flat10MIP: An emissions-driven experiment to diagnose the climate response to positive, zero, and negative CO2 emissions

Abstract. The proportionality between global mean temperature and cumulative emissions of CO₂ predicted in Earth System Models (ESMs) is the foundation of carbon budgeting frameworks. Deviations from this behavior could impact estimates of required net zero timings and negative emissions requirements to meet the Paris Agreement climate targets. However, existing ESM diagnostic experiments do not allow for direct estimation of these deviations as a function of defined emissions pathways. Here we perform a set of climate model diagnostic experiments for the assessment of Transient Climate Response to cumulative CO₂ Emissions (TCRE), Zero Emissions Commitment (ZEC), and climate reversibility metrics in an emissions-driven framework. The emissions-driven experiments provide consistent independent variables simplifying simulation, analysis and interpretation with emissions rates more comparable to recent levels than existing protocols using model-specific compatible emissions from the CMIP DECK 1pctCO2 experiment, where emissions are strongly weighted towards the end of the experiment at significantly greater than present day values. A base experiment, ‘esm-flat10’, has constant emissions of CO₂ of 10 GtC per year (near-present day values), and initial results show that TCRE estimated in this experiment is about 0.1 K less than that obtained using 1pctCO2. A subset of ESMs exhibit land carbon sinks which saturate during this experiment. A branch experiment, esm-flat10-zec, measures ZEC, which we find is reduced by 25–50 % compared with 1pctCO2 branch experiments. A final experiment, esm-flat10-cdr, assesses climate reversibility under negative emissions, where we find that peak warming may occur before or after net zero and residual warming after removal of all greenhouse gases is well described by ZEC in most models and that current Simple Climate Model (SCM) distributions may be over-estimate temperature reversibility compared with ESMs. We propose a set of climate diagnostic indicators to quantify various aspects of climate reversibility. These experiments were suggested as potential candidates in CMIP7 and have since been adopted as “fast track” simulations.