Land Use, Land Use Change, and Forestry🔗
C-ROADS endogenously calculates the land use, land use change, and forestry (LULUCF) net C emissions by explicitly keeping track of each hectare of different land types; the fluxes of changing land types and the use of each land type due to land and energy demands and policies; and the coflow of carbon on the land. The land area is disaggregated by land type, denoted with a "u" suffix, and by region, denoted by an "r" suffix. Variables reflecting a change from one type of land to another type for a given region has the suffix "uur".
The terrestrial biosphere carbon (TBC) cycle accounts for these anthropogenic carbon emissions as well as natural emissions from biomass and soil respiration and releases as CH4, accounted in the CH4 cycle, and primary productivity of each land type.
The TBC cycle reflects that cutting down trees releases carbon, and stops the them from absorbing CO2 from the atmosphere. While harvesting crops also releases carbon, the approximately annual or faster regrowth time allows the related carbon release to be considered net zero.
C-ROADS models different kinds of land that can be converted into the others, and the biomass and soil carbon on the land that can build up or be released. We have four different land uses: Forest, Agriculture, Other, Tundra; with Forest further divided into three cohorts (Young, 0-50 years; Medium, 50-100 year; and Mature, 100+ years) and whether or not it resulted from afforestation (9 total land uses).
Each type of land has carbon flows:
- From the atmosphere to biomass (primary production through photosynthesis)
- From biomass to soil (decomposition et cetera)
- From soil and biomass to the atmosphere (respiration, decay, burning)
- When land use changes, some of the carbon stays on the land and some is released to the atmosphere
We cut trees or remove biomass for two reasons: we want the material or we want the land (or both). The material is involved in concepts like bioenergy, wood products, and forest degradation. Needing the land means concepts like deforestation, afforestation, land use change, and agriculture. Those are the policies and scenarios where you can intervene in En-ROADS with each area described below.
Drivers of Deforestation and Degradation🔗
Land that is converted from forest becomes either Farmland (driven by needs of the food system and bioenergy) or Other Land (non-farm deforestation). With six subcategories of forest (NonAF/AF, Young/Medium/Mature), the model assumes that the fraction of deforestation to farmland and to other is proportional to the land area of each to the total forest land.
The primary driver of deforestation has historically been to expand farmland to meet food demands and by the fraction of farmland expansion that comes from forest. Farmland needs that cannot come from forest comes from Other Land. Farm conversion from other land (mostly grasslands and scrub, but also deserts, barren, urban, etc) has less effect on the carbon cycle than does deforestation. The fraction of farmland expansion that comes from forest is fixed (at 0.6) in the base case based on historical land use changes.
Deforestation to other land reflects forest clearing for development and mining.
A data model uses the LUH land area data and projections from SSP2, aggregated to 20 regions, to determines the land areas changes per year. These land area changes are aggregated to the 7 regions used in C-ROADS. The resulting fluxes are set as default land changes in the model.
Forests are also harvested for bioenergy and for non-fuel wood (lumber, paper, et cetera), and allowed to regrow. Both fuel and non-fuel wood demand for each region are based on history and projections are based on continuing the per capita demand times the population. The regrowing process can remove carbon from the atmosphere and is therefore often considered carbon-neutral. However, it can take decades to repay the carbon debt incurred with forest harvesting. All forests can be harvested for bioenergy or for wood products. The proportion of total harvest from each forest category is a function of available carbon on each category relative to the total carbon on all forests.
The policy to prevent deforestation reduces both deforestation and degradation from the mature forest cohort.
Other Land Decreases and Increases🔗
Afforestation policy, i.e. the action depending on the Afforestation slider of En-ROADS, is implemented as the conversion of other land to forest land, since the land identified to be available for afforestation , excludes existing forests and agricultural land and falls into the other land category. Afforestation, as a policy implementation, is formulated based on a user-defined fraction of the full potential of afforestable land, and its delayed conversion to afforested land, which results in the land flux of Land afforestation rate. This flux is then incorporated into the land use change module as a chain of conversions from the other land to young forests and then aging to medium and mature forests. Deforestation from afforested land to farmland and other land affects the efficacy of this policy. The model captures historic regrowth of other land to nonAF young forest. Other land also decreases with farmland expansion, as only a fraction of the expansion comes from forests.