Editing Geologic temperature record (section)

=== Pleistocene ===
[[File:Five Myr Climate Change.png|thumb|left|350px|Reconstruction of the past 5 million years of climate history, based on [[Δ18O|oxygen isotope fractionation]] in deep sea sediment cores (serving as a proxy for the total global mass of glacial ice sheets), fitted to a model of [[orbital forcing]] (Lisiecki and Raymo 2005)<ref>{{cite journal |url=http://lorraine-lisiecki.com/LisieckiRaymo2005.pdf |title=A Pliocene-Pleistocene stack of 57 globally distributed benthic d<sup>18</sup>O records |journal=Paleoceanography |first=Lorraine E. |last=Lisiecki |author2=Raymo, Maureen E. |volume=20 |issue=1 |pages=PA1003 | date=January 2005 |doi=10.1029/2004PA001071 |bibcode=2005PalOc..20.1003L|hdl=2027.42/149224 |s2cid=12788441 }}
:*Supplement: {{cite journal |title=Pliocene-Pleistocene stack of globally distributed benthic stable oxygen isotope records  |journal=Pangaea |first=L. E. |last=Lisiecki |author2=Raymo, M. E. |year=2005 |doi=10.1594/PANGAEA.704257}}
{{Cite journal| doi = 10.1029/2005PA001164 | title = Correction to "A Pliocene-Pleistocene stack of 57 globally distributed benthic δ<sup>18</sup>O records"| date=May 2005 | last1 = Lisiecki | first1 = L. E.| journal = Paleoceanography| volume = 20| issue = 2| pages = PA2007 | last2 = Raymo | first2 = M. E.|bibcode = 2005PalOc..20.2007L | doi-access = free}}<br/>data: {{doi|10.1594/PANGAEA.704257}}.
</ref> and to the temperature scale derived from [[Vostok Station#Ice core drilling|Vostok]] ice cores following Petit et al. (1999).<ref>{{cite journal |title=Climate and Atmospheric History of the Past 420,000 years from the Vostok Ice Core, Antarctica |journal=Nature |first=J. R. |last=Petit |author2=Jouzel, J. |author3=Raynaud, D. |author4=Barkov, N. I. |author5=Barnola, J. M. |author6=Basile, I. |author7=Bender, M. |author8=Chappellaz, J. |author9=Davis, J. |author10=Delaygue, G. |author11=Delmotte, M. |author12=Kotlyakov, V. M. |author13=Legrand, M. |author14=Lipenkov, V. |author15=Lorius, C. |author16=Pépin, L. |author17=Ritz, C. |author18=Saltzman, E. |author19=Stievenard, M. |volume=399 |issue=6735 |pages=429–436 |year=1999 |doi=10.1038/20859|bibcode=1999Natur.399..429P |s2cid=204993577 |url=http://www.escholarship.org/uc/item/7rx4413n }}</ref> ]]
The [[Pleistocene|last 3 million years]] have been characterized by cycles of glacials and interglacials within a gradually deepening [[ice age]]. [[Holocene|Currently]], the Earth is in an interglacial period, beginning about [[Last Glacial Maximum|20,000 years ago]] (20 kya).

The cycles of glaciation involve the growth and retreat of continental ice sheets in the Northern Hemisphere and involve fluctuations on a number of time scales, notably on the 21 ky, 41 ky and 100 ky scales. Such cycles are usually interpreted as being [[orbital forcing|driven by]] predictable changes in the Earth orbit known as [[Milankovitch cycles]]. At the beginning of the [[Middle Pleistocene]] (0.8 million years ago, close to the [[Brunhes–Matuyama reversal|Brunhes–Matuyama]] [[geomagnetic reversal]]) there has been a [[100,000-year problem|largely unexplained]] switch in the dominant periodicity of glaciations from the 41 ky to the 100 ky cycle.

The gradual intensification of this ice age over the last 3 million years has been associated with declining concentrations of the [[greenhouse gas]] [[carbon dioxide]], though it remains unclear if this change is sufficiently large to have caused the changes in [[Temperature record|temperature]]s. 
Decreased temperatures can cause a decrease in carbon dioxide as, by [[Henry's Law]], carbon dioxide is more soluble in colder waters, which may account for 30ppmv of the 100ppmv decrease in carbon dioxide concentration during the last glacial maximum. {{ref|sigmanboyle2000}}

Similarly, the initiation of this deepening phase also corresponds roughly to the closure of the [[Isthmus of Panama]] by the action of [[plate tectonics]]. This prevented direct ocean flow between the Pacific and Atlantic, which would have had significant effects on ocean circulation and the distribution of heat. However, modeling studies have been ambiguous as to whether this could be the direct cause of the intensification of the present ice age.

This recent period of cycling climate is part of the more extended ice age that began about {{Ma|40}} with the glaciation of [[Antarctica]].