Community Earth System Model Version 2 (CESM2)
Observation and Context
Climate scientists rely on global models to study Earth’s past, present, and future climate. Previous climate models, such as CESM1, provided useful projections but suffered from noticeable errors. For example, CESM1 exhibited severe precipitation biases in the tropics, incorrect cloud reflectivity, and excessive nutrient limitations that caused inaccurate land carbon uptake estimates. To prepare for the Coupled Model Intercomparison Project Phase 6 (CMIP6), researchers needed an updated model that could represent complex Earth systems—including land, ocean, ice, and atmosphere—with far greater physical accuracy.
Hypothesis
Integrating updated physical parameterizations, advanced component sub-models, and consistent atmospheric chemistry into the Community Earth System Model Version 2 (CESM2) will significantly reduce climate biases and yield historical climate simulations that align closer to real-world observations than CESM1.
Experiment and Methodology
Researchers integrated improved sub-models into CESM2, including the CAM6 (“low-top”) and WACCM6 (“high-top”) atmosphere models, CLM5 for land, POP2 for oceans, CICE5 for sea ice, and CISM2.1 for land ice. They conducted long preindustrial control runs (1,200 years for CAM6; 500 years for WACCM6) and historical ensemble integrations spanning 1850 to 2014 using a nominal 1° horizontal grid resolution. WACCM6 interactively calculated chemistry and aerosols, and its output was used to force CAM6 to maintain physical consistency. During testing, researchers adjusted cloud microphysics and boundary layer parameters to resolve developmental challenges, such as unrealistic historical mid-century cooling and excessive sea-ice formation in the Labrador Sea.
Results and Data
- Precipitation Improvements: CESM2 significantly reduced tropical wet biases in precipitation, mitigating the persistent “double ITCZ” problem seen in previous models.
- Cloud Radiative Forcing: Shortwave cloud forcing root-mean-square error decreased from 14.02 W/m² in CESM1 to 9.14 W/m² in CESM2(CAM6).
- Atmospheric Variability: The model accurately captured eastward-propagating Madden-Julian Oscillation (MJO) patterns and produced more realistic El Niño-Southern Oscillation (ENSO) teleconnections.
- Land Carbon Accumulation: Land carbon accumulated appropriately over the late 20th century (~40 Pg C), closely matching observational datasets due to improved nitrogen-carbon cycling in CLM5.
- Climate Sensitivity Metrics: CESM2 demonstrated an Equilibrium Climate Sensitivity (ECS) of 5.1–5.3°C, higher than CESM1’s 4.0°C. However, its Transient Climate Response (TCR) remained stable at 1.9–2.0°C.
Conclusion and Climate Impact
CESM2 represents a major advancement in Earth system modeling. By incorporating updated turbulence schemes, realistic cloud microphysics, and improved land ecosystem dynamics, CESM2 offers a markedly superior historical climate simulation compared to CESM1. The increase in ECS demonstrates that subtle adjustments in cloud microphysics and boundary-layer dynamics strongly dictate long-term climate sensitivity to doubled CO₂. These findings provide critical, high-fidelity data to inform global climate policies and CMIP6 research.
Full Citation
Danabasoglu, G., Lamarque, J.-F., Bacmeister, J., Bailey, D. A., DuVivier, A. K., Edwards, J., et al. (2020). The Community Earth System Model Version 2 (CESM2). Journal of Advances in Modeling Earth Systems, 12(2), e2019MS001916. https://doi.org/10.1029/2019MS001916