Dependence of climate states of gray atmosphere on solar constant: From the runaway greenhouse to the snowball states

Multiple equilibrium solutions of a gray atmosphere are investigated for various values of the solar constant. Two types of models are utilized in order to conduct a comparative study: a general circulation model with simplified physical processes (GCM) and a one-dimensional energy balance model (EBM). For intermediate values of solar constant, both of the models indicate the existence of multiple solutions that include the runaway greenhouse state in addition to the globally ice-covered state and the partially ice-covered state. In the GCM results, there is no partially ice-covered state with the ice line latitude lower than 22°. This indicates that the large ice cap instability discussed in previous EBM studies also occurs in GCM. Compared to the results of EBM, the ice line of the partially ice-covered state of GCM reaches lower latitudes. The appearance of the large ice cap instability in our GCM is impeded by condensation heating at the ice lines. The efficient latitudinal heat transport in the Hadley cell is considered to be a contributing factor to this delay in ice cap expansion.