Evaluating the Land and Ocean Components of the Global Carbon Cycle in the CMIP5 Earth System Models

The authors assess the ability of 18 Earth system models to simulate the land and ocean carbon cycle for the presentclimate. These modelswill be used in the next Intergovernmental Panel onClimate Change(IPCC)Fifth Assessment Report (AR5) for climateprojections, and such evaluationallows identificationof thestrengths andweaknessesofindividualcoupledcarbon‐climatemodelsaswellasidentificationofsystematicbiasesof themodels. Resultsshowthat models correctlyreproducethemain climatic variablescontrolling thespatial and temporal characteristics of the carbon cycle. The seasonal evolution of the variables under examination is well captured. However, weaknesses appear when reproducing specific fields: in particular, considering the land carbon cycle, a general overestimation of photosynthesis and leaf area index is found for most of the models, while the ocean evaluation shows that quite a few models underestimate the primary production.The authors also propose climate and carbon cycle performance metrics in order to assess whether there is a set of consistently better models for reproducing the carbon cycle. Averaged seasonal cycles and probability density functions (PDFs) calculated from model simulations are compared with the corresponding seasonal cycles and PDFs from different observed datasets. Although the metrics used in this study allow identificationofsomemodels as betterorworsethantheaverage, therankingofthisstudyispartiallysubjective because of the choice of the variables under examination and also can be sensitive to the choice of reference data. In addition, it was found that the model performances show significant regional variations.