Base year emissions, under the Kyoto Protocol, are defined as the aggregate anthropogenic carbon dioxide equivalent emissions of the GHGs listed in Annex A sources in a historical base year. For most Annex I Parties, the historical base year is 1990 but Parties included in Annex I undergoing the process of transition to a market economy may choose a year or period other than 1990¹, in accordance with Article 3, paragraph 5². Annex I Parties may choose to use 1995 as the base year for total emissions of hydrofluorocarbons, perfluorocarbons and sulphur hexafluoride (F-gases), in accordance with Article 3, paragraph 8.

If the land use, land-use change and forestry sector constituted a net source of greenhouse gas emissions in the base year or period, then net emissions from the part relating to the conversion of forests (deforestation) will be included in the total national emissions for that year or period.

The table below provides, for Annex I Parties included in Annex B to the Kyoto Protocol:

1. Quantified emission limitation or reduction commitment expressed as per cent of the base year or period;
2. Selection of the base year for CO₂, CH₄ and N₂O;
3. Selection of the base year for F-gases;
4. Base year level of total national emissions as determined by the initial review, expressed in tonnes CO₂ equivalent;
5. Source of information on the base year and base year emissions level.

¹Annex I Parties with the base year other than 1990 are Bulgaria (1988), Hungary (average of 1985-1987), Poland (1988), Romania (1989), Slovenia (1986).
²The Articles here refer to Articles of the Kyoto Protocol unless specified otherwise.