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Abstract.In the following, we test the validity of a one-box climate model as an emulator for atmosphere–ocean general circulation models (AOGCMs). The one-box climate model is currently employed in the integratedassessment models FUND, MIND, and PAGE, widely used in policy making. Our findings are twofold. Firstly,when directly prescribing AOGCMs’ respective equilibrium climate sensitivities (ECSs) and transient climateresponses (TCRs) to the one-box model, global mean temperature (GMT) projections are generically too highby 0.5 K at peak temperature for peak-and-decline forcing scenarios, resulting in a maximum global warmingof approximately 2 K. Accordingly, corresponding integrated assessment studies might tend to overestimatemitigation needs and costs. We semi-analytically explain this discrepancy as resulting from the informationloss resulting from the reduction of complexity. Secondly, the one-box model offers a good emulator of theseAOGCMs (accurate to within 0.1 K for Representative Concentration Pathways, RCPs, namely RCP2.6, RCP4.5,and RCP6.0), provided the AOGCM’s ECS and TCR values are universally mapped onto effective one-boxcounterparts and a certain time horizon (on the order of the time to peak radiative forcing) is not exceeded.Results that are based on the one-box model and have already been published are still just as informative asintended by their respective authors; however, they should be reinterpreted as being influenced by a larger climateresponse to forcing than intended.
