The base year is the period whose emissions a company compares later years against and from which it measures progress toward its reduction targets. To keep the comparison valid, the base year is recalculated when there are significant changes in the company's structure or methodology, or when significant errors are found. Without recalculation, an apparent drop in emissions may simply reflect the sale of a plant rather than any real action.
How to choose a base year
Choose a year for which you have complete, verifiable data and which reflects the company's normal operations. Practical criteria:
- data for all significant Scope 1 and 2 sources has been collected monthly and is backed by documents;
- the year is not distorted by emergency shutdowns, unusual capacity utilization or one-off events;
- the organizational boundaries and methodology are documented and will continue to be applied.
If a single year is not representative, the GHG Protocol allows you to use an average of several consecutive years. Record the choice and its rationale in your methodology.
When to recalculate the base year
Recalculation is needed if changes significantly affect base-year emissions:
- Structural changes: acquisitions, divestments, mergers, and insourcing or outsourcing of activities.
- Changes in methodology: new emission factors, a switch to a different set of GWP values, more accurate calculation methods.
- Discovery of errors: missed sources, wrong units, double entries.
The company sets its own significance threshold and describes it in its recalculation policy; a reporting program or verifier may require a specific value. Organic growth or decline in production does not change the base year: that is the real trend in emissions.
Example: recalculation after a facility sale
Base year 2023: the company's emissions are 5,000 t CO₂e, of which 1,200 t come from a plant sold in 2025. Emissions in 2025 without the plant are 3,900 t CO₂e.
- Without recalculation: a reduction of (5,000 − 3,900) / 5,000 = 22%.
- With recalculation: base = 5,000 − 1,200 = 3,800 t; 3,900 t in 2025 means an increase of 2.6%.
The conclusion is the opposite: the remaining facilities did not reduce their emissions. This is exactly why recalculation is essential for tracking reduction targets correctly.
Recalculation when emission factors change
If you switch to a more accurate grid electricity factor, apply it to the base year as well. Example: base-year consumption is 1,000,000 kWh, the old factor is 0.80 and the new one is 0.726 kg CO₂e/kWh (illustrative values). Base-year emissions with the new factor are 726 t instead of 800 t. Otherwise, a 74 t “reduction” would be nothing more than the effect of a methodology change.
Base year vs. project baseline
Do not confuse an organization's base year with the baseline of a climate project. The base year is used to compare the entire inventory over time, while the project baseline describes the emissions scenario without the project and is used to calculate emission reductions.
How it works in the platform
The platform does not yet have a dedicated base year or reduction target setting. The foundation for a base year is the facilities' annual data: it is entered month by month on the Data entry screen, every record shows the methodology applied, and a change history is kept for each facility's year. A facility's year can be closed by an administrator or by the responsible employee with edit rights, while only an administrator can reopen a closed year. Annual totals, and a comparison with the previous year when both years are complete, are shown in company GHG analytics; for how reliable your source data is, see Data quality.
Frequently asked questions
Can we change the base year?
Yes, if the previous base year is no longer representative or its data turned out to be incomplete. Disclose the reason for the change in your report and revise your targets against the new base year.
Do we need to recalculate the base year when production grows?
No. Organic growth or decline is a real change in emissions and is reported as it is. To compare performance, companies also use intensity metrics, such as t CO₂e per unit of output.
Who sets the significance threshold for recalculation?
The company, in its recalculation policy. If you take part in a reporting program or undergo verification, check its threshold requirements.