cal ibr ated ▁dates ▁for ▁these ▁er as . ▁ ▁The ▁Centre ▁for ▁Ice ▁and ▁Cl imate ▁at ▁the ▁University ▁of ▁C open hagen ▁has ▁proposed ▁" b 2 k " ▁as ▁" year s ▁before ▁AD ▁ 2 0 0 0 ", ▁based ▁on ▁the ▁Green land ▁Ice ▁Core ▁Chron ology ▁ 2 0 0 5 ▁( G IC C 0 5 ) ▁time ▁scale . ▁ ▁Rad i oc ar bon ▁d ating ▁ ▁Rad i oc ar bon ▁d ating ▁was ▁first ▁used ▁in ▁ 1 9 4 0 . ▁Begin ning ▁in ▁ 1 9 5 4 , ▁met ro log ists ▁established ▁ 1 9 5 0 ▁as ▁the ▁origin ▁year ▁for ▁the ▁B P ▁scale ▁for ▁use ▁with ▁radi oc ar bon ▁d ating , ▁using ▁a ▁ 1 9 5 0 - based ▁reference ▁sample ▁of ▁ox al ic ▁acid . ▁According ▁to ▁scient ist ▁A . ▁C urr ie ▁Lloyd : ▁ ▁The ▁year ▁ 1 9 5 0 ▁was ▁chosen ▁because ▁it ▁was ▁the ▁standard ▁astronom ical ▁epo ch ▁at ▁that ▁time . ▁It ▁also ▁marked ▁the ▁publication ▁of ▁the ▁first ▁radi oc ar bon ▁dates ▁in ▁December ▁ 1 9 4 9 , ▁and ▁ 1 9 5 0 ▁also ▁ant ed ates ▁large ▁scale ▁atmos pher ic ▁testing ▁of ▁nuclear ▁weapons , ▁which ▁alter ed ▁the ▁global ▁ratio ▁of ▁carbon - 1 4 ▁to ▁carbon - 1 2 . ▁ ▁Rad i oc ar bon ▁cal ibration ▁▁ ▁D ates ▁determined ▁using