As early as the 1950’s it was recognized that 14C levels in the atmosphere had varied in the past, it was only through developments in dendrochronology and radiocarbon dating that the construction of curves or tables for correction (or calibration) of radiocarbon ages to calendar age estimates was possible.

The first curves were published in the 1960’s and revised through the early 1980’s.

In 1986 the first high-precision calibration curve for terrestrial samples was constructed and based primarily on measurements made in Seattle, Belfast, Heidelberg and Arizona.  The wood samples came mainly from Irish oak and trees from the Pacific coast of North America as well as data from bristlecone pines and German oak.  A marine curve based on an ocean-atmosphere box model was also produced.

The term ‘IntCal’ was coined in 1993 to recognize the curve was an international effort when uranium-thorium dated corals (reservoir age corrected for use in the terrestrial curve) were included to extend the calibration curves to 21,950 cal BP.

In 1998 the curves were updated with more tree-ring measurements and extended to 24,000 years before present (cal BP) by including foraminifera from Cariaco basin varved sediments and additional U-Th dated corals.

In 2001 a grant from the Leverhulme Trust, UK to Queen’s University Belfast (QUB) enabled the formation of the IntCal Working Group (IWG).

The IWG met at Queen’s University Belfast and Woods Hole Oceanographic Institute to establish criteria for calibration data and methods for curve construction.  In 2004 the IWG released new IntCal and Marine curves which had been further refined and extended to 26,000 cal BP.

Meeting of the IntCal Working Group at Woods Hole Oceanographic Institute, USA in 2003

A calibration curve for the Southern Hemisphere was also developed from tree-ring measurements (SHCal04).

In 2006, a NERC grant to QUB and University of Sheffield funded refinements to the statistical methods for curve construction and database development. The IWG extended the curves to 50,000 cal BP in 2009 and updated the curves again in 2013.  The next update of the curves is planned for the end of 2019.

IntCal database
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Meeting of the IntCal Working Group at Queen’s University, Belfast in 2017

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