Geochemical data and models suggest a positive correlation between carbon dioxide changes and climate during the last 540 m.y. The most dramatic exception to this correlation involves the Lage Ordovician (440 Ma) glaciation, which occurred at a time when CO2 levels may have been much greater than present (14--16X?). Since decreased solar luminosity at the time only partially offset increased radiative forcing from CO2, some other factor needs to be considered to explain the glaciation. Prior work with energy balance models (EBMs) suggested that the unique geographic configuration of Gondwanaland at that time may have resulted in a small area of parameter space permitting permanent snow cover and higher CO2 levels. However, the crude snow and sea ice parameterizations in the EBM left these conclusions open to further scrutiny.

Herein we present results from four experiments with the GENESIS general circulation model with CO2 levels 14X greater than present, solar luminosity reduced 4.5%, and an orbital configuration set for minimum summer insolation receipt. We examined the effects of different combinations of ocean heat transport and topography on high-latitude snow cover on Gondwanaland. For the no-elevation simulations we failed to simulate permanent summer snow cover. However, for the slightly elevated topography cases (300--500 m), permanent summer snow cover occurs where geological data indicate the Ordovician ice sheet was present. These results support the hypothesis based on EBM studies. Further results indicate that although average runoff per grid point increases substantially for the Ordovician runs, the decreased land area results in global runoff 10--30% less than present, with largest runoff reductions for flat topography. This response has implications for CO2-runoff/weathering parameterizations in geochemical models. Finally, simulated tropical sea surface temperatures (SSTs) are the same or only marginally warmer than present. This result is consistent with evidence from other warm time intervals indicating small changes in tropical SSTs during time of high CO2.