Uncertainties in modelled CO₂  fluxes from land use and land use change

W. OBERMEIER, C. SCHWINGSHACKL, F. HAVERMANN & J. PONGRATZ

Land use and land-use changes (LULUCs) have greatly influenced the global carbon cycle, primarily through emissions from the conversion of natural forests to agricultural land. Additionally, LULUCs have diminished the potential natural land sink, as the cleared primary forests could have sequestered carbon due to favorable environmental changes (mainly via so-called CO₂ -fertilization). As in the Global Carbon Project’s global carbon budget, CO₂ fluxes from LULUCs can be estimated using models, such as dynamic global vegetation models (DGVMs) and bookkeeping models (BMs). We assess differences in these modelling approaches, from the global to the country-level, particularly focusing on the assumed environmental conditions. BMs do not consider environmental changes as they rely on observed, fixed carbon stocks (similar to inventory-based approaches). In contrast, transient LULUC flux estimates from DGVMs accumulate the loss of additional sink capacity, accounting for C stock changes that would have occurred in the absence of LULUCs that destroyed the natural land sink. We find that the loss of additional sink capacity accounts for ~40% of recent fluxes from LULUCs on the global scale. Additionally, LULUC flux estimates are highly uncertain in some countries, strongly dependent on the assumptions of the environmental conditions and the land use forcing data.