This page explains key terminology related to forest carbon sinks, carbon stocks and the climate effects of forest use.
Absolute emissions and removals
Emission is the quantitative, mass-based unit of emission sources. A sink, the quantitative unit of which is GHG removal, acts as a process opposite to emission sources. Therefore, a sink is any process, action or mechanism that stores GHGs (such as CO2), aerosols and the precursors of GHGs, thus removing them from the atmosphere. As a consequence of a carbon sink, some of the carbon stocks increase. Carbon stocks include oceans, vegetation and the soil. In this context, the atmosphere is not considered a carbon stock as the flows of carbon and changes in the stocks are examined from an atmospheric perspective.
Source of emissions
A source of emissions is any process, action or mechanism that releases GHGs (such as CO2), aerosols or precursors of GHGs into the atmosphere. The effectiveness of a source is measured as emissions.
Sink
A sink is any process, action or mechanism that stores GHGs (such as CO2), aerosols and the precursors of GHGs, thus removing some of them from the atmosphere. The effectiveness of a sink is measured as GHG removals.
Net sink
A net sink refers to a process, action or mechanism including both sources of emissions and sinks, the quantitative units of which (emissions and removals) produce a negative sum (removal exceeds the emissions). The overall land use, land use change and forestry (LULUCF) sector, for example, is considered a net sink if its emissions are lower than its removals. In GHG inventories, emissions and removals account for CO2 and other GHGs, such as methane and nitrous oxide, that are converted into CO2 equivalents. This translates as a net sink of GHGs.
Net source (of emissions)
A net source (of emissions) refers to a process, action or mechanism including both sources of emissions and sinks, the quantitative units of which (emissions and removals) produce a positive sum (emissions exceed the removals) (see Net sink).
Net emissions
Net emissions refers to the difference of emissions and removals.
Carbon sink
The actual meaning of a carbon sink is any process, action or mechanism that stores CO2 from the atmosphere. However, carbon sink may also be used to refer to a sink converted into the carbon or CO2 equivalents of GHGs.
Forest carbon sink
The actual meaning of a forest carbon sink is forests in which carbon stock in increasing as they sequester more carbon than they release. This is considered a net sink as forests always both sequester carbon as a result of photosynthesis and release it as a result of decay and soil respiration and harvest removals. Methane and nitrous oxide are usually taken into consideration as CO2 equivalents and forests are defined as carbon sinks if they act as a net sink for GHGs.
Carbon sink of wood products
A carbon sink of wood products refers to an increase in the carbon stock of wood products. If a specific carbon stock of wood products (for example, that of a state) stores more carbon than it releases over a certain period of time (for example, one year), the carbon stock of the wood products increases and the wood products are reported in the GHG inventory as removal corresponding to a net sink. This is because international agreements state that the CO2 emissions from burning biomass are reported in the energy sector as zero and stock changes of biomass in forests, the soil and in wood products are reported instead of the flows of CO2 sequestered in the biomass or released from it. Harvested wood products cover any wood materials per product category, including timber, wood panels and paper products. Solid, gaseous and liquid biofuels are not usually considered harvested wood products as the carbon stored in them is expected to be released in the year of their production.
Removal
GHG removal refers to the amount of GHGs, aerosols or precursors of GHGs removed from the atmosphere by sinks. Removal is also referred to as negative emissions, describing their opposite nature in relation to emissions.
Carbon balance
Carbon balance refers to the change in the carbon stock at an annual level, in other words the function of the process as a source of carbon or as a sink. If the carbon balance is negative, the carbon stock (of a forest, for example) has been reduced and CO2 has been released into the atmosphere. If the carbon balance is positive, the carbon stock (of a forest, for example) has increased and the stock (a forest, for example) has acted as a carbon sink. The carbon balance can be expanded as GHG balance by taking into account other GHGs, such as methane and nitrous oxide, as carbon (dioxide) equivalents.
Absolute emissions
Absolute emissions refer to any real or assumed (physical) emissions that can be observed, measured or verified upon its realisation.
Forest carbon stock
Forest carbon stock consists of the carbon in the living and dead biomass above and below ground. The growth of the tree stand and the forest litter feeding the soil are some of the factors increasing the carbon stock of forests, whereas the carbon stock is reduced by felling, timber harvesting, decay of the tree stand and soil respiration.
Emissions and removals in proportion to a reference situation
Reference situation
A reference situation (benchmark, counterfactual, baseline scenario) uses assumptions to generate a modelled description of what would take place without the action under review. Determining a reference situation is essential for consistently assessing the impact of any reviewed action. This is particularly important when the reference situation is dynamic. The carbon stock of forests, for example, changes over time even if no felling takes place or if the felling is carried out to a lesser extent than in the reviewed situation. Therefore, the impact of the reviewed felling on the carbon stock of forests is generated in proportion to the reference situation. In the reference situation, the assumption can be that the forests are managed or that they are not managed at all. A consistent choice depends on the perspective of the review, but it is important that the reviewed process (for example, felling) is not included in the reference situation. The reference situation is selected according to the objective of the review.
The reference situation should describe all the emissions and removals that are expected to change as a consequence of the reviewed action. For example, products and energy produced from wood can be used to replace non-renewable materials and energy sources. Consequently, the emissions otherwise generated in the production and use of alternative products and energy are avoided.
Substitution
Substitution refers to an action of replacing a product or a service with another product or service. Substitution is always carried out in proportion to the reference situation. When substituting the use of timber, for example, the reviewed wood products and energy would not be used in the reference situation but would, instead, be replaced by alternative products and energy. Unlike absolute emissions, substitution cannot be observed, measured or verified; it must be modelled using assumptions.
Avoided emissions
The GHG balance of the difference between the reviewed action and its reference situation. For example, the difference of the carbon stock of forests and wood products and the emissions caused by the use of non-renewable materials in the production of a wood product or energy and in its reference situation.
Climate effect of emissions and removals
GHG emissions and removals adjust the radiation balance of the planet and, thus, have either a warming or a cooling effect on the climate. Radiative forcing refers to the imbalance of energy caused by GHG emissions or removals in the climate system. The temporal occurrence of emissions and removals affects the concentrations of GHGs in the atmosphere and, thus, the radiative forcing. As a result, the action that also temporarily causes emissions in the atmosphere (carbon neutral over time), accumulates energy in the climate system and is not, therefore, climate-neutral. The GHG balances of forests and the use of timber, for example, involve dynamics that disable climate neutrality.
Radiative forcing
Radiative forcing refers to the imbalance of energy caused by GHG emissions or removals in the climate system. The increased amount of GHGs reduces the amount of thermal radiation released from the atmosphere into space, causing warming or positive radiative forcing. Particulates, in turn, reduce the warming effect of the sun’s radiation, thus causing cooling or negative radiative forcing on the planet. Radiative forcing is measured by the amount of radiation power (watts) per square metre (W/m2).
Atmospheric CO2
Atmospheric CO2 refers to the amount of carbon in the atmosphere (carbon dioxide is the most significant human-produced GHG by a large margin). It is usually indicated with a ppm value, in other words one part per million (whereas ppb = one part per billion). The values also indicate the share of the particular gaseous molecules in the air. For example, a CO2 concentration of 300 ppm means that, out of one million air molecules, 300 are CO2 molecules. CO2 emissions are slow to exit the atmosphere, often taking decades, and some of the carbon can remain in the atmosphere for centuries.
Carbon neutrality
Carbon neutrality is a state of balance where the emissions and removals of CO2 or GHGs converted into CO2 equivalents resulting from an action are zero. The action can be constantly carbon neutral or carbon neutral over time. Carbon neutrality can be reviewed with the sum of absolute emissions and removals or those determined in proportion to a reference situation. The term GHG neutral is also used to refer carbon equivalent neutrality. Carbon neutrality has been defined in different ways, based on context. To avoid any misunderstandings, the term should always be specifically defined.
Climate neutrality
The actual meaning of climate neutrality is an action that constantly causes zero radiative forcing. Therefore, the action does not have an effect on the climate at any time. In many contexts, however, climate neutrality is used as a synonym for carbon or GHG neutrality. The Paris climate agreement and the European Commission, for example, refer to the balance of GHG emissions and removal with the term climate-neutral. To avoid any misunderstandings, the term should always be specifically defined.