Definition, sourcing, and updating of emissions baselines
The essential framework here is effort-sharing as opposed to a resource sharing. Thus, calculations of fair shares are sensitive to assumptions about baselines – assumptions about “business-as-usual,” or BAU, as projected into the future. This is because effort-sharing approaches, by definition, allocate obligations in terms of reductions below a BAU situation in which no special climate-related efforts are being taken; i.e., a national emissions baseline.
What this means in practice is that a country’s actual mitigation obligation in any given year is relative to a projected baseline. Two countries with the same Responsibility and Capacity Index (or RCI) and thus the same mitigation obligation would have different emission allocations in a given year – say 2020 – if they had different BAU emissions projections for that year.
See Definition, sourcing, and updating of the emissions baselines for more information.