### Carbon cycle

#### Indicators:

Carbon flux Airborne CO2 fraction Carbon flux for each land-cover type Biomass pools Net primary production (NPP) (graph) | Average Primary Production (graph) Average Net Ecosystem Production (graph) Annual net primary production (map) Soil respiration (graph) Soil respiration (map)

The carbon fluxes between the atmosphere, the biosphere and the oceans characterize the dynamics of the carbon cycle and define the changes in atmospheric CO2 concentrations. Several models are involved in calculating the net annual flux.

| #### Carbon flux
| unit: Pg C/yr (Petagram of C per year) dimension: carbon flux

The carbon fluxes include CO2 emissions from the global energy use/production and industrial processes to the atmosphere, and global fluxes between the biosphere and the oceans and the atmosphere. A positive value indicates a flux to the atmosphere (i.e. emissions). A negative value correponds to a flux from the atmosphere to some carbon pool (i.e., uptake by the biosphere and the oceans). The atmospheric increase or decrease is the sum of all C fluxes.

CO2 emissions from energy and industry are the sum of the CO2 emissions from energy use and those from industrial production (mainly cement). These emissions are computed by the TIMER emission model (TEM).

The land-use related carbon fluxes (caused by anthropogenic activities) are calculated by the terrestrial carbon model (TCM) and include:

- emissions caused by deforestation for agricultural expansion* and the timber industry
- emissions caused by the use of traditional biomass as a fuel
- emissions caused by decay of timber products (short-lived with lifetimes up to 10 year, e.g. paper and pulpwood; and long-lived with lifetimes up to 100 years, e.g. industrial roundwood for construction)
- uptake by regrowth of forests after timber extraction or abandonment of agricultural land (NEP of regrowing vegetation)*

The first three land use fluxes are combined as "deforestation" and the fourth flux is called "regrowing vegetation".

Fluxes from natural, full-grown vegetation (NEP of full-grown vegetation) are mainly driven by CO2 fertilization: increasing atmospheric CO2 concentration result in a net uptake by vegetation. This natural flux is calculated by TCM as well.

Carbon uptake by the oceans is the sum of the oceanic uptake of CO2 by dissolution and biological uptake by phytoplankton. These processes are simulated by the ocean carbon model (OCM).

* The simulated slowdown of CO2 emissions from deforestation and increase in carbon uptake by land is in line with the suggestion of the IPCC that a slowdown in deforestation may have taken place in the early 1990s. In the land-cover model abandoned grassland areas are replaced by the natural vegetation type (regrowing forest), which causes an increase in the carbon uptake. This aspect is also in line with the possibility that land has taken up more carbon during the 1990s than during the 1980s.

The reason for the slowdown of deforestation and increased carbon uptake simulated by the land-cover model (LCM) and terrestrial cabon model (TCM) is that the grassland areas show a decrease for some major countries in the early 1990s after a period of constant areas, or of constant increase or decrease. This observation is confirmed by other FAO reports which observe a general trend of gradual substitution of grazing-based livestock production by feed-crop-based production systems, which generally are less land-demanding.

| #### Airborne CO2 fraction
| unit: none (fraction) dimension: none

Airborne fraction is the atmospheric accumulation (i.e., the atmospheric CO2 increase compared to the pre-industrial levels) expressed as a fraction of the total cumulative anthropogenic CO2 emissions since pre-industrial times. Anthropogenic CO2 emissions include those from fossil fuel combustion, industrial production, and CO2 fluxes caused by land use changes (deforestation and regrowing vegetation).

The airborne fraction is the result of CO2 fluxes between the atmosphere, terrestrial biosphere and oceans. An increasing airborne fraction indicates an ongoing saturation of the terrestrial fertilization feedback (CO2 response of uptake according to a logarithmic function) and the oceanic uptake at higher atmospheric CO2 levels.

| #### Carbon fluxes for each land-cover type
| unit: Pg C/yr (Petagram of C per year) dimension: region, land-cover type, carbon flux

Carbon fluxes for land-cover types include those associated with natural vegetation and those related to changes in land use.

Carbon fluxes associated with undisturbed natural vegetation as computed by the terrestrial carbon model (TCM) are mainly driven by CO2 fertilization: increasing atmospheric CO2 concentration results in net C uptake.

The land-use related carbon fluxes include:

- emissions caused by deforestation for agricultural expansion and the timber industry
- emissions caused by the use of traditional biomass as a fuel
- emissions caused by decay of timber products (short-lived with lifetimes up to 10 year, e.g. paper and pulpwood; and long-lived with lifetimes up to 100 years, e.g. industrial roundwood for construction)
- uptake by regrowth of forests after timber extraction or abandoment of agricultural land

The first three land use fluxes are combined as "deforestation" and the fourth flux is called "regrowing vegetation". The net C flux represents the exchange of CO2 between biosphere and atmosphere (a negative value indicates an uptake of the biosphere), determined by the sum of deforestation and uptake by regrowing and full-grown vegetation.

| #### Biomass pools
| unit: Pg C (Petagram of C) dimension: region, land-cover type, biomass pool

Biomass is the total mass of living and dead plants. The three biomass pools in the carbon cycle model are: plants (living biomass in leaves, branches, stems and roots), soil (dead biomass in litter, humus and charcoal) and timber (harvested stems and branches).

Changes in biomass are explicitly calculated in the terrestrial carbon model (TCM) as the difference between carbon formation and outflow. The main driving force of biomass formation is net primary production (NPP), allocated anually to the plant tissues (leaves, branches, stems and roots) and then slowly shifts to the non-living biomass compartments, where it decomposes through soil respiration and returns as CO2 to the atmosphere. The allocation fractions as well as the turnover times are defined for each land-cover type and C compartment. NPP is a function of climate, soil, atmospheric CO2 concentration, altitude, land-cover type and land-cover history.

For timber, the sum of two pools are shown: short lived timber products (e.g., paper and pulpwood with lifetimes of up to 10 years) and long lived timber products (e.g., timber used for construction with lifetimes of up to 100 years, ).

| #### Net primary production (NPP) (graph)
| unit: Pg C/yr (Petagram of C per year) dimension: region, land-cover type

Net primary production (NPP, plant photosynthesis minus plant respiration) is modelled as a function of climate, soil, atmospheric CO2-concentration, altitude, land-cover and land-cover history. Based on pre-defined allocation fractions for each land-cover type, the NPP is allocated to four seperate carbon pools as distinguished: stems, branches, leaves, and roots.

The graph shows the total NPP for different regions and the world total, and for the land-cover types distinguished in IMAGE 2.4.

#### Average net primary production (NPP) (graph) |
unit: 100g C/m2/yr (ton C per hectare per year) dimension: region, land-cover type |

Average net primary production (NPP, plant photosynthesis minus plant respiration) is modelled as a function of climate, soil, atmospheric CO2-concentration, altitude, land-cover and land-cover history. Based on pre-defined allocation fractions for each land-cover type

The graph shows the area weighted average of NPP for different regions and the world total, and for the land-cover types distinguished in IMAGE 2.4.

#### Average net ecosystem production (NPP) (graph) |
unit: 100g C/m2/yr (ton C per hectare per year) dimension: region, land-cover type |

Average net ecosystem production (NEP) is modelled as a function of ....

The graph shows the area weighted average of NEP for different regions and the world total, and for the land-cover types distinguished in IMAGE 2.4.

| #### Annual net primary production (map)
| unit: Mg C/km2 (Megagram of C per square kilometre) dimension: annual NPP

Net primary production (NPP, plant photosynthesis minus plant respiration) is modelled as a function of climate, soil, atmospheric CO2-concentration, altitude, land-cover and land-cover history. Based on pre-defined allocation fractions for each land-cover type, the NPP is allocated to four seperate carbon pools as distinguished: stems, branches, leaves, and roots.

The map shows the spatial patterns of NPP for regions or the globe.

| #### Soil respiration flux (graph)
| unit: Pg C/yr (Petagram of C per year) dimension: soil biomass pool, region, land-cover type

Soil respiration is the flux of carbon to the atmosphere caused by soil organic matter decomposition. Three soil organic matter pools are distinguished:

- litter, consisiting of dead leaves, branches and stems with a short lifetime (1-3 years, depending on the land-cover type)
- humus, easily decomposable soil organic matter with a short lifetime (20-50 years, depending on the land-cover type)
- charcoal, or recalcitrant soil organic matter and charcoal with a long lifetime (500 years)

As litter and dead roots decay, part is transformed into soil humus (determined by the humification fraction), while another (major) part is oxidized to CO2 and lost to the atmosphere. This CO2 flux is shown as the soil respiration flux from litter.

The largest part of the soil humus pool is oxidized to CO2 and lost to the atmosphere, while a small fraction (the carbonization fraction) is transformed into charcoal. The humus oxidation flux is shown as the soil respiration flux from humus.

Charcoal is a major carbon pool in many land-cover types. Its respiration flux is therefore significant, despite its long lifetime.

When viewing the graph two important aspects should be considered:

- Large respiration fluxes occur during the first years after deforestation, because the NPP drops to zero and only slowly recovers, while soil respiration from litter and humus pools continue or increase due to the decomposition of dead roots, leaves and branches remaining after forest clearing (especially in the case of timber extraction)
- Acceleration of soil decomposition occurs under climate warming

| #### Annual soil respiration flux (map)
| unit: Mg C/km2 (Megagram of C per square kilometer) dimension: C respiration flux

Soil respiration is the C flux resulting from transformation of soil organic matter (litter, humus and charcoal). As litter and dead roots decay, part is transformed into soil humus, while another (major) part is oxidized to CO2 and lost to the atmosphere. An important part of the soil humus pool is also oxidized to CO2 and lost to the atmosphere, while a small fraction is transformed into charcoal. Charcoal is a major carbon pool in many land-cover types. Its respiration flux is therefore significant, despite its long lifetime.

When viewing the graph two important aspects should be considered:

- Large respiration fluxes occur during the first years after deforestation, because the NPP drops to zero and only slowly recovers, while soil respiration from litter and humus pools continue or increase due to the decomposition of dead roots, leaves and branches remaining after forest clearing (especially in the case of timber extraction)
- Acceleration of soil decomposition occurs under climate warming