Reduced Complexity Model Intercomparison Project Phase 3: Experimental protocol for coordinated constraining and evaluation of Reduced-Complexity Models

Abstract. Reduced-Complexity Models (RCMs) are a critical tool for synthesising climate science knowledge and providing climate projections for a wide range of emissions scenarios. The Reduced-Complexity Model Intercomparison Project (RCMIP) provides a framework for the coordinated evaluation and application of these models. Here, we introduce the experimental protocol for RCMIP Phase 3 (RCMIP3), which is timed to inform the upcoming seventh assessment cycle of the Intergovernmental Panel on Climate Change (IPCC AR7). Taking stock of lessons from previous phases, RCMIP3 builds on community climate assessment products to support a common framework to compare RCM output against historical climate benchmarks. The experimental design aims to support a comprehensive assessment of RCMs across multiple climate-relevant domains, with a particular focus on carbon cycle dynamics and climate reversibility. The protocol is designed in tandem with the Coupled Model Intercomparison Project Phase 7 (CMIP7), replicating its "Assessment Fast Track" together with complementary experiments which explore wider state dependencies, sampling multiple scenario generations, long timescale response and diverse emissions-driven process representation.