| Title: |
Is the High ECS in CESM2 Degrading Transient Climate Change Projections Over the 21st Century? |
| Authors: |
Duffy, Margaret L.1,2 (AUTHOR) mduffy@ucdavis.edu; Simpson, Isla R.1 (AUTHOR); Medeiros, Brian1 (AUTHOR); Zhu, Jiang1 (AUTHOR); McCluskey, Christina S.1 (AUTHOR); Herrington, Adam R.1 (AUTHOR); Gettelman, Andrew3 (AUTHOR); Otto‐Bliesner, Bette L.1 (AUTHOR); Fasullo, John T.1 (AUTHOR); Lauritzen, Peter H.1 (AUTHOR); Neale, Richard B.1 (AUTHOR); Lawrence, David M.1 (AUTHOR) |
| Source: |
Journal of Advances in Modeling Earth Systems. Jan2026, Vol. 18 Issue 1, p1-20. 20p. |
| Subject Terms: |
*Climate sensitivity; *Climate change; *Paleoclimatology; Atmospheric models; Microphysics |
| Abstract: |
The Community Earth System Model version 2 (CESM2) has a higher equilibrium climate sensitivity (ECS) than previous versions of CESM and many other Coupled Model Intercomparison Project models. Relatedly, CESM2 simulates too‐cold ice‐age and too‐hot warm paleoclimates. An inappropriate ice number limiter in the CESM2 microphysics scheme was discovered, and some simulations indicate that the high ECS may be partially attributable to this inappropriate limiter. In light of those findings, we seek to provide users of CESM2 guidance on the fitness of CESM2 for a variety of applications. We find that despite concerns about its climate sensitivity and simulations of past climates, the transient climate response in CESM2 is moderate relative to the CMIP6 ensemble and robust across different versions of CESM. The changes made between CESM1 and CESM2 and the fixes to the microphysical issues of CESM2 have little impact on its simulated 20th and 21st century climates under SSP3–7.0. As a result, the simulated 20th and 21st century climates of CESM2 fall well within the range of the CMIP6 ensemble and agree well with observations over the historical record. However, hotter and colder paleoclimates simulated by CESM2 are inconsistent with paleoclimate evidence. A modified version of CESM2, PaleoCalibr CESM2, may be suitable for paleoclimate studies. Simulations past the end of the 21st century with default CESM2 and studies of microphysical processes in all GCMs should be analyzed with care. Plain Language Summary: The Community Earth System Model version 2 (CESM2) is a widely‐used, coupled climate model. CESM2 has a higher equilibrium climate sensitivity (ECS) than many CMIP6 models, the previous generation of CESM, and estimates from other lines of evidence. That is, it has a comparatively strong equilibrium response of global‐mean surface temperature to a doubling of atmospheric CO2. Since its release, global‐mean surface temperature changes in CESM2 have been evaluated against paleoclimate evidence which revealed that it simulates a too‐cold Last Glacial Maximum (LGM) unless a set of modifications called "PaleoCalibr" is used. Included is a modification to an inappropriate upper limit on cloud ice number concentration. In addition to better agreement with the LGM, these PaleoCalibr modifications lowered the ECS. In light of these findings, we provide guidance for users of CESM2. We find that the PaleoCalibr modifications do not alter the transient climate response and that the PaleoCalibr modifications do not alter the 20th or 21st century climates under SSP3–7.0 warming scenario. Therefore, any concerns about the high ECS and inappropriate upper limit on cloud ice number concentration do not apply to moderate warming. It is only under more extreme climate changes where CESM2 and PaleoCalibr CESM2 diverge. Key Points: Community Earth System Model version 2's (CESM2's) high equilibrium climate sensitivity (ECS) and too‐cold simulation of ice age climate have raised questions about its skillChanges to CESM2's microphysical representation that improve its ice age climate and ECS do not impact its Transient Climate ResponseCESM2 is appropriate for historical and 21st century climates, and we provide guidance on how to use CESM2 for studies of other climates [ABSTRACT FROM AUTHOR] |
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| Database: |
GreenFILE |