The relevance of soil moisture for land carbon sink projections

During the last century the land biosphere has been a sink of anthropogenic carbon emissions to the atmosphere. Here we analyze future projections of terrestrial carbon fluxes from multiple Earth system models participating in the sixth phase of the Coupled Model Intercomparison (CMIP6) with a focus on the evolution of the land carbon sink under increasing atmospheric carbon concentrations and associated climate change. We find that interannual variability in the land carbon sink given by the net biome production (NBP) is dominantly related to variability in soil moisture across most models and regions. Nevertheless, several models indicate that temperature variations are more strongly related to NBP variations in the core of the Amazon. Model trends in NBP are relatively well explained by trends in both soil moisture and temperature. Finally, the CMIP6 ensemble has a large inter-model spread of the average land carbon sink projected for the end of the century (2071–2100). We show that inter-model differences in soil moisture conditions and in the sensitivity of NBP to soil moisture contribute to explain this spread, particularly in boreal forest regions. Overall, our study highlights the influence of water-carbon interactions on the future evolution of the terrestrial carbon cycle. We suggest that efforts to constrain Earth system model projections should jointly constrain soil moisture and carbon fluxes.