GENERAL-CIRCULATION MODEL SENSITIVITY EXPERIMENTS WITH POLE-CENTERED SUPERCONTINENTS

Although high CO2 levels appear to be required to explain most times o f ice-free climates in Earth history, previous calculations with a two -dimensional energy balance model (EBM) have suggested that the low th ermal inertia associated with pole-centered supercontinents might allo w seasonally snow-free conditions to develop in the absence of high CO 2. This conclusion has been challenged by other EBM calculations with lower geographic resolution but more explicit model physics. To addres s this problem more directly we have undertaken a series of general ci rculation model (GCM) simulations with a new model (GENESIS) developed at the National Center for Atmospheric Research. For idealized pole-c entered supercontinents we obtain mid summer south polar temperatures of 22-degrees-C for the present orbital configuration and 15-degrees-C with a modified orbital configuration favorable to cool summers. Thes e results support the two-dimensional EBM calculations. However, their application to real-world situations of the Paleozoic is hampered by the fact that due to extremely high sea levels and decreased solar lum inosity the best time periods for applying this concept (420-360 Ma) h ave supercontinent areas too small to yield an ice-free climate for al l orbital configurations. Snow-free conditions with summer temperature s at the pole of 25-degrees-C are simulated for a ''hot'' orbital conf iguration for one realistic geography configuration (Carboniferous, 30 0 Ma). These results indicate that while the basic idea of snow-free, pole-centered supercontinents is still theoretically possible, the rea l-world application to specific past time periods requires some modifi cations. Our reframed hypothesis states that for realistic geography a nd high CO2 levels, pole-edged supercontinents could have permanent sn ow cover, but pole-centered supercontinents would be snow-free in summ er.