Robust elements of Snowball Earth atmospheric circulation and oases for life

Abstract

[1] Atmospheric circulation in a Snowball Earth is critical for determining cloud behavior, heat export from the tropics, regions of bare ice, and sea glacier flow. These processes strongly affect Snowball Earth deglaciation and the ability of oases to support photosynthetic marine life throughout a Snowball Earth. Here we establish robust aspects of the Snowball Earth atmospheric circulation by running six general circulation models with consistent Snowball Earth boundary conditions. The models produce qualitatively similar patterns of atmospheric circulation and precipitation minus evaporation. The strength of the Snowball Hadley circulation is roughly double modern at low CO 2 and greatly increases as CO 2 is increased. We force a 1-D axisymmetric sea glacier model with general circulation model (GCM) output and show that, neglecting zonal asymmetry, sea glaciers would limit ice thickness variations to O(10%). Global mean ice thickness in the 1-D sea glacier model is well-approximated by a 0-D ice thickness model with global mean surface temperature as the upper boundary condition. We then show that a thin-ice Snowball solution is possible in the axysymmetric sea glacier model when forced by output from all the GCMs if we use ice optical properties that favor the thin-ice solution. Finally, we examine Snowball oases for life using analytical models forced by the GCM output and find that conditions become more favorable for oases as the Snowball warms, so that the most critical time for the survival of life would be near the beginning of a Snowball Earth episode.

The dynamics of the Snowball Earth Hadley circulation for off-equatorial and seasonally varying insolation

“…The Hadley circulation also affects other suggested deglaciation factors such as atmospheric dust (Abbot and Halevy, 2010) and tropical clouds . Through its impact on the tropical precipitation minus evaporation pattern, the Hadley circulation furthermore influences the flow of thick sea ice and the possibility of open narrow seaways that could serve as oases for the survival of life during Snowball events (Campbell et al, 2011;Tziperman et al, 2012;Abbot et al, 2013). Finally, the Hadley circulation plays an important role during the initiation of Snowball Earth episodes (Voigt and Marotzke, 2010;Abbot et al, 2011;Voigt and Abbot, 2012;Yang et al, 2012a, b) during which it progressively approaches the dry limit discussed here.…”

“…Finally, the Hadley circulation plays an important role during the initiation of Snowball Earth episodes (Voigt and Marotzke, 2010;Abbot et al, 2011;Voigt and Abbot, 2012;Yang et al, 2012a, b) during which it progressively approaches the dry limit discussed here. All of the above factors depend on the strength of the Hadley circulation, which Abbot et al (2013) found to vary by up to a factor of 4 in annual-mean and a factor of 2 in solstice conditions among comprehensive atmosphere general circulation models driven by the same idealized Snowball Earth boundary conditions. The simulations presented here suggest that this difference might, at least partly, be caused by differences in the models' treatment of vertical diffusion of momentum.…”

“…A better characterization of the climate during Snowball Earth episodes also helps to constrain the deglaciation threshold of a hard Snowball Earth and to judge to what extent proposed scenarios for the survival of life during such harsh conditions appear plausible. The low heat capacity of the ice surface allows large seasonal shifts of the Hadley circulation, yielding an indirect annual-mean circulation and, in contrast to Earth's modern climate, a zone of net evaporation around the equator Abbot et al, 2013). The equatorial net evaporation zone lowers the equatorial surface albedo because bare ice has a much lower albedo than snow (Brandt et al, 2005;Warren and Brandt, 2006;Dadic et al, 2013) and because of accumulation of surface dust.…”

“…Both factors facilitate the deglaciation of a hard Snowball Earth Le Hir et al, 2010). While the equatorial net evaporation zone emerges as a robust feature of climate models (Abbot et al, 2013), its detailed characteristics likely depend on the Hadley circulation. The Hadley circulation also affects other suggested deglaciation factors such as atmospheric dust (Abbot and Halevy, 2010) and tropical clouds .…”

Radiative forcing at high concentrations of well‐mixed greenhouse gases

“…Under such large climate changes, substantial nonsmooth climate feedbacks are expected, and the proportionality between radiative forcing and mean surface temperature will weaken. Such climates are an active area of research with general circulation models [e.g., Abbot et al, 2013;Russell et al, 2013]. Nonetheless, radiative forcings and a climate sensitivity parameter of ≈0.5 K∕(W m −2 ) [IPCC, 2001] provide a good first-approximation estimate of climate change and are the best way of comparing the relative efficacy of different greenhouse gases.…”