Editing Ice age (section)

===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.