Editing Geologic temperature record (section)

=== Initial Eocene thermal maxima ===
[[Image:65 Myr Climate Change.png|300px|thumb|Climate change during the last 65 million years. The true magnitude of the PETM is likely to be understated in this figure due to coarse sampling.<ref>{{Cite journal| doi = 10.1126/science.1059412| pmid = 11326091| year = 2001| last1 = Zachos | first1 = J.| last2 = Pagani | first2 = M.| last3 = Sloan | first3 = L.| last4 = Thomas | first4 = E.| last5 = Billups | first5 = K.| title = Trends, rhythms, and aberrations in global climate 65 Ma to present| volume = 292| issue = 5517| pages = 686–693| journal = Science|bibcode = 2001Sci...292..686Z | s2cid = 2365991| url = http://doc.rero.ch/record/13508/files/PAL_E304.pdf}}</ref>]]

In the earliest part of the [[Eocene]] [[geologic period|period]], a series of abrupt thermal spikes have been observed, lasting no more than a few hundred thousand years. The most pronounced of these, the [[Paleocene-Eocene Thermal Maximum]] (PETM) is visible in the figure at right. These are usually interpreted as caused by abrupt releases of [[methane]] from [[clathrates]] (frozen methane ices that accumulate at the bottom of the ocean), though some scientists dispute that methane would be sufficient to cause the observed changes.{{Citation needed|date=April 2008}} During these events, temperatures in the [[Arctic Ocean]] may have reached levels more typically associated with modern temperate (i.e. mid-latitude) oceans.{{Citation needed|date=April 2008}} During the PETM, the global mean temperature seems to have risen by as much as 5&ndash;8&nbsp;°C (9&ndash;14&nbsp;°F) to an average temperature as high as 23&nbsp;°C (73&nbsp;°F), in contrast to the global average temperature of today at just under 15&nbsp;°C (60&nbsp;°F). Geologists and paleontologists think that during much of the Paleocene and early Eocene, the poles were free of ice caps, and palm trees and crocodiles lived above the Arctic Circle, while much of the continental United States had a sub-tropical environment.<ref>{{cite web|last1=NOAA|title=What's the hottest Earth's ever been?|url=https://www.climate.gov/news-features/climate-qa/whats-hottest-earths-ever-been |website=climate.gov |publisher=National Oceanic and Atmospheric Administration|access-date=19 February 2015}}</ref>