The Community Earth System Model: A Framework for Collaborative Research
Observation and Context
Earlier climate models, such as the Community Climate System Model (CCSM), primarily represented physical atmospheric and oceanic fluid dynamics. However, real-world climate change is deeply coupled to complex interactions involving living ecosystems, nutrient limitations, chemical processes, ice sheets, and the upper atmosphere. Most previous models treated ice sheets as static features, left out interactive nitrogen limits on plant growth, and omitted the upper atmosphere above the stratosphere. Scientists required an open-source, modular Earth System Model to explore these interconnected feedbacks collaboratively across research institutions.
Hypothesis
If an open, community-governed Earth System Model integrates interactive aerosol–cloud physics, prognostic carbon–nitrogen biogeochemistry, dynamic land-ice sheets, and whole-atmosphere chemistry, it will provide a more realistic representation of historical climate trends, better capture atmospheric structures and ice mass balances, and yield more accurate projections of future climate sensitivity.
Experiment and Methodology
The National Center for Atmospheric Research (NCAR) and partner institutions developed the Community Earth System Model, version 1.0 (CESM1):
- Atmosphere: Coupled the eighth-generation CAM5 featuring two-moment cloud microphysics, modal aerosols, and indirect aerosol effects; users could also choose the WACCM high-top configuration (extending to ~140 km into the thermosphere) or CAM-CHEM for atmospheric chemistry.
- Land and Ice Sheets: Used CLM4 with prognostic carbon–nitrogen (CN) cycles, transient land-use changes, and interactive dust and permafrost tracking; coupled the dynamic Glimmer-CISM model on a 5-km grid to simulate the Greenland Ice Sheet (GrIS) surface mass balance (SMB).
- Ocean and Sea Ice: Used POP2 ocean physics with upgraded eddy and overflow parameterizations, and CICE4 sea ice with explicit melt-pond radiative physics.
- Biogeochemistry: Configured CESM1(BGC) to predict atmospheric carbon dioxide from prescribed greenhouse gas emissions with nutrient limitations (nitrogen, phosphorus, iron).
- The team conducted pre-industrial control simulations, 20th-century historical runs (1850–2005), and 21st-century Representative Concentration Pathway (RCP) scenario projections as part of the CMIP5 initiative.
Results and Data
- Aerosols and Sensitivity: Introducing indirect aerosol cooling (–1.5 W/m²) in CAM5 reduced 20th-century warm biases, matching observed historical warming trends better than CCSM4. Equilibrium climate sensitivity increased to 4.1°C in CESM1(CAM5) (compared to 3.2°C in CCSM4) due to stronger positive cloud feedbacks.
- Clouds and Dynamics: Cloud optical depth distributions matched satellite data much more closely. Subgrid surface stress parameterizations significantly improved atmospheric blocking frequencies over the Atlantic and Eurasia.
- Upper Atmosphere and Chemistry: WACCM captured Antarctic ozone-hole depletion down to ~100 Dobson units and replicated observed polar lower-stratospheric cooling (–4.6 K/decade).
- Carbon Cycle: CESM1(BGC) demonstrated that terrestrial carbon uptake is strongly constrained by nitrogen colimitation. However, both land and ocean components tended to underestimate modern carbon uptake after 1950, causing prognostic atmospheric carbon dioxide to rise faster than observed.
- Greenland Ice Sheet: Modeled historical Greenland SMB (359 ± 120 Gt/yr) matched regional estimates (376 ± 117 Gt/yr). Under RCP 8.5, SMB dropped to –78 Gt/yr by 2080–2099 due to severe summer melting, driving long-term ice sheet decay.
Conclusion and Climate Impact
The hypothesis was confirmed: CESM1 successfully delivers a flexible, community-driven Earth System Model that bridges physical, chemical, and biological climate processes. The model highlights how aerosol indirect effects and cloud feedbacks strongly govern warming rates, shows that nitrogen limitations restrict natural carbon absorption on land, and demonstrates that dynamic ice sheets will significantly worsen global sea level rise under high emissions. CESM1 serves as a foundational open tool informing international policy assessments, including the IPCC Fifth Assessment Report.
Full Citation
Hurrell, J. W., Holland, M. M., Gent, P. R., Ghan, S., Kay, J. E., Kushner, P. J., Lamarque, J.-F., Large, W. G., Lawrence, D., Lindsay, K., Lipscomb, W. H., Long, M. C., Mahowald, N., Marsh, D. R., Neale, R. B., Rasch, P., Vavrus, S., Vertenstein, M., Bader, D., Collins, W. D., Hack, J. J., Kiehl, J., & Marshall, S. (2013). The Community Earth System Model: A Framework for Collaborative Research. Bulletin of the American Meteorological Society, 94(9), 1339–1360. https://doi.org/10.1175/BAMS-D-12-00121.1