A new process-based carbon cycle for the FaIR simple climate model

Simple Climate Models (SCMs) provide an efficient way to explore potential climate futures by quickly evaluating emissions and mitigation scenarios.  This efficiency enables applications beyond the capabilities of complex Earth System Models (ESMs), such as integration with integrated assessment models and reactive policy analysis. A prominent example of this type of models is the FaIR SCM, which has gained popularity in recent years and been applied in various contexts. However, the current implementation of FaIR’s carbon cycle lacks detail, as it does not resolve the carbon fluxes between different ecosystem components. This limitation reduces the model’s flexibility and prevents it from participating in carbon-focused research.

Here, we present a new simple carbon cycle model that simulates the evolution of the global carbon stocks and fluxes across the atmosphere, ocean, soil and vegetation pools. The model calibration used data from 13 ESMs participating in the 6^th Coupled Model Intercomparison Project (CMIP6), including all model simulations for the Shared Socioeconomic Pathways (SSP) scenarios. We evaluate the model’s performance in emulating ESM carbon cycles and discuss the integration with the FaIR SCM. By using the calibrations to CMIP6 ESMs and sampling the uncertainty parameters in our carbon cycle model, we can obtain posterior sets that compare well with best available observations, such as the growth in land, ocean and atmospheric stocks from the annual Global Carbon Budget. This enhancement to FaIR to include a process-based carbon cycle significantly strengthens its carbon cycle capabilities, unlocking new research opportunities.