The Oceanic Carbon Model (OCM) of IMAGE 2.2 simulates the carbon (C) flux between the atmosphere and ocean with the Bern Carbon Cycle model (Bern-CC) as described by Joos et al. (1996). This model is a box-diffusion type oceanic carbon model. The model is based on a mixed-layer-pulse-response function, which allows for describing time-dependent non-linear effects of seawater chemistry resulting from changes in the atmospheric CO2 concentration. This non-linearity occurs in the chemical equilibria involving the buffering system formed by undissociated CO2 and HCO3- and CO32- ions. The analytical representation of the mixed layer response function of the Princeton 3-D model (Joos et al., 1996) is used In IMAGE 2.2.
The model input and output variables of the OCM are listed below:
| Model input | CO2 emissions from energy and industrial sources (TEM) |
| CO2 emissions from land-use change (TCM) | |
| CO2 uptake by full-grown forest (TCM) | |
| Global mean surface temperature change (UDCM) | |
| Model output | Oceanic CO2 uptake |
| Atmospheric CO2 concentration |
The atmospheric CO2 increase is calculated as follows:
with:
Na = atmospheric increase
Efossil = Energy and industrial emissions
Eland-use = Land-use emissions (including terrestrial uptake by regrowing vegetation)
NEPt = Terrestrial uptake by full-grown vegetation
No = Oceanic uptake.
In the historical period (1765-1970) the model results are validated using data on atmospheric CO2 concentration and the analysis of C fluxes based on Bern-CC presented in the Third Assessment Report of IPCC (IPCC, 2001).
The OCM computes the air-sea flux based on the CO2 concentration in the atmosphere and the surface ocean. Initially (1765) these partial pressures were assumed to be equal (278 ppmv). The perturbation of dissolved inorganic carbon is calculated in ten time steps each year, and used, in turn, to calculate the new partial pressure of CO2 in the ocean mixed layer (upper layer).
A positive feedback loop is caused by temperature increases of the ocean (Joos et al., 1999). Higher temperatures influence the chemical CO2 buffering system which can result in reduced carbon transport from the mixed layer to deeper layers. This may lead to a reduction of oceanic carbon uptake and an increase of the atmospheric CO2 growth. This feedback is taken into account in the OCM by correcting the calculated partial pressure of CO2 in the ocean mixed layer with an exponential function using the temperature increase calculated in the Climate Model of IMAGE 2.2 calculated for this layer.
The calculated partial pressure of CO2 in the surface layer results in the air-sea CO2 flux. After ten time steps covering one year, the carbon flux budget can be made up and the atmospheric CO2 concentration of CO2 can be derived.
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