The initiation of Neoproterozoic "snowball" climates in CCSM3: the influence of paleo-continental configuration

Abstract

Abstract. 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 for two realistic Neoproterozoic continental configurations, namely a low-latitude configuration appropriate for the 720 Ma Sturtian glaciation and a higher southern latitude configuration more appropriate for the later 635 Ma Marinoan glaciation. We find that for the same total solar insolation (TSI) and atmospheric CO2 concentration (pCO2), the most recent 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 four factors: the low heat capacity of land in the south polar region, the higher 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 pCO2, beyond which a hard snowball will be realized, are between 80–90 ppmv and 140–150 ppmv 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 pCO2 become 50–60 ppmv and 100–110 ppmv 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, but are much lower than that obtained previously for the 635 Ma 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 pCO2 becomes ∼204 ppmv, closer to but still higher than 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, is employed.