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 1950-01-01 as the arbitrary origin of the age scale. "BP" may also be considered to be an abbreviation of Before Physics. For example, 1500 BP means 1500 years before 1950, that is, in the year 450.
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 14C 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 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. 
The year 1950 was chosen because it is the year in which calibration curves for radiocarbon dating were established, and also to honor the publication of the first radiocarbon dates in December 1949. 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.
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 ka 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 (also called Libby or raw) and calibrated (also called Cambridge dates). 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 (dating on basis of tree growth-rings) and stratigraphy (dating on the basis of sediment layers in mud or sedimentary rock). Such calibrated dates are expressed as cal BP, where "cal" indicates "calendar years" or "calibrated years".
- Further information: Radiocarbon dating#Calibration
- ^ CalPal software documentation, from CalPal.de
- ^ Dena Ferran Dincauze Environmental Archaeology: Principles and Practice page 110 ISBN 0521310776
- ^ Currie Lloyd A (2004), “The Remarkable Metrological History of Radiocarbon Dating [II]”, Journal of Research of the National Institute of Standards Technology 109: 185-217, <http://nvl.nist.gov/pub/nistpubs/jres/109/2/j92cur.pdf>
- ^ Arnold JR, Libby WF (1949), “Age determinations by radiocarbon content: Checks with samples of known age”, Science 109: 227-228
- ^ AD or BC? from www.ScienceCourseware.org
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