Editing Proxy (climate) (section)

=== Ice cores ===

====Drilling====

[[Image:Icecore 4x.jpg|thumb|Ice Core sample taken from drill. Photo by [[Lonnie Thompson]], [[Byrd Polar Research Center]].]] [[Ice cores]] are [[Core sample|cylindrical samples]] from within [[ice sheets]] in the [[Greenland]], [[Antarctic]], and [[North American]] regions.<ref name="Strom, Robert p. 255">Strom, Robert. ''Hot House.'' p. 255</ref><ref name="Core Location Maps.">[http://nicl.usgs.gov/coresite.htm "Core Location Maps."] {{webarchive|url=https://web.archive.org/web/20091110005705/http://nicl.usgs.gov/coresite.htm |date=2009-11-10 }}</ref> First attempts of extraction occurred in 1956 as part of the [[International Geophysical Year]]. As original means of extraction, the [[Cold Regions Research and Engineering Laboratory|U.S. Army's Cold Regions Research and Engineering Laboratory]] used an {{convert|80|ft|m|adj=on}}-long modified electrodrill in 1968 at [[Camp Century]], [[Greenland]], and [[Byrd Station]], [[Antarctica]]. Their machinery could drill through {{convert|15-20|ft}} of [[ice sheet|ice]] in 40–50 minutes. From 1300 to {{convert|3000|ft|m}} in depth, [[core sample]]s were {{convert|4+1/4|in}} in [[diameter]] and 10 to {{convert|20|ft|m}} long. Deeper [[core sample|samples]] of 15 to {{convert|20|ft|m}} long were not uncommon. Every subsequent [[drilling]] team improves their method with each new effort.<ref>Vardiman, Larry, Ph.D. ''Ice Cores and the Age of the Earth.'' p. 9-13</ref>

====Proxy====

[[File:Vostok deltaD deltaO18.jpg|thumb|left|300px|δ18O<sub>air</sub> and δD<sub>ice</sub> for Vostok, Antarctica ice core.]]

The ratio between the <sup>16</sup>O and <sup>18</sup>O [[Properties of water#Isotopologues|water molecule isotopologues]] in an [[ice core]] helps determine [[Paleoclimatology|past temperatures]] and [[Precipitation (meteorology)#Measurement|snow accumulations]].<ref name="Strom, Robert p. 255"/> The heavier [[isotope]] (<sup>18</sup>O) condenses more readily as [[temperatures]] decrease and falls more easily as [[Precipitation (meteorology)|precipitation]], while the lighter isotope (<sup>16</sup>O) needs colder conditions to precipitate. The farther [[north]] one needs to go to find elevated levels of the <sup>18</sup>O isotopologue, the warmer the period.{{explain|date=April 2019}}<ref>[http://earthobservatory.nasa.gov/Features/Paleoclimatology_OxygenBalance/oxygen_balance.php "Paleoclimatology: the Oxygen Balance."]</ref>

In addition to oxygen isotopes, water contains hydrogen isotopes – <sup>1</sup>H and <sup>2</sup>H, usually referred to as H and D (for [[deuterium]]) – that are also used for temperature proxies. Normally, ice cores from Greenland are analyzed for δ<sup>18</sup>O and those from Antarctica for δ-deuterium.{{Why?|date=November 2011}} Those cores that analyze for both show a lack of agreement.{{citation needed|date=June 2018}} (In the figure, δ<sup>18</sup>O is for the trapped air, not the ice. [[δD]] is for the ice.)

[[Air bubble]]s in the ice, which contain trapped [[greenhouse gases]] such as [[carbon dioxide]] and [[methane]], are also helpful in determining past climate changes.<ref name="Strom, Robert p. 255"/>

From 1989 to 1992, the European Greenland Ice Core Drilling Project drilled in central [[Greenland]] at coordinates 72° 35' N, 37° 38' W. The ices in that core were 3840 years old at a depth of 770 m, 40,000 years old at 2521 m, and 200,000 years old or more at 3029 m [[bedrock]].<ref>[http://www.ncdc.noaa.gov/paleo/icecore/greenland/summit/document/ "The GRIP Coring Effort."]</ref> [[Ice cores]] in Antarctica can reveal the climate records for the past 650,000 years.<ref name="Strom, Robert p. 255"/>

Location [[map]]s and a complete list of U.S. [[ice core]] drilling sites can be found on the website for the [[National Ice Core Laboratory]].<ref name="Core Location Maps."/>