View source for Before Present

{{Short description|Time scale used in scientific disciplines}}
{{Redirect2|YBP|ybp||YBP (disambiguation)}}
'''Before Present''' ('''BP''') or "'''years before present''' ('''YBP''')" is a [[geologic time scale|time scale]] used mainly in [[archaeology]], geology, and other scientific disciplines to specify when events occurred relative to the origin of practical [[radiocarbon dating]] in the 1950s. Because the "present" time changes, standard practice is to use 1{{nbsp}}January 1950 as the commencement date (epoch) of the age scale, with 1950 being labelled as the "standard year". The abbreviation "BP" has been interpreted retrospectively as "Before Physics",<ref>{{Cite journal|last1=Flint|first1=Richard Foster|last2=Deevey|first2=Edward S|year=1962|title=Volume 4 – 1962|url=https://journals.uair.arizona.edu/index.php/radiocarbon/article/view/16458/16133|journal=[[Radiocarbon (journal)|Radiocarbon]]|volume=4 |issue= 1|pages=i}}</ref><ref>{{cite book |last= van der Plicht |first= Johannes |chapter= Radiocarbon, the Calibration Curve and Scythian Chronology |title= NATO Science Series: IV: Earth and Environmental Sciences |publisher= Springer Netherlands |location= Dordrecht |date= January 2004 |volume= 42 |doi= 10.1007/1-4020-2656-0_5 |pages= 45–61 (47) |isbn= 978-1-4020-2655-3 |url= http://cio.eldoc.ub.rug.nl/FILES/root/2004/NATOSciIVvdPlicht/2004NATOSciIVvdPlicht.pdf |archive-url= https://web.archive.org/web/20110724164654/http://cio.eldoc.ub.rug.nl/FILES/root/2004/NATOSciIVvdPlicht/2004NATOSciIVvdPlicht.pdf |archive-date= 2011-07-24 |access-date= 8 August 2024}}</ref> which refers to the time before [[nuclear weapons testing]] artificially altered the proportion of the [[Isotopes of carbon|carbon isotope]]s in the atmosphere, which scientists must account for when using radiocarbon dating for dates of origin that may fall after this year.<ref name="Taylor">{{Cite journal
 |title=The beginnings of radiocarbon dating in ''American Antiquity'': a historical perspective
 |author=Taylor RE
 |journal=American Antiquity
 |year=1985
 |volume=50
 |issue=2
 |pages=309–325
 |doi=10.2307/280489
|jstor=280489
 |s2cid=163900461
 }}</ref><ref>{{Cite book|last=Dincauze|first=Dena |title=Environmental Archaeology: Principles and Practice|year=2000|publisher=Cambridge University Press|location=Cambridge, England|isbn=978-0-5213-1077-2|page=110|chapter=Measuring time with isotopes and magnetism}}</ref>

In a convention that is not always observed, many sources restrict the use of BP dates to those produced with radiocarbon dating; the alternative notation "RCYBP" stands for the explicit "radio carbon years before present".

==Usage==
The BP scale is sometimes used for dates established by means other than radiocarbon dating, such as [[stratigraphy]].<ref name="AGUStyle">{{Cite web|url=http://www.agu.org/pubs/style_guide_intro.html|publisher=American Geophysical Union|title=AGU Editorial Style Guide for Authors|date=21 September 2007|access-date=2009-01-09|archive-url=https://web.archive.org/web/20080714134306/http://www.agu.org/pubs/style_guide_intro.html <!-- Bot retrieved archive --> |archive-date=2008-07-14}}</ref><ref name="Strat">{{Cite journal|author=North American Commission on Stratigraphic Nomenclature|url=https://ngmdb.usgs.gov/Info/NACSN/Code2/code2.html#Article13|title=North American Stratigraphic Code: Article 13 (c)|journal=The American Association of Petroleum Geologists Bulletin|volume=89|issue=11|date=November 2005|pages=1547–1591|doi=10.1306/07050504129|access-date=2009-06-29|archive-date=2014-02-02|archive-url=https://web.archive.org/web/20140202155725/http://ngmdb.usgs.gov/Info/NACSN/Code2/code2.html#Article13|url-status=dead|url-access=subscription}}</ref> This usage differs from the recommendation by van der Plicht & Hogg,<ref>
{{Cite journal|title=A note on reporting radiocarbon|first1=Johannes|last1=van der Plicht|first2=Alan|last2=Hogg|journal=[[Quaternary Geochronology]]|volume=1|year=2006|issue=4|pages=237–240|doi=10.1016/j.quageo.2006.07.001|bibcode=2006QuGeo...1..237V |s2cid=128628228 |url=https://pure.rug.nl/ws/files/6706881/2006QuatGeochronolvdPlicht.pdf}}</ref> followed by the ''[[Quaternary Science Reviews]]'',<ref>
{{Cite journal|title=The use of time units in Quaternary Science Reviews|journal=Quaternary Science Reviews|volume=26|issue=9–10|date=May 2007|pages=1193|doi=10.1016/j.quascirev.2007.04.002|bibcode=2007QSRv...26.1193.}}</ref><ref>{{Cite journal|last=Wolff|first=Eric W.|title=When is the "present"?|journal=Quaternary Science Reviews|volume=26|date=December 2007|issue=25–28|pages=3023–3024|doi=10.1016/j.quascirev.2007.10.008|bibcode=2007QSRv...26.3023W|s2cid=131227900 }}</ref> both of which requested that publications should use the [[a (unit)|unit "a"]] (for "annum", Latin for "year") and reserve the term "BP" for radiocarbon estimations.

Some [[archaeologist]]s use the lowercase letters ''bp'', ''[[Anno Domini|bc]]'' and ''[[Anno Domini|ad]]'' as terminology for uncalibrated dates for these eras.<ref name="Huth1994">{{Cite book|author=Edward J. Huth|title=Scientific Style and Format: The CBE Manual for Authors, Editors, and Publishers|url=https://books.google.com/books?id=PoFJ-OhE63UC&pg=PA495|access-date=4 October 2012|date=25 November 1994|publisher=Cambridge University Press|isbn=978-0-521-47154-1|pages=495–}}</ref>

The Centre for Ice and Climate at the [[University of Copenhagen]] instead uses the unambiguous "b2k", for "years before 2000 AD", often in combination with the [[Greenland]] [[Ice Core]] Chronology 2005 (GICC05) time scale.<ref>{{Cite web|url=http://www.iceandclimate.nbi.ku.dk/research/strat_dating/annual_layer_count/gicc05_time_scale/|title=The GICC05 time scale|date=3 September 2009|publisher=Centre for Ice and Climate – University of Copenhagen|access-date=September 17, 2018|archive-date=18 September 2018|archive-url=https://web.archive.org/web/20180918012233/http://www.iceandclimate.nbi.ku.dk/research/strat_dating/annual_layer_count/gicc05_time_scale/|url-status=dead}}</ref>

Some authors who use the YBP dating format also use "YAP" ("years after present") to denote years after 1950.<ref>{{Cite journal |last=Berger |first=André |date=1988 |title=Milankovitch Theory and Climate |url=https://dial.uclouvain.be/pr/boreal/en/object/boreal:52561 |journal=Reviews of Geophysics |language=en |volume=26 |issue=4 |pages=624–657 |doi=10.1029/RG026i004p00624 |bibcode=1988RvGeo..26..624B |issn=8755-1209}}</ref>

===SI prefixes===
[[Metric prefix|SI prefix multipliers]] may be used to express larger periods of time, e.g. ''ka BP'' (thousand years BP), ''Ma BP'' (million years BP) and [[Year#SI prefix multipliers|many others]].<ref name="Nways">{{Cite web|author = Martin Kölling |title = Numerous ways to say "thousand years" in a scientific paper |date = 2015 |access-date = 2023-03-24 |publisher = Marine Geochemistry - Laboratory Methods |place = Universität Bremen |url = https://www.sedgeochem.uni-bremen.de/kiloyears.html}}</ref>

== Radiocarbon dating ==
{{Main|Radiocarbon dating}}
[[Radiocarbon dating]] was first used in 1949.<ref name="libby49">{{cite journal |last1=Arnold |first1=J.R. |last2=Libby |first2=W.F. |year=1949 |title=Age determinations by radiocarbon content: checks with samples of known age |url=http://hbar.phys.msu.ru/gorm/fomenko/libby.htm |journal=Science |volume=110 |issue=2869 |pages=678–680 |bibcode=1949Sci...110..678A |doi=10.1126/science.110.2869.678 |jstor=1677049 |pmid=15407879|url-access=subscription }}</ref><ref name="Aitken_60">{{cite book |title=Science-based Dating in Archaeology |last=Aitken |first=M.J. |publisher=Longman |year=1990 |isbn=978-0-582-49309-4 |location=London |pages=60–61}}</ref> Beginning in 1954, [[metrologist]]s established 1950 as the origin year for the BP scale for use with radiocarbon dating, using a 1950-based reference sample of [[oxalic acid]]. According to scientist A. Currie Lloyd:

{{blockquote|The problem was tackled by the international radiocarbon community in the late 1950s, in cooperation with the U.S. [[National Bureau of Standards]]. A large quantity of contemporary [[oxalic acid]] [[dihydrate]] was prepared as NBS Standard Reference Material (SRM) 4990B. Its <sup>14</sup>C concentration was about 5% above what was believed to be the natural level, so the standard for radiocarbon dating was defined as 0.95 times the <sup>14</sup>C concentration of this material, adjusted to a <sup>13</sup>C reference value of −19 per mil (PDB). This value is defined as "modern carbon" referenced to AD 1950. Radiocarbon measurements are compared to this modern carbon value, and expressed as "fraction of modern" (fM). "Radiocarbon ages" are calculated from fM using the exponential decay relation and the "Libby half-life" 5568 a. The ages are expressed in years before present (BP) where "present" is defined as AD 1950.<ref name="Currie">{{Cite journal
 |url=http://nvl.nist.gov/pub/nistpubs/jres/109/2/j92cur.pdf
 |author=Currie, Lloyd A
 |title=The Remarkable Metrological History of Radiocarbon Dating &#91;II&#93;
 |journal=Journal of Research of the National Institute of Standards and Technology
 |volume=109
 |issue=2
 |pages=185–217
 |date=March–April 2004
 |doi=10.6028/jres.109.013
 |pmc=4853109
 |pmid=27366605
 |bibcode=2004NISTJ.109..185C
 |access-date=30 October 2019
 |archive-url=https://web.archive.org/web/20101206195414/http://nvl.nist.gov/pub/nistpubs/jres/109/2/j92cur.pdf
 |archive-date=2010-12-06
 |url-status=dead
 }}
 [https://books.google.com/books?id=bKoL_vPa47EC&pg=PA185 "The Remarkable Metrological History of Radiocarbon Dating &#91;II&#93;"] {{Webarchive|url=https://web.archive.org/web/20230101102736/https://books.google.com/books?id=bKoL_vPa47EC&pg=PA185#v=twopage&q&f=false |date=2023-01-01 }} at [[Google Books]] (accessed 30 October 2019).</ref>}}

The year 1950 was chosen because it was the standard [[astronomical epoch]] at that time.{{citation needed|date=November 2021}} It also marked<ref name="Taylor" /> the publication of the first radiocarbon dates in December 1949,<ref name="Arnold">
{{Cite journal
 |author=Arnold JR, Libby WF
 |title=Age determinations by radiocarbon content: Checks with samples of known age
 |journal=Science
 |volume=109
 |issue=2827
 |date=1949-03-04
 |pages=227–228
 |doi=10.1126/science.109.2827.227
 |pmid=17818054
 |bibcode = 1949Sci...109..227L }}</ref> and 1950 also antedates large-scale [[nuclear testing|atmospheric testing of nuclear weapons]], which altered the global ratio of [[carbon-14]] to [[carbon-12]].<ref>{{Cite web |title=Nuclear Bombs Made It Possible to Carbon Date Human Tissue |url=https://www.smithsonianmag.com/smart-news/nuclear-bombs-made-it-possible-to-carbon-date-human-tissue-20074710/ |work=Smithsonian Magazine | date=2013-02-19 | access-date=2020-01-09}}</ref>

== Radiocarbon calibration ==
{{Main|Radiocarbon dating#Calibration}}
Dates determined using radiocarbon dating come as two kinds: ''uncalibrated'' (also called ''Libby'' or ''raw'') and ''calibrated'' (also called ''Cambridge'') dates.<ref name="Archaeology">{{Cite book | last=Greene | first=Kevin | author-link=Kevin Greene (archaeologist)| title=Archaeology: An Introduction | year=2002 | publisher=University of Pennsylvania Press | location=Philadelphia | isbn=0-8122-1828-0 | pages=165–167}}</ref> ''Uncalibrated'' radiocarbon dates should be clearly noted as such by "uncalibrated years BP", because they are not identical to calendar dates. This has to do with the fact that the level of atmospheric radiocarbon ([[carbon-14]] or <sup>14</sup>C) has not been strictly constant during the span of time that can be radiocarbon-dated. Uncalibrated radiocarbon ages can be converted to calendar dates by [[Radiocarbon dating#Calibration|calibration curves]] based on comparison of raw radiocarbon dates of samples independently dated by other methods, such as [[dendrochronology]] (dating based on tree growth-rings) and stratigraphy (dating based on sediment layers in mud or sedimentary rock). Such calibrated dates are expressed as cal BP, where "cal" indicates "calibrated years", or "calendar years", before 1950.

Many scholarly and scientific journals require that published calibrated results be accompanied by the name (standard codes are used) of the laboratory concerned, and other information such as confidence levels, because of differences between the methods used by different laboratories and changes in calibrating methods.

==Conversion==
Conversion from [[Gregorian calendar]] years to Before Present years is by starting with the 1950-01-01 epoch of the Gregorian calendar and increasing the BP year count with each year into the past from that Gregorian date.

For example, 1000 BP corresponds to 950 AD, 1949 BP corresponds to 1 AD, 1950 BP corresponds to 1 BC, 2000 BP corresponds to 51 BC.

<!-- This table is only for illustrative purposes related to the BP time scale. This is not an indiscriminate collection of historical events. -->
{| class="wikitable"
|+ Example milestone years in the BP time scale
! {{Nowrap|Gregorian year}}
! {{Nowrap|BP year}}
! Event
|-
| [[10th millennium BC|9701 BC]]
| {{nowrap|11650 BP}}
| End of the [[Pleistocene]] and beginning of the [[Holocene]] epoch<ref name="jqs">
{{Cite journal
 |last1  = Walker     |first1  = Mike              |last2  = Jonsen     |first2  = Sigfus
 |last3  = Rasmussen  |first3  = Sune Olander      |last4  = Popp       |first4  = Trevor
 |last5  = Steffensen |first5  = Jørgen-Peder      |last6  = Gibbard    |first6  = Phil
 |last7  = Hoek       |first7  = Wim               |last8  = Lowe       |first8  = John
 |last9  = Andrews    |first9  = John
 |last10 = Björck     |first10 = Svante  |author-link10=Svante Björck     
 |last11 = Cwynar     |first11 = Les C. 
 |last12 = Hughen     |first12 = Konrad            |last13 = Kershaw    |first13 = Peter 
 |last14 = Kromer     |first14 = Bernd             |last15 = Litt       |first15 = Thomas
 |last16 = Lowe       |first16 = David J.          |last17 = Nakagawa   |first17 = Takeshi 
 |last18 = Newnham    |first18 = Rewi              |last19 = Schwander  |first19 = Jacob
 |year=2009
 |title=Formal definition and dating of the GSSP (Global Stratotype Section and Point) for the base of the Holocene using the Greenland NGRIP ice core, and selected auxiliary records
 |journal = Journal of Quaternary Science
 |volume=24  |issue= 1  |pages=3–17
 |doi=10.1002/jqs.1227  | doi-access = free
 |bibcode = 2009JQS....24....3W
 |url=http://www.stratigraphy.org/GSSP/Holocene.pdf
 |url-status=live
 |archive-url=https://web.archive.org/web/20131104131948/http://www.stratigraphy.org/GSSP/Holocene.pdf
 |archive-date=2013-11-04
}}
</ref>
|-
| [[5th millennium BC|4714 BC]]
| 6663 BP
| [[Epoch (reference date)|Epoch]] of the [[Julian day]] system: Julian day 0 starts at [[Prime meridian (Greenwich)|Greenwich]] noon on January 1, 4713 BC of the [[proleptic Julian calendar]], which is November 24, 4714 BC in the [[proleptic Gregorian calendar]]<ref name=Dershowitz>{{Cite book |last1=Dershowitz |first1=Nachum |author1-link=Nachum Dershowitz |last2=Reingold |first2=Edward M. |author2-link=Edward M. Reingold |title=Calendrical Calculations |edition=3rd |publisher=Cambridge University Press |year=2008 |isbn=978-0-521-70238-6 |title-link= Calendrical Calculations}}</ref>{{rp|10}}
|-
| [[2251 BC]]
| 4200 BP
| Beginning of the [[Meghalayan]] age, the current and latest of the three stages in the [[Holocene]] era.<ref>{{Cite web|url=http://www.stratigraphy.org/index.php/ics-news-and-meetings/120-ics-chart-containing-the-quaternary-gssps-and-new-stages-v-2018-07-is-now-released|title=ICS chart containing the Quaternary and Cambrian GSSPs and new stages (v 2018/07) is now released!|access-date=February 6, 2019}}</ref><ref>{{Cite web|url=https://earthsky.org/earth/new-name-present-times-era-meghalayan|title=Welcome to the Meghalayan age|last=Conners|first=Deanna|date=September 18, 2018|access-date=February 6, 2019}}</ref>
|-
| [[45 BC]]
| 1994 BP
| Introduction of the [[Julian calendar]]
|-
| [[1 BC]]
| 1950 BP
| [[Year zero]] in [[ISO 8601]]
|-
| [[AD 1]]
| 1949 BP
| Beginning of the [[Common Era]] and [[Anno Domini]], from the estimate by [[Dionysius Exiguus|Dionysius]] of the [[Incarnation (Christianity)|Incarnation of Jesus]]
|-
| 1582
| 368 BP
| Introduction of the [[Gregorian calendar]]<ref name=Dershowitz />{{rp|47}}
|-
| 1950
| 0000 AP
| [[Epoch (reference date)|Epoch]] of the Before Present dating scheme<ref>{{Cite journal
 |url=http://nvl.nist.gov/pub/nistpubs/jres/109/2/j92cur.pdf
 |author=Currie Lloyd A
 |title=The Remarkable Metrological History of Radiocarbon Dating &#91;II&#93;
 |journal=Journal of Research of the National Institute of Standards and Technology
 |volume=109
 |issue=2
 |pages=185–217
 |year=2004
 |doi=10.6028/jres.109.013
 |pmid=27366605
 |pmc=4853109
 |bibcode=2004NISTJ.109..185C
 |access-date=2018-06-24
 |archive-url=https://web.archive.org/web/20101206195414/http://nvl.nist.gov/pub/nistpubs/jres/109/2/j92cur.pdf
 |archive-date=2010-12-06
 |url-status=dead
 }}</ref>{{rp|190}}
|-
|{{CURRENTYEAR}}
| {{padleft:{{#expr:{{CURRENTYEAR}} - 1950}}|04|0}} AP
|Current year
|}
{{Notelist}}

==See also==
* {{anl|Anthropocene}}

== Citations ==
{{Reflist}}

== Sources ==
* {{cite book |last=Aitken |first=M.J. |title=Science-based Dating in Archaeology |publisher=Longman |year=1990 |isbn=978-0-582-49309-4 |location=London |ref=Aitken1990}}

{{Calendars}}
{{Chronology}}

[[Category:1950 introductions]]
[[Category:Calendar eras]]
[[Category:Geochronology]]