Before Present (BP) years is a time scale
used mainly in archaeology
, and other scientific
disciplines to specify when events occurred before the origin of practical radiocarbon dating
in the 1950s. Because the "present" time changes, standard practice is to use 1 January 1950 as the commencement date (epoch
) of the age scale. The abbreviation "BP" has been interpreted retrospectively as "Before Physics"; that refers to the time before nuclear weapons testing
artificially altered the proportion of the carbon isotope
s in the atmosphere, making dating after that time likely to be unreliable.
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 is explicitly Radio Carbon Years Before Present.
The BP scale is sometimes used for dates established by means other than radiocarbon dating, such as stratigraphy
This usage differs with the recommendation by van der Plicht & Hogg, followed by the ''Quaternary Science Reviews'', both of which requested that publications should use the unit "a" (for “annum”, Latin for “year”) and reserve the term "BP" for radiocarbon estimations.
s use the lowercase letters ''bp'', ''bc
'' and ''ad
'' as terminology for uncalibrated dates for these eras.
The Centre for Ice and Climate at the University of Copenhagen
has proposed "b2k" as "years before AD 2000", based on the Greenland Ice Core
Chronology 2005 (GICC05) time scale.
was first used in 1940. 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:
The year 1950 was chosen because it was the standard astronomical epoch
at that time. It also marked
the publication of the first radiocarbon dates in December 1949,
and 1950 also antedates large scale atmospheric testing of nuclear weapons
, which altered the global ratio of carbon-14
Dates determined using radiocarbon dating come as two kinds: ''uncalibrated'' (also called ''Libby'' or ''raw'') and ''calibrated'' (also called ''Cambridge'') dates.
''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
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 means of calibration curves
based on comparison of raw radiocarbon dates of samples independently dated by other methods, such as dendrochronology
(dating on the 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 "calibrated years", or "calendar years", before 1950.
Many scholarly/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.