Uncertainties in global terrestrial biosphere modeling, part II: Global constraints for a process-based vegetation model

The terrestrial biosphere is one of several key components of the global ca rbon cycle. Because the mechanisms by which climate determines terrestrial biosphere carbon fluxes are not well understood, significant uncertainties concerning model results exist even for the current state of the system, wi th important consequences for our ability to predict changes under future c limate change scenarios. We assess how far this uncertainty can be reduced by constraining a global mechanistic model of vegetation activity, either w ith global satellite-derived vegetation index data or with measurements of the seasonal CO2 cycle in the atmosphere. We first show how constraining th e model with satellite data from the National Oceanic and Atmospheric Admin istration advanced very high resolution radiometer reduces the sensitivity to estimated uncertainties in model parameters, and thus the estimated erro r range of net primary productivity. Regionally, the satellite data deliver the largest constraint for vegetation activity in boreal and arctic as wel l as in tropical water-limited environments. In a second analysis through a n atmospheric tracer transport model, we check the consistency of those res ults with the measured seasonal cycle of CO2 at various remote monitoring s ites. While before including the satellite data into model calculations, so me simulations within the error range lead to a CO2 seasonal cycle outside the observations, there is a good agreement with the additional constraint. The conclusion is that the constraint delivered by the satellite data is a t least as significant as that delivered by atmospheric CO2 measurements. W e also show that the CO2 data mainly reflect the activity of northern veget ation, in particular conifers and C3 grasses. This suggests that satellite measurements provide the most useful global data currently available for ch ecking and improving terrestrial vegetation models and that consistency wit h CO2 measurements is a necessary but not a sufficient requirement for thei r realism.