Recent geological studies of the Neoproterozoic era (around 600-800 million years ago), have suggested that the Earth has experienced global glaciation events, termed 'snowball' Earth (Hoffman and Schrag, 2002). Paleomagnetic studies of equatorial carbonate deposits, the prolonged drop in biological activity and the formation of rich of iron rocks, which are formed in the absence of oxygen, indicate widespread glaciation (Kirschvink et al., 2000; Hoffman and Schrag, 2002). Based on the magnetic orientation of mineral grains in glacial deposits, it is hypothesized that the continents were clustered together near the equator during that time. The Neoproterozoic era is also characterized by a reduced solar insolation of 6%, due to the Faint Young Sun. Still, the question exists, whether the Earth was completely ice covered ('hard snowball' Earth) or some ocean areas remained ice free ('slushball' Earth), and what was the mechanism that forced the climate system to escape from the laciated state.

The geological findings and the enigmas connected with this time period provoke climate modelers to test their models under extreme boundary conditions, to produce a global earth glaciation (e.g. Hyde et al., 2000; Chandler and Sohl, 2000; Crowley et al., 2001; Poulsen et al., 2002; Lewis et al., 2003; Donnadieu et al., 2004). Using different types of models, the scientists investigate the role of changed solar insolation, Earth's rotation rate and high obliquity, the contribution of the changed paleogeography and continental geometries, and change of CO2 concentration.

Examples:

• Crowley, T.J., Hyde, W.T., Peltier, W.R., 2001. CO2 levels required for deglaciation of a “Near Snowball” Earth. Geophys. Res. Lett. 28, 283-286.
• Chandler, M.A., Sohl, L.E., 2000. Climate forcings and the initiation of low-latitude ice sheets during Neoproterozoic Varanger glacial interval. J. Geophys. Res. 105, 20,737-20,756.
• Donnadieu, Y. et al. (2004) A 'snowball Earth' climate triggered by continental break-up through changes in runoff. Nature 428: 303-306.
• Hoffman, P.F., Schrag, D.P. 2002. The snowball Earth hypothesis: testing the limits of global change. Terra Nova 14: 129-155.
• Hyde, W.T., Crowley, T.J., Baum, S.K., Peltier, W.R. 2000. Neoproterozoic 'snowball Earth' simulations with a coupled climate/ice-sheet model. Nature 405, 425-429.
• Kirschvink, J.L., Gaidos, E.J., Bertani, E., Beukes, N.J., Gutzmer, J., Maepa, L.N., Steinberger, R.E., 2000. Paleoproterozoic snowball Earth: Extreme climatic and geochemical global change and its biological consequences. PNAS 97, 1400-14005.
• Lewis, J.P., Weaver, A.J., Johnston, S.T., Eby, M., 2003. Neoproterozoic 'snowball Earth': Dynamic sea ice over a quiescent ocean. Paleoceanography 18: 10.1029/2003PA000926
• Poulsen, C.J., Jacob, R.L., Pierrehumbert, R.T., Huynh, T.T., 2002. Testing paleogeographic controls on a Neoproterozoic snowball Earth. Geophys. Res. Lett. 29, 10.1029/2001GL014352.
• Romanova, V., 2005. Stability of the Climate System and Extreme Climates in Model Experiments. Ber. Polarforsch. Meeresforsch. Heft 510. Seiten 1-137. PhDwork pdf (9,5 MB)