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Source: Schimel et al. (1995).

Source: Schimel et al. (1995).

The concentration of CO2 is calculated in carbon cycle models on the basis of the mass balance as described in equation (45.1). The models consist of a well-mixed atmosphere linked to oceanic and terrestrial biospheric compartments. The oceanic component can be formulated as an upwelling-diffusion model (Siegenthaler and Joos 1992) or can be represented by a mathematical function (known as a convolution integral), which can be used to closely replicate the behavior of other oceanic models (Harvey 1989;Wigley 1991) such as in MAGICC (Wigley and Raper, 1992) and meta-IMAGE. The terrestrial component in both models is vertically differentiated into carbon reservoirs such as vegetation biomass, detritus, topsoil, deep soil and stable humus (Harvey 1989). For meta-IMAGE only, this component is also horizontally differentiated into eight land-use types: forests, grasslands, agriculture and other land for the developing and industrialized world (den Elzen 1998), which allows us to analyze the effect of land-use changes such as deforestation on the global carbon cycle.

To obtain a balanced past carbon budget in these models and therefore a good fit between the historical observed and simulated atmospheric CO2 concentration, it is essential to introduce additional terrestrial sinks. Meta-IMAGE uses the CO2fertilization feedback and the temperature feedback on net primary production and soil respiration. Although the N fertilization feedback was included in the earlier version of the metaIMAGE model (den Elzen et al. 1997), this feedback is, because of the consistency requirement with the IMAGE model, now excluded (den Elzen 1998). The parameteriza-tions of these feedbacks have been derived from experiments with the IMAGE 2.1 model.

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