Baseline Year

The historical year, or period, against which a country measures an emissions target — as in “X per cent below 1990 levels by 2035”. Parties use different base years for legitimate reasons: when reliable inventory data begins, when a domestic law was passed, when a target was first set. Some targets are not expressed against a base year at all, but against a projected business-as-usual pathway, or as an emissions-intensity figure per unit of output or GDP.

Why it matters in the room

Two headline percentages are not comparable unless the base year, the gases and sectors covered, and the target type all match. A large share of any technical exchange about ambition is, on inspection, an exchange about baselines.