the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
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.
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Status: open (until 10 Jan 2025)
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AC1: 'Missing Figure 1', Benjamin Sanderson, 15 Nov 2024
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We regret that the first figure in the preprint is erroneously a duplicate of Figure 3. A high resolution version of the correct figure is attached to this comment.
Figure 1: Experiment design. a) and b) show annual and cumulative carbon emissions as a function of time for the four experiments. Panel c) shows global mean surface temperature derived from cumulative emissions, assuming a perfectly linear TCRE relationship, with expected temperature evolution assuming cumulative emissions proportionality using the IPCC AR6 WGI best TCRE estimate (solid line, 1.65°C per 1000 PgC) and likely range (shaded area, 1.0-2.3°C per 1000 PgC) (Intergovernmental Panel on Climate Change (IPCC), 2023b).
Data sets
Global mean output from flat10MIP simulations B. M. Sanderson https://doi.org/10.5281/zenodo.14012042
Model code and software
Model code to reproduce study B. M. Sanderson, N. Steinert, and C. D. Koven https://doi.org/10.5281/zenodo.14012042
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