Carbon, Nitrogen and Water Cycle
In the carbon, nitrogen and water cycle component, IMAGE 2.4 includes models for describing the global carbon cycle (Terrestrial Carbon Model) and the global nitrogen and phosphate cycle.
Terrestrial Carbon Model, land-cover conversionsConversion typesWith regard to the impact of land cover on NPP and NEP, the Terrestrial Carbon Model distinguishes four major land-cover conversions:
Since the terrestrial carbon model keeps track, in time and space, of all major pools and fluxes of carbon in the terrestrial environment, it also consistently handles the effects of land-cover conversions on the global carbon cycle. These effects may be considerable. For example, different transformations occur after conversion of natural vegetation to agricultural land or regrowth forest:
Effect of land-cover conversionsThe conversion from one natural land-cover type to another alters NPP and NEP. The processes involved are strongly influenced by the rate of climate change and the possibility of natural land-cover types to adapt to new conditions. The procedure for calculating this transient response for grid cells is as follows: Compute potential natural vegetation for each cell with the Natural Vegetation Model (NVM). A potential migration zone based on the current vegetation distribution is calculated using vegetation-specific maximum migration distances and rates. If potential vegetation is changed with respect to earlier time steps, the possibility of grid-cell occurrence within the potential migration zone is evaluated. If this happens, the cell starts to convert towards the new vegetation type (next step). If not, the old vegetation type remains, and it is assumed that its NPP cannot increase. If a cell begins to convert to another type, NPP also changes over the assigned transition period. These periods are assumed to be short when plant types disappear from a grid cell, but long when new plant types are introduced. The assumed migration distances and rates are based on individual plant types occurring in each vegetation class. An important assumption, for example, is that grasses grow fast and are widely dispersed, therefore migrating rapidly over long distances. Trees, on the other hand, grow and disperse more slowly, and have smaller migration potentials. The resulting vegetation shifts are therefore a function of distance and growth rates of vegetation types. SummaryIn summary, with the capacities described above, the Natural Carbon Model can:
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