A quasi-three-dimensional global climate model designed to trace the transient evolution of climate is described. The model, which has an interactive ocean, differs from earlier models in including a 5-layer atmospheric structure and in utilizing a 5-day time step. It is essentially a coarse-grid general circulation model with simplified dynamics. The basic nonlinear system of equations used includes a prognostic equation for surface temperature and diagnostic equations for all the other variables, including sea-level pressure and wind velocity. Several 20-year runs were made with the model, primarily to test its stability, its simulation of the real climate, and its sensitivity to variations in the solar constant. The model is quite stable and shows no obvious tendency to drift with time toward either a very cold or a very warm climate. It simulates most of the major features of the atmospheric circulation and the hydrologic cycle. It fails, however, to give strong Northern Hemisphere subtropica... Abstract A quasi-three-dimensional global climate model designed to trace the transient evolution of climate is described. The model, which has an interactive ocean, differs from earlier models in including a 5-layer atmospheric structure and in utilizing a 5-day time step. It is essentially a coarse-grid general circulation model with simplified dynamics. The basic nonlinear system of equations used includes a prognostic equation for surface temperature and diagnostic equations for all the other variables, including sea-level pressure and wind velocity. Several 20-year runs were made with the model, primarily to test its stability, its simulation of the real climate, and its sensitivity to variations in the solar constant. The model is quite stable and shows no obvious tendency to drift with time toward either a very cold or a very warm climate. It simulates most of the major features of the atmospheric circulation and the hydrologic cycle. It fails, however, to give strong Northern Hemisphere subtropica...