Editing Snowball Earth (section)

==Occurrence and timing==
===Palaeoproterozoic===
{{main|Huronian glaciation}}
The Snowball Earth hypothesis has been invoked to explain glacial deposits in the [[Huronian Supergroup]] of [[Canada]], though the palaeomagnetic evidence that suggests ice sheets at low latitudes is contested,<ref name="Williams">{{cite journal
|author1=Williams G.E. |author2=Schmidt P.W. | title=Paleomagnetism of the Paleoproterozoic Gowganda and Lorrain formations, Ontario: low palaeolatitude for Huronian glaciation
| journal=[[Earth and Planetary Science Letters]]
| year=1997
| volume=153
| issue=3
| pages=157–169
| doi = 10.1016/S0012-821X(97)00181-7
| bibcode=1997E&PSL.153..157W}}</ref><ref name="Kopp" /> and stratigraphic evidence clearly shows only three distinct depositions of glacial material (the Ramsay, Bruce and Gowganda Formations) separated by significant periods without. The glacial sediments of the Makganyene formation of South Africa are slightly younger than the Huronian glacial deposits (~2.25 billion years old) and were possibly deposited at tropical latitudes.<ref>{{cite journal |last1=Evans |first1=D. A. |last2=Beukes |first2=N. J. |last3=Kirschvink |first3=J. L. |title=Low-latitude glaciation in the Palaeoproterozoic era |journal=[[Nature (journal)|Nature]] |date=March 1997 |volume=386 |issue=6622 |pages=262–266 |doi=10.1038/386262a0 |bibcode=1997Natur.386..262E |s2cid=4364730 }}</ref> It has been proposed that rise of free oxygen that occurred during the Great Oxygenation Event removed [[atmospheric methane]] through oxidation. As the [[solar irradiance]] was notably weaker at the time, Earth's climate may have relied on methane, a powerful greenhouse gas, to maintain surface temperatures above freezing. In the absence of this methane greenhousing, temperatures plunged and a global glaciation could have occurred between 2.5 and 2.2 [[giga annum|Gya]], during the [[Siderian]] and [[Rhyacian]] periods of the Paleoproterozoic era.<ref name="Kopp">{{cite journal
|author1=Robert E. Kopp |author2=Joseph L. Kirschvink |author3=Isaac A. Hilburn |author4=Cody Z. Nash  |name-list-style=amp | title=The Paleoproterozoic snowball Earth: A climate disaster triggered by the evolution of oxygenic photosynthesis
| journal=[[Proceedings of the National Academy of Sciences of the United States of America]]
| year=2005
| volume=102
| issue=32
| pages=11131–6
| doi=10.1073/pnas.0504878102
| pmid=16061801
| pmc=1183582
| bibcode=2005PNAS..10211131K|doi-access=free }}</ref>

===Neoproterozoic===
{{main|Sturtian glaciation|Marinoan glaciation|Baykonurian glaciation}}
There were three or four significant ice ages during the late Neoproterozoic. Of these, the Marinoan was the most significant, and the Sturtian glaciations were also widespread.<ref name=Stern2006>{{cite journal
| author = Stern, R.J. |author2=Avigad, D. |author3=Miller, N.R. |author4=Beyth, M.
| year = 2006
| title = Geological Society of Africa Presidential Review: Evidence for the Snowball Earth Hypothesis in the Arabian-Nubian Shield and the East African Orogen
| journal = [[Journal of African Earth Sciences]]
| volume = 44
|issue=1 | pages = 1–20
| doi = 10.1016/j.jafrearsci.2005.10.003|bibcode = 2006JAfES..44....1S }}</ref> Even the leading snowball proponent Hoffman agrees that the 350 thousand-year-long<ref name=Pu/> [[Gaskiers glaciation]] did not lead to global glaciation,<ref name=Hoffman2005>{{cite journal
| author = Hoffman, P.F.
| year = 2005
| title = On Cryogenian (Neoproterozoic) ice-sheet dynamics and the limitations of the glacial sedimentary record
| journal = [[South African Journal of Geology]]
| volume = 108
| pages = 557–77
| doi = 10.2113/108.4.557
| issue = 4
}}</ref> although it was probably as intense as the [[Andean-Saharan glaciation|late Ordovician glaciation]]. The status of the [[Kaigas]] "glaciation" or "cooling event" is currently unclear; some scientists do not recognise it as a glacial, others suspect that it may reflect poorly dated strata of Sturtian association, and others believe it may indeed be a third ice age.<ref name=Smith2008>{{Cite journal | last1 = Smith | first1 = A. G. | doi = 10.1144/SP326.2 | title = Neoproterozoic timescales and stratigraphy | journal =  Geological Society, London, Special Publications| volume = 326 | pages = 27–54 | year = 2009 | issue = 1 | bibcode=2009GSLSP.326...27S| s2cid = 129706604 }}</ref> It was certainly less significant than the Sturtian or Marinoan glaciations, and probably not global in extent. Emerging evidence suggests that Earth underwent a number of glaciations during the Neoproterozoic, which would stand strongly at odds with the snowball hypothesis.<ref name=nature_geo/>