The compact Earth system model OSCAR v4

OSCAR is an open-source reduced-complexity Earth system model designed to probabilistically emulate the coupled climate–carbon–chemistry system with low computational cost. Following a preivously published evaluation of OSCAR v3.1 against observations and CMIP6 Earth system models, we present OSCAR v4, which incorporates a range of structural, numerical, and methodological improvements. Key developments include enhanced numerical stability, modularization of the code to allow running submodels independently, revised and streamlined modules, and recalibration using the latest AR6, CMIP6, and TRENDY datasets. Monte Carlo sampling has been improved using continuous probability distributions, and the constraining strategy now leverages Latin-hypercube sampling combined with probability integral transforms to provide more robust probabilistic ensembles compatible with observations. Alongside core model improvements, OSCAR v4 will introduce a suite of user-oriented functionalities and a full online documentation, facilitating broader adoption and reproducibility.

We illustrate the performance of OSCAR v4 through participation in the Reduced Complexity Model Intercomparison Project (RCMIP) phase 3 exercise. This benchmarking demonstrates the model’s ability to reproduce the spread of global temperature and carbon-cycle responses observed in more complex Earth system models, while providing rapid, policy-relevant probabilistic projections. Given it's level of complexity, OSCAR v4 is positioned as a versatile tool bridging comprehensive Earth system models and the simpler reduced-complexity approaches for large-scale climate assessments.