Simulated responses of soil carbon to climate change in CMIP6 Earth system models: the role of false priming

Reliable estimates of soil carbon change are required to determine the carbon budgets consistent with the Paris Agreement climate targets. This study evaluates projections of soil carbon during the 21st century in Coupled Model Intercomparison Project Phase 6 (CMIP6) Earth system models (ESMs) under a range of atmospheric composition scenarios. In general, we find a reduced spread of changes in global soil carbon (Delta Cs) in CMIP6 compared to the previous CMIP5 model generation. However, similar reductions were not seen in the derived contributions to Delta Cs due to both increases in plant net primary productivity (NPP, named Delta Cs,NPP) and reductions in the effective soil carbon turnover time (tau s, named Delta Cs,tau). Instead, we find a strong relationship across the CMIP6 models between these NPP and tau s components of Delta Cs, with more positive values of Delta Cs,NPP being correlated with more negative values of Delta Cs,tau. We show that the concept of "false priming" is likely to be contributing to this emergent relationship, which leads to a decrease in the effective soil carbon turnover time as a direct result of NPP increase and occurs when the rate of increase in NPP is relatively fast compared to the slower timescales of a multi-pool soil carbon model. This finding suggests that the structure of soil carbon models within ESMs in CMIP6 has likely contributed towards the reduction in the overall model spread in future soil carbon projections since CMIP5.