The role of residence time in diagnostic models of global carbon storage capacity: Model decomposition based on a traceable scheme

As a key factor that determines carbon storage capacity, residence time (τ_E) is not well constrained in terrestrial biosphere models. This factor is recognized as an important source of model uncertainty. In this study, to understand how τ_E influences terrestrial carbon storage prediction in diagnostic models, we introduced a model decomposition scheme in the Boreal Ecosystem Productivity Simulator (BEPS) and then compared it with a prognostic model. The result showed that τ_E ranged from 32.7 to 158.2 years. The baseline residence time (τ′_E) was stable for each biome, ranging from 12 to 53.7 years for forest biomes and 4.2 to 5.3 years for non-forest biomes. The spatiotemporal variations in τ_E were mainly determined by the environmental scalar (ζ). By comparing models, we found that the BEPS uses a more detailed pool construction but rougher parameterization for carbon allocation and decomposition. With respect to ζ comparison, the global difference in the temperature scalar (ζ_t) averaged 0.045, whereas the moisture scalar (ζ_w) had a much larger variation, with an average of 0.312. We propose that further evaluations and improvements in τ′_E and ζ_w predictions are essential to reduce the uncertainties in predicting carbon storage by the BEPS and similar diagnostic models.