Carbon sequestration is the uptake and storage of carbon in some medium.
Trees and plants, for example, absorb carbon dioxide, release the oxygen and store the carbon. According to one theory, fossil fuels were at one time biomass and continue to store the carbon until burned.
In addition to biological sequestration in trees and plants, there are forms of sequestration that occur naturally in soil and gas hydrates (in ice). There is also man-made geological sequestration. Major disturbance of soil, ice and rock formations can lead to major releases of gases with high global warming potential, notably methane. The plowing of the Great Plains in the late 19th century was supposedly responsible for the single largest release of carbon into the atmosphere. Shallow plowing techniques are now advised by most rural development experts, for this and other reasons.
Carbon sequestration in plants, soils, and artificially in rock formations is a major means of greenhouse gas control. Suppliers of such sequestration deal with carbon emitters in carbon emission markets, e.g. that in the UK, where utilities are required to either generate electricity with renewables or offset their emissions by purchasing CO2 reserves, e.g. forest replantation or protection in Latin America. There is as yet no carbon audit regime for all such markets globally, and none is specified in the Kyoto Protocol. Each nation is on its own to verify actual carbon emission reductions, and to account for carbon sequestration using some less formal method. Some suggest this is an argument for carbon-based accounting reform.
Scientifically, the carbon leakage or loss potential from various means of carbon sequestration is poorly understood. Initial studies suggest that geological sequestration may in fact be the most reliable given the many ways in which plants may be disturbed, e.g. cutting, forest fires.
However, seismic activity or deliberate human sabotage might release the gas from geological formations rather easily and immediately, whereas an equivalent effort to release the carbon from plants or soil would not be as immediate. Methane sequestration has another name, that being a "natural gas deposit", and in that form is useful as fuel and quite well understood.