market.
Carbon tax	A carbon tax is another way to incentivize entities to stop emitting CO₂ and other GHGs. With a carbon tax, a regulatory government body will put a price on each ton of CO₂ emitted. The polluting entity will have to pay for each ton emitted. Today, there is no carbon tax in the United States, but many organizations are hopeful that one will be implemented soon. Carbon taxes are problematic from an airport perspective because airports operate under grant assurances that, as a public-use airport, require them to operate in a certain way, including the rates and charges for use of the airport. This is described more in Chapter 6, Funding and Financing Considerations.
Negative-emissions technologies (NETs)	Technologies that remove and sequester carbon. In addition to net-zero goals, some organizations have set carbon-negative targets, meaning they will remove more carbon than they emit. NETs and CDR are often used interchangeably for carbon-removal techniques that must be used in addition to emissions-reduction strategies and efforts to meet net-zero goals. Some view the term NETs as generally referring to a technological approach, whereas the CDR is more inclusive of technological and nature-based approaches; however, this use is inconsistent. CDR and NETs will be key in achieving carbon-negative goals. As stated in the introduction, CDR is used for all carbon-removal techniques to be consistent. However, it is important to recognize the overlap in terminology when laying the groundwork for airports to evaluate CDR.
Seasonality of carbon sequestration	Natural carbon-sequestration potential changes seasonally in the Northern Hemisphere. This is due to foliage and trees losing their leaves and becoming dormant during winters. Plants and foliage absorb atmospheric CO₂ during the process of photosynthesis, which does not occur when dormant. Seasonality is important when evaluating those natural CDR techniques for airports.