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. Idealized climate change simulations are used as benchmark experiments to facilitate thecomparison of ensembles of climate models. In the fifth phase of the Coupled ModelIntercomparison Project (CMIP5), the 1 % per yearly compounded change in atmospheric CO2 concentration experiment was used to compare Earth system models with fullrepresentations of the global carbon cycle in the Coupled Climate–Carbon Cycle ModelIntercomparison Project (C 4 MIP). However, this “1 % experiment” was neverintended for such a purpose and implies a rise in atmospheric CO2 concentrationat double the rate of the instrumental record. Here, we examine this choice by using anintermediate complexity climate model to compare the 1 % experiment to an idealized CO2 pathway derived from a logistic function. The comparison shows three keydifferences in model output when forcing the model with the logistic experiment. (1) Themodel forced with the logistic experiment exhibits a transition of the land biospherefrom a carbon sink to a carbon source, a feature absent when forcing the model with the1 % experiment. (2) The ocean uptake of carbon comes to dominate the carbon cycle asemissions decline, a feature that cannot be captured when forcing a model with the1 % experiment, as emissions always increase in that experiment. (3) The permafrostcarbon feedback to climate change under the 1 % experiment forcing is less than halfthe strength of the feedback seen under logistic experiment forcing. Using the logisticexperiment also allows smooth transition to zero or negative emissions states, allowingthese states to be examined without sharp discontinuities in CO2 emissions. Theprotocol for the CMIP6 iteration of C 4 MIP again sets the 1 % experiment as thebenchmark experiment for model intercomparison; however, clever use of the Tier 2experiments may alleviate some of the limitations outlined here. Given the limitations ofthe 1 % experiment as the benchmark experiment for carbon cycle intercomparisons,adding a logistic or similar idealized experiment to the protocol of the CMIP7 iterationof C 4 MIP is recommended.

geoscientific model development

Limitations of the 1 % experiment as the benchmark idealized experiment for carbon cycle intercomparison in C&amp;lt;sup&amp;gt;4&amp;lt;/sup&amp;gt;MIP