The Terrestrial Carbon Model (TCM) simulates the carbon (C) flux between the atmosphere and biosphere as altered by atmospheric CO2 concentrations, climate change and different land cover conversions. The spatial resolution of the calculations is 0.5 degree latitude by 0.5 degree longitude, as in most calculations in the Terrestrial Environment System (TES). Previous versions of the model are described in detail by Klein Goldewijk et al.(1995) and Alcamo et al. (1998). The basic structure of the terrestrial carbon model in IMAGE version 2.2 has not changed in comparison with earlier versions of IMAGE (2.0 and 2.1). An important improvement is that the calibration of the full carbon cycle starts in 1765. Historical anthropogenic CO2 emissions (Marland et al., 2000) and the historical trend of oceanic C uptake (Joos et al., 1996) are used (1765-1970) to obtain a terrestrial uptake in TCM that leads to a CO2 concentration of 325 ppmv in 1970.
| Model input | Historical and updated land cover map (LCM) |
| Potential vegetation (TVM) | |
| CO2 concentration (ACM) | |
| Temperature map (GPS) | |
| Moisture availability (TVM) | |
| Assumptions | Soil fertility and altitude |
| Model output | Net Primary Production (NPP) and Soil respiration fluxes |
| Net Ecosystem Productivity (NEP) and Land use emissions of CO2 |
Each cell is characterized by its monthly climate (temperature and soil moisture), soil, and natural or agricultural land cover. Biomass within the cells is divided into different compartments as follows:
The terrestrial carbon model is driven by Net Primary Productivity (NPP, plant photosynthesis minus plant respiration), a function of climate, soil, atmospheric CO2 concentration, altitude, land-cover type and land-cover history. NPP is allocated over the living biomass compartments, and then slowly shifts to the non-living biomass compartments, where it is decomposed and returns as CO2 to the atmosphere. The allocation fractions and turnover times are defined for each land-cover type and C compartment.
Soil respiration is the C flux to the atmosphere resulting from the transformation of soil organic matter (litter, humus and charcoal). During decay of litter and dead roots, 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.
The net C flux between the atmosphere and biosphere is called 'net ecosystem productivity' (NEP) and is equal to the NPP minus soil respiration. Negative values indicate a net release of CO2, while positive values indicate a net uptake of CO2 by the biosphere. NEP is influenced by land cover (changes) and soil and climatic factors.
Climate feedbacks (effect of temperature and CO2 fertilization on plant growth, and effect of climate on soil respiration) are calculated at monthly intervals by using different response functions dependent of temperature, soil water and species characteristics for each grid cell (Klein Goldewijk et al.,1995). These monthly values are aggregated to annual totals for use in C-balance calculations (using the approach of Melillo et al., 1993). However, the monthly resolution allows different plant responses in different seasons to be taken into account. This temporal resolution is also consistent with the downscaling of monthly climate-change patterns in the atmosphere-ocean system.
With 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:
The 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:
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.
In summary, with the capacities described above, the terrestrial carbon model can:
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