The Community Climate System Model version 3 (CCSM3) is a coupled climate model that includes components for the atmosphere, ocean, sea ice, and land surface, connected by a flux coupler. It is designed to produce realistic simulations across a wide range of spatial resolutions, enabling both inexpensive long-term simulations and detailed studies of continental-scale dynamics. CCSM3 incorporates significant improvements in physical parameterizations, reducing or eliminating several systematic biases in the mean climate from previous versions. These improvements include new treatments of cloud processes, aerosol radiative forcing, land-atmosphere fluxes, ocean mixed layer processes, and sea ice dynamics. CCSM3 shows significant improvements in sea ice thickness, polar radiation budgets, tropical sea surface temperatures, and cloud radiative effects. However, there are still systematic biases in ocean-atmosphere fluxes in coastal regions west of continents, the spectrum of ENSO variability, the spatial distribution of precipitation in the tropical oceans, and continental precipitation and surface air temperatures. Work is ongoing to extend CCSM to a more accurate and comprehensive model of the Earth's climate system. CCSM3 is the third generation of an ongoing series of coupled models developed through international collaboration. The first generation, CSM1, was released in 1996 and did not require adjustments to simulate stable climates. The second generation, CCSM2, was released in 2002 and showed several improvements over CSM1. However, several important deficiencies prompted the development of CCSM3. The main model biases in CCSM2 include a double ITCZ, extended cold tongue, overestimation of winter land surface temperatures, underestimation of tropical tropopause temperatures, erroneous cloud response to SST changes, errors in the east Pacific surface energy budget, and underestimation of tropical variability. CCSM3 has reduced or eliminated some of these biases. CCSM3 includes new versions of all component models: the Community Atmosphere Model version 3 (CAM3), the Community Land Surface Model version 3 (CLM3), the Community Sea Ice Model version 5 (CSIM5), and the ocean based on the Parallel Ocean Program version 1.4.3 (POP). Each component is designed to conserve energy, mass, total water, and freshwater in concert with the other components. The model has been designed to produce simulations with reasonable fidelity over a wide range of resolutions and with a variety of atmospheric dynamical frameworks. The standard version of CAM3 is based upon the Eulerian spectral dynamical core with triangular spectral truncation at 31, 42, and 85 wavenumbers. The ocean model uses a dipole grid with a nominal horizontal resolution of 3° or 1°. The sea ice model is integrated on the same horizontal grid as the ocean model. The three standard configurations of CCSM combine the T31 CAM/CLM with the 3° POP/CSIMThe Community Climate System Model version 3 (CCSM3) is a coupled climate model that includes components for the atmosphere, ocean, sea ice, and land surface, connected by a flux coupler. It is designed to produce realistic simulations across a wide range of spatial resolutions, enabling both inexpensive long-term simulations and detailed studies of continental-scale dynamics. CCSM3 incorporates significant improvements in physical parameterizations, reducing or eliminating several systematic biases in the mean climate from previous versions. These improvements include new treatments of cloud processes, aerosol radiative forcing, land-atmosphere fluxes, ocean mixed layer processes, and sea ice dynamics. CCSM3 shows significant improvements in sea ice thickness, polar radiation budgets, tropical sea surface temperatures, and cloud radiative effects. However, there are still systematic biases in ocean-atmosphere fluxes in coastal regions west of continents, the spectrum of ENSO variability, the spatial distribution of precipitation in the tropical oceans, and continental precipitation and surface air temperatures. Work is ongoing to extend CCSM to a more accurate and comprehensive model of the Earth's climate system. CCSM3 is the third generation of an ongoing series of coupled models developed through international collaboration. The first generation, CSM1, was released in 1996 and did not require adjustments to simulate stable climates. The second generation, CCSM2, was released in 2002 and showed several improvements over CSM1. However, several important deficiencies prompted the development of CCSM3. The main model biases in CCSM2 include a double ITCZ, extended cold tongue, overestimation of winter land surface temperatures, underestimation of tropical tropopause temperatures, erroneous cloud response to SST changes, errors in the east Pacific surface energy budget, and underestimation of tropical variability. CCSM3 has reduced or eliminated some of these biases. CCSM3 includes new versions of all component models: the Community Atmosphere Model version 3 (CAM3), the Community Land Surface Model version 3 (CLM3), the Community Sea Ice Model version 5 (CSIM5), and the ocean based on the Parallel Ocean Program version 1.4.3 (POP). Each component is designed to conserve energy, mass, total water, and freshwater in concert with the other components. The model has been designed to produce simulations with reasonable fidelity over a wide range of resolutions and with a variety of atmospheric dynamical frameworks. The standard version of CAM3 is based upon the Eulerian spectral dynamical core with triangular spectral truncation at 31, 42, and 85 wavenumbers. The ocean model uses a dipole grid with a nominal horizontal resolution of 3° or 1°. The sea ice model is integrated on the same horizontal grid as the ocean model. The three standard configurations of CCSM combine the T31 CAM/CLM with the 3° POP/CSIM