The Coupled Climate-Carbon Cycle Model Intercomparison Project (C4MIP) experimental protocol for CMIP7

Abstract. Model intercomparison projects (MIPs) have become an essential tool for systematically comparing Earth system models (ESMs) by identifying, characterizing, and quantifying common model behaviour through standardized experiments and metrics under idealized, historical, and scenario-based forcing. Over the past few decades, community MIPs have become a cornerstone of the broader climate modelling community and the Coupled Model Intercomparison Project (CMIP). Here, we describe the scientific objectives of the Coupled Climate-Carbon Cycle Model Intercomparison Project (C4MIP) in the context of the seventh phase of CMIP (CMIP7) and present the complete suite of C4MIP experiments, including those contributing to CMIP7's Assessment Fast Track (AFT) and those proposed for subsequent analyses beyond the AFT. The proposed experimental protocol maintains backward compatibility with previous CMIP phases while introducing new experiments. In addition to the traditional biogeochemically (BGC) and radiatively (RAD) coupled versions of the 1pctCO2 experiment, the new experiments include concentration-driven BGC and RAD variants of the abrupt 2×CO2 experiment, a suite of emissions-driven simulations that investigate carbon-cycle behaviour under positive, zero, and negative emissions, and a large pulse experiment to reassess the impulse-response of newer generation ESMs and update the widely-used function from earlier model generations. Whereas earlier phases of C4MIP primarily focused on quantifying the feedback metrics that describe interactions between the coupled carbon cycle and the physical climate system, the scope of C4MIP has expanded to include the estimation of the transient climate response to cumulative emissions (TCRE) and the zero-emissions commitment (ZEC). These policy-relevant metrics are most naturally quantified using emissions-driven ESM simulations. The increased emphasis on emissions-driven simulations reflects both the growing policy relevance of these metrics and the ability of emissions-driven simulations to represent carbon-climate interactions in a bi-directional manner. In particular, changes in carbon sinks and evolution of feedback effects influence the atmospheric CO₂ concentration in emission-driven simulations, whereas these interactions are absent from concentration-driven simulations, in which atmospheric CO₂ concentrations are prescribed. The proposed experimental design attempts to balance scientific ambition with the computational and storage resources required of participating climate modelling centres. The final suite of experiments and associated protocols has been developed through extensive community consultation and guidance from the C4MIP Scientific Steering Committee. As in previous CMIP phases, the experiments are expected to enable a broad range of community-led investigations of the coupled carbon-climate system.