The Community Climate System Model Version 3 (CCSM3)
W. D. Collins, et al.
Observation and Context
Scientists rely on advanced computer models to simulate Earth’s past, present, and future climates. To be accurate, these models must realistically connect different environmental systems, such as the atmosphere, land surface, oceans, and sea ice. The previous version of this model, CCSM2, suffered from several systematic errors, known as “biases.” For example, CCSM2 simulated the polar atmospheres and the upper tropical troposphere as much too cold, overestimated winter temperatures over sub-Arctic land, and struggled to accurately represent tropical wind and rain patterns.
Hypothesis
If developers upgrade the physical equations representing cloud processes, atmospheric aerosols, land-atmosphere heat exchanges, sea ice movements, and ocean mixing, then the updated model (CCSM3) will significantly reduce these systematic temperature biases and produce stable, long-term climate simulations without needing artificial adjustments.
Experiment and Methodology
The researchers built CCSM3 by combining four upgraded, individual model components:
- Atmosphere: The Community Atmosphere Model (CAM3), which includes new physics for clouds, precipitation, and aerosol interactions.
- Land: The Community Land Model (CLM3), which simulates soil columns, vegetation, and land-air heat fluxes.
- Sea Ice: The Community Sea Ice Model (CSIM5), which tracks ice thickness and movement.
- Ocean: An ocean model based on the Parallel Ocean Program (POP), which simulates deep-sea currents and temperatures.
These components run asynchronously and communicate through a central “coupler” program that coordinates and balances the exchanges of heat, water, and momentum. The researchers tested this setup using a high-resolution configuration (T85×1) that pairs a T85 spectral grid for the atmosphere and land with a 1° grid for the ocean and sea ice. They ran a “control” simulation under present-day conditions for 600 years to analyze the model’s stability, comparing the results directly against satellite and weather data.
Results and Data
The new physical equations successfully corrected several major errors from CCSM2:
- Atmospheric Warming: Introducing optically thicker cirrus clouds warmed the upper tropical atmosphere by 2.3 Kelvin, cutting the previous cold bias by 60%. Polar atmospheric temperatures warmed by 2.3 Kelvin in the north and 3.9 Kelvin in the south, reducing those biases by 33%.
- Sea Ice and Ocean Heat: Improved winter radiation calculations led to highly realistic Arctic sea ice thicknesses, averaging 2 to 2.5 meters in the central basin compared to the too-thin 1.5 meters in CCSM2. Additionally, northward heat transport in the Atlantic Ocean increased by 0.3 petawatts, matching real-world observations.
- Simulation Stability: Global surface temperatures remained exceptionally stable, cooling by a minor 0.011 Kelvin per century due to a tiny, acceptable energy loss of 0.21 Watts per square meter at the top of the model.
Despite these major improvements, some biases remained, such as an unrealistic “double ITCZ” rainfall pattern in the tropical Pacific and a compressed two-year cycle for El Niño events.
Conclusion and Climate Impact
The experiment supported the hypothesis. CCSM3 successfully reduced major systematic temperature biases and proved that it could run stable, multi-century climate simulations without drifting or requiring artificial adjustments to its energy budget. By providing highly reliable projections of global temperatures and ice dynamics, CCSM3 serves as a vital tool for assessing future human-caused climate change and helping society prepare for its environmental impacts.
Citation
Collins, W. D., Bitz, C. M., Blackmon, M. L., Bonan, G. B., Bretherton, C. S., Carton, J. A., Chang, P., Doney, S. C., Hack, J. J., Henderson, T. B., Kiehl, J. T., Large, W. G., McKenna, D. S., Santer, B. D., & Smith, R. D. (2006). The Community Climate System Model Version 3 (CCSM3). Journal of Climate, 19(11), 2122–2143.