Before Present (
BP) years is a
time scale used mainly in
geology and other
scientific disciplines to specify when events in the past occurred. Because the "present" time changes, standard practice is to use 1 January 1950 as commencement date of the age scale, reflecting the fact that
radiocarbon dating became practicable in the 1950s. The abbreviation "BP", with the same meaning, has also been interpreted as "Before Physics"; that is, before
nuclear weapons testing artificially altered the proportion of the
carbon isotopes in the atmosphere, making dating after that time likely to be unreliable.