What is rebaselining?
A base year is the reference period against which your organisation measures emissions performance and reduction progress. Rebaselining means recalculating your base year footprint to account for significant changes that would otherwise distort year-on-year comparisons. The GHG Protocol requires organisations to establish a base year and define a rebaselining policy — yet many companies either don't have one or have never triggered it. As your organisation evolves, getting rebaselining right is essential for credible reporting.
When should you rebaseline?
The GHG Protocol identifies several triggers: structural changes like mergers, acquisitions, or divestments that transfer emissions into or out of your boundary; methodology changes such as updated calculation approaches or improved data quality; discovery of significant errors in previous calculations; and changes to your organisational or operational boundary definition. A good rule of thumb is that any change affecting more than 5–10% of your base year footprint warrants rebaselining.
How to rebaseline correctly
Rebaselining isn't simply recalculating your first year with new emission factors. It means adjusting your base year to reflect what your footprint would have been if your current organisational structure had existed then. For an acquisition, add the acquired entity's historical emissions to your base year. For a divestment, remove them. For methodology improvements, apply the improved methodology retroactively. Document every adjustment with clear rationale — your auditor will expect a transparent rebaselining log.
Setting a rebaselining policy
Every organisation should have a written rebaselining policy that defines: what constitutes a significant change (threshold percentage), who has authority to trigger a rebaseline, the process for recalculating the base year, and how rebaselines will be disclosed in reports. This policy should be set before you need it — deciding rebaselining rules retrospectively invites cherry-picking and undermines credibility. Include the policy in your GHG reporting methodology document and review it annually.