The C20C project was originally created by the Hadley Centre. This project has involved the use of both ocean-forced AGCMs and observed data, to study climate variations and changes over the last 130 years, in particular the period since 1949. The project took on an international dimension when a number of groups became interested in using the same SST and sea-ice data for simulating climate variability and change. Many informal bilateral collaborations between the Hadley Centre and groups in the USA, Canada, Australia, New Zealand, and UK universities were formed, along with a few smaller collaborations elsewhere. Many other groups also have copies of the Hadley Centre's global sea-ice and sea surface temperature data set, formerly GISST, now called HadISST, which is the central link in C20C. In late 1994, a very successful workshop was held at the Hadley Centre (Folland and Rowell, 1995, "Workshop on Simulations of the Climate of the Twentieth Century using GISST, 28-30 November 1994, Hadley Centre", CRTN 56), and in 1995 a C20C session was included in the 1st International AMIP Conference. The C20C idea is also mentioned in the CLIVAR plan and has been endorsed by the Working Group on Numerical Experimentation. The Hadley Centre and COLA agreed in 1998 to re-invigorate and formalize the International C20C Project, with the necessary infrastructure to be supplied by COLA. Several research groups making use of AGCMs were invited and have agreed to participate.
The basic scientific idea of C20C is that up to three sets of ensemble simulations are to be run by each participating institute: