1Laboratory for Climate and Ocean-Atmosphere Studies, Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing, China
2Department of Physics, University of Toronto, Toronto, Ontario, Canada
*now at: Department of Physics, University of Toronto, Toronto, Ontario, Canada
Abstract. Geochemical and geological evidence suggested that several global-scale glaciation events occurred during the Neoproterozoic era at 750–580 million years ago. The initiation of these glaciations is thought to have been a consequence of the combined influence of a result of low-level carbon dioxide and an approximately 6% weakening of solar luminosity. The latest version of the Community Climate System Model (CCSM4) is employed herein to explore the detailed combination of forcings required to trigger such extreme glaciation under present-day geography and topography conditions. It is found that runaway glaciation occurs in the model under the following conditions: (1) a 8–9% reduction in solar radiation with 286 ppmv CO2 or (2) a 6% reduction in solar radiation with 70–100 ppmv CO2. These thresholds are only moderately different from those found to be characteristic of the previous CCSM3 model reported recently in Yang et al. (2011a,b) for which the respective critical points corresponded to a 10–10.5% reduction in solar radiation with 286 ppmv CO2 or a 6% reduction in solar radiation with 17.5–20 ppmv CO2. The most important reason for these differences is that the sea-ice/snow albedo in CCSM4 is somewhat higher than in CCSM3. Differences in cloud radiative forcings and oceanic and atmospheric heat transports between CCSM3 and CCSM4 also influence the bifurcation points.
The forcings required to trigger a "hard Snowball" Earth in either CCSM3 or CCSM4 may be not met by the conditions expected to be characteristic of the Neoproterozoic. Furthermore, there exist "soft Snowball" Earth states, in which the sea-ice coverage reaches approximately 60–65%, land masses in low latitudes are covered by perennial snow, and runaway glaciation does not develop. This is also qualitatively consistent with our previous results of the CCSM3 model. These results suggest that a "soft Snowball" solution for the Neoproterozoic is entirely plausible and may in fact be preferred.