Editing Ice core (section)

== History ==
{{See also|History of scientific ice drilling}}

=== Early years ===
[[File:2000 core-repository02 hg.jpg|thumb|A store of core samples|alt=A man on a walkway between two high shelf racks loaded with ice core samples]]

In 1841 and 1842, [[Louis Agassiz]] drilled holes in the [[Unteraargletscher]] in the [[Alps]]; these were drilled with iron rods and did not produce cores.  The deepest hole achieved was 60&nbsp;m.  On [[Erich von Drygalski]]'s Antarctic expedition in 1902 and 1903, 30&nbsp;m holes were drilled in an iceberg south of the Kerguelen Islands and temperature readings were taken.  The first scientist to create a snow sampling tool was [[James E. Church]], described by [[Pavel Talalay]] as "the father of modern snow surveying".  In the winter of 1908–1909, Church constructed steel tubes with slots and cutting heads to retrieve cores of snow up to 3&nbsp;m long.  Similar devices are in use today, modified to allow sampling to a depth of about 9&nbsp;m.  They are simply pushed into the snow and rotated by hand.<ref>{{harvnb|Talalay|2016}}, pp. 9–11.</ref>

The first systematic study of snow and firn layers was by Ernst Sorge, who was part of the [[Alfred Wegener]] Expedition to central Greenland in 1930–1931.  Sorge dug a 15&nbsp;m pit to examine the snow layers, and his results were later formalized into Sorge's Law of Densification by Henri Bader, who went on to do additional coring work in northwest Greenland in 1933.<ref>{{harvnb|Langway|2008}}, pp. 5–6.</ref> In the early 1950s, a [[SIPRE]] expedition took pit samples over much of the Greenland ice sheet, obtaining early oxygen isotope ratio data.  Three other expeditions in the 1950s began ice coring work: a joint [[Norwegian-British-Swedish Antarctic Expedition]] (NBSAE), in [[Queen Maud Land]] in Antarctica; the [[Juneau Ice Field Research Project]] (JIRP), in [[Alaska]]; and [[Expéditions Polaires Françaises]], in central Greenland.  Core quality was poor, but some scientific work was done on the retrieved ice.<ref>{{harvnb|Langway|2008}}, p. 7.</ref>

The [[International Geophysical Year]] (1957–1958) saw increased [[glaciology]] research around the world, with one of the high priority research targets being deep cores in polar regions.  SIPRE conducted pilot drilling trials in 1956 (to 305 m) and 1957 (to 411 m) at Site 2 in Greenland; the second core, with the benefit of the previous year's drilling experience, was retrieved in much better condition, with fewer gaps.<ref>{{harvnb|Langway|2008}}, pp. 9–11.</ref> In Antarctica, a 307&nbsp;m core was drilled at [[Byrd Station]] in 1957–1958, and a 264&nbsp;m core at [[Little America V]], on the [[Ross Ice Shelf]], the following year.<ref>{{harvnb|Langway|2008}}, pp. 14–15.</ref> The success of the IGY core drilling led to increased interest in improving ice coring capabilities, and was followed by a [[CRREL]] project at Camp Century, where in the early 1960s three holes were drilled, the deepest reaching the base of the ice sheet at 1387&nbsp;m in July 1966.<ref>{{harvnb|Langway|2008}}, pp. 17–20.</ref> The drill used at Camp Century then went to Byrd Station, where a 2164&nbsp;m hole was drilled to bedrock before the drill was frozen into the borehole by sub-ice meltwater and had to be abandoned.<ref>{{harvnb|Langway|2008}}, p. 23.</ref>

French, Australian and Canadian projects from the 1960s and 1970s include a 905&nbsp;m core at [[Dome C]] in Antarctica, drilled by [[Centre national de la recherche scientifique|CNRS]]; cores at [[Law Dome]] drilled by [[Australian National Antarctic Research Expeditions|ANARE]], starting in 1969 with a 382&nbsp;m core; and [[Devon Ice Cap]] cores recovered by a Canadian team in the 1970s.<ref name="Jouzel-2013-1" />

=== Antarctica deep cores ===
[[File:Co2 glacial cycles 800k.png|thumb|Composite data for Dome C, CO<sub>2</sub> levels (ppm) going back nearly 800,000 years, and related glacial cycles.|left|alt=Graph showing CO<sub>2</sub> levels, highlit to indicate glacial cycles|300x300px]]

[[Soviet Union|Soviet]] ice drilling projects began in the 1950s, in [[Franz Josef Land]], the [[Urals]], [[Novaya Zemlya]], and at [[Mirny Station|Mirny]] and Vostok in the Antarctic; not all these early holes retrieved cores.<ref>{{harvnb|Ueda|Talalay|2007}}, pp. 3–5.</ref> Over the following decades work continued at multiple locations in Asia.<ref>{{harvnb|Ueda|Talalay|2007}}, pp. 50–58.</ref> Drilling in the Antarctic focused mostly on Mirny and Vostok, with a series of deep holes at Vostok begun in 1970.<ref>{{harvnb|Ueda|Talalay|2007}}, pp. 3–26.</ref> The first deep hole at Vostok reached 506.9&nbsp;m in April 1970; by 1973 a depth of 952&nbsp;m had been reached.  A subsequent hole, Vostok 2, drilled from 1971 to 1976, reached 450&nbsp;m, and Vostok 3 reached 2202&nbsp;m in 1985 after six drilling seasons.<ref>{{harvnb|Ueda|Talalay|2007}}, p. 11.</ref> Vostok 3 was the first core to retrieve ice from the previous glacial period, 150,000 years ago.<ref name="Jouzel-2013-2" /> Drilling was interrupted by a fire at the camp in 1982, but further drilling began in 1984, eventually reaching 2546&nbsp;m in 1989. A fifth Vostok core was begun in 1990, reached 3661&nbsp;m in 2007, and was later extended to 3769&nbsp;m.<ref name="Jouzel-2013-1">{{harvnb|Jouzel|2013}}, p. 2527.</ref><ref name="Jouzel-2013-2" /> The estimated age of the ice is 420,000 years at 3310&nbsp;m depth; below that point it is difficult to interpret the data reliably because of mixing of the ice.<ref name="Jouzel-2013-3">{{harvnb|Jouzel|2013}}, p. 2529.</ref>

[[File:EPICA delta D plot.svg|thumb|The EPICA Dome C and Vostok ice cores compared|400px]]

[[EPICA]], a European ice coring collaboration, was formed in the 1990s, and two holes were drilled in East Antarctica: one at Dome C, which reached 2871&nbsp;m in only two seasons of drilling, but which took another four years to reach bedrock at 3260 m; and one at [[Kohnen Station]], which reached bedrock at 2760&nbsp;m in 2006.  The Dome C core had very low accumulation rates, which mean that the climate record extended a long way; by the end of the project the usable data extended to 800,000 years ago.<ref name="Jouzel-2013-3" />

Other deep Antarctic cores included a Japanese project at [[Dome F]], which reached 2503&nbsp;m in 1996, with an estimated age of 330,000 years for the bottom of the core; and a subsequent hole at the same site which reached 3035&nbsp;m in 2006, estimated to reach ice 720,000 years old.<ref name="Jouzel-2013-3" /> US teams drilled at [[McMurdo Station]] in the 1990s, and at [[Taylor Dome]] (554&nbsp;m in 1994) and [[Siple Dome]] (1004&nbsp;m in 1999), with both cores reaching ice from the last glacial period.<ref name="Jouzel-2013-3" /><ref>{{Cite journal|last1=Bentley|first1=Charles R.|last2=Koci|first2=Bruce R.|year=2007|title=Drilling to the beds of the Greenland and Antarctic ice sheets: a review|journal=Annals of Glaciology|volume=47|issue=1|pages=3–4|doi=10.3189/172756407786857695|bibcode=2007AnGla..47....1B|doi-access=free}}</ref> The [[West Antarctic Ice Sheet]] (WAIS) project, completed in 2011, reached 3405 m; the site has high snow accumulation so the ice only extends back 62,000 years, but as a consequence, the core provides high resolution data for the period it covers.<ref name="Jouzel-2013-4">{{harvnb|Jouzel|2013}}, p. 2530.</ref> A 948&nbsp;m core was drilled at [[Berkner Island]] by a project managed by the [[British Antarctic Survey]] from 2002 to 2005, extending into the last glacial period;<ref name="Jouzel-2013-4" /> and an Italian-managed [[ITASE]] project completed a 1620&nbsp;m core at [[Talos Dome]] in 2007.<ref name="Jouzel-2013-4" /><ref>{{Cite web|url=http://www.taldice.org/site/index.php|title=TALos Dome Ice CorE – TALDICE|last=Iaccarino|first=Tony|publisher=Talos Dome Ice Core|access-date=28 May 2017}}</ref>

In 2016, cores were retrieved from the [[Allan Hills]] in Antarctica in an area where old ice lay near the surface.  The cores were dated by potassium-argon dating; traditional ice core dating is not possible as not all layers were present.  The oldest core was found to include ice from 2.7 million years ago—by far the oldest ice yet dated from a core.<ref>{{Cite news|url=https://www.science.org/content/article/record-shattering-27-million-year-old-ice-core-reveals-start-ice-ages|title=Record-shattering 2.7-million-year-old ice core reveals start of the ice ages|date=14 August 2017|work=Science |publisher= AAAS|access-date=30 August 2017|language=en}}</ref>

=== Greenland deep cores ===
In 1970, scientific discussions began which resulted in the [[Greenland Ice Sheet Project]] (GISP), a multinational investigation into the Greenland ice sheet that lasted until 1981.  Years of field work were required to determine the ideal location for a deep core; the field work included several intermediate-depth cores, at Dye 3 (372&nbsp;m in 1971), Milcent (398&nbsp;m in 1973) and Crete (405&nbsp;m in 1974), among others.  A location in north-central Greenland was selected as ideal, but financial constraints forced the group to drill at Dye 3 instead, beginning in 1979.  The hole reached bedrock at 2037&nbsp;m, in 1981.  Two holes, 30&nbsp;km apart, were eventually drilled at the north-central location in the early 1990s by two groups: [[Greenland Ice Core Project|GRIP]], a European consortium, and GISP-2, a group of US universities.  GRIP reached bedrock at 3029&nbsp;m in 1992, and GISP-2 reached bedrock at 3053&nbsp;m the following year.<ref>{{harvnb|Langway|2008}}, pp. 27–28.</ref> Both cores were limited to about 100,000 years of climatic information, and since this was thought to be connected to the topography of the rock underlying the ice sheet at the drill sites, a new site was selected 200&nbsp;km north of GRIP, and a new project, [[North Greenland Ice Core Project|NorthGRIP]], was launched as an international consortium led by Denmark.  Drilling began in 1996; the first hole had to be abandoned at 1400&nbsp;m in 1997, and a new hole was begun in 1999, reaching 3085&nbsp;m in 2003.  The hole did not reach bedrock, but terminated at a subglacial river.  The core provided climatic data back to 123,000 years ago, which covered part of the last interglacial period.  The subsequent North Greenland Eemian ([[NEEM Camp|NEEM]]) project retrieved a 2537&nbsp;m core in 2010 from a site further north, extending the climatic record to 128,500 years ago;<ref name="Jouzel-2013-2">{{harvnb|Jouzel|2013}}, p. 2528.</ref> NEEM was followed by [[East Greenland Ice-Core Project|EastGRIP]], which began in 2015 in east Greenland and was planned to be completed in 2020.<ref>{{Cite web|url=http://eastgrip.org/documentation/|title=Documentation|last=Madsen|first=Martin Vindbæk|date=15 March 2016|publisher=East Greenland Ice Core Project|language=en|access-date=17 March 2017|archive-url=https://web.archive.org/web/20170318090450/http://eastgrip.org/documentation/|archive-date=18 March 2017|url-status=dead}}</ref>
In March 2020, the 2020 EGRIP field campaign was cancelled due to the ongoing [[COVID-19 pandemic]]. EastGRIP reopened for field work in 2022, where the CryoEgg reached new depths in the ice, under pressures in excess of 200 bar and temperatures of around -30c.<ref>{{cite web|url=https://eastgrip.org/FieldDiaries/2022diaries/2022-05-07.html|title=Finally, put in at EGRIP|access-date=2023-08-21}}</ref><ref>{{Cite web|url=https://protronix.co.uk/surviving-harsh-operating-conditions-how-protronix-ems-is-powering-cutting-edge-research-in-east-greenland/|title=Surviving Harsh Operating Conditions: How Protronix EMS is Powering Cutting-Edge Research in East Greenland &#124; Protronix EMS|website=protronix.co.uk/}}</ref>

=== Non-polar cores ===
Ice cores have been drilled at locations away from the poles, notably in the [[Himalayas]] and the [[Andes]].  Some of these cores reach back to the last glacial period, but they are more important as records of [[El Niño]] events and of [[monsoon season]]s in south Asia.<ref name="Jouzel-2013-4" /> Cores have also been drilled on [[Mount Kilimanjaro]],<ref name="Jouzel-2013-4" /> in the Alps,<ref name="Jouzel-2013-4" /> and in Indonesia,<ref>{{harvnb|MacKinnon|1980}}, p. 41.</ref> New Zealand,<ref>{{harvnb|MacKinnon|1980}}, p. 42.</ref> Iceland,<ref>{{harvnb|MacKinnon|1980}}, p. 36.</ref> Scandinavia,<ref>{{harvnb|MacKinnon|1980}}, p. 39.</ref> Canada,<ref>{{harvnb|MacKinnon|1980}}, p. 26-29.</ref> and the US.<ref>{{harvnb|MacKinnon|1980}}, p. 30.</ref>