Duffy et al. (2025) Is the High ECS in CESM2 Degrading Transient Climate Change Projections Over the 21st Century?

⚠️ Warning: This summary was generated from the abstract only, as the full text was not available.

Identification

• Journal: Journal of Advances in Modeling Earth Systems
• Year: 2025
• Date: 2025-12-29
• Authors: Margaret L. Duffy, Isla R. Simpson, Brian Medeiros, Jiang Zhu, Christina S Mccluskey, Adam R. Herrington, Andrew Gettelman, Bette L. Otto‐Bliesner, John Fasullo, P. H. Lauritzen, Richard Neale, David M. Lawrence
• DOI: 10.1029/2025ms004967

Research Groups

• Community Earth System Model (CESM) development team (likely associated with the National Center for Atmospheric Research - NCAR)

Short Summary

This paper evaluates the fitness of CESM2 for various applications, concluding that despite a high equilibrium climate sensitivity, it accurately simulates 20th and 21st-century transient climates but is unreliable for extreme paleoclimates or long-term projections beyond the 21st century.

Objective

• To provide users of CESM2 guidance on the fitness of the model for a variety of applications, considering its high equilibrium climate sensitivity and issues with paleoclimate simulations.

Study Configuration

• Spatial Scale: Global (Earth System Model)
• Temporal Scale: 20th century, 21st century (under SSP3–7.0 scenario), paleoclimates (ice-age, warm paleoclimates), and projections past the end of the 21st century.

Methodology and Data

• Models used: Community Earth System Model version 2 (CESM2), CESM1, PaleoCalibr CESM2. Analysis of CESM2's microphysics scheme.
• Data sources: Coupled Model Intercomparison Project Phase 6 (CMIP6) ensemble for comparison, historical observations, and paleoclimate evidence.

Main Results

• CESM2 exhibits a higher equilibrium climate sensitivity (ECS) than previous CESM versions and many other CMIP models.
• CESM2 simulates too-cold ice-age and too-hot warm paleoclimates, inconsistent with paleoclimate evidence.
• An inappropriate ice number limiter was discovered in the CESM2 microphysics scheme, potentially contributing to the high ECS.
• The transient climate response (TCR) in CESM2 is moderate relative to the CMIP6 ensemble and robust across different CESM versions.
• Changes between CESM1 and CESM2, and fixes to microphysical issues, have minimal impact on simulated 20th and 21st-century climates under SSP3–7.0.
• Simulated 20th and 21st-century climates of CESM2 fall within the CMIP6 ensemble range and agree well with historical observations.
• A modified version, PaleoCalibr CESM2, may be suitable for paleoclimate studies.
• Caution is advised for simulations past the 21st century with default CESM2 and for studies of microphysical processes in all GCMs.

Contributions

• Provides critical guidance for the user community on the appropriate applications and limitations of CESM2, particularly regarding its climate sensitivity and paleoclimate performance.
• Identifies a specific microphysical scheme issue in CESM2 and its potential implications for ECS.
• Differentiates CESM2's performance for transient 20th/21st-century climates versus extreme paleoclimates and long-term projections.

Funding

• Not mentioned in the abstract.

Citation

@article{Duffy2025Is,
  author = {Duffy, Margaret L. and Simpson, Isla R. and Medeiros, Brian and Zhu, Jiang and Mccluskey, Christina S and Herrington, Adam R. and Gettelman, Andrew and Otto‐Bliesner, Bette L. and Fasullo, John and Lauritzen, P. H. and Neale, Richard and Lawrence, David M.},
  title = {Is the High ECS in CESM2 Degrading Transient Climate Change Projections Over the 21st Century?},
  journal = {Journal of Advances in Modeling Earth Systems},
  year = {2025},
  doi = {10.1029/2025ms004967},
  url = {https://doi.org/10.1029/2025ms004967}
}