Terminology used to describe the Carbon Cycle

In this article, we will review terminology used to describe the carbon cycle.

Reservoir: A delimited compartment of the Earth System used as homogeneous global unit to quantify the stocks and fluxes of matter of a given element. The limits of a reservoir can also be defined by the state of the considered element. For example, within the same physical boundaries, two different reservoirs can be defined by a pool of particulate and a pool of dissolved matter or, in the case of carbon a pool of inorganic and a pool of organic matter.

Stock: The amount of a given element (carbon for example) in a given reservoir.

Flux: A quantity of mater or energy per unit of time transferred from a reservoir to another. A flux may consist in a change of state of the element. For example, transformation of organic into inorganic carbon or of particulate matter into dissolved material. Note that several fluxes can connect the same two reservoirs; the sum of all these fluxes will the called ‘net flux’. 

Biogeochemical cycle: All reservoirs and fluxes involving a chemical substance through the different compartments of the Earth System.

Petagram: A unit of mass corresponding to 10¹⁵ grams, often used to quantify very large amounts of material, such as the global stocks of carbon. A petagram is equivalent to a gigaton (i.e. 10⁹ tons or one billion tons). Note that fluxes of CO₂ can either be expressed in Pg C, which refers to the mass of carbon alone, or in Pg CO₂ refers to the mass of CO₂ and both units are regularly used. 

Mass balance: A concept stating that, within a bounded system (like the Earth for example), matter cannot be lost or created and the sum of all the reservoirs of a system must be constant over time or that change of this sum must equal the flux coming in or out of the system.

Sinks and sources: Terms characterizing if a flux going in or out of a reservoir represents a gain or a loss for the reservoir. In the context of the global carbon cycle, it is most commonly used to referring to the atmosphere. A flux removing carbon from the atmosphere will be called a sink of atmospheric carbon (uptake by the ocean) while a flux introducing additional carbon to the atmosphere will be referred to as a source (i.e. fossil fuel burning)

Steady state: A situation in which all fluxes entering each reservoir of a system are exactly compensated by the fluxes leaving each of these reservoirs. At steady state, the size of all the reservoirs of a given system (or biogeochemical cycle) will not change over time.

Residence time: A representation of the average time an element spends in a given reservoir. A residence time (usually expressed in years) can be calculated by dividing the size of a reservoir by all the fluxes. Note that the residence time of carbon in a reservoir can be calculated for the present-day or the anthropic carbon cycle. In the latter case, it will be referred to as the residence time of anthropic carbon in the reservoir.

Short term carbon cycle: A part of the global carbon cycle (fluxes and reservoirs) which is characterized by residence times equal or shorter than the average human life span.

Long term carbon cycle: A part of the global carbon cycle (fluxes and reservoirs) which is characterized by residence times larger than the average human life span (usually by several orders of magnitude).