Abstract
Abstract. We study the initiation of a Marinoan Snowball Earth (~635 million years before present) with the state-of-the-art atmosphere-ocean general circulation model ECHAM5/MPI-OM. This is the most sophisticated model ever applied to Snowball initiation. A comparison with a pre-industrial control climate shows that the change of surface boundary conditions from present-day to Marinoan, including a shift of continents to low latitudes, induces a global-mean cooling of 4.6 K. Two thirds of this cooling can be attributed to increased planetary albedo, the remaining one third to a weaker greenhouse effect. The Marinoan Snowball Earth bifurcation point for pre-industrial atmospheric carbon dioxide is between 95.5 and 96% of the present-day total solar irradiance (TSI), whereas a previous study with the same model found that it was between 91 and 94% for present-day surface boundary conditions. A Snowball Earth for TSI set to its Marinoan value (94% of the present-day TSI) is prevented by doubling carbon dioxide with respect to its pre-industrial level. A zero-dimensional energy balance model is used to predict the Snowball Earth bifurcation point from only the equilibrium global-mean ocean potential temperature for present-day TSI. We do not find stable states with sea-ice cover above 55%, and land conditions are such that glaciers could not grow with sea-ice cover of 55%. Therefore, none of our simulations qualifies as a "slushball" solution. While uncertainties in important processes and parameters such as clouds and sea-ice albedo suggest that the Snowball Earth bifurcation point differs between climate models, our results contradict previous findings that Snowball Earth initiation would require much stronger forcings.
Highlights
The apparent existence of low-latitude land glaciers at sea level during at least two episodes of the Neoproterozoic era, the Sturtian (∼710 million years before present, Ma) and the Marinoan (∼635 Ma) (Evans, 2000; Trindade and Macouin, 2007; Macdonald et al, 2010), has led to the proposal that these glaciations were accompanied by completely ice-covered oceans
This is sometimes cited as evidence that atmosphereocean general circulation models cannot exhibit a runaway ice-albedo feedback in the Neoproterozoic (Ridgwell and Kennedy, 2004; Chumakov, 2008), even though Poulsen and Jacob (2004) corrected this misconception by showing that Fast Ocean Atmosphere Model (FOAM) does exhibit a runaway ice-albedo feedback when total solar irradiance (TSI) is further reduced to 91%
Our study demonstrates that Snowball Earth initiation for Marinoan total solar irradiance (94% of the present-day value) in the most sophisticated climate model hitherto applied is possible at similar or even higher carbon dioxide levels than in simpler models without ocean dynamics (Donnadieu et al, 2004; Micheels and Montenari, 2008; Chandler and Sohl, 2000; Pollard and Kasting, 2004)
Summary
The apparent existence of low-latitude land glaciers at sea level during at least two episodes of the Neoproterozoic era, the Sturtian (∼710 million years before present, Ma) and the Marinoan (∼635 Ma) (Evans, 2000; Trindade and Macouin, 2007; Macdonald et al, 2010), has led to the proposal that these glaciations were accompanied by completely ice-covered oceans These states have become popular under the term “(hard) Snowball Earth” and are at the heart of the Snowball Earth hypothesis (Kirschvink, 1992; Hoffman et al, 1998), which yields explanations for banded iron formations and cap carbonates (Hoffman and Schrag, 2002). We investigate the total solar irradiance (TSI) and atmospheric CO2 level that cause a Snowball Earth bifurcation as well as the maximum stable sea-ice cover By comparing these simulations to previous simulations with the same model for present-day surface boundary conditions (Voigt and Marotzke, 2010), we find that low-latitude continents favor Snowball Earth initiation, as suggested by Kirschvink (1992). The appendix points at an imbalance of the diagnosed global-mean top of atmosphere and surface energy fluxes found in some of our simulations
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