Assessment of Equilibrium Climate Sensitivity of the Community Earth System Model Version 2 Through Simulation of the Last Glacial Maximum
Zhu, Jiang; Otto‐bliesner, Bette L.; Brady, Esther C.; Poulsen, Christopher J.; Tierney, Jessica E.; Lofverstrom, Marcus; DiNezio, Pedro
2021-02
Citation
Zhu, Jiang; Otto‐bliesner, Bette L. ; Brady, Esther C.; Poulsen, Christopher J.; Tierney, Jessica E.; Lofverstrom, Marcus; DiNezio, Pedro (2021). "Assessment of Equilibrium Climate Sensitivity of the Community Earth System Model Version 2 Through Simulation of the Last Glacial Maximum." Geophysical Research Letters 48(3): n/a-n/a.
Abstract
The upper end of the equilibrium climate sensitivity (ECS) has increased substantially in the latest Coupled Model Intercomparison Projects phase 6 with eight models (as of this writing) reporting an ECS > 5°C. The Community Earth System Model version 2 (CESM2) is one such high- ECS model. Here we perform paleoclimate simulations of the Last Glacial Maximum (LGM) using CESM2 to examine whether its high ECS is realistic. We find that 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 (CESM1). The large LGM cooling in CESM2 is attributed to a strong shortwave cloud feedback in the newest atmosphere model. Our results indicate that the high ECS of CESM2 is incompatible with LGM constraints and that the projected future warming in CESM2, and models with a similarly high ECS, is thus likely too large.Plain Language SummaryEquilibrium climate sensitivity (ECS) is one of the most important metrics in climate science. It measures the amount of global warming over hundreds of years after a doubling of the atmospheric CO2 concentration. An ECS range of 1.5°C- 4.5°C has been consistently supported by climate models over the past 40 years. However, this has changed in the latest generation of climate models with eight (as of this writing) showing an ECS > 5°C. Such a high ECS implies that future warming will be much greater than previously thought for the same amount of greenhouse gas emissions, making it more challenging for human and natural systems to adapt. This study examines whether the ECS in one - high- ECS- model- Community Earth System Model version 2 (CESM2)- is realistic. Our approach is to perform a paleoclimate simulation of the culmination of the last glacial period, which was colder and had a lower atmospheric CO2 level than today. We find that the magnitude of cooling in the CESM2 simulation is much larger than supported by the observational evidence, indicating the model’s ECS is too large. We further find that the high ECS of CESM2 is caused by the treatment of clouds in the model.Key PointsCommunity Earth System Model version 2 simulates an Last Glacial Maximum (LGM) global temperature at least 5°C colder than the proxy estimate, indicating its equilibrium climate sensitivity is too highThe large LGM cooling is caused by a strong shortwave cloud feedback in the new atmosphere modelThe shortwave cloud feedback in LGM simulations is connected to that in abrupt 4à CO2 simulations from the present- day climatePublisher
Wiley Periodicals, Inc. Cambridge University Press
ISSN
0094-8276 1944-8007
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