Above-Ground CO₂ Storage: The carbon stored in plants, not including roots and soil. In a typical forest, 80% of the total carbon is above ground. Only 10% of the total carbon is above ground in grasslands.

Absorbent, Amine: Amines are a class of chemicals that are used to capture CO₂ and/or sulfur dioxide (SO₂) from gaseous process streams. Amines capture CO₂ from a gas by a chemical reaction in which the CO₂ becomes attached to the amine. Heat is required to reverse the reaction and release the captured CO₂.

Absorbent, Metal: Solid materials containing sodium or other metals that can chemically absorb and desorb CO₂ in a regenerable cycle. Leading concepts introduce adsorbent contacted to the CO₂ in a fluidized bed. Such systems have potential for low energy use per unit of CO₂ captured compared to conventional water-based amine absorbents, because they are dry and water does not have to be heated up and cooled down in the absorption/desorption cycle.

Acid Gas: Chemical species, principally oxides of sulfur (SOx) and nitrogen (NOx), contained in flue gas and other process streams that combine with water vapor or water droplets to form acid. Acid gases can corrode pipelines and other process equipment and confound CO₂ separation technologies, especially at temperatures low enough for water to condense.

Adsorbent, Carbon: Amorphous solid carbon materials (as opposed to diamonds) physically attract and bind CO₂ onto their surface and can be used to capture CO₂ from a gas stream. Amorphous carbon materials produced in different ways have different surface structures and different adsorption properties. Potentially, carbon absorbents can capture and release CO₂ with very little energy required because there is no chemical reaction involved.

Avoided GHG Emissions: A measurement of greenhouse gas (GHG) emissions reduction that takes into account the reduced capacity of power plants and other industrial facilities due to the addition of CO₂ capture systems. For example, if a capture system were installed at a power plant that emitted 100 units of CO₂ per year, and the capture system captured 90% of the CO₂ but reduced the power plant's output from 50 units/yr to 40 units/yr, the reduction in terms of avoided emissions would be 10/40 versus 100/50 - a reduction of 87.5%, which is less than 90%. Often avoided-emissions calculations assume a method of making up lost capacity. In the above example, if the lost capacity was made up with a power plant without CO₂ controls (100/50), then the new emissions would be 30 units CO₂ per year (10 + (10/50)*100), which gives an avoided emissions rate of 70 units CO₂ per year --- again less than the 90 units of CO₂ per year that are captured.

Bio-Accelerated Sequestration: A concept of using microbial organisms (microscopic plants or animals) with CO₂ in geologic formations to sequester the CO₂ and/or convert it to methane. A key challenge in this area is to identity microbial organisms that can survive the harsh subsurface conditions (i.e., 10,000 pounds per square inch (psi), 125ºC, pH 3-10, and salt concentrations of 30% or more in water).

Brine: Any solution of water and salt (sodium chloride), usually containing other salts also. Ocean water contains 3% salt. Brines in underground formations can contain 20% salt or more. Brine must be desalinated before it can be used for drinking, and underground formations containing brines are not regarded as drinking water supplies.

Carbon Dioxide (CO₂): A colorless, odorless gas that is produced when animals (including humans) breathe or when carbon-containing materials (including fossil fuels) are burned. Carbon dioxide is essential to the photosynthesis process that sustains plant and animal life, however, it can accummulate in the air and trap heat near the Earth's surface (the "greenhouse effect").

Carbon Sequestration: The capture and storage of carbon dioxide and other greenhouse gases that would otherwise be emitted to the atmosphere. The greenhouse gases can be captured at the point of emission, or they can be removed from the air. The captured gases can be stored in underground reservoirs, dissolved in deep oceans, converted to rock-like solid materials, or contained in trees, grasses, soils, or algae.

CO₂ Capture: The removal of CO₂ from a process stream or from the atmosphere to produce a highly pure stream of CO₂ amenable for conversion or storage. CO₂ capture systems are assessed on the purity of the captured CO₂, the percent of total CO₂ that is captured, and the capital cost and energy use per unit of CO₂ captured.

CO₂ Enhanced CBM Recovery: It is common for coal beds to have methane trapped in pore spaces and adsorbed onto the surface of the coal. In the case of seams that are too deep for economical mining, this coal bed methane (CBM) can be recovered and sold. CBM recovery can be enhanced by injecting CO₂ into the coal seam, as CO₂ preferentially adsorbs onto the coal surface and displaces the methane. Nitrogen can also be injected into unmineable coal seams to enhance CBM production.

CO₂ Enhanced Oil Recovery: The injection of CO₂ into depleting oil reservoirs to recover addition oil beyond that which would have been recovered by conventional drilling. In the most common CO₂ enhanced oil recovery (EOR) method, miscible CO₂ EOR, CO₂ becomes dissolved in the crude oil and both causes the oil to swell and reduces its viscosity - which drives and enables the oil to slip through the rock formation toward the oil recovery well. At 200,772 barrels of oil per day in 2000, CO2 EOR accounted for 5% of total U.S. oil production. It is estimated that 3,000 to 8,000 standard cubic feet of CO₂ remains sequestered in a formation for every barrel of enhanced oil recovered.

CO₂ Depleting Oil and Gas Reservoirs: Reservoirs that have been producing for a while using primary recovery methods, for which the rate of resource production is starting to decline. Economically viable production from such formations can be extended by methods to enhance production rates including water and CO2 floods.

Fossil Fuel: Any naturally occurring fuel of an organic nature formed by the decomposition of plants or animals; includes coal, natural gas, and petroleum.

Geologic Formation: A contiguous underground material which is sufficiently homogeneous to be considered a single unit. Geologic formations with a certain structure and porosity present an opportunity for underground CO₂ storage, as evidenced by existing formations which have been storing CO₂ for millions of years. Examples of formations with CO₂ sequestration potential include depleted oil reservoirs, depleted gas reservoirs, unmineable coal seams, saline formations, and shale formations.

Global Warming Potential (GWP): A measure of the magnitude of the heat-trapping effect resulting from the addition of 1 kilogram of a gas to the atmosphere relative to that of 1 kilogram of carbon dioxide. GWP is a function of two factors (1) the instantaneous heat-absorbing ability of the gas, and (2) the length of time that emissions of the gas persist in the atmosphere, on average. Gases differ substantially in both heat-trapping ability and atmospheric lifetime. For example, methane is over 100 times more capable of heat trapping than CO₂, but methane's average atmospheric lifetime is only 12 years, compared to CO₂ which persists anywhere between 5 and 200 years. Most GWPs are reported as the average heat trapping effect over 100 years. The 100-year GWP for methane is 23; the 100-year GWP for CO₂, by definition, equals 1.

Greenhouse Effect: The warming of the Earth's surface and lower atmosphere caused by the trapping of radiated heat, much the same way the coated window panes of an agricultural greenhouse keep heat inside the greenhouse. Several gases, such as carbon dioxide and methane, can keep heat from escaping from the Earth into space and are called "greenhouse gases."

Greenhouse Gas (GHG): A gas which does not absorb radiation of wavelengths in the visible light spectrum, but does absorb infrared (heat) radiation. In the atmosphere these gases allow energy from the sun to reach the earth's surface, but limit infrared energy (heat) from escaping. This effect is called radiative forcing. Greenhouse gases absorb 90% of infrared energy radiating from the Earth. Water vapor is the primary GHG, and CO₂ is the most important GHG emitted to the atmosphere as a result of human activities. CO₂ accounts for over 80% of the anthropogenic GHG effect. Other GHGs include: methane CH₄, ozone (O₃), CFCs (CFC-11, CFC-12, CFC-113), HFCs (HCFC-22) and nitrous oxide (N₂O), CCl4, methyl chloroform, sulfur hexafluoride, trifluoromethyl sulfur pentafluoride, and perfluoroethane. Each gas has a different global warming potential and longevity in the atmosphere.

Hydrates: A hydrate is a naturally occurring, ice-like crystalline compound in which a crystal lattice of water molecules encloses a molecule of some other substance. The compounds are very dense and insoluble in water. CO₂ hydrates are being investigated for use in CO₂ capture and storage.

Membrane: A material that is selectively permeable to one or more chemical species and can therefore be used to separate that species from a gas stream. Selective separation is driven by the partial pressure difference across the membrane surface. Membrane materials are being developed for the separation of CO₂ from hydrogen, natural gas, and flue gas.

Measurement, Montoring, and Verification (MM&V): MM&V is defined as the capability to measure the amount of CO₂ stored at a specific sequestration site, to monitor the site for leaks or other deterioration of storage integrity over time, and to verify that the CO₂ is stored and unharmful to the host ecosystem. MM&V capability will ensure safe permanent storage, reduce the risk associated with buying or selling credits for sequestered CO₂, and help satisfy regulators and local government officials who must approve large sequestration projects. MM&V will also provide valuable feedback for continual refinement of injection and management practices.

Micro-Seismic: Micro-seismic sensors detect sonic-bursts (sounds) generated by sudden small-scale slips/cracking, or changes in temperature, volume, pressure, or stress in, rock mass formations and geomaterials. These sensors can be used to monitor oil and gas reservoirs for deformation, cracking, and other changes in condition.

Migration of CO₂: The movement of CO₂ through a geologic formation, driven by density or a pressure differential.

Mineral Carbonization: A process in which CO₂ reacts with magnesium or calcium oxide to form mineral carbonates. The mineral carbonates are unreactive solids - highly permanent carbon storage. Challenges include slow reaction rates and the large tonnage of mineral-rich earth that must be mined for each unit of CO₂ sequestered.

Ocean Fertilization: A concept for ocean sequestration in which infertile waters are seeded with iron or other nutrients to enhance the growth of plankton and consequently increase the uptake of CO₂ into the ocean waters.

Ocean Injection: A concept for ocean sequestration in which CO₂ is injected directly into the mid-or deep ocean waters, where it dissolves into the ocean water.

Ocean Sequestration: Storage of CO₂ in ocean waters. Oceans are an important part of the natural carbon cycle because they store, release, and absorb large quantities of CO₂ to and from the atmosphere. Research in this area is focused on learning more about the ocean carbon cycle, deep ocean ecosystems, and the safety and potential environmental impacts of CO₂ storage.

Ocean Combustion: An approach to fossil fuel conversion in which the fuel is combusted in pure oxygen rather than air, which is 78% nitrogen and 21% oxygen. Oxygen combustion systems exhaust a stream of highly pure CO₂ as an inherent part of their operation.

Ozone: A bluish, toxic gas with a pungent odor, formed by three oxygen atoms rather than the usual two. Ozone occurs in the stratosphere and plays a role in filtering out ultraviolet radiation from the sun's rays. At ground level, ozone is a major component of smog.

Post-Combustion Capture: A system for CO₂ capture from a fossil fuel conversion where the fuel is combusted in air and resulting CO₂ is scrubbed, absorbed, or otherwise captured from the flue gas, which is primarily CO₂ and nitrogen.

Pre-Combustion Capture: A system for CO₂ capture from a fossil fuel conversion where the fuel is decarbonized via gasification, pyrolysis, or reforming prior to combustion. The synthesis gas from decarbonatization is primarily a mixture of CO₂ and hydrogen. The CO₂ is captured from the hydrogen before the hydrogen is combusted.

Reservoir: A subsurface, porous, permeable rock body surrounded by impermeable rock and containing oil, gas, or water. Most reservoir rocks consist individually or collectively of limestone, dolomites, or sandstone.

Saline Formations: Geologic formation of porous rock that is filled with brine.

Soil Carbon: Organic and inorganic carbon contained in soil. CO₂ is converted into organic carbon by plants during photosynthesis, and is then deposited in the soil through their roots and in plant residue. Organic carbon is found in the top layer of soil. Inorganic soil carbons are carbonates that form through non-biological interactions. They are a minor component compared to organic carbon.

Soil carbon sequestration occurs through direct and indirect fixation of atmospheric CO₂. Direct soil carbon sequestration occurs by inorganic chemical reactions that convert CO₂ into soil inorganic carbon compounds such as calcium and magnesium carbonates. Direct plant carbon sequestration occurs as plants photosynthesize atmospheric CO₂ into plant biomass. Subsequently, some of this plant biomass is indirectly sequestered as soil organic carbon (SOC) during decomposition processes. Worldwide, SOC in the top 1 meter of soil comprises about 3/4 of the earth's terrestrial carbon.

Terrestrial Carbon Sequestration: The absorption and storage of carbon by vegetation and soils in terrestrial ecosystems.

Unmineable Coal Seams: Coal seams which are inaccessible and/or uneconomical to mine due to depth, coal quality, and technological or land use restrictions.