Editing Paleoclimatology (section)

==Reconstructing ancient climates==
[[File:Earth's average surface temperature over the past 500 million years.png|thumb|upright=2|Preliminary results from a [[National Museum of Natural History|Smithsonian Institution]] project, showing Earth's average surface temperature over the past 500 million years<ref>{{Cite web |date=2023-11-22 |title=What's the hottest Earth's ever been? |publisher=[[NOAA]] |url=http://www.climate.gov/news-features/climate-qa/whats-hottest-earths-ever-been |access-date=2024-06-03 |website=www.climate.gov |language=en}}</ref><ref>{{Cite web |last=Soul | first=Laura |work=Smithsonian Magazine |title=Leading Scientists Convene to Chart 500M Years of Global Climate Change |url=http://www.smithsonianmag.com/blogs/national-museum-of-natural-history/2018/04/24/leading-scientists-convene-to-chart-500m-years-of-global-climate-change/ |date=2018-04-24 |access-date=2024-06-03 |language=en}}</ref>]]
[[File:All palaeotemps.svg|thumb|right|upright=3|Palaeotemperature graphs placed together]]
[[File:Sauerstoffgehalt-1000mj2.png|thumb|upright=2|The oxygen content in the atmosphere over the last billion years]]
{{Main|Proxy (climate)}}
Paleoclimatologists employ a wide variety of techniques to deduce ancient climates. The techniques used depend on which variable has to be reconstructed (this could be [[temperature]], [[precipitation]], or something else) and how long ago the climate of interest occurred. For instance, the deep marine record, the source of most isotopic data, exists only on oceanic plates, which are eventually [[Subduction|subducted]]; the oldest remaining material is {{ma|200|million years}} old. Older sediments are also more prone to corruption by [[diagenesis]]. This is due to the millions of years of disruption experienced by the rock formations, such as pressure, tectonic activity, and fluid flowing. These factors often result in a lack in quality or quantity of data, which causes resolution and confidence in the data decrease over time.

Specific techniques used to make inferences on ancient climate conditions are the use of lake sediment cores and speleothems. These utilize an analysis of sediment layers and rock growth formations respectively, amongst element-dating methods utilizing oxygen, carbon and uranium.

===Proxies for climate===

====Direct Quantitative Measurements====
The Direct Quantitative Measurements method is the most direct approach to understand the change in a climate. Comparisons between recent data to older data allows a researcher to gain a basic understanding of weather and climate changes within an area. There is a disadvantage to this method. Data of the climate only started being recorded in the mid-1800s. This means that researchers can only utilize 150 years of data. That is not helpful when trying to map the climate of an area 10,000 years ago. This is where more complex methods can be used. <ref>{{cite book |last1=Saltzman |first1=Barry |title=Dynamical Paleoclimatology: Generalized Theory of Global Climate Change |date=2002 |publisher=Academic Press |location=Google Scholar |isbn=978-0-12-617331-4 |url=https://books.google.com/books?id=kJkE52UtpXcC&dq=paleoclimatology&pg=PP1 |access-date=1 April 2024}}</ref>

====Ice====
Mountain [[glacier]]s and the polar [[ice caps]]/[[ice sheets]] provide much data in paleoclimatology. Ice-coring projects in the ice caps of [[Greenland]] and [[Antarctica]] have yielded data going back several hundred thousand years, over 800,000 years in the case of the [[European Project for Ice Coring in Antarctica|EPICA]] project.

* Air trapped within fallen [[snow]] becomes encased in tiny bubbles as the snow is compressed into ice in the glacier under the weight of later years' snow. The trapped air has proven a tremendously valuable source for direct measurement of the composition of air from the time the ice was formed.
* Layering can be observed because of seasonal pauses in ice accumulation and can be used to establish chronology, associating specific depths of the core with ranges of time.
* Changes in the layering thickness can be used to determine changes in precipitation or temperature.
* [[Oxygen-18]] quantity changes ({{delta|18|O|link}}) in ice layers represent changes in average ocean surface temperature. Water molecules containing the heavier O-18 evaporate at a higher temperature than water molecules containing the normal [[Oxygen-16]] isotope. The ratio of O-18 to O-16 will be higher as temperature increases but it also depends on factors such as water salinity and the volume of water locked up in ice sheets. Various cycles in isotope ratios have been detected.
* [[Pollen#In the fossil record|Pollen]] has been observed in the ice cores and can be used to understand which plants were present as the layer formed. Pollen is produced in abundance and its distribution is typically well understood. A pollen count for a specific layer can be produced by observing the total amount of pollen categorized by type (shape) in a controlled sample of that layer. Changes in plant frequency over time can be plotted through statistical analysis of pollen counts in the core. Knowing which plants were present leads to an understanding of precipitation and temperature, and types of fauna present. [[Palynology]] includes the study of pollen for these purposes.
* [[Volcanic ash]] is contained in some layers, and can be used to establish the time of the layer's formation. Volcanic events distribute ash with a unique set of properties (shape and color of particles, chemical signature). Establishing the ash's source will give a time period to associate with the layer of ice.

A multinational consortium, the [[European Project for Ice Coring in Antarctica]] (EPICA), has drilled an ice core in Dome C on the East Antarctic ice sheet and retrieved ice from roughly 800,000 years ago.<ref>{{cite journal|last1=Jouzel|first1=Jean|title=Orbital and Millennial Antarctic Climate Variability over the Past 800,000 Years|journal=Science|date=10 August 2007|volume=317|pages=793–796|doi=10.1126/science.1141038|last2=Masson-Delmotte|first2=V.|last3=Cattani|first3=O.|last4=Dreyfus|first4=G.|last5=Falourd|first5=S.|last6=Hoffmann|first6=G.|last7=Minster|first7=B.|last8=Nouet|first8=J.|last9=Barnola|first9=J. M.|last10=Chappellaz|first10=J.|last11=Fischer|first11=H.|last12=Gallet|first12=J. C.|last13=Johnsen|first13=S.|last14=Leuenberger|first14=M.|last15=Loulergue|first15=L.|last16=Luethi|first16=D.|last17=Oerter|first17=H.|last18=Parrenin|first18=F.|last19=Raisbeck|first19=G.|last20=Raynaud|first20=D.|last21=Schilt|first21=A.|last22=Schwander|first22=J.|last23=Selmo|first23=E.|last24=Souchez|first24=R.|last25=Spahni|first25=R.|last26=Stauffer|first26=B.|last27=Steffensen|first27=J. P.|last28=Stenni|first28=B.|last29=Stocker|first29=T. F.|last30=Tison|first30=J. L.|issue=5839|pmid=17615306|bibcode=2007Sci...317..793J|s2cid=30125808|display-authors=8|url=https://epic.awi.de/id/eprint/16356/1/Fis2007b.pdf}}</ref> The international ice core community has, under the auspices of International Partnerships in Ice Core Sciences (IPICS), defined a priority project to obtain the oldest possible ice core record from Antarctica, an ice core record reaching back to or towards 1.5 million years ago.<ref name="Page 1 1 International Partnerships in Ice Core Sciences (IPICS)">{{cite web|title=Page 1 1 International Partnerships in Ice Core Sciences (IPICS) The oldest ice core: A 1.5 million year record of climate and greenhouse gases from Antarctica|url=https://docs.google.com/viewer?a=v&q=cache:q-s3FmzPx2IJ:www.pages-igbp.org/ipics/data/ipics_oldest_sip_final.pdf+oldest+ice+core+taken+was+from+the+Antarctic&hl=en&gl=us&pid=bl&srcid=ADGEEShU7yNImhQRe0OT1sDyHElyttKBXnyOnXCGkoaJmbd5ocjrRfY5_2e2LYTOf-SPPzfuZ6KM82kfyeSkoKQzCH3OSKAXsfeim00_D5l6gwdDCa0VtDqqLKqXzI0dlH1vcGdpF6_I&sig=AHIEtbSRsko6dyE5t8K9yXxvk6GYPekLCg&pli=1|access-date=22 September 2011}}</ref>

====Dendroclimatology====
{{Main|Dendroclimatology}}
Climatic information can be obtained through an understanding of changes in tree growth. Generally, trees respond to changes in climatic variables by speeding up or slowing down growth, which in turn is generally reflected by a greater or lesser thickness in growth rings. Different species, however, respond to changes in climatic variables in different ways. A tree-ring record is established by compiling information from many living trees in a specific area. This is done by comparing the number, thickness, ring boundaries, and pattern matching of tree growth rings.

The differences in thickness displayed in the growth rings in trees can often indicate the quality of conditions in the environment, and the fitness of the tree species evaluated. Different species of trees will display different growth responses to the changes in the climate. An evaluation of multiple trees within the same species, along with one of trees in different species, will allow for a more accurate analysis of the changing variables within the climate and how they affected the surrounding species.<ref>{{Cite journal |last1=Frank |first1=David |last2=Esper |first2=Jan |last3=Zorita |first3=Eduardo |last4=Wilson |first4=Rob |date=14 May 2010 |title=A noodle, hockey stick, and spaghetti plate: a perspective on high-resolution paleoclimatology |url=https://wires.onlinelibrary.wiley.com/doi/10.1002/wcc.53 |journal=WIREs Climate Change |language=en |volume=1 |issue=4 |pages=507–516 |doi=10.1002/wcc.53 |bibcode=2010WIRCC...1..507F |issn=1757-7780}}</ref>

Older intact wood that has escaped decay can extend the time covered by the record by matching the ring depth changes to contemporary specimens. By using that method, some areas have tree-ring records dating back a few thousand years. Older wood not connected to a contemporary record can be dated generally with radiocarbon techniques. A tree-ring record can be used to produce information regarding precipitation, temperature, hydrology, and fire corresponding to a particular area.

====Sedimentary content====
On a longer time scale, geologists must refer to the sedimentary record for data.

* Sediments, sometimes lithified to form rock, may contain remnants of preserved vegetation, animals, plankton, or [[Palynology|pollen]], which may be characteristic of certain climatic zones.
* Biomarker molecules such as the [[alkenones]] may yield information about their temperature of formation.
* Chemical signatures, particularly [[Mg/Ca]] ratio of [[calcite]] in [[Foraminifera]] tests, can be used to reconstruct past temperature.
* Isotopic ratios can provide further information. Specifically, the {{delta|18|O|link}} record responds to changes in temperature and ice volume, and the {{delta|13|C|link}} record reflects a range of factors, which are often difficult to disentangle.

[[File:Core+Repository+core samples2.jpg|right|thumb|Sea floor core sample labelled to identify the exact spot on the sea floor where the sample was taken. Sediments from nearby locations can show significant differences in chemical and biological composition.]]
;[[Sedimentary facies]]
On a longer time scale, the rock record may show signs of [[sea level]] rise and fall, and features such as [[Dune#Lithified dunes|"fossilised" sand dunes]] can be identified. Scientists can get a grasp of long-term climate by studying [[sedimentary rock]] going back billions of years. The division of Earth history into separate periods is largely based on visible changes in sedimentary rock layers that demarcate major changes in conditions. Often, they include major shifts in climate.

====Sclerochronology====

;Corals (see also [[sclerochronology]])
Coral "rings<nowiki>''</nowiki> share similar evidence of growth to that of trees, and thus can be dated in similar ways. A primary difference is their environments and the conditions within those that they respond to. Examples of these conditions for coral include water temperature, freshwater influx, changes in pH, and wave disturbances. From there, specialized equipment, such as the Advanced Very High Resolution Radiometer (AVHRR) instrument, can be used to derive the [[sea surface temperature]] and water salinity from the past few centuries. The [[Δ18O|δ<sup>18</sup>O]] of [[Coralline algae|coralline]] red algae provides a useful proxy of the combined sea surface temperature and sea surface salinity at high latitudes and the tropics, where many traditional techniques are limited.<ref name=Geology2008>{{cite journal|last1=Halfar|first1=J.|year=2008|doi=10.1130/G24635A.1|title=Coralline red algae as high-resolution climate recorders|journal=Geology|volume=36|pages=463|last2=Steneck|first2=R.S.|last3=Joachimski|first3=M.|last4=Kronz|first4=A.|last5=Wanamaker|first5=A.D.|issue=6|bibcode=2008Geo....36..463H|s2cid=129376515 }}</ref><ref name=Cobb>{{cite journal|last1=Cobb|first1=K.|last2=Charles|first2=C. D.|last3=Cheng|first3=H|last4=Edwards|first4=R. L.|year=2003|title=El Nino/Southern Oscillation and tropical Pacific climate during the past millennium|journal=Nature|volume=424|issue=6946|pages=271–6|pmid=12867972|doi=10.1038/nature01779|bibcode=2003Natur.424..271C|s2cid=6088699}}</ref>

====Landscapes and landforms====
Within [[climatic geomorphology]], one approach is to study [[Relict (geology)|relict landforms]] to infer ancient climates.<ref name=Gutix2>{{cite book|chapter-url=https://www.researchgate.net/publication/285964375|chapter=Climatic Geomorphology|title=Treatise on Geomorphology|volume=13|author-last=Gutiérrez|first1=Mateo|author-link=Mateo Gutiérrez|last2=Gutiérrez|first2=Francisco|year=2013|pages=115–131}}</ref> Being often concerned about past climates climatic geomorphology is considered sometimes to be a theme of [[historical geology]].<ref>{{cite book|date=2005|chapter=Chapter 1 Climatic geomorphology|title=Developments in Earth Surface Processes|editor-last=Gutiérrez|editor-first=Mateo|editor-link=Mateo Gutiérrez|volume=8|pages=3–32|doi=10.1016/S0928-2025(05)80051-3|isbn=978-0-444-51794-4}}</ref> Evidence of these past climates to be studied can be found in the landforms they leave behind. Examples of these landforms are those such as glacial landforms (moraines, striations), desert features (dunes, desert pavements), and coastal landforms (marine terraces, beach ridges).<ref>{{Cite web |last1=Douglas |first1=Peter |last2=Brenner |first2=Mark |last3=Curtis |first3=Jason |date=27 February 2016 |title=Methods and future directions for paleoclimatology in the Maya Lowlands. Global and Planetary Change. |doi=10.1016/j.gloplacha.2015.07.008 |url=https://doi.org/10.1016/j.gloplacha.2015.07.008 }}</ref> Climatic geomorphology is of limited use to study recent ([[Quaternary]], [[Holocene]]) large climate changes since there are seldom discernible in the geomorphological record.<ref name=Goudie2004>{{cite encyclopedia|last=Goudie|first=A.S.|editor-last=Goudie|editor-first=A.S.|author-link=Andrew Goudie (geographer)|encyclopedia=Encyclopedia of Geomorphology|title=Climatic geomorphology|year=2004|pages=162–164}}</ref>

===Timing of proxies===
The field of [[geochronology]] has scientists working on determining how old certain proxies are. For recent proxy archives of tree rings and corals the individual year rings can be counted, and an exact year can be determined. [[Radiometric dating]] uses the properties of radioactive elements in proxies. In older material, more of the radioactive material will have decayed and the proportion of different elements will be different from newer proxies. One example of radiometric dating is [[radiocarbon dating]]. In the air, [[cosmic ray]]s constantly convert nitrogen into a specific radioactive carbon isotope, [[Carbon-14|<sup>14</sup>C]]. When plants then use this carbon to grow, this isotope is not replenished anymore and starts decaying. The proportion of 'normal' carbon and Carbon-14 gives information of how long the plant material has not been in contact with the atmosphere.<ref>{{harvnb|Cronin|2010|pp=32–34}}.</ref>