Editing Ice age (section)

==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===
Another important contribution to ancient climate regimes is the variation of ocean currents, 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 |url-status=dead }}</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 |url-status=dead |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 |url-status=dead }} 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|url-status=dead}}</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|url-status=dead|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.