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{{Short description|Period of long-term reduction in temperature of Earth's surface and atmosphere}}
{{About|glacial periods in general|specific recent glacial periods often referred to as the "Ice Age"|Last Glacial Period|and|Pleistocene|and|Quaternary glaciation|the current ice age|Late Cenozoic Ice Age|other uses|Ice Age (disambiguation){{!}}Ice Age}}
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[[File:IceAgeEarth.jpg|thumb|upright=1.35|An artist's impression of ice age Earth at [[Pleistocene]] glacial maximum]]

An '''ice age''' is a term describing periods of time when the reduction in the temperature of [[Earth]]'s surface and atmosphere results in the presence or expansion of continental and polar [[ice sheet]]s and alpine [[glacier]]s. The term is applied in several different senses to very long and comparatively short periods of cooling. Colder periods are called glacials or ice ages, and warmer periods are called interglacials.

Earth's climate alternates between [[Greenhouse and icehouse Earth|icehouse and greenhouse periods]] based on whether there are glaciers on the planet, and for most of Earth's history it has been in a greenhouse period with little or no permanent ice. Over the very long term, Earth is currently in an icehouse period called the [[Late Cenozoic Ice Age]], which started 34 million years ago. There have been colder and warmer periods within this ice age, and the term is also applied to the [[Quaternary glaciation]], which started 2.58 million years ago. Within this period, the [[Last Interglacial]] ended 115,000 years ago, and was followed by the [[Last Glacial Period]] (LGP), which gave way to the current warm [[Holocene]], which started 11,700 years ago. The most severe cold period of the LGP was the [[Last Glacial Maximum]], which reached its maximum between 26,000 and 20,000 years ago. The most recent glaciation was the [[Younger Dryas]] between 12,800 and 11,700 years ago

==History of research==
{{See also|History of climate change science}}

In 1742, Pierre Martel (1706–1767), an engineer and geographer living in [[Geneva]], visited the valley of [[Chamonix]] in the [[Alps]] of [[Savoy]].<ref>{{cite journal |vauthors=Rémy F, Testut L |title=Mais comment s'écoule donc un glacier ? Aperçu historique |journal=Comptes Rendus Geoscience |language=fr |volume=338 |issue=5 |pages=368–385 |year=2006 |doi=10.1016/j.crte.2006.02.004 |url=http://remy.omp.free.fr/FTP/histoire_de_la_glaciologie/ecoulement_glacier.pdf |bibcode=2006CRGeo.338..368R |access-date=2009-06-23 |archive-date=2012-04-26 |archive-url=https://web.archive.org/web/20120426050144/http://remy.omp.free.fr/FTP/histoire_de_la_glaciologie/ecoulement_glacier.pdf |url-status=live }} Note: p. 374</ref><ref>{{harvnb|Montgomery|2010}}</ref> Two years later he published an account of his journey. He reported that the inhabitants of that valley attributed the dispersal of [[glacial erratic|erratic boulders]] to the glaciers, saying that they had once extended much farther.<ref>{{cite book |author=Martel, Pierre |chapter=Appendix: Martel, P. (1744) An account of the glacieres or ice alps in Savoy, in two letters, one from an English gentleman to his friend at Geneva; the other from Pierre Martel, engineer, to the said English gentleman |editor=Mathews, C.E. |title=The annals of Mont Blanc |chapter-url=https://books.google.com/books?id=oestAAAAYAAJ&pg=PA327 |year=1898 |publisher=Unwin |location=London |page=327}} See {{harv|Montgomery|2010}} for a full bibliography</ref><ref>{{cite book |last=Krüger |first=Tobias |year=2013 |title=Discovering the Ice Ages. International Reception and Consequences for a Historical Understanding of Climate (German edition: Basel 2008) |location=Leiden, Netherlands |publisher=Brill |page=47 |isbn=978-90-04-24169-5 |oclc=968318929 }}</ref> Later similar explanations were reported from other regions of the Alps. In 1815 the carpenter and [[chamois]] hunter Jean-Pierre Perraudin (1767–1858) explained erratic boulders in the Val de Bagnes in the Swiss canton of Valais as being due to glaciers previously extending further.<ref>{{harvnb|Krüger|2013|pp=78–83}}</ref> An unknown woodcutter from Meiringen in the Bernese Oberland advocated a similar idea in a discussion with the Swiss-German geologist [[Jean de Charpentier]] (1786–1855) in 1834.<ref>{{harvnb|Krüger|2013|p=150}}</ref> Comparable explanations are also known from the Val de Ferret in the Valais and the Seeland in western Switzerland<ref>{{harvnb|Krüger|2013|pp=83, 151}}</ref> and in [[Johann Wolfgang von Goethe|Goethe]]'s [[Goethean science|scientific work]].<ref>Goethe, Johann Wolfgang von: Geologische Probleme und Versuch ihrer Auflösung, Mineralogie und Geologie in Goethes Werke, Weimar 1892, {{ISBN|3-423-05946-X}}, book 73 (WA II, 9), pp. 253, 254.</ref> Such explanations could also be found in other parts of the world. When the Bavarian naturalist [[Ernst von Bibra]] (1806–1878) visited the Chilean Andes in 1849–1850, the natives attributed fossil [[moraine]]s to the former action of glaciers.<ref>{{harvnb|Krüger|2013|p=83}}</ref>

Meanwhile, European scholars had begun to wonder what had caused the dispersal of erratic material. From the middle of the 18th century, some discussed ice as a means of transport. The Swedish mining expert Daniel Tilas (1712–1772) was, in 1742, the first person to suggest drifting sea ice was a cause of the presence of erratic boulders in the Scandinavian and Baltic regions.<ref>{{harvnb|Krüger|2013|p=38}}</ref> In 1795, the Scottish philosopher and gentleman naturalist, [[James Hutton]] (1726–1797), explained erratic boulders in the Alps by the action of glaciers.<ref>{{harvnb|Krüger|2013|pp=61–2}}</ref> Two decades later, in 1818, the Swedish botanist [[Göran Wahlenberg]] (1780–1851) published his theory of a glaciation of the Scandinavian peninsula. He regarded glaciation as a regional phenomenon.<ref>{{harvnb|Krüger|2013|pp=88–90}}</ref>

<!--[[File:AntarcticaDomeCSnow.jpg|thumb|left|upright=1.15|The [[Antarctic ice sheet]]. Ice sheets expand during an ice age.]]
[[File:Vostok Petit data.svg|thumb|left|upright=1.15|Variations in temperature, {{CO2}}, and dust from the [[Vostok, Antarctica|Vostok]] ice core over the last 400,000 years]]-->
[[File:Haukalivatnet.JPG|thumb|Haukalivatnet lake (50 meters above sea level) where [[Jens Esmark]] in 1823 discovered similarities to [[moraine]]s near existing glaciers in the high mountains]]

Only a few years later, the Danish-Norwegian geologist [[Jens Esmark]] (1762–1839) argued for a sequence of worldwide ice ages. In a paper published in 1824, Esmark proposed changes in climate as the cause of those glaciations. He attempted to show that they originated from changes in Earth's orbit.<ref>{{harvnb|Krüger|2013|pp=91–6}}</ref> Esmark discovered the similarity between moraines near [[Haukalivatnet]] lake near sea level in [[Rogaland]] and moraines at branches of [[Jostedalsbreen]]. Esmark's discovery were later attributed to or appropriated by [[Theodor Kjerulf]] and [[Louis Agassiz]].<ref>{{Cite journal|last=Hestmark|first=Geir|date=2018|title=Jens Esmark's mountain glacier traverse 1823 − the key to his discovery of Ice Ages|journal=Boreas|language=en|volume=47|issue=1|pages=1–10|doi=10.1111/bor.12260|bibcode=2018Borea..47....1H |issn=1502-3885|quote=The discovery of Ice Ages is one of the most revolutionary advances made in the Earth sciences. In 1824 Danish-Norwegian geoscientist Jens Esmark published a paper stating that there was indisputable evidence that Norway and other parts of Europe had previously been covered by enormous glaciers carving out valleys and fjords, in a cold climate caused by changes in the eccentricity of Earth's orbit. Esmark and his travel companion Otto Tank arrived at this insight by analogous reasoning: enigmatic landscape features they observed close to sea level along the Norwegian coast strongly resembled features they observed in the front of a retreating glacier during a mountain traverse in the summer of 1823.|doi-access=free|hdl=10852/67376|hdl-access=free}}</ref><ref>{{Citation|last=Berg|first=Bjørn Ivar|title=Jens Esmark|date=2020-02-25|url=http://nbl.snl.no/Jens_Esmark|work=Norsk biografisk leksikon|language=nb|access-date=2021-02-28|archive-date=2021-03-07|archive-url=https://web.archive.org/web/20210307220710/https://nbl.snl.no/Jens_Esmark|url-status=live}}</ref><ref>{{Cite web|last=Hverven|first=Tom Egil|title=Isens spor|url=https://arkiv.klassekampen.no/article/20170805/ARTICLE/170809976|access-date=2021-02-28|website=Klassekampen|archive-date=2021-04-17|archive-url=https://web.archive.org/web/20210417172110/https://arkiv.klassekampen.no/article/20170805/ARTICLE/170809976|url-status=live}}</ref>

During the following years, Esmark's ideas were discussed and taken over in parts by Swedish, Scottish and German scientists. At the University of Edinburgh [[Robert Jameson]] (1774–1854) seemed to be relatively open to Esmark's ideas, as reviewed by Norwegian professor of glaciology [[Bjørn G. Andersen]] (1992).<ref>{{cite journal |first=Bjørn G. |last=Andersen |year=1992 |title=Jens Esmark—a pioneer in glacial geology |journal=[[Boreas (journal)|Boreas]] |volume=21 |pages=97–102 |doi=10.1111/j.1502-3885.1992.tb00016.x|title-link=Jens Esmark |issue=1 |bibcode=1992Borea..21...97A }}</ref> Jameson's remarks about ancient glaciers in Scotland were most probably prompted by Esmark.<ref>{{cite book |author=Davies, Gordon L. |title=The Earth in Decay. A History of British Geomorphology 1578–1878 |url=https://archive.org/details/earthindecayhist0000herr |url-access=registration |location=London |year=1969 |pages=267f|publisher=New York, American Elsevier Pub. Co |isbn=978-0-444-19701-6 }}<br />{{cite book |author=Cunningham, Frank F. |title=James David Forbes. Pioneer Scottish Glaciologist |publisher=Scottish Academic Press |location=Edinburgh |year=1990 |isbn=978-0-7073-0320-8 |page=15}}</ref> In Germany, Albrecht Reinhard Bernhardi (1797–1849), a geologist and professor of forestry at an academy in Dreissigacker (since incorporated in the southern [[Thuringia]]n city of [[Meiningen]]), adopted Esmark's theory. In a paper published in 1832, Bernhardi speculated about the polar ice caps once reaching as far as the temperate zones of the globe.<ref>{{harvnb|Krüger|2013|pp=142–47}}</ref>

In [[Val de Bagnes]], a valley in the [[Swiss Alps]], there was a long-held local belief that the valley had once been covered deep in ice, and in 1815 a local chamois hunter called Jean-Pierre Perraudin attempted to convert the geologist [[Jean de Charpentier]] to the idea, pointing to deep striations in the rocks and giant erratic boulders as evidence. Charpentier held the general view that these signs were caused by vast floods, and he rejected Perraudin's theory as absurd. In 1818 the engineer [[Ignatz Venetz]] joined Perraudin and Charpentier to examine a [[proglacial lake]] above the valley created by an ice dam as a result of the [[1815 eruption of Mount Tambora]], which threatened to cause a catastrophic flood when the dam broke. Perraudin attempted unsuccessfully to convert his companions to his theory, but when the dam finally broke, there were only minor erratics and no striations, and Venetz concluded that Perraudin was right and that only ice could have caused such major results. In 1821 he read a prize-winning paper on the theory to the Swiss Society, but it was not published until Charpentier, who had also become converted, published it with his own more widely read paper in 1834.<ref>{{cite book|last=Wood |first=Gillen D'Arcy  |title=Tambora, the Eruption that Changed the World|pages=160–167 |publisher=Princeton University Press |location =Princeton, NJ |year=2014|isbn=978-0-691-16862-3}}</ref>

In the meantime, the German botanist [[Karl Friedrich Schimper]] (1803–1867) was studying mosses which were growing on erratic boulders in the alpine upland of Bavaria. He began to wonder where such masses of stone had come from. During the summer of 1835 he made some excursions to the Bavarian Alps. Schimper came to the conclusion that ice must have been the means of transport for the boulders in the alpine upland. In the winter of 1835–36 he held some lectures in Munich. Schimper then assumed that there must have been global times of obliteration ("Verödungszeiten") with a cold climate and frozen water.<ref>{{harvnb|Krüger|2013|pp=155–59}}</ref> Schimper spent the summer months of 1836 at Devens, near Bex, in the Swiss Alps with his former university friend [[Louis Agassiz]] (1801–1873) and Jean de Charpentier. Schimper, Charpentier and possibly Venetz convinced Agassiz that there had been a time of glaciation. During the winter of 1836–37, Agassiz and Schimper developed the theory of a sequence of glaciations. They mainly drew upon the preceding works of Venetz, Charpentier and on their own fieldwork. Agassiz appears to have been already familiar with Bernhardi's paper at that time.<ref>{{harvnb|Krüger|2013|pp=167–70}}</ref> At the beginning of 1837, Schimper coined the term "ice age" (''"Eiszeit"'') for the period of the glaciers.<ref>{{harvnb|Krüger|2013|p=173}}</ref> In July 1837 Agassiz presented their synthesis before the annual meeting of the Swiss Society for Natural Research at Neuchâtel. The audience was very critical, and some were opposed to the new theory because it contradicted the established opinions on climatic history. Most contemporary scientists thought that Earth had been gradually cooling down since its birth as a molten globe.<ref>{{harvnb|Krüger|2013|pp=177–78}}</ref>

In order to persuade the skeptics, Agassiz embarked on geological fieldwork. He published his book ''Study on Glaciers'' ("Études sur les glaciers") in 1840.<ref>{{cite book |first1=Louis |last1=Agassiz |author-link=Louis Agassiz |first2=Joseph |last2=Bettannier |title=Études sur les glaciers. Ouvrage accompagné d'un atlas de 32 planches, Neuchâtel |url=https://books.google.com/books?id=fTMAAAAAQAAJ |year=1840 |publisher=H. Nicolet}}</ref> Charpentier was put out by this, as he had also been preparing a book about the glaciation of the Alps. Charpentier felt that Agassiz should have given him precedence as it was he who had introduced Agassiz to in-depth glacial research.<ref>{{harvnb|Krüger|2013|pp=223–4.  Charpentier, Jean de: ''Essais sur les glaciers et sur le terrain erratique du bassin du Rhône,'' Lausanne 1841.}}</ref> As a result of personal quarrels, Agassiz had also omitted any mention of Schimper in his book.<ref>{{harvnb|Krüger|2013|pp=181–84}}</ref>

It took several decades before the ice age theory was fully accepted by scientists. This happened on an international scale in the second half of the 1870s, following the work of [[James Croll]], including the publication of ''Climate and Time, in Their Geological Relations'' in 1875, which provided a credible explanation for the causes of ice ages.<ref>{{harvnb|Krüger|2013|pp=458–60}}</ref>

==Evidence==
There are three main types of evidence for ice ages: geological, chemical, and paleontological.

''Geological'' evidence for ice ages comes in various forms, including rock scouring and scratching, [[glacial moraine]]s, [[drumlin]]s, valley cutting, and the deposition of [[till]] or tillites and [[glacial erratic]]s. Successive glaciations tend to distort and erase the geological evidence for earlier glaciations, making it difficult to interpret. Furthermore, this evidence was difficult to date exactly; early theories assumed that the glacials were short compared to the long interglacials. The advent of sediment and ice cores revealed the true situation: glacials are long, interglacials short. It took some time for the current theory to be worked out.

The ''chemical'' evidence mainly consists of variations in the ratios of [[isotope]]s in fossils present in sediments and [[sedimentary rock]]s and [[ocean sediment]] cores. For the most recent glacial periods, [[ice core]]s provide climate [[Proxy (climate)|proxies]], both from the ice itself and from atmospheric samples provided by included bubbles of air. Because water containing lighter isotopes has a lower [[heat of evaporation]], its proportion decreases with warmer conditions.<ref>{{cite journal |title=How are past temperatures determined from an ice core? |journal=Scientific American |date=2004-09-20 |url=http://www.scientificamerican.com/article.cfm?id=how-are-past-temperatures |access-date=2011-04-04 |archive-date=2013-05-20 |archive-url=https://web.archive.org/web/20130520182757/http://www.scientificamerican.com/article.cfm?id=how-are-past-temperatures |url-status=live }}</ref> This allows a temperature record to be constructed. This evidence can be confounded, however, by other factors recorded by isotope ratios.

The ''paleontological'' evidence consists of changes in the geographical distribution of fossils. During a glacial period, cold-adapted organisms spread into lower latitudes, and organisms that prefer warmer conditions become extinct or retreat into lower latitudes. This evidence is also difficult to interpret because it requires:
#sequences of sediments covering a long period of time, over a wide range of latitudes and which are easily correlated;
#ancient organisms which survive for several million years without change and whose temperature preferences are easily diagnosed; and
#the finding of the relevant fossils.

Despite the difficulties, analysis of ice core and ocean sediment cores<ref>{{cite journal |title=Glacier advance in southern middle-latitudes during the Antarctic Cold Reversal |first1=Aaron E. |last1=Putnam |first2=George H. |last2=Denton |first3=Joerg M. |last3=Schaefer |first4=David J. A. |last4=Barrell |first5=Bjørn G. |last5=Andersen |first6=Robert C. |last6=Finkel |first7=Roseanne |last7=Schwartz |first8=Alice M. |last8=Doughty |first9=Michael R. |last9=Kaplan |first10=Christian |last10=Schlüchter |year=2010 |journal=[[Nature Geoscience]] |volume=3 |pages=700–704 |issue=10 |doi=10.1038/ngeo962|bibcode=2010NatGe...3..700P }}</ref> has provided a credible record of glacials and interglacials over the past few million years. These also confirm the linkage between ice ages and continental crust phenomena such as glacial moraines, drumlins, and glacial erratics. Hence the continental crust phenomena are accepted as good evidence of earlier ice ages when they are found in layers created much earlier than the time range for which ice cores and ocean sediment cores are available.

==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=978-0-19-965306-5 }}{{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 |url-access=subscription }}</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 |article-number=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 |url-access=subscription }}</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 |url-access=subscription }}</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.

==Glacials and interglacials==
<!-- This section is redirected to by [[Glacial cycle]]. If the section name is changed or the section is removed, an anchor should be placed for the redirect. -->
{{See also|Glacial period|Interglacial}}
[[File:Ice Age Temperature.png|right|thumb|upright=1.35|Pattern of temperature and ice volume changes associated with recent glacials and interglacials]]
{{multiple image
| align = right
| direction = vertical
| width = 230
| header = Minimum and maximum glaciation
| image1 = Iceage north-intergl glac hg.png
| caption1 = Minimum (interglacial, black) and maximum (glacial, grey) glaciation of the [[Northern Hemisphere|northern hemisphere]]
| image2 = Iceage south-intergl glac hg.png
| caption2 = Minimum (interglacial, black) and maximum (glacial, grey) glaciation of the [[Southern Hemisphere|southern hemisphere]]
}}

Within the current glaciation, more temperate and more severe periods have occurred. The colder periods are called ''glacial periods'', the warmer periods ''interglacials'', such as the [[Eemian|Eemian Stage]].<ref name="ehlers-gibbard-2011">{{cite book|year=2011|doi=10.1007/978-90-481-2642-2_423|title = Encyclopedia of Snow, Ice and Glaciers|pages=873–882|series = Encyclopedia of Earth Sciences Series|last1 = Ehlers|first1 = Jürgen|last2=Gibbard|first2=Philip|chapter=Quaternary Glaciation |isbn=978-90-481-2641-5}}</ref> There is evidence that similar '''glacial cycles''' occurred in previous glaciations, including the Andean-Saharan<ref>{{cite journal |last1=Ghienne |first1=Jean-François |title=Late Ordovician sedimentary environments, glacial cycles, and post-glacial transgression in the Taoudeni Basin, West Africa |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |date=January 2003 |volume=189 |issue=3–4 |pages=117–145 |doi=10.1016/S0031-0182(02)00635-1|bibcode=2003PPP...189..117G }}</ref> and the late Paleozoic ice house.  The glacial cycles of the late Paleozoic ice house are likely responsible for the deposition of [[cyclothems]].<ref>{{cite book |last1=Heckel |first1=P.H. |year=2008 |chapter=Pennsylvanian cyclothems in Midcontinent North America as far-field effects of waxing and waning of Gondwana ice sheets |title=Resolving the Late Paleozoic Ice Age in Time and Space |editor-last1=Fielding |editor-first1=C.R. |editor-last2=Frank |editor-first2=T.D. |editor-last3=Isbell |editor-first3=J.L. |pages=275–290}}</ref>

Glacials are characterized by cooler and drier climates over most of Earth and large land and sea ice masses extending outward from the poles. Mountain glaciers in otherwise unglaciated areas extend to lower elevations due to a lower [[snow line]]. Sea levels drop due to the removal of large volumes of water above sea level in the icecaps. There is evidence that ocean circulation patterns are disrupted by glaciations. The glacials and interglacials coincide with changes in [[orbital forcing]] of climate due to [[Milankovitch cycles]], which are periodic changes in Earth's orbit and the tilt of Earth's rotational axis.

Earth has been in an interglacial period known as the [[Holocene]] for around 11,700 years,<ref name="Walker, M. 2009. pp. 3">{{cite journal |last1 = Walker |first1 = M. |last2 = Johnsen |first2 = S. |last3 = Rasmussen |first3 = S. O. |last4 = Popp |first4 = T. |last5 = Steffensen |first5 = J.-P. |last6 = Gibbard |first6 = P. |last7 = Hoek |first7 = W. |last8 = Lowe |first8 = J. |last9 = Andrews |first9 = J. |last10 = Bjo |last11 = Cwynar |first11 = L. C. |last12 = Hughen |first12 = K. |last13 = Kershaw |first13 = P. |last14 = Kromer |first14 = B. |last15 = Litt |first15 = T. |last16 = Lowe |first16 = D. J. |last17 = Nakagawa |first17 = T. |last18 = Newnham |first18 = R. |last19 = Schwander |first19 = J. |year = 2009 |title = Formal definition and dating of the GSSP (Global Stratotype Section and Point) for the base of the Holocene using the Greenland NGRIP ice core, and selected auxiliary records |url = http://www.stratigraphy.org/GSSP/Holocene.pdf |journal = J. Quaternary Sci. |volume = 24 |issue = 1 |pages = 3–17 |doi = 10.1002/jqs.1227 |bibcode = 2009JQS....24....3W |doi-access = free |access-date = 2017-07-26 |archive-date = 2013-11-04 |archive-url = https://web.archive.org/web/20131104131948/http://www.stratigraphy.org/GSSP/Holocene.pdf |url-status = live }}</ref> and an article in ''Nature'' in 2004 argues that it might be most analogous to a previous interglacial that lasted 28,000 years.<ref>{{cite journal
 |title=Eight glacial cycles from an Antarctic ice core
 |journal=Nature
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 }}</ref> Predicted changes in orbital forcing suggest that the next glacial period would begin at least 50,000 years from now. Moreover, anthropogenic forcing from increased [[greenhouse gas]]es is estimated to potentially outweigh the orbital forcing of the Milankovitch cycles for hundreds of thousands of years.<ref>{{cite web |url=https://www.sciencedaily.com/releases/2007/08/070829193436.htm |title=Next Ice Age Delayed By Rising Carbon Dioxide Levels |access-date=2008-02-28 |year=2007 |website=ScienceDaily |archive-date=2008-03-02 |archive-url=https://web.archive.org/web/20080302083828/http://www.sciencedaily.com/releases/2007/08/070829193436.htm |url-status=live }}</ref><ref name="PIK2016">{{cite web|url=https://www.pik-potsdam.de/news/press-releases/human-made-climate-change-suppresses-the-next-ice-age|title=Human-made climate change suppresses the next ice age|year=2016|publisher=Potsdam Institute for Climate Impact Research in Germany|access-date=2019-01-07|archive-date=2020-08-18|archive-url=https://web.archive.org/web/20200818202438/https://www.pik-potsdam.de/news/press-releases/human-made-climate-change-suppresses-the-next-ice-age}}</ref><ref name="LiveScience2007">{{cite web|url=https://www.livescience.com/1846-global-warming-good-news-ice-ages.html|title=Global Warming Good News: No More Ice Ages|year=2007|publisher=LiveScience|last1=Thomson|first1=Andrea|access-date=2019-01-07|archive-date=2020-11-12|archive-url=https://web.archive.org/web/20201112000544/https://www.livescience.com/1846-global-warming-good-news-ice-ages.html|url-status=live}}</ref>

==Feedback processes==
Each glacial period is subject to [[positive feedback]] mechanisms, which makes it more severe, and [[negative feedback]] which dampens the overall climate response to different types of forcing. In the case of Quaternary ice ages, Earth's high [[albedo]] from ice sheets and atmospheric dust as well as low concentrations of atmospheric {{CO2}} contributed to cold glacial climates.<ref>{{Citation | vauthors=((Hain, M. P.)), ((Chalk, T. B.)) | year=2025 | title=Greenhouse gas effects on Quaternary climates | publisher=Elsevier | url=https://earth-system-biogeochemistry.net/wp-content/uploads/2025/01/Hain-and-Chalk-2024-EQS-Milankovitch-vs-CO2.pdf | doi=10.1016/b978-0-323-99931-1.00271-3}}</ref>
[[File:Fig3 Q-climate-CO2-feedbacks V2.png|thumb|Diagram of key climate-carbon cycle feedbacks linking Quaternary climates and temperatures, Generalized Milankovitch Theory (GMT), to atmospheric {{CO2}} and ice sheets.<ref>{{Citation | vauthors=((Hain, M. P.)), ((Chalk, T. B.)) | year=2025 | title=Greenhouse gas effects on Quaternary climates | publisher=Elsevier | url=https://earth-system-biogeochemistry.net/wp-content/uploads/2025/01/Hain-and-Chalk-2024-EQS-Milankovitch-vs-CO2.pdf | doi=10.1016/b978-0-323-99931-1.00271-3}}</ref> Positive feedbacks amplify and negative feedbacks dampen environmental change, with slow-acting responses shown as dashed arrows.]]
===Positive===
An important form of feedback is provided by Earth's [[albedo]], which is how much of the sun's energy is reflected rather than absorbed by Earth. Ice and snow increase Earth's albedo, while [[Boreal forest|forests]] reduce its albedo. When the air temperature decreases, ice and snow fields grow, and they reduce forest cover. This continues until competition with a negative feedback mechanism forces the system to an equilibrium.

One theory is that when glaciers form, two things happen: the ice grinds rocks into dust, and the land becomes dry and arid. This allows winds to transport iron rich dust into the open ocean, where it acts as a fertilizer that causes massive algal blooms that pulls large amounts of {{CO2}} out of the atmosphere. This in turn makes it even colder and causes the glaciers to grow more.<ref>{{Cite web |url=https://www.smithsonianmag.com/science-nature/complicated-role-iron-ocean-health-and-climate-change-180973893/ |title=The Complicated Role of Iron in Ocean Health and Climate Change |access-date=2022-08-02 |archive-date=2022-08-02 |archive-url=https://web.archive.org/web/20220802125320/https://www.smithsonianmag.com/science-nature/complicated-role-iron-ocean-health-and-climate-change-180973893/ |url-status=live }}</ref>

In 1956, Ewing and Donn<ref>{{Cite journal|last1=Ewing|first1=M.|last2=Donn|first2=W. L.|date=1956-06-15|title=A Theory of Ice Ages|journal=Science|volume=123|issue=3207|pages=1061–1066|doi=10.1126/science.123.3207.1061|issn=0036-8075|pmid=17748617|bibcode=1956Sci...123.1061E}}</ref> hypothesized that an ice-free Arctic Ocean leads to increased snowfall at high latitudes. When low-temperature ice covers the Arctic Ocean there is little evaporation or [[Sublimation (phase transition)|sublimation]] and the polar regions are quite dry in terms of precipitation, comparable to the amount found in mid-latitude [[desert]]s. This low precipitation allows high-latitude snowfalls to melt during the summer. An ice-free Arctic Ocean absorbs solar radiation during the long summer days, and evaporates more water into the Arctic atmosphere. With higher precipitation, portions of this snow may not melt during the summer and so glacial ice can form at lower altitudes ''and'' more southerly latitudes, reducing the temperatures over land by increased albedo as noted above. Furthermore, under this hypothesis the lack of oceanic pack ice allows increased exchange of waters between the Arctic and the North Atlantic Oceans, warming the Arctic and cooling the North Atlantic. (Current projected consequences of [[global warming]] include [[Arctic sea ice decline#Ice-free summer vs. ice-free winter|a brief ice-free Arctic Ocean period by 2050]].) Additional fresh water flowing into the North Atlantic during a warming cycle may also [[shutdown of thermohaline circulation|reduce]] the [[Thermohaline circulation|global ocean water circulation]]. Such a reduction (by reducing the effects of the [[Gulf Stream]]) would have a cooling effect on northern Europe, which in turn would lead to increased low-latitude snow retention during the summer.<ref>{{cite book|last=Garrison|first=Tom|title=Oceanography: An Invitation to Marine Science|publisher=Cengage Learning|edition=7th|date=2009|page=582|isbn=978-0-495-39193-7}}</ref><ref>{{cite journal|doi=10.1038/nature04385 |author=Bryden, H.L. |author2=H.R. Longworth |author3=S.A. Cunningham |title=Slowing of the Atlantic meridional overturning circulation at 25° N|journal=Nature|issue=7068|pages=655–657|year=2005|pmid=16319889|volume=438|bibcode = 2005Natur.438..655B |s2cid=4429828 }}</ref><ref>{{cite journal|doi=10.1126/science.1109477 |author=Curry, R. |author2=C. Mauritzen|author2-link= Cecilie Mauritzen |title=Dilution of the northern North Atlantic in recent decades|journal=Science|volume=308|issue=5729 |pages=1772–1774|year=2005|bibcode = 2005Sci...308.1772C|pmid=15961666|s2cid=36017668 }}</ref> It has also been suggested{{by whom|date=November 2020}} that during an extensive glacial, glaciers may move through the [[Gulf of Saint Lawrence]], extending into the North Atlantic Ocean far enough to block the Gulf Stream.

===Negative===
Ice sheets that form during glaciations erode the land beneath them. This can reduce the land area above sea level and thus diminish the amount of space on which ice sheets can form. This mitigates the albedo feedback, as does the rise in sea level that accompanies the reduced area of ice sheets, since open ocean has a lower albedo than land.<ref>{{Cite book|last1=Huddart|first1=David|url=https://books.google.com/books?id=_64G6cYuz3AC&q=%22lowering+in+sea+level+that+accompanies+the+formation+of+ice+sheets%22&pg=PT1624|title=Earth Environments: Past, Present and Future|last2=Stott|first2=Tim A.|date=2013-04-16|publisher=John Wiley & Sons|isbn=978-1-118-68812-0|language=en}}</ref>

Another negative feedback mechanism is the increased aridity occurring with glacial maxima, which reduces the precipitation available to maintain glaciation. The glacial retreat induced by this or any other process can be amplified by similar [[inverse positive feedback]]s as for glacial advances.<ref>{{Cite book|url=https://archive.org/details/glacialgeologyic00benn_0|url-access=registration|quote=Another factor is the increased aridity occurring with glacial maxima, which reduces the precipitation available to maintain glaciation. The glacial retreat induced by this or any other process can be amplified by similar inverse positive feedbacks as for glacial advances.|title=Glacial Geology: Ice Sheets and Landforms|last1=Bennett|first1=Matthew M.|last2=Glasser|first2=Neil F.|date=2010-03-29|publisher=Wiley|isbn=978-0-470-51690-4|language=en}}</ref>

According to research published in ''[[Nature Geoscience]]'', human emissions of [[carbon dioxide|carbon dioxide (CO<sub>2</sub>)]] will defer the next glacial period. Researchers used data on Earth's orbit to find the historical warm interglacial period that looks most like the current one and from this have predicted that the next glacial period would usually begin within 1,500 years. They go on to predict that emissions have been so high that it will not.<ref>{{cite news|last=Black|first=Richard|title=Carbon emissions 'will defer Ice Age'|url=https://www.bbc.co.uk/news/science-environment-16439807|work=BBC News|access-date=10 August 2012|date=9 January 2012|archive-date=18 August 2012|archive-url=https://web.archive.org/web/20120818115004/http://www.bbc.co.uk/news/science-environment-16439807|url-status=live}}</ref>

==Causes==
The causes of ice ages are not fully understood for either the large-scale ice age periods or the smaller ebb and flow of glacial–interglacial periods within an ice age. The consensus is that several factors are important: [[Atmosphere of Earth|atmospheric composition]], such as the concentrations of [[carbon dioxide]] and [[methane]] (the specific levels of the previously mentioned gases are now able to be seen with the new ice core samples from the European Project for Ice Coring in Antarctica (EPICA) Dome C in Antarctica over the past 800,000 years); changes in Earth's orbit around the [[Sun]] known as [[Milankovitch cycles]]; the motion of [[Plate tectonics|tectonic plates]] resulting in changes in the relative location and amount of continental and oceanic crust on Earth's surface, which affect wind and [[ocean current]]s; variations in [[solar variation|solar output]]; the orbital dynamics of the Earth–Moon system; the impact of relatively large [[meteorite]]s and volcanism including eruptions of [[supervolcano]]es.<ref>{{cite journal|last=Luthi|first=Dieter|title=High-resolution carbon dioxide concentration record 650,000–800,000 years before present|journal=Nature|date=2008-03-17|volume=453|pages=379–382|doi=10.1038/nature06949|pmid=18480821|issue=7193|bibcode=2008Natur.453..379L|s2cid=1382081|display-authors=et al.|url=https://epic.awi.de/id/eprint/18281/1/Lth2008a.pdf|doi-access=free|access-date=2019-08-16|archive-date=2019-08-28|archive-url=https://web.archive.org/web/20190828222006/https://epic.awi.de/id/eprint/18281/1/Lth2008a.pdf|url-status=live}}</ref>{{Citation needed|date=April 2010}}

Some of these factors influence each other. For example, changes in Earth's atmospheric composition (especially the concentrations of greenhouse gases) may alter the climate, while climate change itself can change the atmospheric composition (for example by changing the rate at which [[weathering]] removes {{CO2}}).

[[Maureen Raymo]], [[William Ruddiman]] and others propose that the [[Tibetan Plateau|Tibetan]] and [[Colorado Plateau]]s are immense {{CO2}} "scrubbers" with a capacity to remove enough {{CO2}} from the global atmosphere to be a significant causal factor of the 40 million year [[Cenozoic#Climate|Cenozoic Cooling]] trend. They further claim that approximately half of their uplift (and {{CO2}} "scrubbing" capacity) occurred in the past 10 million years.<ref>{{cite journal | last1 = Ruddiman | first1 = W.F. | last2 = Kutzbach | first2 = J.E. | year = 1991 | title = Plateau Uplift and Climate Change | journal = Scientific American | volume = 264 | issue = 3| pages = 66–74 | doi = 10.1038/scientificamerican0391-66 |bibcode = 1991SciAm.264c..66R}}</ref><ref name="Raymo 649–653">{{Cite journal|last1=Raymo|first1=Maureen E.|last2=Ruddiman|first2=William F.|last3=Froelich|first3=Philip N.|date=1988-07-01|title=Influence of late Cenozoic mountain building on ocean geochemical cycles|journal=Geology|language=en|volume=16|issue=7|pages=649–653|doi=10.1130/0091-7613(1988)016<0649:IOLCMB>2.3.CO;2|issn=0091-7613|bibcode=1988Geo....16..649R}}</ref>

===Changes in Earth's atmosphere===
There is evidence that [[greenhouse gas]] levels fell at the start of ice ages and rose during the retreat of the ice sheets, but it is difficult to establish cause and effect (see the notes above on the role of weathering). Greenhouse gas levels may also have been affected by other factors which have been proposed as causes of ice ages, such as the movement of continents and volcanism.

The [[Snowball Earth]] hypothesis maintains that the severe freezing in the late [[Proterozoic]] was ended by an increase in {{CO2}} levels in the atmosphere, mainly from volcanoes, and some supporters of Snowball Earth argue that it was caused in the first place by a reduction in atmospheric {{CO2}}. The hypothesis also warns of future Snowball Earths.

In 2009, further evidence was provided that changes in solar [[insolation]] provide the initial trigger for Earth to warm after an Ice Age, with secondary factors like increases in greenhouse gases accounting for the magnitude of the change.<ref>{{Cite journal |first1=Peter U. |last1=Clark |first2=Arthur S. |last2=Dyke |first3=Jeremy D. |last3=Shakun |first4=Anders E. |last4=Carlson |first5=Jorie |last5=Clark |first6=Barbara |last6=Wohlfarth |first7=Jerry X. |last7=Mitrovica |first8=Steven W. |last8=Hostetler |first9=A. Marshall |last9=McCabe |name-list-style=amp|author-link6=Barbara Wohlfarth |year=2009 |title=The Last Glacial Maximum |journal=Science |volume=325 |issue=5941 |pages=710–714 |doi=10.1126/science.1172873 |pmid=19661421 |bibcode = 2009Sci...325..710C|s2cid=1324559 }}</ref>

===Position of the continents===
The geological record appears to show that ice ages start when the continents are in [[Continental drift|positions]] which block or reduce the flow of warm water from the equator to the poles and thus allow ice sheets to form. The ice sheets increase Earth's [[albedo|reflectivity]] and thus reduce the absorption of solar radiation. With less radiation absorbed the atmosphere cools; the cooling allows the ice sheets to grow, which further increases reflectivity in a [[positive feedback]] loop. The ice age continues until the reduction in weathering causes an increase in the [[greenhouse effect]].

There are three main contributors from the layout of the continents that obstruct the movement of warm water to the poles:<ref>Lee Hannah, ''Climate Change Biology'', 2nd ed. (Amsterdam: Academic Press, 2014), 23–28. {{ISBN|012799923X}}</ref>

* A continent sits on top of a pole, as [[Antarctica]] does today.
* A polar sea is almost land-locked, as the Arctic Ocean is today.
* A supercontinent covers most of the equator, as [[Rodinia]] did during the [[Cryogenian]] period.

Since today's Earth has a continent over the South Pole and an almost land-locked ocean over the North Pole, geologists believe that Earth will continue to experience glacial periods in the geologically near future.

Some scientists believe that the [[Himalayas]] are a major factor in the current ice age, because these mountains have increased Earth's total rainfall and therefore the rate at which carbon dioxide is washed out of the atmosphere, decreasing the greenhouse effect.<ref name="Raymo 649–653"/> The Himalayas' formation started about 70 million years ago when the [[Indo-Australian Plate]] collided with the [[Eurasian Plate]], and the Himalayas are still rising by about 5&nbsp;mm per year because the Indo-Australian plate is still moving at 67&nbsp;mm/year. The history of the Himalayas broadly fits the long-term decrease in Earth's average temperature since the [[Eocene|mid-Eocene]], 40 million years ago.

===Fluctuations in ocean currents===
{{Seealso|Abrupt climate change|Atlantic meridional overturning circulation}}
Another important contribution to ancient climate regimes is the variation of [[ocean current]]s, which are modified by continent position, sea levels and salinity, as well as other factors. They have the ability to cool (e.g. aiding the creation of Antarctic ice) and the ability to warm (e.g. giving the British Isles a temperate as opposed to a boreal climate). The closing of the [[Isthmus of Panama]] about 3 million years ago may have ushered in the present period of strong glaciation over North America by ending the exchange of water between the tropical Atlantic and Pacific Oceans.<ref>{{cite journal |url=http://discovermagazine.com/1996/apr/weareallpanamani743 |title=We are all Panamanians |author=Svitil, K. A. |date=April 1996 |journal=Discover |access-date=2012-04-23 |archive-date=2014-02-03 |archive-url=https://web.archive.org/web/20140203183832/http://discovermagazine.com/1996/apr/weareallpanamani743 |url-status=live }}—formation of Isthmus of Panama may have started a series of climatic changes that led to evolution of hominids</ref>

Analyses suggest that ocean current fluctuations can adequately account for recent glacial oscillations. During the last glacial period the sea-level fluctuated 20–30&nbsp;m as water was sequestered, primarily in the [[Northern Hemisphere]] ice sheets. When ice collected and the sea level dropped sufficiently, flow through the [[Bering Strait]] (the narrow strait between Siberia and Alaska is about 50&nbsp;m deep today) was reduced, resulting in increased flow from the North Atlantic. This realigned the [[thermohaline circulation]] in the Atlantic, increasing heat transport into the Arctic, which melted the polar ice accumulation and reduced other continental ice sheets. The release of water raised sea levels again, restoring the ingress of colder water from the Pacific with an accompanying shift to northern hemisphere ice accumulation.<ref name=Hu2010>{{Cite journal |last1=Hu |first1=Aixue |last2=Meehl |first2=Gerald A. |author2-link=Gerald Meehl |last3=Otto-Bliesner |first3=Bette L. |author-link3=Bette Otto-Bliesner |last4=Waelbroeck |first4=Claire |author5=Weiqing Han |last6=Loutre |first6=Marie-France |last7=Lambeck |first7=Kurt |last8=Mitrovica |first8=Jerry X. |last9=Rosenbloom |first9=Nan |title=Influence of Bering Strait flow and North Atlantic circulation on glacial sea-level changes |journal=Nature Geoscience |volume=3 |issue=2 |pages=118–121 |year=2010 |doi=10.1038/ngeo729 |bibcode=2010NatGe...3..118H |hdl=1885/30691 |url=http://www.cgd.ucar.edu/ccr/publications/ngeo729.pdf |citeseerx=10.1.1.391.8727 |access-date=2017-10-24 |archive-date=2017-08-11 |archive-url=https://web.archive.org/web/20170811021943/http://www.cgd.ucar.edu/ccr/publications/ngeo729.pdf }}</ref>

According to a study published in ''[[Nature (journal)|Nature]]'' in 2021, all [[glacial period]]s of ice ages over the last 1.5 million years were associated with northward shifts of melting Antarctic icebergs which changed ocean circulation patterns, [[Oceanic carbon cycle|leading to more CO<sub>2</sub> being pulled out of the atmosphere]]. The authors suggest that this process may be disrupted in the future as the [[Southern Ocean]] will become too warm for the icebergs to travel far enough to trigger these changes.<ref>{{cite news |title=Melting icebergs key to sequence of an ice age, scientists find |url=https://phys.org/news/2021-01-icebergs-key-sequence-ice-age.html |access-date=12 February 2021 |work=phys.org |language=en |archive-date=27 January 2021 |archive-url=https://web.archive.org/web/20210127163116/https://phys.org/news/2021-01-icebergs-key-sequence-ice-age.html |url-status=live }}</ref><ref>{{cite journal |last1=Starr |first1=Aidan |last2=Hall |first2=Ian R. |last3=Barker |first3=Stephen |last4=Rackow |first4=Thomas |last5=Zhang |first5=Xu |last6=Hemming |first6=Sidney R. |last7=Lubbe |first7=H. J. L. van der |last8=Knorr |first8=Gregor |last9=Berke |first9=Melissa A. |last10=Bigg |first10=Grant R. |last11=Cartagena-Sierra |first11=Alejandra |last12=Jiménez-Espejo |first12=Francisco J. |last13=Gong |first13=Xun |last14=Gruetzner |first14=Jens |last15=Lathika |first15=Nambiyathodi |last16=LeVay |first16=Leah J. |last17=Robinson |first17=Rebecca S. |last18=Ziegler |first18=Martin |title=Antarctic icebergs reorganize ocean circulation during Pleistocene glacials |journal=Nature |date=January 2021 |volume=589 |issue=7841 |pages=236–241 |doi=10.1038/s41586-020-03094-7 |pmid=33442043 |bibcode=2021Natur.589..236S |hdl=10261/258181 |s2cid=231598435 |url=https://www.nature.com/articles/s41586-020-03094-7 |access-date=12 February 2021 |language=en |issn=1476-4687 |hdl-access=free |archive-date=4 February 2021 |archive-url=https://web.archive.org/web/20210204185828/https://www.nature.com/articles/s41586-020-03094-7 |url-status=live }}</ref>

===Uplift of the Tibetan plateau===
[[Matthias Kuhle]]'s geological theory of Ice Age development was suggested by the existence of an ice sheet covering the [[Tibetan Plateau]] during the Ice Ages ([[Last Glacial Maximum]]?). According to Kuhle, the plate-tectonic uplift of Tibet past the snow-line has led to a surface of c. 2,400,000 square kilometres (930,000 sq mi) changing from bare land to ice with a 70% greater [[albedo]]. The reflection of energy into space resulted in a global cooling, triggering the [[Pleistocene]] Ice Age. Because this highland is at a subtropical latitude, with four to five times the insolation of high-latitude areas, what would be Earth's strongest heating surface has turned into a cooling surface.

Kuhle explains the [[interglacial]] periods by the 100,000-year cycle of radiation changes due to variations in Earth's orbit. This comparatively insignificant warming, when combined with the lowering of the Nordic inland ice areas and Tibet due to the weight of the superimposed ice-load, has led to the repeated complete thawing of the inland ice areas.<ref>{{cite journal |author=Kuhle, Matthias |title=The Pleistocene Glaciation of Tibet and the Onset of Ice Ages — An Autocycle Hypothesis |journal=GeoJournal |volume=17 |issue=4 |pages=581–595 |date=December 1988 |jstor=41144345|doi=10.1007/BF00209444 |bibcode=1988GeoJo..17..581K |s2cid=189891305 }}</ref><ref>2c (Quaternary Glaciation — Extent and Chronology, Part III: South America, Asia, Africa, Australia, Antarctica{{cite book |author=Kuhle, M. |chapter=The High Glacial (Last Ice Age and LGM) ice cover in High and Central Asia |chapter-url=https://books.google.com/books?id=2xpIEPH7RW4C&pg=PA175 |editor1=Ehlers, J. |editor2=Gibbard, P.L. |title=Quaternary Glaciations: South America, Asia, Africa, Australasia, Antarctica |publisher=Elsevier |location=Amsterdam |year=2004 |isbn=978-0-444-51593-3 |pages=175–199 |url=https://books.google.com/books?id=2xpIEPH7RW4C |series=Development in Quaternary Science: Quaternary Glaciations: Extent and Chronology Vol. 3}}</ref><ref>{{cite journal |author=Kuhle, M. |title=Reconstruction of an approximately complete Quaternary Tibetan inland glaciation between the Mt. Everest- and Cho Oyu Massifs and the Aksai Chin. A new glaciogeomorphological SE–NW diagonal profile through Tibet and its consequences for the glacial isostasy and Ice Age cycle |journal=GeoJournal |volume=47 |issue=1–2 |pages=3–276 |year=1999 |doi=10.1023/A:1007039510460|bibcode=1999GeoJo..47....3K |s2cid=128089823 }}</ref><ref>{{cite book |author=Kuhle, M. |chapter=Ice Age Development Theory |editor1=Singh, V.P. |editor2=Singh, P. |editor3=Haritashya, U.K. |title=Encyclopedia of Snow, Ice and Glaciers |publisher=Springer |year=2011 |pages=576–581}}</ref>

===Variations in Earth's orbit===
[[File:SummerSolstice65N-future.png|thumb|upright=2.25|Past and future of daily average insolation at top of the atmosphere on the day of the summer solstice, at 65 N latitude]]

The [[Milankovitch cycles]] are a set of cyclic variations in characteristics of Earth's orbit around the Sun. Each cycle has a different length, so at some times their effects reinforce each other and at other times they (partially) cancel each other.

There is strong evidence that the Milankovitch cycles affect the occurrence of glacial and interglacial periods within an ice age. The present ice age is the most studied and best understood, particularly the last 400,000 years, since this is the period covered by [[ice core]]s that record atmospheric composition and proxies for temperature and ice volume. Within this period, the match of glacial/interglacial frequencies to the Milanković orbital forcing periods is so close that orbital forcing is generally accepted. The combined effects of the changing distance to the Sun, the precession of Earth's [[axis of rotation|axis]], and the changing tilt of Earth's axis redistribute the sunlight received by Earth. Of particular importance are changes in the tilt of Earth's axis, which affect the intensity of seasons. For example, the amount of solar influx in July at [[65th parallel north|65 degrees north]] [[latitude]] varies by as much as 22% (from 450 W/m<sup>2</sup> to 550 W/m<sup>2</sup>). It is widely believed that ice sheets advance when summers become too cool to melt all of the accumulated snowfall from the previous winter. Some believe that the strength of the orbital forcing is too small to trigger glaciations, but feedback mechanisms like {{CO2}} may explain this mismatch.

While Milankovitch forcing predicts that cyclic changes in Earth's [[orbital elements]] can be expressed in the glaciation record, additional explanations are necessary to explain which cycles are observed to be most important in the timing of glacial–interglacial periods. In particular, during the last 800,000 years, the dominant period of glacial–interglacial oscillation has been 100,000 years, which corresponds to [[Perturbation (astronomy)|changes]] in Earth's [[orbital eccentricity]] and orbital [[inclination]]. Yet this is by far the weakest of the three frequencies predicted by Milankovitch. During the period 3.0–0.8 million years ago, the dominant pattern of glaciation corresponded to the 41,000-year period of changes in Earth's [[obliquity]] (tilt of the axis). The reasons for dominance of one frequency versus another are poorly understood and an active area of current research, but the answer probably relates to some form of resonance in Earth's climate system.  Recent work suggests that the 100K year cycle dominates due to increased southern-pole sea-ice increasing total solar reflectivity.<ref>{{cite web|url=https://news.brown.edu/articles/2017/01/iceages|title=Earth's orbital variations and sea ice synch glacial periods|access-date=2017-01-29|archive-date=2019-02-17|archive-url=https://web.archive.org/web/20190217084915/https://news.brown.edu/articles/2017/01/iceages|url-status=live}}</ref><ref>{{cite web|url=http://www.sciforums.com/threads/ice-age-explanation.158750/|title=Ice-Age Explanation - Sciforums|website=www.sciforums.com|date=28 January 2017|access-date=29 January 2017|archive-date=2 February 2017|archive-url=https://web.archive.org/web/20170202051228/http://www.sciforums.com/threads/ice-age-explanation.158750/|url-status=live}}</ref>

The "traditional" Milankovitch explanation struggles to explain the dominance of the 100,000-year cycle over the last 8 cycles. [[Richard A. Muller]], [[Gordon J. F. MacDonald]],<ref>{{Cite journal|last1=Muller|first1=R. A.|last2=MacDonald|first2=G. J.|date=1997-08-05|title=Spectrum of 100-kyr glacial cycle: orbital inclination, not eccentricity|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=94|issue=16|pages=8329–8334|doi=10.1073/pnas.94.16.8329|issn=0027-8424|pmc=33747|pmid=11607741|bibcode=1997PNAS...94.8329M|doi-access=free}}</ref><ref>{{cite web |author=Richard A. Muller |url=http://muller.lbl.gov/pages/glacialmain.htm |title=A New Theory of Glacial Cycles |publisher=Muller.lbl.gov |access-date=2012-08-07 |archive-date=2013-04-29 |archive-url=https://web.archive.org/web/20130429203041/http://muller.lbl.gov/pages/glacialmain.htm |url-status=live }}</ref><ref>{{Cite journal|last=Muller|first=R. A.|date=1997-07-11|title=Glacial Cycles and Astronomical Forcing|journal=Science|volume=277|issue=5323|pages=215–218|doi=10.1126/science.277.5323.215|bibcode=1997Sci...277..215M|url=https://zenodo.org/record/1231114|access-date=2020-05-03|archive-date=2020-08-01|archive-url=https://web.archive.org/web/20200801205823/https://zenodo.org/record/1231114|url-status=live}}</ref> and others have pointed out that those calculations are for a two-dimensional orbit of Earth but the three-dimensional orbit also has a 100,000-year cycle of orbital inclination. They proposed that these variations in orbital inclination lead to variations in insolation, as Earth moves in and out of known dust bands in the [[Solar System]]. Although this is a different mechanism to the traditional view, the "predicted" periods over the last 400,000 years are nearly the same. The Muller and MacDonald theory, in turn, has been challenged by Jose Antonio Rial.<ref>{{cite journal |author=Rial, J.A. |title=Pacemaking the ice ages by frequency modulation of Earth's orbital eccentricity |journal=Science |volume=285 |issue=5427 |pages=564–8 |date=July 1999 |pmid=10417382 |url=http://pangea.stanford.edu/Oceans/GES290/Rial1999.pdf |doi=10.1126/science.285.5427.564 |archive-url=https://web.archive.org/web/20081015123309/http://pangea.stanford.edu/Oceans/GES290/Rial1999.pdf |archive-date=2008-10-15 }}</ref>

[[William Ruddiman]] has suggested a model that explains the 100,000-year cycle by the [[modulating]] effect of eccentricity (weak 100,000-year cycle) on precession (26,000-year cycle) combined with greenhouse gas feedbacks in the 41,000- and 26,000-year cycles. Yet another theory has been advanced by [[Peter Huybers]] who argued that the 41,000-year cycle has always been dominant, but that Earth has entered a mode of climate behavior where only the second or third cycle triggers an ice age. This would imply that the 100,000-year periodicity is really an illusion created by averaging together cycles lasting 80,000 and 120,000 years.<ref>{{Cite journal|last1=Huybers|first1=Peter|last2=Wunsch|first2=Carl|date=2005-03-24|title=Obliquity pacing of the late Pleistocene glacial terminations|journal=Nature|volume=434|issue=7032|pages=491–494|doi=10.1038/nature03401|issn=1476-4687|pmid=15791252|bibcode=2005Natur.434..491H|s2cid=2729178|url=http://nrs.harvard.edu/urn-3:HUL.InstRepos:3382978|hdl=1912/555|hdl-access=free}}</ref> This theory is consistent with a simple empirical multi-state model proposed by [[Didier Paillard]].<ref>{{cite journal |author=Paillard, D. |title=The timing of Pleistocene glaciations from a simple multiple-state climate model |journal=Nature |volume=391 |issue=6665 |pages=378–381 |date=22 January 1998 |doi=10.1038/34891 |bibcode = 1998Natur.391..378P|s2cid=4409193 }}</ref> Paillard suggests that the late Pleistocene glacial cycles can be seen as jumps between three quasi-stable climate states. The jumps are induced by the [[orbit]]al forcing, while in the early Pleistocene the 41,000-year glacial cycles resulted from jumps between only two climate states. A dynamical
model explaining this behavior was proposed by Peter Ditlevsen.<ref>{{cite journal |author=Ditlevsen, P.D. |title=Bifurcation structure and noise-assisted transitions in the Pleistocene glacial cycles |journal=Paleoceanography |volume=24 |pages=PA3204 |year=2009 |doi=10.1029/2008PA001673 |url=http://www.agu.org/pubs/crossref/2009/2008PA001673.shtml |bibcode=2009PalOc..24.3204D |issue=3 |arxiv=0902.1641 |access-date=2012-06-09 |archive-date=2012-11-01 |archive-url=https://web.archive.org/web/20121101101821/http://www.agu.org/pubs/crossref/2009/2008PA001673.shtml }} as [http://www.gfy.ku.dk/~pditlev/papers/2008PA001673.pdf PDF] {{Webarchive|url=https://web.archive.org/web/20110927153529/http://www.gfy.ku.dk/~pditlev/papers/2008PA001673.pdf |date=2011-09-27 }}</ref> This is in support of the suggestion that the late [[Pleistocene]] glacial cycles are not due to the weak 100,000-year eccentricity cycle, but a non-linear response to mainly the 41,000-year obliquity cycle.

===Variations in the Sun's energy output===
There are at least two types of variation in the Sun's energy output:<ref>{{cite book |last1=Guinan |first1=E.F. |last2=Ribas |first2=I. |chapter=Our Changing Sun: The Role of Solar Nuclear Evolution and Magnetic Activity on Earth's Atmosphere and Climate |title=The Evolving Sun and its Influence on Planetary Environments |year=2002 |isbn=1-58381-109-5 |page=85|publisher=Astronomical Society of the Pacific }}</ref>

* In the very long term, astrophysicists believe that the Sun's output increases by about 7% every one billion years.
* Shorter-term variations such as [[sunspot|sunspot cycles]], and longer episodes such as the [[Maunder Minimum]], which occurred during the coldest part of the [[Little Ice Age]].

The long-term increase in the Sun's output cannot be a cause of ice ages.

===Volcanism===
Volcanic eruptions may have contributed to the inception and/or the end of ice age periods. At times during the paleoclimate, carbon dioxide levels were two or three times greater than today. Volcanoes and movements in continental plates contributed to high amounts of CO<sub>2</sub> in the atmosphere. Carbon dioxide from volcanoes probably contributed to periods with highest overall temperatures.<ref>{{cite web|last=Rieke|first=George|title=Long Term Climate|url=http://ircamera.as.arizona.edu/NatSci102/NatSci102/lectures/climate.htm|access-date=25 April 2013|archive-date=2 June 2015|archive-url=https://web.archive.org/web/20150602033750/http://ircamera.as.arizona.edu/NatSci102/NatSci102/lectures/climate.htm}}</ref> One suggested explanation of the [[Paleocene–Eocene Thermal Maximum]] is that undersea volcanoes released [[methane]] from [[clathrate]]s and thus caused a large and rapid increase in the [[greenhouse effect]].<ref>{{Cite web|url=https://www.wunderground.com/climate/PETM.asp|title=PETM: Global Warming, Naturally |website=Weather Underground |access-date=2016-12-02|archive-url=https://web.archive.org/web/20161202234346/https://www.wunderground.com/climate/PETM.asp|archive-date=2016-12-02}}</ref>  There appears to be no geological evidence for such eruptions at the right time, but this does not prove they did not happen.

==Recent glacial and interglacial phases==
{{Main|Timeline of glaciation}}
[[File:Northern icesheet hg.png|thumb|upright=1.25|Northern hemisphere glaciation during the last ice ages. The setup of 3 to 4 kilometer thick ice sheets caused a [[sea level rise|sea level lowering]] of about 120&nbsp;m.]]

The current geological period, the [[Quaternary]], which began about 2.6 million years ago and extends into the present,<ref name="ICSchart2013">{{cite web |last1=Cohen |first1=K .M. |last2=Finney |first2=S. C. |last3=Gibbard |first3=P. L. |last4=Fan |first4=J.-X. |title=International Chronostratigraphic Chart 2013 |url=http://www.stratigraphy.org/icschart/chronostratchart2013-01.pdf |website=stratigraphy.org |publisher=ICS |access-date=7 January 2019 |ref=ICS2013 |archive-date=17 July 2013 |archive-url=https://web.archive.org/web/20130717121504/http://www.stratigraphy.org/ICSchart/ChronostratChart2013-01.pdf |url-status=live }}</ref> is marked by warm and cold episodes, cold phases called [[Glacial period|glacials]] ([[Quaternary glaciation|Quaternary ice age]]) lasting about 100,000 years, and warm phases called [[interglacial]]s lasting 10,000–15,000 years. The last cold episode of the [[Last Glacial Period]] ended about 10,000 years ago.<ref>{{cite magazine|url=https://www.nationalgeographic.com/science/prehistoric-world/quaternary|archive-url=https://web.archive.org/web/20170320053318/http://www.nationalgeographic.com/science/prehistoric-world/quaternary/|archive-date=March 20, 2017|title=Quaternary Period|magazine=National Geographic|date=2017-01-06}}</ref> Earth is currently in an interglacial period of the Quaternary, called the [[Holocene]].

===Glacial stages in North America===
{{See also|Glacial history of Minnesota}}
The major glacial stages of the current ice age in North America are the [[Illinoian (stage)|Illinoian]], [[Eemian]], and [[Wisconsin glaciation]]. The use of the Nebraskan, Afton, Kansan, and Yarmouthian stages to subdivide the ice age in North America has been discontinued by Quaternary geologists and geomorphologists. These stages have all been merged into the [[Pre-Illinoian]] in the 1980s.<ref name="Hallberg1">{{cite journal |author=Hallberg, G.R. |title=Pre-Wisconsin glacial stratigraphy of the Central Plains region in Iowa, Nebraska, Kansas, and Missouri |journal=Quaternary Science Reviews |volume=5 |pages=11–15 |year=1986 |doi=10.1016/0277-3791(86)90169-1 |bibcode = 1986QSRv....5...11H}}</ref><ref name="RichmondOther1">{{cite journal |author1=Richmond, G.M. |author2=Fullerton, D.S. |title=Summation of Quaternary glaciations in the United States of America |journal=Quaternary Science Reviews |volume=5 |pages=183–196 |year=1986 |doi=10.1016/0277-3791(86)90184-8 |bibcode = 1986QSRv....5..183R}}</ref><ref name="GibbardOthers2007">Gibbard, P.L., S. Boreham, K.M. Cohen and A. Moscariello, 2007, [http://www.quaternary.stratigraphy.org.uk/correlation/POSTERSTRAT_v2007b_small.jpg ''Global chronostratigraphical correlation table for the last 2.7 million years v. 2007b.''] {{Webarchive|url=https://web.archive.org/web/20080910122430/http://www.quaternary.stratigraphy.org.uk/correlation/POSTERSTRAT_v2007b_small.jpg |date=2008-09-10 }}, jpg version 844 KB. Subcommission on Quaternary Stratigraphy, Department of Geography, University of Cambridge, Cambridge, England</ref>

During the most recent North American glaciation, during the latter part of the [[Last Glacial Maximum]] (26,000 to 13,300 years ago), ice sheets extended to about [[45th parallel north]]. These sheets were {{convert|3 to 4|km}} thick.<ref name="RichmondOther1"/>
[[File:Glacial lakes.jpg|thumb|right|Stages of [[proglacial lake]] development in the region of the current North American [[Great Lakes]]]]

This Wisconsin glaciation left widespread impacts on the North American landscape. The [[Great Lakes]] and the [[Finger Lakes]] were carved by ice deepening old valleys. Most of the lakes in Minnesota and Wisconsin were gouged out by glaciers and later filled with glacial meltwaters. The old [[Teays River]] drainage system was radically altered and largely reshaped into the [[Ohio River]] drainage system. Other rivers were dammed and diverted to new channels, such as [[Niagara Falls]], which formed a dramatic waterfall and gorge, when the waterflow encountered a limestone escarpment. Another similar waterfall, at the present [[Clark Reservation State Park]] near [[Syracuse, New York]], is now dry.

The area from [[Long Island]] to [[Nantucket, Massachusetts]] was formed from glacial [[till]], and the plethora of lakes on the [[Canadian Shield]] in northern Canada can be almost entirely attributed to the action of the ice. As the ice retreated and the rock dust dried, winds carried the material hundreds of miles, forming beds of [[loess]] many dozens of feet thick in the [[Missouri River|Missouri Valley]]. [[Post-glacial rebound]] continues to reshape the Great Lakes and other areas formerly under the weight of the ice sheets.

The [[Driftless Area]], a portion of western and southwestern Wisconsin along with parts of adjacent [[Minnesota]], [[Iowa]], and [[Illinois]], was not covered by glaciers.

===Last Glacial Period in the semiarid Andes around Aconcagua and Tupungato===
A specially interesting climatic change during glacial times has taken place in the semi-arid Andes. Beside the expected cooling down in comparison with the current climate, a significant precipitation change happened here. So, researches in the presently semiarid subtropic Aconcagua-massif (6,962&nbsp;m) have shown an unexpectedly extensive glacial glaciation of the type "ice stream network".<ref>{{cite journal |author=Kuhle, M. |title=Spuren hocheiszeitlicher Gletscherbedeckung in der Aconcagua-Gruppe (32–33° S) |journal=Zentralblatt für Geologie und Paläontologie, Teil I |volume=11/12 |pages=1635–46 |year=1984 |issn=0340-5109}} Verhandlungsblatt des Südamerika-Symposiums 1984 in Bamberg.</ref><ref>{{cite journal |author=Kuhle, M. |title=Die Vergletscherung Tibets und die Entstehung von Eiszeiten |journal=Spektrum der Wissenschaft |issue=9/86 |pages=42–54 |year=1986 |issn=0170-2971}}</ref><ref>{{cite journal |author=Kuhle, Matthias |title=Subtropical Mountain- and Highland-Glaciation as Ice Age Triggers and the Waning of the Glacial Periods in the Pleistocene |journal=GeoJournal |volume=14 |issue=4 |pages=393–421 |date=June 1987 |jstor=41144132 |doi=10.1007/BF02602717|bibcode=1987GeoJo..14..393M |s2cid=129366521 }}</ref><ref name=Kuhle04>{{cite book |author=Kuhle, M. |chapter=The Last Glacial Maximum (LGM) glacier cover of the Aconcagua group and adjacent massifs in the Mendoza Andes (South America) |chapter-url=https://books.google.com/books?id=2xpIEPH7RW4C&pg=PA75 |editor1=Ehlers, J. |editor2=Gibbard, P.L. |title=Quaternary Glaciations: South America, Asia, Africa, Australasia, Antarctica |publisher=Elsevier |location=Amsterdam |year=2004 |isbn=978-0-444-51593-3 |pages=75–81 |url=https://books.google.com/books?id=2xpIEPH7RW4C |series=Development in Quaternary Science}}</ref><ref name=Kuhle11>{{cite book |author=Kuhle, M. |chapter=Ch 53: The High-Glacial (Last Glacial Maximum) Glacier Cover of the Aconcagua Group and Adjacent Massifs in the Mendoza Andes (South America) with a Closer Look at Further Empirical Evidence |chapter-url=https://books.google.com/books?id=Jv4uA1lHezEC&pg=PA735 |editor1=Ehlers, J. |editor2=Gibbard, P.L. |editor3=Hughes, P.D. |title=Quaternary Glaciations – Extent and Chronology: A Closer Look |publisher=Elsevier |location=Amsterdam |year=2011 |isbn=978-0-444-53447-7 |pages=735–8 |url=https://books.google.com/books?id=Jv4uA1lHezEC |series=Development in Quaternary Science}}</ref> The connected valley glaciers exceeding 100&nbsp;km in length, flowed down on the East-side of this section of the Andes at 32–34°S and 69–71°W as far as a height of 2,060&nbsp;m and on the western luff-side still clearly deeper.<ref name=Kuhle11/><ref>{{cite journal |author=Brüggen, J. |title=Zur Glazialgeologie der chilenischen Anden |journal=Geol. Rundsch. |volume=20 |issue=1 |pages=1–35 |year=1929 |doi=10.1007/BF01805072|bibcode=1929GeoRu..20....1B |s2cid=128436981 }}</ref> Where current glaciers scarcely reach 10&nbsp;km in length, the snowline (ELA) runs at a height of 4,600&nbsp;m and at that time was lowered to 3,200&nbsp;m [[Above sea level|asl]], i.e. about 1,400&nbsp;m. From this follows that—beside of an annual depression of temperature about c. 8.4&nbsp;°C— here was an increase in precipitation. Accordingly, at glacial times the humid climatic belt that today is situated several latitude degrees further to the S, was shifted much further to the N.<ref name=Kuhle04/><ref name=Kuhle11/>

==Effects of glaciation==
{{See also|Glacial landform}}
[[File:Scandinavia.TMO2003050.jpg|right|thumb|[[Scandinavia]] exhibits some of the typical effects of ice age glaciation such as [[fjord]]s and lakes.]]

Although the last glacial period ended more than 8,000 years ago, its effects can still be felt today. For example, the moving ice carved out the landscape in Canada (See [[Canadian Arctic Archipelago]]), Greenland, northern Eurasia and Antarctica. The [[erratic boulder]]s, [[till]], [[drumlin]]s, [[esker]]s, [[fjord]]s, [[kettle lake]]s, [[moraine]]s, [[cirque]]s, [[Glacial horn|horns]], etc., are typical features left behind by the glaciers. The weight of the ice sheets was so great that they deformed Earth's crust and mantle. After the ice sheets melted, the ice-covered land [[Post-glacial rebound|rebounded]]. Due to the high [[viscosity]] of [[Earth's mantle]], the flow of mantle rocks which controls the rebound process is very slow—at a rate of about 1&nbsp;cm/year near the center of rebound area today.

During glaciation, water was taken from the oceans to form the ice at high latitudes, thus global sea level dropped by about 110 meters, exposing the continental shelves and forming land-bridges between land-masses for animals to migrate. During [[deglaciation]], the melted ice-water returned to the oceans, causing sea level to rise. This process can cause sudden shifts in coastlines and hydration systems resulting in newly submerged lands, emerging lands, collapsed [[Proglacial lake|ice dams]] resulting in [[salinity|salination]] of lakes, new ice dams creating vast areas of freshwater, and a general alteration in regional weather patterns on a large but temporary scale. It can even cause temporary [[reglaciation]]. This type of chaotic pattern of rapidly changing land, ice, saltwater and freshwater has been proposed as the likely model for the [[Baltic region|Baltic]] and [[Scandinavia]]n regions, as well as much of central North America at the end of the last glacial maximum, with the present-day coastlines only being achieved in the last few millennia of prehistory. Also, the effect of elevation on Scandinavia submerged a vast continental plain that had existed under much of what is now the North Sea, connecting the British Isles to Continental Europe.<ref>{{cite book |last1=Andersen |first1=Bjørn G. |last2=Borns |first2=Harold W. Jr. |title=The Ice Age World: an introduction to quaternary history and research with emphasis on North America and Northern Europe during the last 2.5 million years |year=1997 |url=http://www.universitetsforlaget.no/boker/realfagogit/biologi_geologi_og_miljoefag/katalog?productId=674197 |archive-url=https://archive.today/20130112093533/http://www.universitetsforlaget.no/boker/realfagogit/biologi_geologi_og_miljoefag/katalog?productId=674197 |archive-date=2013-01-12 |publisher=[[Universitetsforlaget]] |location=Oslo |isbn=978-82-00-37683-5 |access-date=2013-10-14 }}</ref>

The redistribution of ice-water on the surface of Earth and the flow of mantle rocks causes changes in the [[Gravity of Earth|gravitational field]] as well as changes to the distribution of the [[moment of inertia]] of Earth. These changes to the moment of inertia result in a change in the [[angular velocity]], [[Axis of rotation|axis]], and wobble of Earth's rotation.

The weight of the redistributed surface mass loaded the [[lithosphere]], caused it to [[Bending|flex]] and also induced [[Stress (mechanics)|stress]] within Earth. The presence of the glaciers generally suppressed the movement of [[Fault (geology)|faults]] below.<ref>{{cite book |author=Johnston, A. |chapter=The effect of large ice sheets on earthquake genesis |editor1=Gregersen, S. |editor2=Basham, P. |title=Earthquakes at North-Atlantic passive margins: Neotectonics and postglacial rebound |publisher=Kluwer |location=Dordrecht |year=1989 |isbn=978-0-7923-0150-9 |pages=581–599}}</ref><ref>{{Cite journal|last1=Wu|first1=Patrick|last2=Hasegawa|first2=Henry S.|date=October 1996|title=Induced stresses and fault potential in eastern Canada due to a realistic load: a preliminary analysis|journal=Geophysical Journal International|language=en|volume=127|issue=1|pages=215–229|doi=10.1111/j.1365-246X.1996.tb01546.x|bibcode=1996GeoJI.127..215W|doi-access=free}}</ref><ref>{{cite journal | last1 = Turpeinen | first1 = H. | last2 = Hampel | first2 = A. | last3 = Karow | first3 = T. | last4 = Maniatis | first4 = G. | year = 2008 | title = Effect of ice sheet growth and melting on the slip evolution of thrust faults | journal = [[Earth and Planetary Science Letters]] | volume = 269 | issue = 1–2 | pages = 230–241 |bibcode = 2008E&PSL.269..230T |doi = 10.1016/j.epsl.2008.02.017}}</ref> During [[deglaciation]], the faults experience accelerated slip triggering [[earthquake]]s. Earthquakes triggered near the ice margin may in turn accelerate [[ice calving]] and may account for the [[Heinrich events]].<ref>{{Cite journal|last1=Hunt|first1=A. G.|last2=Malin|first2=P. E.|date=May 1998|title=Possible triggering of Heinrich events by ice-load-induced earthquakes|journal=Nature|language=en|volume=393|issue=6681|pages=155–158|doi=10.1038/30218|issn=0028-0836|bibcode=1998Natur.393..155H|s2cid=4393858}}</ref> As more ice is removed near the ice margin, more [[intraplate earthquake]]s are induced and this positive feedback may explain the fast collapse of ice sheets.

In Europe, glacial erosion and [[isostasy|isostatic]] sinking from the weight of ice made the [[Baltic Sea]], which before the Ice Age was all land drained by the [[Eridanos (geology)|Eridanos River]].

==Future ice ages==
{{Main|Next glacial period}}

Based on past estimates for interglacial durations of about 10,000 years, there was some concern in the 1970s that the next glacial period would be imminent.<ref>{{cite news |title=The Fiction Of Climate Science |url=https://www.forbes.com/2009/12/03/climate-science-gore-intelligent-technology-sutton.html |work=Forbes |date=4 December 2009}}</ref> [[Human impact on the environment|Human impact]] is now seen as possibly extending what would already be an unusually long warm period.<ref>{{cite news |title=Our next ice age is due in 10,000 years, but there's a catch |url=https://www.dw.com/en/earths-next-ice-age-is-due-in-10000-years-but-theres-a-catch/a-71786018 |work=Deutsche Welle |date=3 March 2025}}</ref><ref>{{cite news |title=Study Shows How Earth's Orbit Affects Ice Ages |url=https://learningenglish.voanews.com/a/study-shows-how-earth-s-orbit-affects-ice-ages/7997495.html |work=[[Voice of America]] |date=10 March 2025}}</ref> Ice ages go through cycles of about 100,000 years, but the next one may well be avoided due to human carbon dioxide emissions.<ref name="PIK2016" /> According to Stephen Barker of [[Cardiff University]], without human interference, the next glaciation of the Earth would "occur within the next 11,000 years, and it would end in 66,000 years' time."<ref>{{cite news |title=Human-caused emissions have delayed Earth's next ice age, study says. But by how long? |url=https://www.euronews.com/green/2025/02/28/human-caused-emissions-have-delayed-earths-next-ice-age-study-says-but-by-how-long |work=Euronews |date=28 February 2025}}</ref>

A 2015 report by the Past Global Changes Project says simulations show that a new glaciation is unlikely to happen within the next approximately 50,000 years, before the next strong drop in Northern Hemisphere summer insolation occurs "if either atmospheric {{CO2}} concentration
remains above 300 ppm or cumulative carbon emissions exceed 1000 Pg C" (i.e. 1,000 gigatonnes carbon). "Only for an atmospheric {{CO2}} content below the preindustrial level may a glaciation occur within the next 10 ka. ... Given the continued anthropogenic {{CO2}} emissions, glacial inception is very unlikely to occur in the next 50 ka, because the timescale for {{CO2}} and temperature reduction toward unperturbed values in the absence of active removal is very long [IPCC, 2013], and only weak precessional forcing occurs in the next two precessional cycles." (A [[Milankovitch cycles#Apsidal precession|precessional cycle]] is around 21,000 years, the time it takes for the [[perihelion]] to move all the way around the [[tropical year]].)<ref>{{Cite journal|last=Interglacial Working Group Of PAGES|title=Interglacials Of The Last 800,000 years|url=http://discovery.ucl.ac.uk/1474024/1/Past%20Interglacials%20Working%20Group%20of%20PAGES%20Interglacials%20of%20the%20last%20800%2C000%20years%20VoR.pdf |bibcode-access=free |hdl-access=free |journal=Reviews of Geophysics|date=November 20, 2015|volume=54|issue=1|pages=162–219|bibcode=2016RvGeo..54..162P|doi=10.1002/2015RG000482|hdl=2078.1/175429|doi-access=free |via=UCL Discovery |url-status=live |archive-url= https://web.archive.org/web/20180718233716/http://discovery.ucl.ac.uk/1474024/1/Past%20Interglacials%20Working%20Group%20of%20PAGES%20Interglacials%20of%20the%20last%20800%2C000%20years%20VoR.pdf |archive-date= Jul 18, 2018 }}</ref>

==See also==
{{Portal|Geology|Paleontology}}
{{Div col|colwidth=30em}}
* {{annotated link|Geologic temperature record}}
* {{annotated link|Global cooling}}
* {{annotated link|International Union for Quaternary Research}}
* {{annotated link|Irish Sea Glacier}}
* {{annotated link|Last Glacial Maximum}}
* [[List of Ice Age species preserved as permafrost mummies]]
* {{annotated link|Little Ice Age}}
* {{annotated link|Post-glacial rebound}}
* {{annotated link|Timeline of glaciation}}
{{Div col end}}

==References==
{{Reflist}}

===Works cited===
* {{cite web |first=Keith |last=Montgomery |title=Development of the glacial theory, 1800–1870 |year=2010 |url=https://www.glacialtheory.net }}{{Dead link|date=June 2025 |bot=InternetArchiveBot |fix-attempted=yes }} Historical Simulation

==External links==
{{Wikibooks|Historical Geology|Ice ages}}
{{Commons category|Ice ages}}
{{NSRW poster|Ice-Age, The|Ice age}}
* [https://www.pbs.org/wgbh/nova/ice/ Cracking the Ice Age] {{Webarchive|url=https://web.archive.org/web/20170904171146/http://www.pbs.org/wgbh/nova/ice/ |date=2017-09-04 }} from PBS
* {{cite web |title=Scientists unveil 'best-preserved Ice Age animal ever found' |author=Rina Torchinsky |website=AccuWeather |date=9 Aug 2021 |url=https://www.accuweather.com/en/weather-news/28000-year-old-lion-cub-best-preserved-ice-age-animal-ever-found/995878#:~:text=28%2C000%2Dyear%2Dold%20Lion%20cub,AccuWeather |access-date=9 August 2021 |archive-date=9 August 2021 |archive-url=https://web.archive.org/web/20210809172900/https://www.accuweather.com/en/weather-news/28000-year-old-lion-cub-best-preserved-ice-age-animal-ever-found/995878#:~:text=28%2C000%2Dyear%2Dold%20Lion%20cub,AccuWeather |url-status=live }}
* {{cite web |author=Raymo, M. |title=Overview of the Uplift-Weathering Hypothesis |date=July 2011 |url=http://www.moraymo.us/uplift_overview.php |archive-url=https://web.archive.org/web/20081022085754/http://www.moraymo.us/uplift_overview.php |archive-date=2008-10-22 }}
* [https://ice.tsu.ru/index.php?option=com_content&view=category&layout=blog&id=43&Itemid=88&limitstart=5 Eduard Y. Osipov, Oleg M. Khlystov. Glaciers and meltwater flux to Lake Baikal during the Last Glacial Maximum.] {{Webarchive|url=https://web.archive.org/web/20160312054118/http://ice.tsu.ru/index.php?id=43&itemid=88&layout=blog&limitstart=5&option=com_content&view=category |date=2016-03-12 }}
* {{cite news |author=Black, R. |title=Carbon emissions 'will defer Ice Age' |publisher=[[BBC News]] |department=Science and Environment |date=9 January 2012 |url=https://www.bbc.co.uk/news/science-environment-16439807 |access-date=20 June 2018 |archive-date=23 October 2018 |archive-url=https://web.archive.org/web/20181023223334/https://www.bbc.co.uk/news/science-environment-16439807 |url-status=live }}

{{Ice ages}}
{{Greenhouse and Icehouse Earth}}
{{Continental Glaciations}}
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[[Category:Ice ages| ]]
[[Category:Geological history of the Great Lakes]]
[[Category:Glaciology]]
[[Category:History of climate variability and change]]
[[Category:History of science]]