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The quantity of greenhouse gas (GHG) emissions have been reduced or removed as the results of an offset or transfer project. In quatitative terms, additionality is the difference between the baseline emissions (emissions that have not been subjected to any intentional reductions) and the volume of emissions released after the implementation of an offset project. 


The planting of new forests on lands that historically have not contained forests.

Alternative Energy

Energy derived from nontraditional sources (e.g., compressed natural gas, solar, hydroelectric, wind).


Made by people or resulting from human activities. Usually used in the context of emissions that are produced as a result of human activities.


The multilayer gaseous envelope surrounding the Earth. The dry atmosphere consists almost entirely of nitrogen (78.1% volume mixing ratio) and oxygen (20.9% volume mixing ratio), together with a number of trace gases, such as argon (0.93% volume mixing ratio), helium, radiatively active greenhouse gases such as carbon dioxide (0.035% volume mixing ratio), and ozone. In addition, the atmosphere contains water vapor, the amount of which is highly variable, but typically 1% volume mixing ratio; clouds; and aerosols.

Atmospheric Lifetime

The average time that a molecule resides in the atmosphere before it is removed by chemical reaction or deposition. This can also be thought of as the time that it takes after a human-caused emission of a gas for the concentrations of that gas in the atmosphere to return to natural levels. Greenhouse gas lifetimes can range from a few years to a few thousand years.


Gas or liquid fuel made from plant materials, including wood, wood waste, wood liquors, peat, railroad ties, wood sludge, spent sulfite liquors, agricultural waste, straw, tires, fish oils, tall oil, sludge waste, waste alcohol, municipal solid waste, landfill gases, ethanol blended into gasoline and other waste. 


Materials that are biological in origin, including organic material (both living and dead) from above and below ground, e.g., trees, crops, grasses, tree litter, roots, and animals and animal waste.


The part of the Earth system comprising all ecosystems and living organisms in the atmosphere, on land (terrestrial biosphere) or in the oceans (marine biosphere), including derived dead organic matter, such as litter, soil organic matter and oceanic detritus.

Cap and Trade

A policy under which a regulatory or international body sets a limit (cap) on the amount of pollution (e.g., greenhouse gases) that can be emitted during a certain period by certain entities. Depending on the regulatory authority, these entities might represent industrial sectors or a group of nations. The cap is divided into permits for the right to emit a part of the capped pollution. The permits have transferable titles (ownership), which allows for exchange of permits. (Not to be confused with offsetting. See carbon offset.)


A chemical element found in many greenhouse gases. Carbon dioxide (CO2) accounts for about 80% of the total greenhouse gas (GHG) mix; methane (also carbon-based) is another significant GHG component.

Carbon Capture and Sequestration

Carbon capture and sequestration (CCS) is a set of technologies that can greatly reduce carbon dioxide emissions from new and existing coal- and gas-fired power plants, industrial processes and other stationary sources of carbon dioxide (CO2). It is a three-step process that includes capture of carbon dioxide from power plants or industrial sources; transport of the captured and compressed carbon dioxide (usually in pipelines); and underground injection and geologic sequestration (permanent storage) of that carbon dioxide in rock formations that contain tiny openings or pores that trap and hold the gas.

Carbon Credit

Represents the right to emit one tonne of carbon dioxide. Carbon credits can be exchanged between an offset project owner and a company or other entity that requires a credit to offset its emissions. Carbon credits also can be bought and sold on the international market at the current market price. (See carbon offset.)

Carbon Cycle

All parts (reservoirs) and fluxes of carbon. The cycle is usually thought of as four main reservoirs of carbon interconnected by pathways of exchange. The reservoirs are the atmosphere, terrestrial biosphere (land—and usually includes freshwater systems), oceans, and sediments (which include fossil fuels). The annual movements of carbon—the carbon exchanges between reservoirs—occur because of various chemical, physical, geological and biological processes. The ocean contains the largest pool of carbon near the surface of the Earth, but most of that pool is not involved with rapid exchange with the atmosphere. 

Carbon Dioxide (CO2)

A naturally occurring gas that is also produced as a byproduct of burning fossil fuels and biomass as well as by land-use changes and other industrial processes. It is the principal human-caused greenhouse gas that affects the Earth’s radiative balance. It is the reference gas against which other greenhouse gases are measured and, therefore, has a global warming potential (GWP) of 1.

Carbon Dioxide Equivalent

A metric measure used to compare the emissions from various greenhouse gases, based upon their global warming potential (GWP). Carbon dioxide equivalents are commonly expressed as “million metric tons of carbon dioxide equivalents” (MMTCO2e ). The carbon dioxide equivalent of a gas is derived by multiplying the metric tons of the gas by the associated GWP.

MMTCO2e = (million metric tons of a gas) x (GWP of the gas)

Carbon Finance

Investments in GHG emission reduction projects and the creation of financial instruments that are tradeable on the carbon market.

Carbon Footprint

The total amount of greenhouse gases that are emitted into the atmosphere each year by a person, family, building, organization or company. A person’s carbon footprint includes greenhouse gas (GHG) emissions from fuel that an individual burns, such as by heating a home or riding in a car. It also includes greenhouse gases that come from producing the goods or services that the individual uses, including emissions from power plants that make electricity, factories that make products, and landfills where waste is sent.

Carbon Offset

An instrument designed to allow the continuing release of carbon in one place in exchange for reducing the carbon release in another place. Carbon offsets are measured and credits are given for the amount that is reduced. One carbon credit represents the reduction of one metric ton (tonne) of carbon dioxide or its carbon dioxide equivalent (CO2e) in other greenhouse gases. Carbon offsets are issued by various bodies. Some are only accepted in voluntary markets. Only those issued by the Kyoto Protocol are accepted in the EU emissions trading system (EUETS).

Carbon Trading

The sale and purchase of greenhouse gas or carbon accounting tokens (permits and credits), including transactions and securities based on these tokens.

Certified Emissions Reductions (CERs)

A unit of greenhouse gas (GHG) emission reductions issued pursuant to the Clean Development Mechanism of the Kyoto Protocol. A CER is measured in metric tons of carbon dioxide equivalent (CO2e). One CER represents a GHG emissions reduction equal to one CO2e.

Chlorofluorocarbons (CFCs)

Gases covered under the 1987 Montreal Protocol, used for refrigeration, air conditioning, packaging, insulation, solvents or aerosol propellants. Since they are not destroyed in the lower atmosphere, CFCs drift into the upper atmosphere where, given suitable conditions, they break down the ozone layer. CFCs are greenhouse gases and considered ozone depleting substances (ODSs). As such, they are being replaced by other compounds: hydrochlorofluorocarbons, an interim replacement for CFCs that are also covered under the Montreal Protocol, and hydrofluorocarbons (weak ODSs), which are covered under the Kyoto Protocol. 


In a narrow sense, climate is usually defined as the “average weather” or, more rigorously, as the statistical description in terms of the mean and variability of relevant quantities—most often of surface variables such as temperature, precipitation and wind—over a period of time ranging from months to thousands of years. The classical period is three decades, as defined by the World Meteorological Organization (WMO). In a wider sense, climate is the state, including a statistical description, of the climate system.

Climate Change

Climate change refers to any significant change in the measures of climate that lasts for an extended period of time. In other words, climate change includes major changes in temperature, precipitation or wind patterns, among others, that occur over several decades or longer.


The benefits of policies that are implemented for various reasons at the same time, including climate change mitigation, acknowledging that most policies designed to address greenhouse gas mitigation also have other, at least equally important, rationales (e.g., related to objectives of development, sustainability and equity).


Carbon dioxide equivalent.

Coal Mine Methane

Coal mine methane is the subset of coalbed methane that is released from coal seams during the process of coal mining. 

Coalbed Methane

Coalbed methane is methane contained in coal seams. It is often referred to as virgin coalbed methane or coal seam gas.


The practices or processes of clearing forests and the end result of such clearing. Deforestation contributes to increased carbon dioxide (CO2) concentrations in two ways: 1) the release of carbon dioxide from the burning or decomposition of wood and 2) the absence of carbon dioxide removal, which occurs through photosynthesis, by the trees that are no longer present.


Land degradation in arid, semiarid and dry sub-humid areas resulting from various factors, including climatic variations and human activities.

Dryland Farming

A technique that uses soil moisture conservation and seed selection to optimize production under dry conditions.


Any natural unit or entity including living and nonliving parts that interact to produce a stable system through cyclic exchange of materials. 


The release of a substance (usually a gas when referring to the subject of climate change) into the atmosphere.

Emissions Factor

A unique value for scaling emissions to activity data in terms of a standard rate of emissions per unit of activity (e.g., grams of carbon dioxide emitted per barrel of fossil fuel consumed or per pound of product produced). 

Emissions Reduction Purchase Agreement (ERPA)

A contract between a buyer and a seller of project-based offset credits under the Kyoto Protocol, which stipulates the firm intent and method of purchase of credits eventually awarded to the project owners. The contract will also cover such events as failure to deliver. A pro forma has been developed by the International Emissions Trading Association that reflects the needs of its members mostly on the purchasing side; however, the terms are free to be set according to each project’s needs.

Emissions Trading

The sale and purchase of airborne pollution accounting tokens (permits and credits) including transactions and securities based on these tokens. 

Enhanced Greenhouse Effect

The concept that the natural greenhouse effect has been amplified by increased atmospheric concentrations of greenhouse gases (such as CO2 and methane) emitted as a result of human activities. These added greenhouse gases cause the Earth to warm.

Enteric Fermentation

The process by which livestock, especially cattle, produce methane as part of their digestion. It represents approximately one third of the total CO2e emissions emanating from the agriculture sector.

Fluorinated Gases

Powerful synthetic greenhouse gases such as hydrofluorocarbons, perfluorocarbons and sulfur hexafluoride that are emitted from a variety of industrial processes.


Carbon-fluorine compounds that often contain other elements such as hydrogen, chlorine or bromine. Common fluorocarbons include chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs).

Fossil Fuel

A general term for organic materials formed from decayed plants and animals that have been converted to crude oil, coal, natural gas or heavy oils by exposure to heat and pressure in the Earth’s crust over hundreds of millions of years.

Fuel Switching

In general, this refers to substituting one type of fuel for another, such as switching from coal to natural gas. With regard to climate change, it is implicit that the substituted fuel generates lower carbon emissions per unit energy than the original fuel.

Global Warming

The recent and ongoing global average increase in temperature near the Earth’s surface.

Global Warming Potential (GWP)

A measure of the total energy that a gas absorbs over a particular period of time (usually 100 years), compared to carbon dioxide.

Greenhouse Effect

The trapping and buildup of heat in the atmosphere nearest the Earth’s surface, known as the troposphere. Some of the heat flowing toward space from the Earth’s surface is absorbed by water vapor, carbon dioxide, ozone and several other gases in the atmosphere and then reradiated back toward the Earth’s surface. If the atmospheric concentrations of these greenhouse gases rise, the average temperature of the troposphere will gradually increase.

Greenhouse Gas (GHG)

Any gas that absorbs infrared radiation in the atmosphere. Greenhouse gases include carbon dioxide, methane, nitrous oxide, ozone, chlorofluorocarbons, hydrochlorofluorocarbons, hydrofluorocarbons, perfluorocarbons and sulfur hexafluoride.


Compounds containing either chlorine, bromine or fluorine and carbon. Such compounds can act as powerful greenhouse gases in the atmosphere. The chlorine and bromine containing halocarbons are also involved in the depletion of the ozone layer. 


Substances containing only hydrogen and carbon. Fossil fuels are made up of hydrocarbons.

Hydrochlorofluorocarbons (HCFCs)

Compounds containing hydrogen, fluorine, chlorine and carbon atoms. Although they are ozone depleting substances and, by association, greenhouse gases, they are less potent at destroying stratospheric ozone than chlorofluorocarbons (CFCs). They have been introduced as temporary replacements for CFCs.

Hydrofluorocarbons (HFCs)

Compounds containing only hydrogen, fluorine and carbon atoms. They were introduced as alternatives to ozone-depleting substances in serving many industrial, commercial, and personal needs. They do not significantly deplete the stratospheric ozone layer, but they are powerful greenhouse gases.

Indirect Emissions

The emissions of greenhouse gases that occur as a result of the generation of electricity used in homes and buildings. These emissions are called “indirect” because the actual emissions occur at the power plant that generates the electricity, not at the building using the electricity.

Kyoto Protocol

An international agreement linked to the United Nations Framework Convention on Climate Change (UNFCCC). The Kyoto Protocol sets binding targets for industrialized countries which are signatories to the protocol as listed in Annex 1, for reducing greenhouse gas (GHG) emissions amounting to an average of a 5% reduction against 1990 levels over the five-year period 2008-12. The UNFCCC encourages industrialized countries to stabilize GHG emissions; the Kyoto Protocol commits them to do so.


A project is considered listed once the Climate Action Reserve (CAR) has satisfactorily reviewed all project submittal forms and tentatively accepted the project. The project will now appear in the public interface of the CAR system.

Methane (CH4)

A hydrocarbon that is a greenhouse gas with a global warming potential (GWP) most recently estimated at 25 times that of carbon dioxide (CO2). Methane is produced through several means, including anaerobic (without oxygen) decomposition of waste in landfills; as a product of animal digestion; by decomposition of animal wastes; by production and distribution of natural gas and petroleum; through coal production; and through incomplete fossil fuel combustion.

Metric Ton (Tonne)

The common international measurement of the quantity of greenhouse gas emissions. A metric ton, often expressed simply as “tonne,” is equal to 2,205 lbs or 1.1 short tons.

Natural Gas

Underground deposits of gases consisting of 50% to 90% methane (CH4) and small amounts of heavier gaseous hydrocarbon compounds such as propane (C3H8) and butane (C4H10).

Nitrogen Oxides (NOx)

Gases consisting of one molecule of nitrogen and varying numbers of oxygen molecules. Nitrogen oxides are produced from the emissions of vehicle exhausts and power stations. Considered pollutants, nitrogen oxides in the atmosphere can contribute to the formation of photochemical ozone, (smog), impair visibility and cause health consequences. 

Nitrous Oxide (N2O)

A powerful greenhouse gas with a global warming potential (GWP) of 298 times that of carbon dioxide. Major sources of nitrous oxide include soil cultivation practices (especially the use of commercial and organic fertilizers), fossil fuel combustion, nitric acid production and biomass burning. 

Offsets / Offsetting

See carbon offset.


A gaseous atmospheric constituent in the troposphere that is created by photochemical reactions with gases that result from both natural sources and human activities (photochemical smog). In high concentrations, tropospheric ozone can be harmful to a wide range of living organisms. Tropospheric ozone acts as a greenhouse gas and is not the same as the ozone layer, which is part of the stratosphere.

Ozone Depleting Substances (ODSs)

A family of man-made compounds that includes, but is not limited to, chlorofluorocarbons (CFCs), bromofluorocarbons (halons), methyl chloroform, carbon tetrachloride, methyl bromide and hydrochlorofluorocarbons (HCFCs). These compounds are potent greenhouse gases.

Ozone Layer

A layer of the atmosphere that begins approximately 15 km above the Earth and thins to an almost negligible amount at about 50 km. The ozone layer shields the Earth from the sun’s harmful ultraviolet radiation.

Ozone Precursors

Chemical compounds such as carbon monoxide, methane, non-methane hydrocarbons and nitrogen oxides, that, in the presence of solar radiation, react with other chemical compounds to form ozone, mainly in the troposphere. 

Particulate Matter (PM)

Very tiny pieces of solid or liquid matter, such as particles of soot, dust, fumes, mists or aerosols.

Perfluorocarbons (PFCs)

A group of chemicals composed of carbon and fluorine only. These chemicals (predominantly CF4 and C2F6) were introduced as alternatives, along with hydrofluorocarbons, to ozone depleting substances.

Project Developer

An organization or individual that develops projects for the purpose of generating emission reductions or removals. 

Project Protocol/Methodology

A document that includes the eligibility rules, greenhouse gas assessment boundary, quantification methodologies, monitoring and reporting parameters, etc. for a specific project type. 


A verified decrease in greenhouse gas (GHG) emissions caused by a project, as measured against an appropriate forward-looking estimate of baseline emissions for the project.


The replanting of forests on lands that have previously contained forests but that have been converted to some other use.


A project is considered registered when it has been verified by an approved third-party verifier and approved by the registration entity such as the Climate Action Reserve or the American Carbon Registry.

Renewable Energy

Energy resources that are naturally replenishing such as biomass, hydro, geothermal, solar, wind, ocean thermal, wave action and tidal action.


A carbon offset is retired when it has been used to offset an equivalent tonne of emissions or when it otherwise has been removed from further transactions on behalf of the environment.

Soil Carbon

A major component of the terrestrial biosphere pool in the carbon cycle. The amount of carbon in the soil is a function of the historical vegetative cover and productivity, which in turn is dependent in part upon climatic variables.


The region of the atmosphere between the troposphere and mesosphere, having a lower boundary of approximately 8 km at the poles to 15 km at the equator and an upper boundary of approximately 50 km. The ozone layer is in the stratosphere.

Sulfur Hexafluoride (SF6)

A very powerful greenhouse gas used primarily in electrical transmission and distribution systems and as a dielectric in electronics. The global warming potential of SF6 is 22,800.


An organization or individual that transfers and manages environmental commodities but does not develop its own projects.


The lowest part of the atmosphere from the Earth’s surface to about 10 km in altitude in mid-latitudes (ranging from 9 km in high latitudes to 16 km in the tropics, on average) where clouds and weather phenomena occur. In the troposphere temperatures generally decrease with height.

Verification Body

An organization or company that has been accredited by the American National Standards Institute (ANSI) to ISO 14065:2007 standards or approved by a registration entity to perform greenhouse gas verification activities for specific project protocols.


An individual that is employed by or subcontracted to an ANSI-accredited or registration-entity-approved verification body and is qualified to provide verification services for specific project protocols.


The date (year) of the initial issue of a credit or permit from an offset development project.

Voluntary Emissions Reduction (VER)

A form of offset that is produced primarily for sale in the voluntary offset markets.


The definitions in this glossary have been reprinted or adapted from the EPA Climate Change Glossary and the Climate Action Reserve Dictionary.