Editing Snowball Earth (section)

==Mechanisms==
[[Image:SnowballSimulations.jpg|350px|thumb|right|One computer simulation of conditions during a snowball Earth period<ref name=Hyde2000 />]]

The initiation of a snowball Earth event would involve some initial cooling mechanism, which would result in an increase in Earth's coverage of snow and ice. The increase in Earth's coverage of snow and ice would in turn increase Earth's [[albedo]], which would result in positive feedback for cooling. If enough snow and ice accumulates, run-away cooling would result. This positive feedback is facilitated by an equatorial continental distribution, which would allow ice to accumulate in the regions closer to the equator, where [[solar radiation]] is most direct.<ref>{{Cite web |date=2019-02-11 |title=How planets die: climate catastrophe! – planetplanet |url=https://planetplanet.net/2019/02/11/how-planets-die-climate-catastrophe/ |access-date=2025-01-25 |language=en-US}}</ref>

Many possible triggering mechanisms could account for the beginning of a snowball Earth, such as a reduction in the atmospheric concentration of [[greenhouse gas]]es including [[methane]] and/or carbon dioxide, the eruption of a [[supervolcano]], changes in [[solar variation|Solar energy output]], or perturbations of [[Earth's orbit]]. The start of the [[Cryogenian]] period is known to correspond with a rapid rise in atmospheric oxygen known as the [[Neoproterozoic Oxygenation Event]] (NOE).  This rise in atmospheric oxygen resulted in a reduction in atmospheric methane, a potent greenhouse gas. Regardless of the trigger, initial cooling results in an increase in the area of Earth's surface covered by ice and snow, and the additional ice and snow reflects more solar energy back to space, further cooling Earth and further increasing the area of Earth's surface covered by ice and snow. This [[positive feedback]] loop could eventually produce a frozen equator as cold as modern Antarctica.<ref name="Hyde20002">{{cite journal |last1=Hyde |first1=William T. |last2=Crowley |first2=Thomas J. |last3=Baum |first3=Steven K. |last4=Peltier |first4=W. Richard |date=May 2000 |title=Neoproterozoic 'snowball Earth' simulations with a coupled climate/ice-sheet model |journal=[[Nature (journal)|Nature]] |volume=405 |issue=6785 |pages=425–429 |bibcode=2000Natur.405..425H |doi=10.1038/35013005 |pmid=10839531 |s2cid=1672712}}</ref>

Global warming associated with large accumulations of carbon dioxide in the atmosphere over millions of years, emitted primarily by volcanic activity, is the proposed trigger for melting a snowball Earth. Due to positive feedback for melting, the eventual melting of the snow and ice covering most of Earth's surface would require as little as a millennium.

===Initiation of glaciation===
A tropical distribution of the continents is, perhaps counter-intuitively, necessary to allow the initiation of a snowball Earth.<ref name=Hoffman2005/> Tropical continents are more reflective than open ocean and so absorb less of the Sun's heat: most absorption of solar energy on Earth today occurs in tropical oceans.<ref name=Jacobsen2001>{{cite journal
| author = Jacobsen, S.B.
| year = 2001
| title = Earth science. Gas hydrates and deglaciations
| journal = [[Nature (journal)|Nature]]
| volume = 412
| issue = 6848
| pages = 691–3
| doi = 10.1038/35089168
| pmid = 11507621
| bibcode = 2001Natur.412..691J
| s2cid = 4339151
}}</ref> Further, tropical continents are subject to more rainfall, which leads to increased river discharge and erosion. When exposed to air, [[silicate]] rocks undergo weathering reactions which remove carbon dioxide from the atmosphere. These reactions proceed in the general form
: Rock-forming mineral + CO<sub>2</sub> + H<sub>2</sub>O → cations + bicarbonate + SiO<sub>2</sub>
An example of such a reaction is the weathering of [[wollastonite]]:
: CaSiO<sub>3</sub> + 2 CO<sub>2</sub> + H<sub>2</sub>O → Ca<sup>2+</sup> + SiO<sub>2</sub> + 2 {{chem|HCO|3|−}}

The released [[calcium]] cations react with the dissolved [[bicarbonate]] in the ocean to form [[calcium carbonate]] as a chemically precipitated sedimentary rock. This transfers carbon dioxide, a greenhouse gas, from the air into the [[geosphere]], and, in steady-state on geologic time scales, offsets the carbon dioxide emitted from volcanoes into the atmosphere.

As of 2003, a precise continental distribution during the Neoproterozoic was difficult to establish because there were too few suitable sediments for analysis.<ref name=Meert2004>{{cite book
| author = Meert, J.G.
|author2=Torsvik, T.H.
 | year = 2004
| title = Paleomagnetic Constraints on Neoproterozoic 'Snowball Earth' Continental Reconstructions
|journal=Washington DC American Geophysical Union Geophysical Monograph Series
 | editor = GS Jenkins
| editor2 = MAS McMenamin
| editor3 = CP McKey
| editor4 = CP Sohl
| editor5 = L Sohl
| publisher=American Geophysical Union
| volume = 146
| pages = 5–11
| doi = 10.1029/146GM02
| series = Geophysical Monograph Series
| isbn = 978-0-87590-411-5
|bibcode = 2004GMS...146....5M |citeseerx=10.1.1.368.2259
 }}</ref> Some reconstructions point towards polar continents—which have been a feature of all other major glaciations, providing a point upon which ice can nucleate. Changes in ocean circulation patterns may then have provided the trigger of snowball Earth.<ref name=Smith2003>{{cite journal
| author = Smith, A.G.
|author2=Pickering, K.T.
 | year = 2003
| title = Oceanic gateways as a critical factor to initiate icehouse Earth
| journal = [[Journal of the Geological Society]]
| volume = 160
| issue= 3
| pages= 337–40
| doi = 10.1144/0016-764902-115
|bibcode=2003JGSoc.160..337S
 |s2cid=127653725
 }}</ref>

Additional factors that may have contributed to the onset of the Neoproterozoic snowball include the introduction of atmospheric free oxygen, which may have reached sufficient quantities to react with [[Atmospheric methane|methane in the atmosphere]], oxidizing it to carbon dioxide, a much weaker greenhouse gas,<ref name=Kerr1999>{{cite journal
| author = Kerr, R.A.
| year = 1999
| title = Early life thrived despite earthly travails
| journal = [[Science (journal)|Science]]
| volume = 284
| issue = 5423
| pages = 2111–3
| doi = 10.1126/science.284.5423.2111
| pmid = 10409069
| s2cid = 32695874
}}</ref> and a younger—thus fainter—Sun, which would have emitted 6 percent less radiation in the Neoproterozoic.<ref name=Eyles2004 />

Normally, as Earth gets colder due to natural climatic fluctuations and changes in incoming solar radiation, the cooling slows these weathering reactions. As a result, less carbon dioxide is removed from the atmosphere and Earth warms as this greenhouse gas accumulates—this '[[negative feedback]]' process limits the magnitude of cooling. During the Cryogenian, however, Earth's continents were all at [[Tropics|tropical]] latitudes, which made this moderating process less effective, as high weathering rates continued on land even as Earth cooled. This caused ice to advance beyond the polar regions. Once ice advanced to within 30° of the equator,<ref name=Kirschvink2002>{{cite journal
| author = Kirschvink, J.L.
| year = 2002
| title = When All of the Oceans Were Frozen
| journal = [[La Recherche]]
| volume = 355
| pages = 26–30
| url = http://web.gps.caltech.edu/~jkirschvink/pdfs/laRechercheEnglish.pdf
}}</ref> a positive feedback could ensue such that the increased reflectiveness (albedo) of the ice led to further cooling and the formation of more ice, until the whole Earth is ice-covered.

Polar continents, because of low rates of [[evaporation]], are too dry to allow substantial carbon deposition—restricting the amount of atmospheric carbon dioxide that can be removed from the [[carbon cycle]]. A gradual rise of the proportion of the isotope <sup>13</sup>C relative to <sup>12</sup>C in sediments pre-dating "global" glaciation indicates that {{co2}} draw-down before snowball Earths was a slow and continuous process.<ref name=Schrag2002>{{cite journal |last1=Schrag |first1=Daniel P. |last2=Berner |first2=Robert A. |last3=Hoffman |first3=Paul F. |last4=Halverson |first4=Galen P. |title=On the initiation of a snowball Earth |journal=[[Geochemistry, Geophysics, Geosystems]] |date=June 2002 |volume=3 |issue=6 |page=1036 |doi=10.1029/2001GC000219 |doi-access=free |bibcode=2002GGG.....3.1036S }}</ref> The start of snowball Earths are marked by a sharp downturn in the δ<sup>13</sup>C value of sediments,<ref name="Hoffman1998">{{cite journal
| author = Hoffman, P.F.
|author2=Kaufman, A.J. |author3=Halverson, G.P. |author4=Schrag, D.P. 
| date = 28 August 1998
| title = A Neoproterozoic Snowball Earth
| journal = [[Science (journal)|Science]]
| volume = 281
| issue = 5381
| pages = 1342–6
| doi = 10.1126/science.281.5381.1342
| pmid = 9721097
| bibcode=1998Sci...281.1342H
|s2cid=13046760 |url=https://semanticscholar.org/paper/37b1ffb6aadc8b436c029fb8a3311c0b26e30d4e }}</ref> a hallmark that may be attributed to a crash in biological productivity as a result of the cold temperatures and ice-covered oceans.

In January 2016, Gernon et al. proposed a "shallow-ridge hypothesis" involving the breakup of Rodinia, linking the eruption and rapid alteration of [[hyaloclastite]]s along shallow ridges to massive increases in alkalinity in an ocean with thick ice cover. Gernon et al. demonstrated that the increase in alkalinity over the course of glaciation is sufficient to explain the thickness of cap carbonates formed in the aftermath of Snowball Earth events.<ref>{{cite journal |last1=Gernon |first1=T. M. |last2=Hincks |first2=T. K. |last3=Tyrrell |first3=T. |last4=Rohling |first4=E. J. |last5=Palmer |first5=M. R. |title=Snowball Earth ocean chemistry driven by extensive ridge volcanism during Rodinia breakup |journal=[[Nature Geoscience]] |date=18 January 2016 |volume=9 |issue=3 |pages=242–8 |doi=10.1038/ngeo2632 |bibcode=2016NatGe...9..242G |s2cid=1642013 |url=https://research-information.bristol.ac.uk/files/60401094/snowball_NG_gernon_final.pdf}}
*{{cite web |author=Thomas Gernon |date=18 January 2016 |title=How 'Snowball Earth' volcanoes altered oceans to help kickstart animal life |website=The Conversation |url=https://theconversation.com/how-snowball-earth-volcanoes-altered-oceans-to-help-kickstart-animal-life-53280}}</ref>

Dating of the Sturtian glaciation's onset has found it to be coeval with the emplacement of a large igneous province in the tropics. Weathering of this equatorial large igneous province is believed to have sucked enough carbon dioxide out of the air to enable the development of major glaciation.<ref>{{cite journal |last1=Hoffman |first1=Paul F. |last2=Abbot |first2=Dorian S. |last3=Ashkenazy |first3=Yosef |last4=Benn |first4=Douglas I. |last5=Brocks |first5=Jochen J. |last6=Cohen |first6=Phoebe A. |last7=Cox |first7=Grant M. |last8=Creveling |first8=Jessica R. |last9=Donnadieu |first9=Yannick |last10=Erwin |first10=Douglas H. |last11=Fairchild |first11=Ian J. |last12=Ferreira |first12=David |last13=Goodman |first13=Jason C. |last14=Halverson |first14=Galen P. |last15=Jansen |first15=Malte F. |last16=Le Hir |first16=Guillaume |last17=Love |first17=Gordon D. |last18=MacDonald |first18=Francis A. |last19=Maloof |first19=Adam G. |last20=Partin |first20=Camille A. |last21=Ramstein |first21=Gilles |last22=Rose |first22=Brian E. J. |last23=Rose |first23=Catherine V. |last24=Sadler |first24=Peter M. |last25=Tziperman |first25=Eli |last26=Voigt |first26=Aiko |last27=Warren |first27=Stephen G. |date=8 November 2017 |title=Snowball Earth climate dynamics and Cryogenian geology-geobiology |journal=[[Science Advances]] |volume=3 |issue=11 |pages=e1600983 |doi=10.1126/sciadv.1600983 |pmid=29134193 |pmc=5677351 |bibcode=2017SciA....3E0983H }}</ref>

===During the frozen period===
[[Image:AntarcticaDomeCSnow.jpg|thumb|250px|Global ice sheets may have created the bottleneck required for the evolution of multicellular life.<ref name=Kirschvink1992 />]]
Global temperature fell so low that the equator was as cold as modern-day Antarctica.<ref name=Hyde2000>{{cite journal |last1=Hyde |first1=William T. |last2=Crowley |first2=Thomas J. |last3=Baum |first3=Steven K. |last4=Peltier |first4=W. Richard |title=Neoproterozoic 'snowball Earth' simulations with a coupled climate/ice-sheet model |journal=[[Nature (journal)|Nature]] |date=May 2000 |volume=405 |issue=6785 |pages=425–429 |doi=10.1038/35013005 |pmid=10839531 |bibcode=2000Natur.405..425H |s2cid=1672712 }}</ref> This low temperature was maintained by the high albedo of the ice sheets, which reflected most incoming solar energy into space. A lack of heat-retaining clouds, caused by water vapor freezing out of the atmosphere, amplified this effect. Degassing of carbon dioxide has been speculated to have been unusually low during the Cryogenian, enabling the persistence of global glaciation.<ref>{{cite journal |last1=Mills |first1=Benjamin J. W. |last2=Scotese |first2=Christopher R. |last3=Walding |first3=Nicholas G. |last4=Shields |first4=Graham A. |last5=Lenton |first5=Timothy M. |date=24 October 2017 |title=Elevated CO2 degassing rates prevented the return of Snowball Earth during the Phanerozoic |journal=[[Nature Communications]] |volume=8 |issue=1 |page=1110 |doi=10.1038/s41467-017-01456-w |pmid=29062095 |pmc=5736558 }}</ref>

===Breaking out of global glaciation===
The carbon dioxide levels necessary to thaw Earth have been estimated as being 350 times what they are today, about 13% of the atmosphere.<ref name=Crowley2001>{{cite journal
| author = Crowley, T.J.
|author2=Hyde, W.T. |author3=Peltier, W.R. 
| year = 2001
| title = CO 2 levels required for deglaciation of a 'near-snowball' Earth
| journal = [[Geophysical Research Letters]]
| volume = 28
| pages = 283–6
| doi = 10.1029/2000GL011836
| bibcode=2001GeoRL..28..283C
| issue = 2
| doi-access = 
|s2cid=129246869 }}</ref> Since Earth was almost completely covered with ice, carbon dioxide could not be withdrawn from the atmosphere by release of alkaline metal ions weathering out of [[siliceous rock]]s. Over 4 to 30 million years, enough {{co2}} and methane, mainly emitted by volcanoes but also produced by microbes converting organic carbon trapped under the ice into the gas,<ref>{{cite web| url = http://www.antarcticglaciers.org/modern-glaciers/glacier-ecosystems/| title = Glacier ecosystems| date = 17 July 2020}}</ref> would accumulate to finally cause enough greenhouse effect to make surface ice melt in the tropics until a band of permanently ice-free land and water developed; this would be darker than the ice and thus absorb more energy from the Sun—initiating a "positive feedback".<ref name=Pierrehumbert2004>{{cite journal
| author = Pierrehumbert, R.T.
| year = 2004
| title = High levels of atmospheric carbon dioxide necessary for the termination of global glaciation
| journal = [[Nature (journal)|Nature]]
| volume = 429
| pages = 646–9
| doi = 10.1038/nature02640
| pmid = 15190348
| issue = 6992
|bibcode = 2004Natur.429..646P | s2cid = 2205883
}}</ref>

The first areas to become free of permanent ice cover may have been in the mid-latitudes rather than in the tropics, because a rapid hydrological cycle would have inhibited the melting of ice at low latitudes. As these mid-latitude regions became ice free, dust from them blew over onto ice sheets elsewhere, decreasing their albedo and accelerating the process of deglaciation.<ref>{{cite journal |last1=De Vrese |first1=Philipp |last2=Stacke |first2=Tobias |last3=Rugenstein |first3=Jeremy Caves |last4=Goodman |first4=Jason |last5=Brovkin |first5=Victor |date=14 May 2021 |title=Snowfall-albedo feedbacks could have led to deglaciation of snowball Earth starting from mid-latitudes |journal=[[Communications Earth & Environment]] |volume=2 |issue=1 |page=91 |doi=10.1038/s43247-021-00160-4 |doi-access=free |bibcode=2021ComEE...2...91D }}</ref> Destabilization of substantial deposits of [[methane hydrate]]s locked up in low-latitude [[permafrost]] may also have acted as a trigger and/or strong positive feedback for deglaciation and warming.<ref>{{cite journal |last1=Kennedy |first1=Martin |last2=Mrofka |first2=David |last3=von der Borch |first3=Chris |title=Snowball Earth termination by destabilization of equatorial permafrost methane clathrate |journal=[[Nature (journal)|Nature]] |date=29 May 2008 |volume=453 |issue=7195 |pages=642–645 |doi=10.1038/nature06961 |pmid=18509441 |bibcode=2008Natur.453..642K |s2cid=4416812 }}</ref>

Methanogens were an important contributor to the deglaciation of the Marinoan Snowball Earth. The return of high primary productivity in surficial waters fueled extensive microbial sulphur reduction, causing deeper waters to become highly euxinic. [[Euxinia]] caused the formation of large amounts of methyl sulphides, which in turn was converted into methane by methanogens. A major negative nickel isotope excursion confirms high methanogenic activity during this period of deglaciation and global warming.<ref>{{cite journal |last1=Zhao |first1=Zhouqiao |last2=Shen |first2=Bing |last3=Zhu |first3=Jian-Ming |last4=Lang |first4=Xianguo |last5=Wu |first5=Guangliang |last6=Tan |first6=Decan |last7=Pei |first7=Haoxiang |last8=Huang |first8=Tianzheng |last9=Ning |first9=Meng |last10=Ma |first10=Haoran |date=11 February 2021 |title=Active methanogenesis during the melting of Marinoan snowball Earth |journal=[[Nature Communications]] |volume=12 |issue=1 |page=955 |doi=10.1038/s41467-021-21114-6 |pmid=33574253 |pmc=7878791 |bibcode=2021NatCo..12..955Z }}</ref>

On the continents, the melting of glaciers would release massive amounts of glacial deposit, which would erode and weather. The resulting sediments supplied to the ocean would be high in nutrients such as [[phosphorus]], which combined with the abundance of {{co2}} would trigger a [[cyanobacteria]] population explosion, which would cause a relatively rapid reoxygenation of the atmosphere and may have contributed to the rise of the [[Ediacaran biota]] and the subsequent [[Cambrian explosion]]—a higher oxygen concentration allowing large [[Multicellular organism|multicellular]] lifeforms to develop. Although the positive feedback loop would melt the ice in geological short order, perhaps less than 1,000 years, replenishment of atmospheric oxygen and depletion of the {{co2}} levels would take further millennia. 

It is possible that carbon dioxide levels fell enough for Earth to freeze again; this cycle may have repeated until the [[continental drift|continents had drifted]] to more polar latitudes.<ref name="Hoffman1999">{{cite journal
| author = Hoffman, P.F.
| year = 1999
| title = The break-up of Rodinia, birth of Gondwana, true polar wander and the snowball Earth
| journal = [[Journal of African Earth Sciences]]
| volume = 28
| issue = 1
| pages = 17–33
| doi = 10.1016/S0899-5362(99)00018-4
| bibcode=1999JAfES..28...17H
}}</ref> More recent evidence suggests that with colder oceanic temperatures, the resulting higher ability of the oceans to dissolve gases led to the carbon content of sea water being more quickly oxidized to carbon dioxide. This leads directly to an increase of atmospheric carbon dioxide, enhanced greenhouse warming of Earth's surface, and the prevention of a total snowball state.<ref>{{cite journal | last1 = Peltier | last2 = Richard | first2 = W. | last3 = Liu | first3 = Yonggang | last4 = Crowley | first4 = John W. | year = 2007 | title = Snowball Earth prevention by dissolved organic carbon remineralization | journal = [[Nature (journal)|Nature]] | volume = 450 | issue = 7171| pages = 813–818 | doi = 10.1038/nature06354 | pmid = 18064001| bibcode = 2007Natur.450..813P | s2cid = 4406636 }}</ref>

During millions of years, [[cryoconite]] would have accumulated on and inside the ice. [[Psychrophile|Psychrophilic]] microorganisms, volcanic ash and dust from ice-free locations would settle on ice covering several million square kilometers. Once the ice started to melt, these layers would become visible and darken the icy surfaces, helping to accelerate the process.<ref>{{cite journal| pmid=27422766 | doi=10.1111/gbi.12191 | volume=14 | issue=6 | title=Cryoconite pans on Snowball Earth: supraglacial oases for Cryogenian eukaryotes? | year=2016 | journal=[[Geobiology (journal)|Geobiology]]| pages=531–542 | author=Hoffman PF| bibcode=2016Gbio...14..531H | s2cid=21261198 }}</ref> Also, ultraviolet light from the Sun [[Hydrogen_peroxide#Other_sources|produced hydrogen peroxide]] (H<sub>2</sub>O<sub>2</sub>) when it hit water molecules. Normally H<sub>2</sub>O<sub>2</sub> breaks down in sunlight, but some would have been trapped inside the ice. When the glaciers started to melt, it would have been released in both the ocean and the atmosphere, where it was split into water and oxygen molecules, increasing atmospheric oxygen.<ref>{{cite web| url = https://www.newscientist.com/article/dn10662-did-snowball-earths-melting-let-oxygen-fuel-life/| title = Did snowball Earth's melting let oxygen fuel life?}}</ref>

===Slushball Earth hypothesis===
[[File:SIM neoproto.ogv|thumb|thumbtime=0:20|right|Simulation with liquid water around the equator]]
While the presence of glaciers is not disputed, the idea that the entire planet was covered in ice is more contentious, leading some scientists to posit a "slushball Earth", in which a band of ice-free, or ice-thin, waters remains around the equator, allowing for a continued hydrologic cycle. This hypothesis appeals to scientists who observe certain features of the sedimentary record that can only be formed under open water or rapidly moving ice (which would require somewhere ice-free to move to). Recent research observed geochemical cyclicity in [[clastic rocks]], showing that the snowball periods were punctuated by warm spells, similar to ice age cycles in recent Earth history. Attempts to construct computer models of a snowball Earth have struggled to accommodate global ice cover without fundamental changes in the laws and constants which govern the planet.

A less extreme snowball Earth hypothesis involves continually evolving continental configurations and changes in ocean circulation.<ref name="Harland2007">{{cite journal
| last1=Harland
| first1=W. B.
| year= 2007
| title=Origin and assessment of Snowball Earth hypotheses
| journal=[[Geological Magazine]]
| volume=144
| issue=4
| pages=633–42
| doi=10.1017/S0016756807003391| bibcode=2007GeoM..144..633H
| s2cid=10947285
}}</ref> Synthesised evidence has produced slushball Earth models<ref name="Fairchild2007">{{cite journal
| last1=Fairchild
| first1=I. J.
| last2=Kennedy
| first2=M. J.
| year=2007
| title=Neoproterozoic glaciations in the Earth System
| journal=[[Journal of the Geological Society]]
| volume=164
| pages=895–921
| doi=10.1144/0016-76492006-191
| issue=5
| citeseerx=10.1.1.211.2233
| bibcode=2007JGSoc.164..895F
| s2cid=16713707
}}</ref> where the stratigraphic record does not permit postulating complete global glaciations.<ref name="Harland2007" /> Kirschvink's original hypothesis<ref name="Kirschvink" /> had recognised that warm tropical puddles would be expected to exist in a snowball Earth.

A more extreme hypothesis, the Waterbelt Earth hypothesis, suggests that ice-free areas of ocean continued to exist even as tropical continents were glaciated.<ref>{{Cite journal |last1=Bechstädt |first1=Thilo |last2=Jäger |first2=Hartmut |last3=Rittersbacher |first3=Andreas |last4=Schweisfurth |first4=Bolko |last5=Spence |first5=Guy |last6=Werner |first6=Georg |last7=Boni |first7=Maria |date=February 2018 |title=The Cryogenian Ghaub Formation of Namibia – New insights into Neoproterozoic glaciations |journal=[[Earth-Science Reviews]] |language=en |volume=177 |pages=678–714 |doi=10.1016/j.earscirev.2017.11.028 |doi-access=free |bibcode=2018ESRv..177..678B }}</ref>