The Initiation of Modern “Soft Snowball” and “Hard Snowball” Climates in CCSM3. Part I: The Influences of Solar Luminosity, CO2 Concentration, and the Sea Ice/Snow Albedo Parameterization

Abstract

The ''Snowball Earth'' hypothesis, proposed to explain the Neoproterozoic glacial episodes in the period 750-580 million years ago, suggested that the earth was globally covered by ice/snow during these events. This study addresses the problem of the forcings required for the earth to enter such a state of complete glaciation using the Community Climate System Model, version 3 (CCSM3). All of the simulations performed to address this issue employ the geography and topography of the present-day earth and are employed to explore the combination of factors consisting of total solar luminosity, CO 2 concentration, and sea ice/snow albedo parameterization that would be required for such an event to occur. The analyses demonstrate that the critical conditions beyond which runaway ice-albedo feedback will lead to global freezing include 1) a 10%-10.5% reduction in solar radiation with preindustrial greenhouse gas concentrations; 2) a 6% reduction in solar radiation with 17.5 ppmv CO 2 ; or 3) 6% less solar radiation and 286 ppmv CO 2 if sea ice albedo is equal to or greater than 0.60 with a snow albedo of 0.78, or if sea ice albedo is 0.58 with a snow albedo equal to or greater than 0.80. These bifurcation points are very sensitive to the sea ice and snow albedo parameterizations. Moreover, ''soft Snowball'' solutions are found in which tropical open water oceans stably coexist with yearround snow-covered low-latitude continents, implying that tropical continental ice sheets would actually be present. The authors conclude that a ''soft Snowball'' is entirely plausible, in which the global sea ice fraction may reach as high as 76% and sea ice margins may extend to 108S(N) latitudes.

Role of ocean heat transport in climates of tidally locked exoplanets around M dwarf stars

“…Simulation with a comprehensive Earth atmospheric general circulation model (AGCM) coupled to a slab ocean, without dynamic ocean heat transport, revealed an "eyeball" climate state, with a round area of open ocean centered at the substellar point and complete ice coverage on the nightside, even for very high CO 2 concentrations (4). In the presence of sea ice, ocean heat transport is likely to be especially important, because it is known from studies of the Snowball Earth phenomenon in Earth-like conditions that ocean heat transport is very effective in holding back the advance of the sea-ice margin (12)(13)(14). The distribution of sea ice on tidally locked exoplanets is only an issue for M stars, because planets with sufficiently dense atmospheres orbiting hotter stars in orbits close enough to yield tidal locking are likely to be too hot to permit ice and may even be too hot to retain water.…”

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

“…However, in other models, it is found that a state with tropical open-water may be stable even if the sea ice edge reaches as low as 10 • latitude (Chandler and Sohl, 2000;Baum and Crowley, 2001;Pierrehumbert et al, 2011;Abbot et al, 2011;Yang et al, 2012a,b). One of the reasons for the absence of runaway glaciation in these models may be associated with the relatively low sea ice and/or snow albedo employed, which restricts the strength of the albedo feedback Yang et al, 2012a). For instance, the sea ice albedo in EBM/ice sheet models has been set to be as low as 0.45 (Hyde et al, 2000;Peltier et al, 2004Peltier et al, , 2007Peltier, 2010, 2011), and the snow albedo in CAM3 and CCSM3 is set to 0.66-0.78 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).…”

“…For instance, the sea ice albedo in EBM/ice sheet models has been set to be as low as 0.45 (Hyde et al, 2000;Peltier et al, 2004Peltier et al, , 2007Peltier, 2010, 2011), and the snow albedo in CAM3 and CCSM3 is set to 0.66-0.78 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). 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).…”

“…Furthermore, in previous models, the parameterization for sea ice and snow albedo has been overly simplified (Briegleb and Light, 2007;Yang et al, 2012a). The dependencies of surface albedo on the solar zenith angle, snow grain radius, cloud cover and the incidence of melt ponds have been neglected, which have significant influence on temporal and spatial variations of the surface albedo (Wiscombe and Warren, 1980;Perovich et al, 2002).…”

“…Table 1. Typical albedos for sea ice (>1 m), sea glacier (compressed snow over ocean), snow and melt pond in CCSM3, CCSM4 (see Briegleb and Light, 2007;Yang et al, 2012a) and observations (see Perovich, 1996;Warren et al, 2002;Warren and Brandt, 2006 The primary purpose of this paper is to investigate the initiation of a modern Snowball Earth with CCSM4 as a function of a reduction in solar constant and/or carbon dioxide concentration. We will determine the thresholds for the occurrence of a "hard Snowball" and the conditions for the existence of a "soft Snowball" and compare them with our previous results obtained using CCSM3 (Yang et al, 2012a,b).…”