Radiocarbon vs. Calendar vs. Years Ago

Three timelines or scales are shown side-by-side, radiocarbon years, calendar years in BC/AD, and years ago.

The years ago scale on the right is directly correlated to the calendar years scale and is given here for convenience. A recent informal survey among the undergraduate students of an introductory archeology course at one of Texas' major universities showed that many students seem to have difficulty in translating calendar years B.C. (before Christ) to years ago. The following rule of thumb may be helpful: to figure out how about many years ago any calendric age in years B.C. occurred, simply add 2,000 years (for instance 2,000 B.C. = 4,000 years ago).

The calendar years scale in the middle measures time using the Gregorian calendar, the familiar international standard for secular use that was developed in the Christian world. We follow the conventional use of "A.D." for those years after Christ was born and "B.C." for those years before Christ. (The year 1 B.C. was followed by the year 1 A.D.). Today, some people prefer to use the more generic terms C.E. and B.C.E., meaning "Common Era" and "Before Common Era" instead of A.D. and B.C., respectively.

The radiocarbon timescale on the left is very important for scientists because radiocarbon dating is the most accurate dating method available for the entire prehistoric era in the region. The radiocarbon dating technique is a complex process that yields statistical age estimates (called "assays") rather than definite calendar dates. Radiocarbon assays are expressed in the number of years B.P. (before present) coupled with an error estimate (for example, 10,300 +/- 160 B.P.). By convention, “B.P.” actual means "before 1950." The technique was developed in the late 1940s by Willard Libby. The year 1950 was an arbitrary reference point chosen to avoid the problem of constantly having to update and recalculate dates as time marches forward. To avoid confusion with general age estimates relative to the present, the proper term is RCYBP or Radiocarbon Years Before the (1950) Present.

But why, at many points in time, don't the two timescales match closely one another? For instance, why isn't 4000 RCBP about the same as 2,000 B.C. (4,000 calendar years ago)? This is because the relationship between radiocarbon ages and calendar years varies through time due to factors such as atmospheric variation in the abundance of carbon-14 (14C), the radioactive isotope upon which the dating technique is based. Thus, radiocarbon dates must be calibrated using sets of dates obtained from tree rings from very long-lived tree species such as the bristlecone pine.

If you want to learn more, see the TBH Special Exhibit Radiocarbon Dating Understood.

Online CalPal
You can use this Interactive radiocarbon calibration tool to gain a quick approximation of the calendric age of any radiocarbon assay, created by German researchers, as part of the Cologne Radiocarbon Calibration and Paleoclimatic Research Package.