Snowball Earth Background:
- Hypothesis that the Earth was entirely frozen over on 3 separate occasions (Marinoan, Sturtian, Kaigas) during the Cryogenian period (850-630 million years ago).
- Evidence for the hypothesis comes from world glacial rock formations which date to the Cryogenian period.
- Mikhail Budyko, a Russian climatologist, in the 1960s developed a climate model to investigate the effect of ice cover on global climate.
- Budyko's results suggested that if ice sheets advanced far enough out of the polar regions a feedback ensued where the increased reflectiveness (albedo) of the ice led to further cooling & the formation of more ice.
- Eventually the the entire Earth could be covered in ice and stabilized in a new ice-covered equilibrium.
- Budyko (1969) concluded that this event could never happened, because his model suggested no way to escape from the simulated outcome. 
- "Snowball Earth" was first coined by Joseph Kirschvink (professor of geobiology at the California Institute of Technology) in 1992.
- The hypothesis was originally considered to help explain the apparent presence of glaciers at tropical latitudes.
- A recent report suggests that the Earth may have been more mudball than snowball.
- It was hypothesised that CO2 levels may have risen during the snowball period, leading to a thaw in the Earth, though CO2 levels seem to have been too low for this.
- Recent research suggests that areas with a higher level of volcanic dust or weathered rock, may have been able to absorb sunlight enough to cause a rewarming.
- Organism size & complexity increased markedly after the end of the Snowball Earth glaciations.
- Evolutionary pressures may have been increased by "icehouse-hothouse" cycles, leading to multicellular organism development.
- Fluctuations in nutrient levels and oxygen may have contributed to evolutionary selective pressures.
- It is hypothesised that kin selection (with organ-like differentiation) may have been a driver for more complex life; this occurring at the expense of individual cell reproduction.
- The evolutionary pressure producing high relatedness (i.e. kin selection) associated with glaciations may have helped overcome the reproductive cost of forming a complex animal.
 - M.I. Budyko (1969). "Effect of solar radiation variation on climate of Earth". Tellus 21 (5): 611 – 1969.
 - Kirschvink, Joseph (1992). "Late Proterozoic low-latitude global glaciation: the Snowball Earth". in J. W. Schopf; C. Klein. The Proterozoic Biosphere: A Multidisciplinary Study. Cambridge University Press.
 - Harland, W.B. (1964). "Critical evidence for a great infra-Cambrian glaciation" (PDF). International Journal of Earth Sciences 54 (1): 45–61. http://www.springerlink.com/index/KW2790433113J4LX.pdf.
 - Dorian Abbot & Raymond Pierrehumbert (University of Chicago). Journal of Geophysical Research – Atmospheres, DOI: 10.1029/2009jd012007, in press.
Albedo effect Nasa image (public domain)
Snowball Earth by guano (cc)
Posted by ALCHEssMIST.
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