A perturbed parameter ensemble for the coupled Earth system model NorESM3

Quantifying parametric uncertainty is important for effective Earth system model development and calibration. Here, we present a 75-member perturbed parameter ensemble (PPE) with the coupled Earth system model NorESM3. As part of operational model calibration, we used the PPE to evaluate parameter sensitivities, identify bias‑reducing parameter choices and trade‑offs, and estimate parametric uncertainty within the model. The PPE was constructed by branching preindustrial (year 1850) simulations from a baseline run, perturbing parameters in the atmosphere, land, ocean, and sea ice modules within expert-informed ranges using Latin hypercube sampling. To study long-term behaviour, we continued a subset of members that met energy-balance criteria while preserving ensemble spread. We evaluated ensemble members against observational and reanalysis datasets to help distinguish biases responsive to parameter tuning from ones likely stemming from model structure.