Editing Geologic temperature record (section)

=== Fluctuations during the remainder of the Phanerozoic ===
[[Image:Phanerozoic Climate Change.png|thumb|300px|500 million years of climate change<ref>{{Cite journal| journal = Chemical Geology| volume = 161| issue = 1–3| pages = 59–88| first1 = J.| last1 = Veizer| title = <sup>87</sup>Sr/<sup>86</sup>Sr, δ<sup>13</sup>C and δ<sup>18</sup>O evolution of Phanerozoic seawater| year = 1999| doi = 10.1016/S0009-2541(99)00081-9| bibcode = 1999ChGeo.161...59V| doi-access = free}}</ref>]]

The [[Phanerozoic]] [[eon (geology)|eon]], encompassing the last 542 million years and almost the entire time since the origination of complex multi-cellular life, has more generally been a period of fluctuating temperature between ice ages, such as the current age, and "[[climate optima]]", similar to what occurred in the Cretaceous. Roughly 4 such cycles have occurred during this time with an approximately 140 million year separation between climate optima. In addition to the present, ice ages have occurred during the [[Permian]]-[[Carboniferous]] interval and the late [[Ordovician]]-early [[Silurian]]. There is also a "cooler" interval during the [[Jurassic]] and early Cretaceous, with evidence of increased sea ice, but the lack of continents at either pole during this interval prevented the formation of continental ice sheets and consequently this is usually not regarded as a full-fledged ice age. In between these cold periods, warmer conditions were present and often referred to as climate optima. However, it has been difficult to determine whether these warmer intervals were actually hotter or colder than occurred during the Cretaceous optima.