Editing Proxy (climate) (section)

===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.