Editing Proxy (climate) (section)

==Proxies==
{{Main| Proxy (statistics)}}
In science, it is sometimes necessary to study a variable which cannot be measured directly. This can be done by "proxy methods," in which a variable which correlates with the variable of interest is measured, and then used to infer the value of the variable of interest. Proxy methods are of particular use in the study of the past climate, beyond times when direct measurements of temperatures are available.

Most proxy records have to be calibrated against independent temperature measurements, or against a more directly calibrated proxy, during their period of overlap to estimate the relationship between temperature and the proxy. The longer history of the proxy is then used to reconstruct temperature from earlier periods.

=== 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."/>

===Tree rings===
[[Image:Tree rings.jpg|thumb|left|Tree rings seen in a cross section of a trunk of a tree.]]
{{main|Dendroclimatology}}

Dendroclimatology is the science of determining past climates from trees, primarily from properties of the annual [[tree rings]]. Tree rings are wider when conditions favor growth, narrower when times are difficult. Two primary factors are temperature and humidity / water availability. Other properties of the annual rings, such as maximum latewood density (MXD) have been shown to be better proxies than simple ring width. Using tree rings, scientists have estimated many local climates for hundreds to thousands of years previous. By combining multiple tree-ring studies (sometimes with other climate proxy records), scientists have estimated past regional and global climates (see [[Temperature record of the past 1000 years]]).

===Fossil leaves===
Paleoclimatologists often use leaf teeth to reconstruct mean annual temperature in past climates, and they use leaf size as a proxy for mean annual precipitation.<ref>{{cite journal|volume=92 |author=[[Dana L. Royer]] |author2=Peter Wilf |author3=David A. Janesko |author4=Elizabeth A. Kowalski |author5=David L. Dilcher |issue=7|pages=1141–1151|date=1 July 2005|doi=10.3732/ajb.92.7.1141|pmid = 21646136|title=Correlations of climate and plant ecology to leaf size and shape: potential proxies for the fossil record|journal=American Journal of Botany|doi-access=free}}</ref> In the case of mean annual precipitation reconstructions, some researchers believe [[taphonomic]] processes cause smaller leaves to be overrepresented in the fossil record, which can bias reconstructions.<ref>{{Citation|pages=44–59|url=https://www.researchgate.net/publication/263087169 |author=David R. Greenwood|title=Palaeobotanical evidence for Tertiary climates|journal=History of the Australian Vegetation: Cretaceous to Recent|year=1994}}</ref> However, recent research suggests that the leaf fossil record may not be significantly biased toward small leaves.<ref>{{cite journal|volume=34 |author=Eric R. Hagen |author2=[[Dana Royer]] |author3=Ryan A. Moye |author4=Kirk R. Johnson |issue=1|pages=43–48|date=9 January 2019|doi=10.2110/palo.2018.091|title=No large bias within species between the reconstructed areas of complete and fragmented fossil leaves|journal=PALAIOS|bibcode=2019Palai..34...43H|s2cid=133599753}}</ref> New approaches retrieve data such as {{CO2|link=yes}} content of past atmospheres from fossil leaf [[stoma]]ta and isotope composition, measuring cellular CO<sub>2</sub> concentrations. A 2014 study was able to use the [[carbon-13]] [[isotope ratio]]s to estimate the CO<sub>2</sub> amounts of the past 400 million years, the findings hint at a higher [[climate sensitivity]] to CO<sub>2</sub> concentrations.<ref>{{cite journal|url=http://droyer.web.wesleyan.edu/Franks_et_al_2014_GRL_new_stomatal-CO2_proxy.pdf <!--[http://onlinelibrary.wiley.com/doi/10.1002/2014GL060457/abstract Abstract] -->|format=PDF|volume=31 |author=Peter J. Franks |author2=[[Dana Royer]] |author3=[[David J. Beerling]] |author4=Peter K. Van de Water |author5=David J. Cantrill |author6=Margaret M. Barbour |author7=Joseph A. Berry |issue=13|pages=4685–4694|date=16 July 2014|doi=10.1002/2014GL060457|title=New constraints on atmospheric CO2 concentration for the Phanerozoic|journal=Geophysical Research Letters|bibcode=2014GeoRL..41.4685F|hdl=10211.3/200431|s2cid=55701037 |hdl-access=free|access-date=31 July 2014|archive-url=https://web.archive.org/web/20140812061706/http://droyer.web.wesleyan.edu/Franks_et_al_2014_GRL_new_stomatal-CO2_proxy.pdf|archive-date=12 August 2014}}</ref>

===Boreholes===

[[Borehole]] temperatures are used as temperature proxies. Since heat transfer through the ground is slow, temperature measurements at a series of different depths down the borehole, adjusted for the effect of rising heat from inside the Earth, can be "[[Invertible matrix|inverted]]" (a mathematical formula to solve matrix equations) to produce a non-unique series of surface temperature values. The solution is "non-unique" because there are multiple possible surface temperature reconstructions that can produce the same borehole temperature profile. In addition, due to physical limitations, the reconstructions are inevitably "smeared", and become more smeared further back in time. When reconstructing temperatures around 1500 AD, boreholes have a temporal resolution of a few centuries. At the start of the 20th century, their resolution is a few decades; hence they do not provide a useful check on the [[instrumental temperature record]].<ref name="Boreholes_in_Glacial_Ice_p80">{{Cite book | url=http://www.nap.edu/openbook.php?record_id=11676&page=80 | doi=10.17226/11676| title=Surface Temperature Reconstructions for the Last 2,000 Years| year=2006| isbn=978-0-309-10225-4| last1=Council| first1=National Research| last2=Studies| first2=Division on Earth Life| last3=Climate| first3=Board on Atmospheric Sciences and| last4=Committee On Surface Temperature Reconstructions For The Last 2| first4=000 Years| citeseerx=10.1.1.178.5968}}</ref><ref name="pollack et al">{{cite journal|last1=Pollack|first1=H. N.|last2=Huang|first2=S.|last3=Shen|first3=P. Y.|title=Temperature trends over the past five centuries reconstructed from borehole temperatures|journal=Nature|date=2000|volume=403|issue=6771|pages=756–758|doi=10.1038/35001556|pmid=10693801|bibcode=2000Natur.403..756H|url=https://deepblue.lib.umich.edu/bitstream/2027.42/62610/1/403756a0.pdf|hdl=2027.42/62610|s2cid=4425128|hdl-access=free}}</ref> However, they are broadly comparable.<ref name="unisci.com"/> These confirmations have given paleoclimatologists the confidence that they can measure the temperature of 500 years ago. This is concluded by a depth scale of about {{convert|492|ft|m|abbr=off|sp=us}} to measure the temperatures from 100 years ago and {{convert|1,640|ft|m|abbr=off|sp=us}} to measure the temperatures from 1,000 years ago.<ref name="archives.cnn.com">[http://archives.cnn.com/2000/NATURE/02/17/boreholes.enn/ Environmental News Network staff. "Borehole temperatures confirm global warming."] {{webarchive|url=https://web.archive.org/web/20091029185720/http://archives.cnn.com/2000/NATURE/02/17/boreholes.enn/|date=2009-10-29}}</ref>

Boreholes have a great advantage over many other proxies in that no calibration is required: they are actual temperatures. However, they record surface temperature not the near-surface temperature (1.5 meter) used for most "surface" weather observations. These can differ substantially under extreme conditions or when there is surface snow. In practice the effect on borehole temperature is believed to be generally small. A second source of error is contamination of the well by groundwater may affect the temperatures, since the water "carries" more modern temperatures with it. This effect is believed to be generally small, and more applicable at very humid sites.<ref name="Boreholes_in_Glacial_Ice_p80" /> It does not apply in ice cores where the site remains frozen all year.

More than 600 boreholes, on all continents, have been used as proxies for reconstructing surface temperatures.<ref name ="pollack et al" /> The highest concentration of boreholes exist in [[North America]] and [[Europe]]. Their depths of drilling typically range from 200 to greater than 1,000 meters into the [[Crust (geology)|crust]] of the Earth or ice sheet.<ref name="archives.cnn.com"/>

A small number of boreholes have been drilled in the ice sheets; the purity of the ice there permits longer reconstructions. Central Greenland borehole temperatures show "a warming over the last 150 years of approximately 1°C ± 0.2°C preceded by a few centuries of cool conditions. Preceding this was a warm period centered around A.D. 1000, which was warmer than the late 20th century by approximately 1°C." A borehole in the Antarctica icecap shows that the "temperature at A.D. 1 [was] approximately 1°C warmer than the late 20th century".<ref name="Boreholes_in_Glacial_Ice_p81">[http://www.nap.edu/openbook.php?record_id=11676&page=81 BOREHOLES IN GLACIAL ICE] Surface Temperature Reconstructions for the Last 2,000 Years (2006), pp 81,82 Board on Atmospheric Sciences and Climate (BASC), National Academy of Science, {{ISBN|978-0-309-10225-4}}</ref>

Borehole temperatures in Greenland were responsible for an important revision to the isotopic temperature reconstruction, revealing that the former assumption that "spatial slope equals temporal slope" was incorrect.

===Corals===
[[File:Bleached Coral.png|thumb|Coral bleached due to changes in ocean water properties]]
[[Ocean]] [[coral]] [[skeletal]] rings, or bands, also share paleoclimatological information, similarly to tree rings. In 2002, a report was published on the findings of Drs. Lisa Greer and Peter Swart, associates of [[University of Miami]] at the time, in regard to [[Isotopes of oxygen|stable oxygen isotopes]] in the [[calcium carbonate]] of coral. Cooler temperatures tend to cause coral to use heavier isotopes in its structure, while warmer temperatures result in more normal [[oxygen isotopes]] being built into the coral structure. Denser water [[salinity]] also tends to contain the heavier isotope. Greer's coral sample from the [[Atlantic Ocean]] was taken in 1994 and dated back to 1935. Greer recalls her conclusions, "When we look at the averaged annual data from 1935 to about 1994, we see it has the shape of a [[sine wave]]. It is periodic and has a significant pattern of [[oxygen isotope]] composition that has a peak at about every twelve to fifteen years." [[Sea surface temperature|Surface water temperatures]] have coincided by also peaking every twelve and a half years. However, since recording this temperature has only been practiced for the last fifty years, correlation between recorded water temperature and coral structure can only be drawn so far back.<ref>[http://earthobservatory.nasa.gov/Newsroom/view.php?id=22843 "Coral Layers Good Proxy for Atlantic Climate Cycles."] {{webarchive|url=https://web.archive.org/web/20100316220809/http://earthobservatory.nasa.gov/Newsroom/view.php?id=22843 |date=2010-03-16 }}</ref>

===Pollen grains===
Pollen can be found in sediments. Plants produce [[pollen]] in large quantities and it is extremely resistant to decay. It is possible to identify a plant species from its pollen grain. The identified plant community of the area at the relative time from that sediment layer, will provide information about the climatic condition. The abundance of pollen of a given [[vegetation]] period or year depends partly on the weather conditions of the previous months, hence pollen density provides information on short-term climatic conditions.<ref>Bradley, R. S. and Jones, P. D. (eds) 1992: Climate since AD 1500. London: Routledge.</ref> The study of prehistoric pollen is [[palynology]].

===Dinoflagellate cysts===
[[File:Peridinium ovatum cyst.png|thumb|Cyst of a dinoflagellate ''Peridinium ovatum'']]
Dinoflagellates occur in most aquatic environments and during their life cycle, some species produce highly resistant organic-walled [[Dinocyst|cysts]] for a dormancy period when environmental conditions are not appropriate for growth. Their living depth is relatively shallow (dependent upon light penetration), and closely coupled to diatoms on which they feed. Their distribution patterns in surface waters are closely related to physical characteristics of the water bodies, and nearshore assemblages can also be distinguished from oceanic assemblages. The distribution of dinocysts in sediments has been relatively well documented and has contributed to understanding the average sea-surface conditions that determine the distribution pattern and abundances of the taxa (<ref>{{cite journal|last1=de Vernal|first1=A.|last2=Eynaud|first2=F.|last3=Henry|first3=M.|last4=Hillaire-Marcel|first4=C.|last5=Londeix|first5=L.|last6=Mangin|first6=S.|last7=Matthiessen|first7=J.|last8=Marret|first8=F.|last9=Radi|first9=T.|last10=Rochon|first10=A.|last11=Solignac|first11=S.|last12=Turon|first12=J. -L.|title=Reconstruction of sea-surface conditions at middle to high latitudes of the Northern Hemisphere during the Last Glacial Maximum (LGM) based on dinoflagellate cyst assemblages|journal=Quaternary Science Reviews|date=1 April 2005|volume=24|issue=7–9|pages=897–924|doi=10.1016/j.quascirev.2004.06.014|bibcode=2005QSRv...24..897D}}</ref>). Several studies, including <ref>{{cite journal|last1=Radi|first1=Taoufik|last2=de Vernal|first2=Anne|title=Dinocyst distribution in surface sediments from the northeastern Pacific margin (40–60°N) in relation to hydrographic conditions, productivity and upwelling|journal=Review of Palaeobotany and Palynology|date=1 January 2004|volume=128|issue=1–2|pages=169–193|doi=10.1016/S0034-6667(03)00118-0|bibcode=2004RPaPa.128..169R }}</ref> and <ref>{{cite journal|last1=Pospelova|first1=Vera|last2=de Vernal|first2=Anne|last3=Pedersen|first3=Thomas F.|title=Distribution of dinoflagellate cysts in surface sediments from the northeastern Pacific Ocean (43–25°N) in relation to sea-surface temperature, salinity, productivity and coastal upwelling|journal=Marine Micropaleontology|date=1 July 2008|volume=68|issue=1–2|pages=21–48|doi=10.1016/j.marmicro.2008.01.008|bibcode=2008MarMP..68...21P}}</ref> have compiled box and gravity cores in the North Pacific analyzing them for palynological content to determine the distribution of dinocysts and their relationships with sea surface temperature, salinity, productivity and upwelling. Similarly,<ref>{{cite journal|last1=Pospelova|first1=Vera|last2=Pedersen|first2=Thomas F.|last3=de Vernal|first3=Anne|title=Dinoflagellate cysts as indicators of climatic and oceanographic changes during the past 40 kyr in the Santa Barbara Basin, southern California|journal=Paleoceanography|date=1 June 2006|volume=21|issue=2|pages=PA2010|doi=10.1029/2005PA001251|language=en|issn=1944-9186|bibcode=2006PalOc..21.2010P}}</ref> and <ref>{{cite journal|last1=Bringué|first1=Manuel|last2=Pospelova|first2=Vera|last3=Field|first3=David B.|title=High resolution sedimentary record of dinoflagellate cysts reflects decadal variability and 20th century warming in the Santa Barbara Basin|journal=Quaternary Science Reviews|date=1 December 2014|volume=105|pages=86–101|doi=10.1016/j.quascirev.2014.09.022|bibcode=2014QSRv..105...86B}}</ref> use a box core at 576.5 m of water depth from 1992 in the central Santa Barbara Basin to determine oceanographic and climatic changes during the past 40 kyr in the area.

===Lake and ocean sediments===

Similar to their study on other proxies, paleoclimatologists examine [[oxygen isotopes]] in the contents of ocean [[sediments]]. Likewise, they measure the layers of [[varve]] (deposited fine and coarse silt or clay)<ref>[http://www.merriam-webster.com/dictionary/varve "Varve."]</ref> laminating lake sediments. Lake varves are primarily influenced by:

* Summer temperature, which shows the energy available to melt seasonal snow and ice
* Winter snowfall, which determines the level of disturbance to sediments when melting occurs
* Rainfall<ref>[http://www.grida.no/publications/other/ipcc_tar/?src=/climate/ipcc_tar/wg1/068.htm "Climate Change 2001: 2.3.2.1 Palaeoclimate proxy indicators"] {{webarchive|url=https://web.archive.org/web/20091204100159/http://www.grida.no/publications/other/ipcc_tar/?src=%2Fclimate%2Fipcc_tar%2Fwg1%2F068.htm |date=2009-12-04 }}</ref>

[[Diatom]]s, [[foraminifera]], [[radiolaria]]ns, [[ostracod]]s, and [[coccolithophore]]s are examples of biotic proxies for lake and ocean conditions that are commonly used to reconstruct past climates. The distribution of the species of these and other aquatic creatures preserved in the sediments are useful proxies. The optimal conditions for species preserved in the sediment act as clues. Researchers use these clues to reveal what the climate and environment was like when the creatures died.<ref>{{cite web|url=http://serc.carleton.edu/microbelife/topics/proxies/paleoclimate.html|title=Paleoclimatology: How Can We Infer Past Climates?|last1=Bruckner|first1=Monica|website=Montana State University.}}</ref> The oxygen isotope ratios in their shells can also be used as proxies for temperature.<ref>{{Cite journal|last1=Shemesh|first1=A.|last2=Charles|first2=C. D.|last3=Fairbanks|first3=R. G.|date=1992-06-05|title=Oxygen Isotopes in Biogenic Silica: Global Changes in Ocean Temperature and Isotopic Composition|journal=Science|language=en|volume=256|issue=5062|pages=1434–1436|doi=10.1126/science.256.5062.1434|issn=0036-8075|pmid=17791613|bibcode=1992Sci...256.1434S|s2cid=38840484}}</ref>

===Water isotopes and temperature reconstruction===
[[File:Climate sensitivity sea level and atmospheric carbon dioxide. Hansen et al 2013.png|thumb|335x335px]]
Ocean water is mostly H<sub>2</sub><sup>16</sup>O, with small amounts of HD<sup>16</sup>O and H<sub>2</sub><sup>18</sup>O, where D denotes [[deuterium]], i.e. hydrogen with an extra neutron. In [[Vienna Standard Mean Ocean Water]] (VSMOW) the ratio of D to H is 155.76x10<sup>−6</sup> and O-18 to O-16 is 2005.2x10<sup>−6</sup>.  [[Isotope fractionation]] occurs during changes between condensed and vapour phases: the vapour pressure of heavier isotopes is lower, so vapour contains relatively more of the lighter isotopes and when the vapour condenses the precipitation preferentially contains heavier isotopes. The difference from VSMOW is expressed as δ<sup>18</sup>O = 1000‰ <math display="inline">\times \left( \frac{([{}^{18}O]/[{}^{16}O])}{([{}^{18}O]/[{}^{16}O])_{\mathrm{VSMOW}}} - 1\right)</math>; and a similar formula for δD. δ values for precipitation are always negative.<ref>{{Cite book | url=http://books.nap.edu/openbook.php?record_id=11676&page=66 | doi=10.17226/11676| title=Surface Temperature Reconstructions for the Last 2,000 Years| year=2006| isbn=978-0-309-10225-4| last1=Council| first1=National Research| last2=Studies| first2=Division on Earth Life| last3=Climate| first3=Board on Atmospheric Sciences and| last4=Committee On Surface Temperature Reconstructions For The Last 2| first4=000 Years| citeseerx=10.1.1.178.5968}}</ref>  The major influence on δ is the difference between ocean temperatures where the moisture evaporated and the place where the final precipitation occurred; since ocean temperatures are relatively stable the δ value mostly reflects the temperature where precipitation occurs. Taking into account that the precipitation forms above the [[inversion (meteorology)|inversion]] layer, we are left with a linear relation:

'''δ <sup>18</sup>O = aT + b'''

This is empirically calibrated from measurements of temperature and δ as a = 0.67 [[‰]]/°C for [[Greenland]] and 0.76 ‰/°C for East [[Antarctica]]. The calibration was initially done on the basis of ''spatial'' variations in temperature and it was assumed that this corresponded to ''temporal'' variations.<ref>Jouzel and Merlivat, 1984) ''Deuterium and oxygen 18 in precipitation: Modeling of the isotopic effects during snow formation,'' Journal of Geophysical Research: Atmospheres, Volume 89, Issue D7, Pages 11589–11829</ref> More recently, [[#Boreholes|borehole thermometry]] has shown that for glacial-interglacial variations, a = 0.33 ‰/°C,<ref>Cuffey et al., 1995, ''Large Arctic temperature change at the Wisconsin–''
Holocene glacial transition,'' Science 270: 455–458''</ref> implying that glacial-interglacial temperature changes were twice as large as previously believed.

A study published in 2017 called the previous methodology to reconstruct paleo ocean temperatures 100 million years ago into question, suggesting it has been relatively stable during that time, much colder.<ref>{{Cite journal|last1=Bernard|first1=S.|last2=Daval|first2=D.|last3=Ackerer|first3=P.|last4=Pont|first4=S.|last5=Meibom|first5=A.|date=2017-10-26|title=Burial-induced oxygen-isotope re-equilibration of fossil foraminifera explains ocean paleotemperature paradoxes|journal=Nature Communications|language=en|volume=8|issue=1|page=1134|doi=10.1038/s41467-017-01225-9|pmid=29070888 |issn=2041-1723|pmc=5656689|bibcode=2017NatCo...8.1134B }}</ref>

===Membrane lipids===
A novel climate proxy obtained from [[peat]] ([[lignites]], ancient peat) and soils, [[membrane lipids]] known as [[glycerol dialkyl glycerol tetraether]] (GDGT) is helping to study paleo environmental factors, which control relative distribution of differently branched GDGT [[isomers]]. The study authors note, "These branched membrane lipids are produced by an as yet unknown group of anaerobic soil bacteria."<ref>{{cite journal|title=Environmental controls on bacterial tetraether membrane lipid distribution in soils|doi=10.1016/j.gca.2006.10.003|journal=Geochimica et Cosmochimica Acta|volume=71|issue=3|pages=703–713 |author=Johan W.H. Weijers |author2=Stefan Schouten |author3=Jurgen C. van den Donker |author4=Ellen C. Hopmans |author5=Jaap S. Sinninghe Damsté |year=2007|bibcode=2007GeCoA..71..703W|s2cid=84677185 }}</ref> {{as of|2018}}, there is a decade of research demonstrating that in mineral soils the degree of [[methylation]] of bacteria (brGDGTs), helps to calculate mean annual air temperatures. This proxy method was used to study the [[Climate across Cretaceous–Paleogene boundary|climate of the early Palaeogene]], at the Cretaceous–Paleogene boundary, and researchers found that annual air temperatures, over land and at mid-latitude, averaged about 23–29 °C (± 4.7 °C), which is 5–10 °C higher than most previous findings.<ref>{{cite journal|doi=10.1038/s41561-018-0199-0|title=High temperatures in the terrestrial mid-latitudes during the early Palaeogene|year=2018 |author=B. D. A. Naafs |author2=M. Rohrssen |author3=G. N. Inglis |author4=O. Lähteenoja |author5=S. J. Feakins |author6=M. E. Collinson |author7=E. M. Kennedy |author8=P. K. Singh |author9=M. P. Singh |author10=D. J. Lunt |author11=R. D. Pancost |journal=Nature Geoscience|volume=11|issue=10|pages=766–771|bibcode=2018NatGe..11..766N|hdl=1983/82e93473-2a5d-4a6d-9ca1-da5ebf433d8b|s2cid=135045515|url=https://research-information.bristol.ac.uk/files/159708447/Combined_manuscript_submitted_June_2018.pdf}}</ref><ref>{{cite news |last1=University of Bristol |title=Ever-increasing CO2 levels could take us back to the tropical climate of Paleogene period |work=ScienceDaily |date=30 July 2018}}</ref>

===Pseudoproxies===

The skill of algorithms used to combine proxy records into an overall hemispheric temperature reconstruction may be tested using a technique known as "[[pseudoproxy|pseudoproxies]]". In this method, output from a [[climate model]] is sampled at locations corresponding to the known proxy network, and the temperature record produced is compared to the (known) overall temperature of the model.<ref>{{Citation 
| last1 = Mann | first1 = M. E. | last2 = Rutherford | first2 = S. 
| doi = 10.1029/2001GL014554 
| title = Climate reconstruction using 'Pseudoproxies' 
| journal = Geophysical Research Letters 
| volume = 29 
| issue = 10 
| date = 31 May 2002 
| pmid =  
| pmc =
 | bibcode=2002GeoRL..29.1501M
 | pages=139–1–139–4
| doi-access = free}}</ref>