The historical emissions for Annex I countries are provided from 1990 onwards using the UNFCCC National Inventory Submissions, Common Reporting Format Tables (CRF).
The data tables are processed for the CAT by the Potsdam Institute for Climate Impact Research. Data reported in the CRF tables are calculated according to IPCC 2006 Guidelines, but the categorisation of data in the tables does not directly match either the 1996 or 2006 Guidelines. The CAT currently uses the CRF data converted to IPCC 1996 categories. The process of data conversion is described in Jeffery et al., 2018 Earth System Science Data Discussions, and the historic data is available from here.
For non-Annex I countries, we use data from national inventories, in most of the cases those that have been submitted to the UNFCCC.
All CAT emissions data uses global warming potentials from the Second Assessment Report of the IPCC. Where countries use different metrics in their reporting, CAT converts the data to the extent possible.