When Is the Right Time to Rebaseline Your GHG Emissions Reporting?

What is rebaselining?

Your emissions baseline is the reference point against which you measure all progress. It represents the first reporting period in which you completed a comprehensive GHG inventory — and every subsequent year's performance is measured against it. If you reported 10,000 tCO₂e in your base year and 8,500 tCO₂e this year, you can claim a 15% reduction.

But businesses are not static. They acquire other companies, sell divisions, open new sites, close old ones, change calculation methodologies, and restructure operations. Over time, these changes can make your original baseline unrepresentative of your current business. When that happens, year-on-year comparisons become misleading — you might appear to have reduced emissions when in reality you just sold a high-emitting subsidiary, or appear to have increased emissions when you simply acquired new operations.

Rebaselining means recalculating your historical baseline to reflect these structural changes, so that the comparison between then and now remains fair, meaningful, and auditable. It is not about changing the numbers to look better — it is about ensuring the numbers tell an honest story.

When should you rebaseline?

The GHG Protocol Corporate Standard provides clear guidance on when rebaselining is appropriate. The general principle is that you should recalculate your base year emissions when significant structural or methodological changes occur that would otherwise distort the trend in your reported emissions.

The most commonly cited threshold is the significance threshold — typically set at 5% of base year emissions. If a structural change would have caused your base year emissions to change by more than 5%, you should recalculate. The specific triggers include:

• Mergers, acquisitions, and divestitures that transfer emissions sources into or out of your organisational boundary • Outsourcing or insourcing of activities that were previously included or excluded from your inventory • Changes in calculation methodology — for example, moving from spend-based to activity-based Scope 3 estimation, or adopting new emission factors • Discovery of significant errors or omissions in your historical data that materially affect the base year figure • Changes in the categories or gases included in your inventory • Structural changes within your organisation, such as creating or dissolving subsidiaries, or transferring operations between entities

Importantly, organic growth or decline is not a trigger for rebaselining. If your emissions went up because you expanded production, that is a real increase and should be reported as such. Rebaselining is specifically for structural changes that alter the comparability of the baseline, not for changes in operational performance.

Setting a rebaselining policy

The GHG Protocol recommends that organisations establish a base year recalculation policy before they need one. This policy should define:

1. Your significance threshold — most organisations use 5%, but you can set a lower or higher threshold depending on your context 2. The types of changes that trigger recalculation 3. The process for conducting and approving a recalculation 4. How recalculated figures will be disclosed and communicated to stakeholders

Having this policy documented and approved in advance removes the risk of ad hoc decisions that might appear to be cherry-picking favourable numbers. When an auditor or stakeholder asks why you recalculated, you can point to a pre-existing policy with clear criteria — that is the difference between a credible rebaseline and one that raises questions.

For organisations subject to SECR, the Companies Act does not prescribe a specific rebaselining methodology, but it does require that comparisons with previous years are meaningful. Adopting the GHG Protocol's rebaselining guidance is considered best practice and will be expected under the anticipated UK SRS.

Climatise maintains full historical data across all reporting periods, making rebaselining a straightforward process. When a structural change occurs, the platform can recalculate your baseline across all affected periods, update your intensity ratios, and regenerate reports — preserving the full audit trail of what changed and why.