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Increasing Earth System Sensitivity in mid-Pliocene simulations from CCSM4 to CESM2
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  • Ran Feng,
  • Otto-Bliesner Bette L.,
  • Esther C. Brady,
  • Nan A. Rosenbloom
Ran Feng
University of Connecticut

Corresponding Author:[email protected]

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Otto-Bliesner Bette L.
Climate and Global Dynamics Division, National Center for Atmospheric Research (NCAR), Boulder, CO 80305, USA
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Esther C. Brady
National Center for Atmospheric Research (UCAR)
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Nan A. Rosenbloom
National Center for Atmospheric Research (UCAR)
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Three new equilibrium Mid-Pliocene (MP) simulations are implemented with the Community Climate System Model version 4 (CCSM4), Community Earth System Model version 1.2 (CESM1.2), and 2 (CESM2). All simulations are carried out with the same boundary and forcing conditions following the protocol of Pliocene Model Intercomparison Project Phase 2. These simulations reveal amplified MP climate change relative to preindustrial going from CCSM4 to CESM2, seen in global mean and polar amplification of surface warming, sea ice reduction in both Arctic and Antarctic, and weakened Hadley circulation. The enhanced global mean warming arises from both enhanced Earth System Sensitivity (ESS) and Equilibrium Climate Sensitivity (ECS) to CO forcing. ESS is amplified by up to 70% in CCSM4, and up to 100% in CESM1.2 and CESM2 relative to ECSs of respective models. Simulations also agree on the strengthened Atlantic Meridional Overturning Circulation, but disagree on several other climate metrics. Compared to preindustrial, CCSM4 features small increase in both low and high cloud cover and no change in the mean climate state of the equatorial Pacific. Whereas, both CESM1.2 and 2 show reduction of cloud cover at all heights, and an anomalous El Niño-like state of the equatorial Pacific. The performances of MP simulations are assessed with a new compilation of paleo-observations of sea surface temperature (SST). CESM1.2 and 2 show better skills than CCSM4 in simulating MP global mean warming and amplified SST warming in the northern middle and high latitudes, supporting the amplified ESS compared to the CCSM4.