Late Paleozoic glaciation on Gondwana is associated with changes in geography, solar luminosity, and estimated CO2 levels. To assess the relative importance of these boundary conditions, we conducted a suite of climate model simulations for the periods before, during, and after peak mid-Carboniferous (∼300 Ma) glaciation (340, 300, and 255 and 225 Ma, respectively). Orbital insolation values favorable for glaciation and interglaciation were used for each time interval. Results indicate that changes in geography cause significant changes in snow area, but the temporal trend is not consistent with the geologic record for glaciation. Combined CO2-plus-geography changes yield the best agreement with observations. In addition, interglacial orbital configurations result in almost ice-free conditions for the glacial interval at 300 Ma, at a time of low CO2. The large simulated glacial-interglacial snowline fluctuations for Permian- Carboniferous time may explain cyclothem fluctuations at these times. Overall, results support the importance of the CO2 paradigm, but also indicate that a fuller understanding of past climate change requires consideration of paleogeographic, luminosity, and orbital insolation changes.

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