Soil carbon in CMIP6 Earth System Models

The response of soil carbon represents one of the key uncertainties in future climate change due to competing soil carbon driven feedbacks. The ability of Earth System Models (ESMs) to simulate both present day and future soil carbon is therefore vital for reliably estimating global carbon budgets required for Paris agreement targets. In this presentation, the simulation of both present day and future soil carbon is investigated within CMIP6 ESMs.

The ability of CMIP6 ESMs to simulate present day soil carbon is evaluated against empirical datasets, where a lack of consistency in modelled soil carbon remains from the previous generation of models (CMIP5). This underestimation is particularly dominant in the northern high latitude soil carbon stocks. The results suggest much of the uncertainty associated with modelled soil carbon stocks can be attributed to the simulation of below ground processes, and greater emphasis is required on improving the representation of below ground soil processes in future developments of models.

Projections of soil carbon during the 21^st century are also evaluated to quantify future soil carbon changes in CMIP6 ESMs and to assess the uncertainty of the soil carbon induced feedback to climate change. The response of soil carbon is broken down into changes due to increases in Net Primary Productivity (NPP) and reductions in soil carbon turnover time (τ_s), with the aim of isolating the differing responses which influence changes in future soil carbon storage. A reduction in the spread of soil carbon projections is identified in CMIP6 compared to CMIP5. However, similar reductions are not seen in the components due respectively to changes in NPP and τ_s. The relationship between the induced soil carbon changes due to NPP and τ_s is investigated and their overall effect on the future soil carbon response is presented.