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	==References==		==References==
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[[Category:Done 2017-12-31]]
[[File:snowball_earth.jpg|thumbnail|400px| Figure 1: An artist's depiction of what snow ball Earth may have looked like near the end of the Proterozoic Eon.<ref>QED Science, "Catastrophe!(6) Snowball Earth". [Online]. Available: https://qedscience.wordpress.com/2013/05/26/catastrophe-6-snowball-earth/. [Accessed: Feb 18, 2018]</ref>]]
<onlyinclude>An '''ice age''' is a period of time where global [[Temperature|temperatures]] drop so significantly that [[Glacier|glaciers]] advance and encompass over one third of [[Earth|Earth’s]] surface both [[Latitude|laterally]] and [[longitude|longitudinally]].<ref name="ref1">S. G. Philander, Ed., Encyclopedia of global warming and climate change, vol. 2, 3 vols. Los Angeles: SAGE, 2008. pp. 519-522. </ref></onlyinclude> As a whole, ice ages can last up to hundreds of millions of years, the longest ice age reaching approximately 300 million years. During an ice age, a [[glacial]] is the period of time where glacial advancement occurs. An [[interglacial]] is the warmer break in between two ice ages where glaciers retreat.<ref>“Glacial-Interglacial Cycles | National Centers for Environmental Information (NCEI) formerly known as National Climatic Data Center (NCDC).” [Online]. Available: https://www.ncdc.noaa.gov/abrupt-climate-change/Glacial-Interglacial%20Cycles. [Accessed: 09-Feb-2018].</ref>

== Triggers ==

There are many different causes as to why glaciation occurs; the most prominent can be placed into three categories: astronomical, atmospheric, and terrestrial.<ref name="ref1"/>

=== Astronomical ===

Astronomically triggered ice ages are the most common. [[Milankovitch cycle|The Milankovitch Cycles]] are the three ways in which the Earth’s orbit around the sun changes over the course of tens of thousands of years. One of the three factors that rules the Milankotich cycle are the variations in [[Milankovitch cycle|eccentricity]] of Earth’s orbit. It is the path of Earth’s orbit around the sun which ultimately affects how much [[Sunlight|sunlight]] and [[Radiation|radiation]] hits Earth’s surface, and is the driving factor for Earth’s [[Season|seasonal]] changes. Over a period of 105,000 years, the eccentricity of Earth’s orbit varies minimally, however it is enough to cause the growth and melt of [[ice sheet]]s.<ref name="ref2">“What Causes Ice Ages?” [Online]. Available: http://www.arctic.uoguelph.ca/cpe/environments/climate/climate_past/ice_cause.htm. [Accessed: 12-Feb-2018].</ref>

=== Atmospheric ===

Atmospheric triggered ice ages are a result of either too much or too little [[greenhouse gas]]es like [[Carbon dioxide| carbon dioxide]] (CO2) in the [[atmosphere| Atmosphere]]. Too little CO2 results in the atmosphere being unable to trap [[Heat|heat]] and warm Earth's surface, therefore resulting in major global temperature drops.<ref name="ref1"/> Alternatively, a major volcanic eruption can result in the release of [[Aerosol| aerosols]] into the [[Atmosphere|atmosphere]] which, once suspended in the atmosphere for a long period of time, reflect sunlight and ultimately cool Earth’s surface.<ref>B. Allen, “Atmospheric Aerosols: What Are They, and Why Are They So Important?,” NASA, 06-Apr-2015. [Online]. Available: http://www.nasa.gov/centers/langley/news/factsheets/Aerosols.html. [Accessed: 09-Feb-2018].</ref>

=== Terrestrial ===

Terrestrial triggered ice ages are a result of [[plate tectonics|plate tectonic]] movement disturbing [[weather]] patterns. When continents move such that they limit the warm currents from the tropics reaching the poles, the result is the building of large glaciers and the development of an ice age. <ref>Utah Geological Survey. “Glad You Asked: Ice Ages – What are they and what causes them?". [Online]. Available: https://geology.utah.gov/map-pub/survey-notes/glad-you-asked/ice-ages-what-are-they-and-what-causes-them/.[Accessed: 12-Feb-2018].</ref><ref name="ref2/>
 

The video below represents how plate tectonics have historically been a direct cause of major glaciation events:<ref>Scotese, C.R., “Plate Tectonics & Ice Ages – Scotese Animation 022116a”, Feb 21, 2016, YouTube. [Online]. Available: https://www.youtube.com/watch?v=Jo5GofO5Syw. [Accessed: 02-Mar-2018]</ref>
<HTML5video width="800" height="450" autoplay="false" loop="true">Plate_techtonics_and_Ice_ages</HTML5video>

== Previous Ice Ages ==
It is speculated that there have been a total of six major ice ages since the beginning of Earth’s history to present day.
[[File:major-past-glaciations.png|800px|left|thumbnail|Figure 3: Geological time scale with Earth's major ice ages.<ref> Earle, S., "Physical Geology: Glacial Periods in Earth's History", BC Open Textbook Project, ch. 16, sec. 1. [Online]. Available: https://opentextbc.ca/geology/chapter/16-1-glacial-periods-in-earths-history/. [Accessed: Mar-16-2018].</ref>]]
{| class="wikitable"
|-
! Glacial !! Era !! Years Ago (Ga)
|-
| Pongola<ref>Grant M. Young, Victor von Brunn, Digby J. C. Gold, and W. E. L. Minter, "Earth's Oldest Reported Glaciation: Physical and Chemical Evidence from the Archean Mozaan Group (∼2.9 Ga) of South Africa," The Journal of Geology 106, no. 5 (September 1998): 523-538.</ref> || Neoarchaean & Mesoarchaean || ~2.9
|-
| Huronian<ref>A. Bekker, “Huronian Glaciation,” in Encyclopedia of Astrobiology, M. Gargaud, R. Amils, J. C. Quintanilla, H. J. Cleaves, W. M. Irvine, D. L. Pinti, and M. Viso, Eds. Berlin, Heidelberg: Springer Berlin Heidelberg, 2011, pp. 768–772.</ref> || Paeleoproterozoic || ~2.45
|-
| Cryogenian<ref>IUGS, "International Chronostratigraphic Chart", International Commission on Stratigraphy, 2017. [Online]. Available: http://www.stratigraphy.org/ICSchart/ChronostratChart2017-02.jpg. [Accessed: 12-Feb-18].</ref> || Neoproterozoic || ~0.72
|-
| Andean-Saharan<ref>Sumanarathna, A.R., et al., AN ASSESSEMENT OFGEOLOGICAL FORMATION OF THE RAKWANA-PANNILA MOUNTAIN OF SRI LANKA; Journal of Eco Astronomy (2017), Vol 01, Issue 01, PP 32-42</ref> || Paleozoic || ~0.46
|-
| Karoo || Paleozoic || ~0.36
|-
| Quarternary || Cenozoic || ~2.6x10-3
|}

=== Looking Ahead ===
Earth’s most recent glaciation occurred during the Pleistocene epoch within the Quarternary period. The Pleistocene epoch experienced around nine glacier advancements and retreats globally. Presently, geologists mark the ending of the last major glaciation as the beginning of the [[Holocene]] period, which began approximately 11,000 years ago. Since the average Pleistocene interglacials have historically lasted around 10,000 years, it is expected that a new glaciation should be occurring soon. However, due to [[Natural vs anthropogenic climate change|human induced climate change]], the next glaciation may take longer to occur than predicted, if at all. <ref> S. Marshak, Earth: portrait of a planet, 3rd ed. New York: W.W. Norton, 2008. Part VI, pp. 791-797.</ref>

==References==
{{reflist}}

Ice age - Revision history

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