Climate modelers have increased the equilibrium climate sensitivity (ECS) of their models for the IPCC next report. Eight models report ECS greater than 5 °C. In contrast, the ECS estimated by using the measured temperature record, when accounting for the natural and urban warming, indicates an ECS of about 1.0 °C. This study tests the Community Earth System Model version 2 (CESM2) for the Last Glacial Maximum (LGM) to examine whether its high ECS is realistic. The simulated LGM global mean temperature decrease exceeds 11°C, greater than both the cooling estimated from proxies and simulated by an earlier model version. The large discrepancy between the LGM temperature in the model and proxies is attributed to strong shortwave cloud feedbacks in the model. The abstract says “We find that the amount of cooling [during the LGM] in the CESM2 simulation is much larger than supported by the observational evidence, indicating the model's ECS is too large.”

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https://friendsofscience.org/assets/documents/Climate_Sensitivity_Energy_Balance_Gregory-2020v2.pdf
https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020GL091220

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