The initiation of Neoproterozoic "snowball" climates in CCSM3: the influence of paleocontinental configuration
We identify the "hard snowball" bifurcation point at which total sea-ice cover of the oceans is expected by employing the comprehensive coupled climate model CCSM3 (Community Climate System Model version 3) for two realistic Neoproterozoic continental configurations, namely a low-latitude configuration appropriate for the 720 Ma Sturtian glaciation and a higher southern latitude configuration reconstructed for 570 Ma but which has often been employed in the past to study the later 635 Ma Marinoan glaciation. Contrary to previous suggestions, we find that for the same total solar insolation (TSI) and atmospheric CO 2 concentration ( pCO 2), the 570 Ma continental configuration is characterized by colder climate than the 720 Ma continental configuration and enters the hard snowball state more easily on account of the following three factors: the higher effective albedo of the snow-covered land compared to that of sea ice, the more negative net cloud forcing near the ice front in the Northern Hemisphere (NH), and, more importantly, the more efficient sea-ice transport towards the Equator in the NH due to the absence of blockage by continents. Beside the paleogeography, we also find the optical depth of aerosol to have a significant influence on this important bifurcation point. When the high value (recommended by CCSM3 but demonstrated to be a significant overestimate) is employed, the critical values of pCO 2, beyond which a hard snowball will be realized, are between 80 and 90 ppmv (sea-ice fraction 55%) and between 140 and 150 ppmv (sea-ice fraction 50%) for the Sturtian and Marinoan continental configurations, respectively. However, if a lower value is employed that enables the model to approximately reproduce the present-day climate, then the critical values of pCO 2 become 50–60 ppmv (sea-ice fraction 57%) and 100–110 ppmv (sea-ice fraction 48%) for the two continental configurations, respectively. All of these values are higher than previously obtained for the present-day geography (17–35 ppmv) using the same model, primarily due to the absence of vegetation, which increases the surface albedo, but are much lower than that obtained previously for the Marinoan continental configuration using the ECHAM5/MPI-OM model in its standard configuration (~500 ppmv). However, when the sea-ice albedo in that model was reduced from 0.75 to a more appropriate value of 0.45, the critical pCO 2 becomes ~204 ppmv, closer to the values obtained here. Our results are similar to those obtained with the present-day geography (70–100 ppmv) when the most recent version of the NCAR model, CCSM4, was employed.