The model only values changes in carbon storage (emission/sequestration) over time. This is due to: 1) the way social cost of carbon methods and carbon markets value carbon; 2) estimates of the value of non-marginal changes to carbon storage generally become more implausible as they get bigger, as with all ecosystem service values (Bockstael et al., 2000)
If you insist on valuing the total carbon stock, I believe the model can accommodate if you simulate future landscapes as having no carbon storage potential.