Editing Orbital forcing (section)

==Overview==
[[File:Vostok Petit data.svg|thumb|upright=1.75|Ice core data. Note length of glacial cycles averages ~100,000 years. Blue curve is temperature, green curve is CO<sub>2</sub>, and red curve is windblown glacial dust (loess). Today's date is on the right side of the graph.]]
It is sometimes asserted that the length of the current interglacial temperature peak will be similar to that of the preceding interglacial peak ([[Eemian Stage|Sangamonian/Eem Stage]]). Therefore, we might be nearing the end of this warm period. However, this conclusion is probably mistaken: the lengths of previous interglacials were not particularly regular (see graphic at right). Berger and Loutre (2002) argue that “with or without human perturbations, the current warm climate may last another 50,000 years. The reason is a minimum in the eccentricity of Earth's orbit around the Sun.”<ref name="Berger2002">{{cite journal|last=Berger|first=A.|author2=Loutre, M. F.|title=An Exceptionally Long Interglacial Ahead?|journal=Science|date=23 August 2002|volume=297|issue=5585|pages=1287–1288|doi=10.1126/science.1076120|pmid=12193773|s2cid=128923481}}</ref> Also, Archer and Ganopolski (2005) report that probable future CO<sub>2</sub> emissions may be enough to suppress the glacial cycle for the next 500 kyr.<ref name="Archer2005">{{cite journal|last1=Archer|first1=David|author-link=David Archer (scientist)|last2=Ganopolski|first2=Andrey|title=A Movable Trigger: Fossil Fuel CO<sub>2</sub> And The Onset Of The Next Glaciation|journal=Geochemistry, Geophysics, Geosystems|date=5 May 2005|volume=6|issue=5|pages=Q05003|doi=10.1029/2004GC000891|bibcode=2005GGG.....6.5003A |doi-access=free}}</ref>

Note in the graphic, the strong [[100,000 year problem|100,000 year periodicity]] of the cycles, and the striking asymmetry of the curves. This asymmetry is believed to result from complex interactions of feedback mechanisms. It has been observed that
ice ages deepen in progressive steps. However, the recovery to interglacial conditions occurs in a single large step.

Orbital mechanics require that the length of the seasons be proportional to the swept areas of the seasonal quadrants, so when the eccentricity is extreme, the seasons on the far side of the orbit can last substantially longer. Today, when autumn and winter in the Northern Hemisphere occur at closest approach, the Earth is moving at its maximum velocity and therefore autumn and winter are slightly shorter than spring and summer. 
[[File:SeasonDuration.png|400px|left|The length of the seasons is proportional to the area of the Earth's orbit swept between the solstices and equinoxes.]]
Today in the Northern Hemisphere, summer is 4.66 days longer than winter and spring is 2.9 days longer than autumn.<ref name="Benson2007">{{cite web|url=https://sites.google.com/site/bensonfamilyhomepage/Home/ice-age-and-global-warming|title=Global Warming, Ice Ages, and Sea Level Changes: Something new or an astronomical phenomenon occurring in present day?|last=Benson|first=Gregory|date=11 December 2007}}</ref> As [[axial precession]] changes the place in the Earth's orbit where the [[solstices]] and [[equinoxes]] occur, Northern Hemisphere winters will get longer and summers will get shorter, eventually creating conditions believed to be favourable for triggering the next glacial period.

The arrangements of land masses on the Earth's surface are believed to reinforce the orbital forcing effects. Comparisons of [[plate tectonic]] continent [[Plate reconstruction|reconstructions]] and paleoclimatic studies show that the [[Milankovitch cycles]] have the greatest effect during [[geologic era]]s when landmasses have been concentrated in polar regions, as is the case today. [[Greenland]], [[Antarctica]], and the northern portions of [[Europe]], [[Asia]], and [[North America]] are situated such that a minor change in solar energy will tip the balance in the [[climate of the Arctic]], between [[Arctic ice pack|year-round snow/ice preservation]] and complete summer melting. The presence or absence of snow and ice is a well-understood [[positive feedback]] mechanism for climate.