Reply on RC1

Abstract

The global carbon budget (GCB) – including fluxes of CO2 between atmosphere, land and ocean, and its atmospheric growth rate – show large interannual to decadal variations. Reconstructing and predicting the variable GCB is essential for tracing the fate of carbon and understanding the global carbon cycle in the changing climate. We use a novel approach to reconstruct and predict the next-years’ variations in GCB based on our decadal prediction system enhanced with an interactive carbon cycle. By assimilating physical atmospheric and oceanic data products into the Max Planck Institute Earth system model (MPI-ESM), we can well reproduce the annual mean historical GCB variations from 1970–2018, with high correlations relative to the assessments from the Global Carbon Project of 0.75, 0.75 and 0.97 for atmospheric CO2 growth, air-land CO2 fluxes and air-sea CO2 fluxes, respectively. Such a fully coupled decadal prediction system, with an interactive carbon cycle enables representation of the GCB within a closed Earth system, and therefore provides an additional line of evidence for the ongoing assessments of the anthropogenic GCB. Retrospective predictions initialized from the assimilation simulation show high confidence in predicting the following year’s GCB. The predictive skill is up to 5 years for the air-sea CO2 fluxes, and 2 years for the air-land CO2 fluxes and atmospheric carbon growth rate. This is the first study investigating the GCB variations and predictions with an emission-driven prediction system, such a system also enables the reconstruction and prediction of the evolution of atmospheric CO2 concentration changes. The earth system predictions in this study provide valuable inputs for understanding the global carbon cycle and informing climate relevant policy.