We keep buying units. Should our reduction target use a fixed or a rolling base year?
With frequent acquisitions a rolling target base year, usually moved forward one year, is the easier fit. When a unit joins, only the year before the change is recalculated. A fixed base year recalculates every year back to the base and may force you to estimate data the acquired unit never kept.
The difference in one table
- Target wording. Fixed: "we will emit X% less in year B than in year A". Rolling: "we will cut emissions Y% every year compared with the previous year".
- Recalculation after a structural change. Fixed: all years back to the base year. Rolling: only the year before the change, or the year of the change, which then becomes the base year.
- Data burden. Fixed: an acquired unit needs data for the base year, which may mean backcasting and a less reliable base. Rolling: data is needed for the year before the acquisition, or only from the acquisition onward.
- Like-with-like view. Fixed: the whole series compares like with like. Rolling: only over two years at a time.
What does not change
The triggers for recalculation are the same under both approaches, as set out in chapter 5 of the standard. The difference is how far back you go.
The trade-off
A rolling base year never gives a clean line from the starting year to the completion year. You can still state reductions together over several years, for example "1% a year from 2026 to 2030", but you cannot restate the whole period on one footing.
Choose fixed when
Acquisitions are rare and you have reliable data for the base year. The standard says to consider rolling when reliable, verifiable data for a fixed base year is hard to keep, for example because of frequent acquisitions.
Example with made-up numbers: a stabilisation target (keep emissions flat)
- Company A emits 1,000 t. Company B, 200 t in year 1 and 260 t in year 2 because it grew.
- A buys B at the start of year 3. Year 3: A emits 1,000 t and B emits 260 t, total 1,260 t.
- Fixed base year: the base is recalculated to include B, 1,000 + 200 = 1,200 t. 1,260 is above 1,200, so the target is missed.
- Rolling base year: the comparison year is year 2 with B included, 1,000 + 260 = 1,260 t. 1,260 equals 1,260, so the target is met.
- B's growth before the purchase does not show as an increase for A under the rolling approach.
This follows the standard's own acquisition example. Growth or decline of a divested unit before the sale does count under the rolling approach.
What Senseible does here
Senseible keeps dated energy and emission records per site, so a newly acquired unit can be brought in from its first bills without rebuilding old years.
Sources: GHG Protocol, Corporate Accounting and Reporting Standard, chapter 11 (Setting a GHG target)
Last reviewed 2026-11-03.