Editing Ice age (section)

==Major ice ages==
{{For timeline|Timeline of glaciation}}
[[File:GlaciationsinEarthExistancelicenced annotated.jpg|thumb|upright=2.75|right|Timeline of glaciations, shown in blue]]

There have been at least five major ice ages in Earth's history (the [[Huronian glaciation|Huronian]], [[Cryogenian]], [[Andean-Saharan]], [[late Paleozoic icehouse|late Paleozoic]], and the latest [[Quaternary glaciation|Quaternary Ice Age]]). Outside these ages, Earth was previously thought to have been ice-free even in high latitudes;<ref>{{cite journal |author=Lockwood, J.G. |title=The Antarctic Ice-Sheet: Regulator of Global Climates?: Review |journal=The Geographical Journal |volume=145 |issue=3 |pages=469–471 |date=November 1979 |jstor=633219 |doi=10.2307/633219 |last2=Zinderen-Bakker |first2=E. M. van|author-link2=Eduard Meine van Zinderen-Bakker}}</ref><ref>{{cite book |url=https://books.google.com/books?id=ihny39BvVhIC&pg=PA289 |title=Evaporites: sediments, resources and hydrocarbons |first=John K. |last=Warren |publisher=Birkhäuser |year=2006 |isbn=978-3-540-26011-0 |page=289}}</ref> such periods are known as [[Greenhouse and icehouse Earth#Greenhouse Earth|greenhouse periods]].<ref>{{cite book |last=Allaby |first=Michael |date=January 2013 |title=A Dictionary of Geology and Earth Sciences |edition=Fourth |url=https://oxfordindex.oup.com/view/10.1093/acref/9780199653065.013.3641 |access-date=17 Sep 2019 |publisher=Oxford University Press |isbn=9780199653065 }}{{Dead link|date=May 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> However, other studies dispute this, finding evidence of occasional glaciations at high latitudes even during apparent greenhouse periods.<ref name=":0">{{Cite journal |last1=Bornemann |first1=André |last2=Norris |first2=Richard D. |last3=Friedrich |first3=Oliver |last4=Beckmann |first4=Britta |last5=Schouten |first5=Stefan |last6=Damsté |first6=Jaap S. Sinninghe |last7=Vogel |first7=Jennifer |last8=Hofmann |first8=Peter |last9=Wagner |first9=Thomas |date=2008-01-11 |title=Isotopic Evidence for Glaciation During the Cretaceous Supergreenhouse |url=https://www.science.org/doi/10.1126/science.1148777 |journal=Science |language=en |volume=319 |issue=5860 |pages=189–192 |doi=10.1126/science.1148777 |pmid=18187651 |bibcode=2008Sci...319..189B |s2cid=206509273 |issn=0036-8075 |access-date=2023-10-26 |archive-date=2023-11-25 |archive-url=https://web.archive.org/web/20231125035757/https://www.science.org/doi/10.1126/science.1148777 |url-status=live }}</ref><ref name=":1">{{Cite journal |last1=Ladant |first1=Jean-Baptiste |last2=Donnadieu |first2=Yannick |date=2016-09-21 |title=Palaeogeographic regulation of glacial events during the Cretaceous supergreenhouse |journal=Nature Communications |language=en |volume=7 |issue=1 |pages=12771 |doi=10.1038/ncomms12771 |pmid=27650167 |pmc=5036002 |bibcode=2016NatCo...712771L |issn=2041-1723|doi-access=free }}</ref>

[[File:EisrandlagenNorddeutschland.png|thumb|right|Ice age map of northern Germany and its northern neighbours. Red: maximum limit of [[Weichselian]] glacial; yellow: [[Saale glaciation|Saale]] glacial at maximum (Drenthe stage); blue: [[Anglian glaciation|Elster]] glacial maximum glaciation.]]Rocks from the earliest well-established ice age, called the [[Huronian]], have been dated to around 2.4 to 2.1 billion years ago during the early [[Proterozoic]] Eon. Several hundreds of kilometers of the [[Huronian Supergroup]] are exposed {{convert|10 to 100|km|0|sp=us}} north of the north shore of Lake Huron, extending from near [[Sault Ste. Marie, Ontario|Sault Ste. Marie]] to Sudbury, northeast of Lake Huron, with giant layers of now-lithified till beds, [[dropstone]]s, [[varve]]s, [[glacial outwash|outwash]], and scoured basement rocks. Correlative Huronian deposits have been found near [[Marquette, Michigan]], and correlation has been made with Paleoproterozoic glacial deposits from Western Australia. The Huronian ice age was caused by the elimination of [[atmospheric methane]], a [[greenhouse gas]], during the [[Great Oxygenation Event]].<ref>{{Cite journal|last=Kopp|first=Robert|date=14 June 2005|title=The Paleoproterozoic snowball Earth: A climate disaster triggered by the evolution of oxygenic photosynthesis|journal=PNAS|volume=102|issue=32|pages=11131–6|doi=10.1073/pnas.0504878102|pmid=16061801|pmc=1183582|bibcode=2005PNAS..10211131K|doi-access=free}}</ref>

The next well-documented ice age, and probably the most severe of the last billion years, occurred from 720 to 630 million years ago (the [[Cryogenian]] period) and may have produced a [[Snowball Earth]] in which glacial ice sheets reached the equator,<ref>{{cite journal |vauthors=Hyde WT, Crowley TJ, Baum SK, Peltier WR |author-link4=William Richard Peltier |title=Neoproterozoic 'snowball Earth' simulations with a coupled climate/ice-sheet model |journal=Nature |volume=405 |issue=6785 |pages=425–9 |date=May 2000 |pmid=10839531 |doi=10.1038/35013005 |url=http://www.meteo.mcgill.ca/~tremblay/Courses/ATOC530/Hyde.et.al.Nature.2000.pdf |bibcode=2000Natur.405..425H |s2cid=1672712 |access-date=2012-06-16 |archive-date=2013-07-01 |archive-url=https://web.archive.org/web/20130701054742/http://www.meteo.mcgill.ca/~tremblay/Courses/ATOC530/Hyde.et.al.Nature.2000.pdf |url-status=live }}</ref> possibly being ended by the accumulation of [[greenhouse gas]]es such as {{CO2}} produced by volcanoes. "The presence of ice on the continents and pack ice on the oceans would inhibit both [[Carbonate–silicate cycle|silicate weathering]] and [[photosynthesis]], which are the two major sinks for {{CO2}} at present."<ref>{{cite web |author=Chris Clowes |date=2003|url=http://www.palaeos.com/Proterozoic/Neoproterozoic/Cryogenian/Snowballs.html|archive-url=https://web.archive.org/web/20090615181543/http://www.palaeos.com/Proterozoic/Neoproterozoic/Cryogenian/Snowballs.html |archive-date=15 June 2009 |title="Snowball" Scenarios of the Cryogenian |work= Paleos: Life through deep time}}</ref> It has been suggested that the end of this ice age was responsible for the subsequent [[Ediacaran]] and [[Cambrian explosion]], though this model is recent and controversial.

The [[Andean-Saharan]] occurred from 460 to 420 million years ago, during the [[Late Ordovician]] and the [[Silurian]] period.

[[File:Five Myr Climate Change.svg|thumb|upright=1.35|right|Sediment records showing the fluctuating sequences of glacials and interglacials during the last several million years]]

The evolution of land plants at the onset of the [[Devonian]] period caused a long term increase in planetary oxygen levels and reduction of {{CO2}} levels, which resulted in the [[late Paleozoic icehouse]]. Its former name, the Karoo glaciation, was named after the glacial tills found in the Karoo region of South Africa. There were extensive polar [[ice cap]]s at intervals from 360 to 260 million years ago in South Africa during the [[Carboniferous]] and [[Cisuralian|early Permian]] periods. Correlatives are known from Argentina, also in the center of the ancient supercontinent [[Gondwanaland]].

Although the [[Mesozoic|Mesozoic Era]] retained a greenhouse climate over its timespan and was previously assumed to have been entirely glaciation-free, more recent studies suggest that brief periods of glaciation occurred in both hemispheres during the [[Early Cretaceous]]. Geologic and palaeoclimatological records suggest the existence of glacial periods during the [[Valanginian]], [[Hauterivian]], and [[Aptian]] stages of the Early Cretaceous. [[Ice rafting|Ice-rafted]] glacial [[dropstone]]s indicate that in the [[Northern Hemisphere]], ice sheets may have extended as far south as the [[Iberian Peninsula]] during the Hauterivian and Aptian.<ref>{{Cite journal |last1=Rodríguez-López |first1=Juan Pedro |last2=Liesa |first2=Carlos L. |last3=Pardo |first3=Gonzalo |last4=Meléndez |first4=Nieves |last5=Soria |first5=Ana R. |last6=Skilling |first6=Ian |date=2016-06-15 |title=Glacial dropstones in the western Tethys during the late Aptian–early Albian cold snap: Palaeoclimate and palaeogeographic implications for the mid-Cretaceous |url=https://www.sciencedirect.com/science/article/pii/S003101821630058X |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |volume=452 |pages=11–27 |doi=10.1016/j.palaeo.2016.04.004 |bibcode=2016PPP...452...11R |issn=0031-0182 |access-date=2023-10-26 |archive-date=2017-09-26 |archive-url=https://web.archive.org/web/20170926035812/http://www.sciencedirect.com/science/article/pii/S003101821630058X |url-status=live }}</ref><ref>{{Cite journal |last1=Rodríguez-López |first1=Juan Pedro |last2=Liesa |first2=Carlos L. |last3=Luzón |first3=Aránzazu |last4=Muñoz |first4=Arsenio |last5=Mayayo |first5=María J. |last6=Murton |first6=Julian B. |last7=Soria |first7=Ana R. |date=2023-10-10 |title=Ice-rafted dropstones at midlatitudes in the Cretaceous of continental Iberia |journal=Geology |volume=52 |pages=33–38 |doi=10.1130/g51725.1 |issn=0091-7613|doi-access=free }}</ref><ref>{{Cite journal |last1=Wang |first1=Tianyang |last2=He |first2=Songlin |last3=Zhang |first3=Qinghai |last4=Ding |first4=Lin |last5=Farnsworth |first5=Alex |last6=Cai |first6=Fulong |last7=Wang |first7=Chao |last8=Xie |first8=Jing |last9=Li |first9=Guobiao |last10=Sheng |first10=Jiani |last11=Yue |first11=Yahui |date=2023-05-26 |title=Ice Sheet Expansion in the Cretaceous Greenhouse World |journal=Fundamental Research |volume=4 |issue=6 |pages=1586–1593 |doi=10.1016/j.fmre.2023.05.005 |issn=2667-3258 |doi-access=free |pmid=39734516 |pmc=11670679 }}</ref> Although ice sheets largely disappeared from Earth for the rest of the period (potential reports from the [[Turonian]], otherwise the warmest period of the Phanerozoic, are disputed),<ref name=":0" /><ref name=":1" /> ice sheets and associated sea ice appear to have briefly returned to Antarctica near the very end of the [[Maastrichtian]] just prior to the [[Cretaceous–Paleogene extinction event|Cretaceous-Paleogene extinction event]].<ref name=":1" /><ref>{{Cite journal |last1=Bowman |first1=Vanessa C. |last2=Francis |first2=Jane E. |last3=Riding |first3=James B. |date=December 1, 2013 |title=Late Cretaceous winter sea ice in Antarctica? |url=https://pubs.geoscienceworld.org/geology/article/41/12/1227/131088/Late-Cretaceous-winter-sea-ice-in-Antarctica |access-date=2023-10-26 |journal=Geology |volume=41 |issue=12 |pages=1227–1230 |doi=10.1130/g34891.1 |bibcode=2013Geo....41.1227B |s2cid=128885087 |archive-date=2023-10-26 |archive-url=https://web.archive.org/web/20231026040033/https://pubs.geoscienceworld.org/geology/article/41/12/1227/131088/Late-Cretaceous-winter-sea-ice-in-Antarctica |url-status=live }}</ref>

The [[Quaternary glaciation|Quaternary Glaciation / Quaternary Ice Age]] started about 2.58 million years ago at the beginning of the [[Quaternary|Quaternary Period]] when the spread of ice sheets in the Northern Hemisphere began. Since then, the world has seen cycles of glaciation with ice sheets advancing and retreating on 40,000- and 100,000-year time scales called [[glacial period]]s, glacials or glacial advances, and [[interglacial]] periods, interglacials or glacial retreats. Earth is currently in an interglacial, and the [[Younger Dryas|last glacial period]] ended about 11,700 years ago. All that remains of the continental [[ice sheet]]s are the [[Greenland ice sheet|Greenland]] and [[Antarctic ice sheet]]s and smaller glaciers such as on [[Baffin Island]].

The definition of the [[Quaternary]] as beginning 2.58 Ma is based on the formation of the [[Arctic ice cap]]. The [[Antarctic ice sheet]] began to form earlier, at about 34 Ma, in the mid-[[Cenozoic]] ([[Eocene–Oligocene extinction event|Eocene-Oligocene Boundary]]). The term [[Late Cenozoic Ice Age]] is used to include this early phase.<ref name="UHCL">University of Houston-Clear Lake - Disasters Class Notes - Chapter 12: Climate Change sce.uhcl.edu/Pitts/disastersclassnotes/chapter_12_Climate_Change.doc</ref>

Ice ages can be further divided by location and time; for example, the names ''Riss'' (180,000–130,000 years [[Before Present|bp]]) and ''[[Würm glaciation|Würm]]'' (70,000–10,000 years bp) refer specifically to glaciation in the [[Alps|Alpine region]]. The maximum extent of the ice is not maintained for the full interval. The scouring action of each glaciation tends to remove most of the evidence of prior ice sheets almost completely, except in regions where the later sheet does not achieve full coverage.