ORCHIDEE_3_r7267

Earth System Models (ESM) represent the time evolution of the biophysical (energy, water cycles) and biogeochemical (carbon cycle) components of the Earth. When used for near-future projections in the context of the Coupled Model Intercomparison Project (CMIP), they use as forcings the evolution of greenhouse gas and other pollutant concentrations and land-use changes simulated by an ensemble of Integrated Assessment Models (IAMs) for a combination of socio-economic pathways and mitigation targets (SSPs). More precisely, only one IAM output is used as representative of a single SSP while the inter-IAM spread is large for ammonia emissions and land-use changes, for instance. This makes the comparison of key ESM diagnostics among SSPs significantly noisy, without the capacity of disentangling SSP-driven and IAM-driven factors. In this paper, we quantify the projected change in land carbon store (CLCS) for the different SSPs with an advanced version of a land surface model embedded into IPSL-CM6 ESM. Through a set of land-only factorial simulations, we specifically aim at estimating the CLCS uncertainties associated with land-use change and nitrogen deposition trajectories. We showed that the spread of the simulated change in global land carbon store induced by the uncertainty on land-use changes is slightly larger than the one associated with the uncertainty on atmospheric CO2. Globally, uncertainty associated with N depositions is responsible for a spread in CLCS lower by a factor three, than the one driven by atmospheric CO2 or land-use changes. Our study calls for making available additional IAM scenarios for each SSP to be used in the next CMIP exercise, in order to specifically assess the IAM-related uncertainty impacts on the carbon cycle and the climate system.