Projected changes in land carbon store over the 21st century: what contributions from land use change and atmospheric nitrogen deposition?

Earth system models (ESMs) represent the time evolution of the biophysical (energy and 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 socioeconomic pathways and mitigation targets (SSPs). More precisely, only one IAM output is used as representative of a single SSP. This makes the comparison of key ESM diagnostics among SSPs significantly noisy and 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 dispersions 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 in the land use changes is slightly larger than the one associated with the uncertainty in the atmospheric CO2. Globally, the uncertainty associated with N depositions is responsible for a spread in CLCS that is lower by a factor of 3 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.