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Before Present (BP) years are a time scale used in archaeology, geology, and other scientific disciplines to specify when events in the past occurred. Because the "present" time changes, standard practice is to use 1950 as the arbitrary benchmark of what's considered "present". "BP" may also be considered to be an abbreviation of Before Physics.^[1] For example, 1500 BP means 1500 years before 1950, that is, in the year 450.

Beginning in 1954, metrologists established 1950 as the origin year for the BP scale for use with radiocarbon dating using a 1950-based reference sample of oxalic acid:

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 di-hydrate was prepared as NBS Standard Reference Material (SRM) 4990B. Its ¹⁴C concentration was ca. 5 % above what was believed to be the natural level, so the standard for radiocarbon dating was defined as 0.95 times the ¹⁴C concentration of this material, adjusted to a ¹³C 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); and “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. ^[2]

The year 1950 was chosen because it is the year in which calibration curves for radiocarbon dating were established, and also to honor^{[citation needed]} the publication of the first radiocarbon dates in December 1949^[3]. The year 1950 is also convenient because it predates large scale atmospheric testing of nuclear weapons, which altered the global ratio of carbon-14 to carbon-12.^[4]

The BP scale is now in common use for dates established by means other than radiocarbon dating. The practice of anchoring "the present" at 1950 is generally followed, although times in the distant past (e.g., 500,000 BP) typically have uncertainties high enough that the difference between 1950 and the actual present year is insignificant.

Dates determined using radiocarbon dating come in two kinds: uncalibrated (or "raw") and calibrated. Uncalibrated radiocarbon dates may be expressed using BP years; however, they are not identical to calendar dates. This has to do with the fact that the level of atmospheric radiocarbon (carbon-14 or ¹⁴C) 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 means of calibration curves based on comparison of raw radiocarbon dates of samples independently dated by other methods, such as dendrochronology (the examination of tree growth-rings). Such calibrated dates are expressed as cal BP, where "cal" indicates "calendar years" or "calibrated years".

Further information: Radiocarbon dating#Calibration

• Geochronology
• Dating methodology (archaeology)
• Radiometric dating
• Megaannum

1. ^ CalPal software documentation, from CalPal.de
2. ^ Lloyd A. Currie, The Remarkable Metrological History of Radiocarbon Dating [II] in J. Res. Natl. Inst. Stand. Technol. vol.109, pp.185-217 (2004)
3. ^ J. R. Arnold and W. F. Libby, Age determinations by radiocarbon content: Checks with samples of known age, Science vol. 109 (1949), pp. 227-228
4. ^ AD or BC? from www.ScienceCourseware.org