The initiation of modern soft and hard Snowball Earth climates in CCSM4

J Yang,Y Hu,W R Peltier

doi:10.5194/cp-8-907-2012

Abstract

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. We demonstrate that there exist ''soft Snowball'' Earth states, in which the fractional 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 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.

Highlights

Based on observations of glacial deposits on the continents at low latitudes, the associated extreme carbon isotope fluctuations, the existence of banded iron formations (BIFs) and post-glacial cap carbonates in glacial deposits of the Neoproterozoic era, it is generally thought that two significant and global-scale glaciations occurred at ∼716 Ma and at ∼635 Ma, during which land-based ice sheets reached deep into tropical latitudes
The sea ice albedo in energy balance models (EBMs)/ice sheet models has been set to be as low as 0.45 (Hyde et al, 2000; Peltier et al, 2004, 2007; Liu and Peltier, 2010, 2011), and the snow albedo in CAM3 and CCSM3 is set to 0.66–0.78 (Pierrehumbert et al, 2011; Abbot et al, 2011; Yang et al, 2012a), both of which are smaller than observations, 0.47–0.52 for bare sea ice, 0.55–0.66 for sea glacier and 0.75–0.87 for snow (Perovich, 1996; Warren et al, 2002; Warren and Brandt, 2006)
We begin by comparing the results obtained using Community Climate System Model version 4 (CCSM4) and CCSM3 for the purpose of the pre-industrial control experiments and in the 6 % reduced solar radiation experiments; through these comparisons, we will establish the main differences between CCSM4 and CCSM3

Summary

Introduction

Based on observations of glacial deposits on the continents at low latitudes, the associated extreme carbon isotope fluctuations, the existence of banded iron formations (BIFs) and post-glacial cap carbonates in glacial deposits of the Neoproterozoic era, it is generally thought that two significant and global-scale glaciations occurred at ∼716 Ma and at ∼635 Ma, during which land-based ice sheets reached deep into tropical latitudes. It is still disputed as to whether the entire ocean was covered by thick sea ice, the so-called “Snowball” or “hard Snowball” Earth hypothesis (Kirschvink, 1992; Hoffman et al, 1998; Hoffman and Schrag, 2002), or whether tropical open-water oceans coexisted with low-latitude continental ice sheets, the so-called “Slushball” or “soft Snowball” Earth hypothesis (Hyde et al, 2000; Peltier et al, 2007; Allen and Etienne, 2008) Aside from these major variations upon what is generally agreed to have been extreme glacial conditions, additional suggestions. As the sea ice and/or snow albedo increases, it will clearly be easier to enter a globally ice-covered state (Lewis et al, 2003; Pierrehumbert et al, 2011; Yang et al, 2012a)

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