The initiation of modern soft and hard Snowball Earth climates in CCSM4 1Laboratory for Climate and Ocean-Atmosphere Studies, Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing, China
16 May 2012
2Department of Physics, University of Toronto, Toronto, Ontario, Canada
*formerly at: Department of Physics, University of Toronto, Toronto, Ontario, Canada
Received: 11 November 2011 – Published in Clim. Past Discuss.: 02 January 2012 Abstract. Geochemical and geological evidence has suggested that several global-scale glaciation events occurred
during the Neoproterozoic Era in the interval from 750–580 million years ago. The initiation of these glaciations
is thought to have been a consequence of the combined influence of a low level of atmospheric carbon
dioxide concentration 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 conditions under present-day circumstances of geography
and topography. It is found that runaway glaciation occurs in the model under the following
conditions: (1) an 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 moderately different
from those found to be characteristic of the previously employd CCSM3 model reported
recently in Yang et al. (2012a,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 parameterization employed in CCSM4 is believed to be more realistic
than that in CCSM3. Differences in cloud radiative forcings and ocean and atmosphere heat
transports also influence the bifurcation points. These results are potentially very important, as they are
to serve as control on further calculations which will be devoted to an investigation of the impact of continental configuration.
Revised: 02 May 2012 – Accepted: 02 May 2012 – Published: 16 May 2012
We demonstrate that there exist ''soft Snowball'' Earth states, in which the fractional sea ice coverage
60–65%, land masses in low latitudes are covered by perennial snow, and runaway glaciation
does not develop. This is consistent with our previous results based upon CCSM3. Although our results cannot exclude
the possibility of a ''hard Snowball'' solution, it is suggested that a ''soft Snowball''
solution for the Neoproterozoic remains entirely plausible.
Citation: Yang, J., Peltier, W. R., and Hu, Y.: The initiation of modern soft and hard Snowball Earth climates in CCSM4, Clim. Past, 8, 907-918, doi:10.5194/cp-8-907-2012, 2012.