The Global Carbon Project (GCP) has been publishing estimates of global and national fossil CO2 emissions since 2001. In the first instance these were simple republications of data from another source, but over subsequent years refinements have been made in response to feedback and identification of inaccuracies. In this article we describe the history of this process leading up to the methodology used in the 2021 release of the GCP’s fossil CO2 dataset. The fossil CO2 emissions dataset is included in both its standard, absolute form, and per capita, with associated metadata files in JSON format. A file indicating the source(s) of each data point is also provided (this is a work in progress).