Projected Land Carbon Sink Uncertainties Related to Water-Carbon Interactions

Abstract:

Land has been a sink for approximately 30% of anthropogenic carbon emissions to the atmosphere during recent years, although interannual variability can be substantial. Here we analyze future projections of terrestrial carbon fluxes from multiple Earth System Models with a focus on the evolution of the land carbon sink under increasing atmospheric carbon concentrations. The CMIP6 ensemble has a large inter-model spread of the land carbon sink based on the cumulative net biome production (NBP). We hypothesize that this uncertainty in projected NBP is related to model differences in (i) soil moisture conditions, (ii) the sensitivity of NBP to changes in soil moisture, and (iii) the fraction of evapotranspiration corresponding to transpiration. First results support our hypothesis. We find that models show contrasting projections of cumulative terrestrial NBP in the tropics, with some simulating a strong sink and others a net source of carbon from the land into the atmosphere. Those models projecting a loss of carbon from land also show a strong reduction of soil moisture in tropical regions. On the other hand, models with high global land carbon sink projections are characterized by high carbon uptake in the extratropics. Overall, this study highlights the influence of water-carbon interactions on the future evolution of the terrestrial carbon cycle. We suggest that efforts to constrain climate model projections should jointly constrain soil moisture and carbon fluxes.