Abstract
Abstract. The Snowball Earth bifurcation, or runaway ice-albedo feedback, is defined for particular boundary conditions by a critical CO2 and a critical sea-ice cover (SI), both of which are essential for evaluating hypotheses related to Neoproterozoic glaciations. Previous work has shown that the Snowball Earth bifurcation, denoted as (CO2, SI)*, differs greatly among climate models. Here, we study the effect of bare sea-ice albedo, sea-ice dynamics and ocean heat transport on (CO2, SI)* in the atmosphere–ocean general circulation model ECHAM5/MPI-OM with Marinoan (~ 635 Ma) continents and solar insolation (94% of modern). In its standard setup, ECHAM5/MPI-OM initiates a~Snowball Earth much more easily than other climate models at (CO2, SI)* ≈ (500 ppm, 55%). Replacing the model's standard bare sea-ice albedo of 0.75 by a much lower value of 0.45, we find (CO2, SI)* ≈ (204 ppm, 70%). This is consistent with previous work and results from net evaporation and local melting near the sea-ice margin. When we additionally disable sea-ice dynamics, we find that the Snowball Earth bifurcation can be pushed even closer to the equator and occurs at a hundred times lower CO2: (CO2, SI)* ≈ (2 ppm, 85%). Therefore, the simulation of sea-ice dynamics in ECHAM5/MPI-OM is a dominant determinant of its high critical CO2 for Snowball initiation relative to other models. Ocean heat transport has no effect on the critical sea-ice cover and only slightly decreases the critical CO2. For disabled sea-ice dynamics, the state with 85% sea-ice cover is stabilized by the Jormungand mechanism and shares characteristics with the Jormungand climate states. However, there is no indication of the Jormungand bifurcation and hysteresis in ECHAM5/MPI-OM. The state with 85% sea-ice cover therefore is a soft Snowball state rather than a true Jormungand state. Overall, our results demonstrate that differences in sea-ice dynamics schemes can be at least as important as differences in sea-ice albedo for causing the spread in climate models' estimates of the Snowball Earth bifurcation. A detailed understanding of Snowball Earth initiation therefore requires future research on sea-ice dynamics to determine which model's simulation is most realistic.
Highlights
The Neoproterozoic glaciations (∼ 715 Ma and ∼ 635 Ma) are characterized by active, wide-spread continental glaciers in the tropics that reached down to sea level (Evans, 2000; Trindade and Macouin, 2007; Macdonald et al, 2010)
From a climate dynamics point of view, one of the most important questions concerning the Snowball Earth hypothesis is the location of the bifurcation that is associated with the onset of the runaway ice-albedo feedback
We investigate the sensitivity of Snowball Earth initiation to bare sea-ice albedo, sea-ice dynamics, and ocean heat transport
Summary
The Neoproterozoic glaciations (∼ 715 Ma and ∼ 635 Ma) are characterized by active, wide-spread continental glaciers in the tropics that reached down to sea level (Evans, 2000; Trindade and Macouin, 2007; Macdonald et al, 2010). Using FOAM, ocean heat transport was identified as a factor that makes Snowball Earth initiation more difficult (Poulsen et al, 2001), especially through the winddriven component of ocean circulation (Poulsen and Jacob, 2004) Consistent with these FOAM results, other coupled AOGCMs showed substantial ocean heat transport towards the sea-ice margin (Voigt and Marotzke, 2010) or even beyond the sea-ice margin to higher latitudes (Yang et al, 2012a,c). Concerning the critical CO2 our interest is in the relative effect of bare seaice albedo, sea-ice dynamics and ocean heat transport rather than in the absolute CO2 estimate This is because uncertainties in the boundary conditions (e.g. continental configuration, aerosols, greenhouse gases other than CO2, precise level of solar constant, land surface albedo) and model physics (e.g. precise sea-ice albedo, effect of dust) translate to uncertainty in the absolute CO2.
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