"BP" may also be considered to be an abbreviation of Before Physics.
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) typically have uncertainties high enough that the difference between 1950 and the actual present year is insignificant.