Re-baselining Carbon Emissions: When to Recalculate Your Base Year

If you’re concerned that the emissions you’re comparing today aren’t measured on the same basis as your baseline, you might be considering re-baselining.

The point isn’t to change the story or fudge the numbers, but to keep your reporting consistent, so year-on-year comparisons stay meaningful.

This guide covers:

What a baseline is for
When the GHG Protocol expects re-baselining
When re-baselining isn’t appropriate
How to communicate re-baselining clearly, without misleading

We'll lean on guidance from the GHG Protocol Corporate Standard.

What Is a Carbon Baseline (Base Year) and Why Does It Matter?

A baseline (or base year) is the reference point you use to track emissions performance over time.

It’s the foundation for:

Year-on-year reporting
Reduction targets
Credible updates to customers, investors, and regulators

A baseline only works if the data stays comparable year-on-year. When this comparability breaks down, you may need to re-baseline – either by changing your baseline year, or (where feasible) by recalculating historic data.

Base Year Changes vs Real Emissions Change

Carbon totals change for two key reasons. Let's take a look at each in turn.

Real-world emissions changes (don’t re-baseline)

These are changes to your carbon footprint as a result of business activity, such as:

Decreased Scope 2 emissions via solar installations
Decreased emissions through a revised business travel policy
Increased Scope 1 emissions through growth in product volumes

These are real, year-on-year changes your footprint that shouldn’t be “smoothed out” by recalculating past years or moving the goalposts. Regardless of whether the number goes up (e.g. through business growth) or down (e.g. through decarbonisation strategies), your aim here is to record and share a true, accurate representation of your carbon footprint data.

Reporting changes

These are changes that affect year-on-year comparability, such as:

Acquisitions or divestments
Organisational boundary changes
Methodology upgrades that materially affect results
Correcting significant errors

The point here is that it's not your underlying footprint that has changed, but instead what you're measuring, or how you're measuring it. Changes like these can prevent a fair, year-on-year comparison of carbon data.

How to Choose a Sensible Baseline Year in the First Place

A useful baseline is relevant to the business going forward (or at least expected to be). As such, it provides a fair and useful basis for comparison.

When Should You Re-baseline Your Carbon Footprint?

A good best practice tip is to define upfront how you plan to handle changes to your baseline year, and apply this consistently. In practice, the most common triggers fall into a few common groups.

1) Re-baselining when your baseline year isn’t representative (e.g., Covid year)

Whilst choosing a representative baseline year is the aim, we all know that some things can't be predicted. Considerable, one-off shocks to the economy might influence your business to the extent that the year isn't representative of ‘normal’ operations. If your chosen base year was unusually high or low because the business was operating in a fundamentally different way, year-on-year comparisons can become less meaningful.

Covid is the most common example. For many teams, that period changed operational emissions drivers but only for a short period – for example:

little or no commuting
a shift to fully remote working
a sharp drop in business travel

In situations like this, it may be more meaningful to change the baseline approach (for example, using a multi-year average base year) so targets and progress tracking stay like for like.

2) Re-baselining after acquisitions, mergers or divestments

A significant structural change can trigger a re-baselining decision, because the historic GHG inventory may no longer represent the same organisation.

Typical examples:

Acquiring a company/business unit
Selling or divesting a major part of the business
Merging with another organisation

Practical tip: If a significant structural change happens mid-year, many teams recalculate the whole year to avoid repeated recalculations later.

3) Re-baselining after outsourcing or insourcing

Outsourcing can shift emissions between scopes.

In general:

If you report the relevant indirect emissions (Scope 2 and/or Scope 3), outsourcing/insourcing usually doesn’t break comparability.
If Scope 3 isn’t reported (or is very incomplete), outsourcing can move emissions out of Scope 1 and make year-on-year comparisons misleading – which may justify re-baselining.

Example: Outsourcing owned transport can move emissions from Scope 1 to Scope 3. If Scope 3 isn’t captured in your baseline, the trend may look like a reduction when nothing changed in the real world.

4) Re-baselining after methodology or data improvements

This is one of the most common situations, especially as Scope 3 coverage improves.

Recalculate the base year when:

Updated calculation methods materially change results
An approach is corrected after being incomplete or inconsistent, resulting in a material difference to your footprint
You shift from a highly limited, partial footprint, to a comprehensive full-scope GHG inventory
You redefine the organisational boundary used as the basis of measurement

The decision comes down to comparability: is the change significant enough that your base year and current year are no longer like-for-like. According to the GHG Protocol Corporate Standard:

“Whether base year emissions are recalculated depends on the significance of the changes.”

Important nuance: Remember, if the change reflects a real operational improvement (e.g., lower-carbon fuels or technology), that should show up as progress – not as a recalculated baseline.

Starting with Partial Measurement (and Why It Triggers the Baseline Question)

Many organisations start with:

Scopes 1 and 2, then expand into Scope 3 over time, or
rough Scope 3 estimates, then upgrade to better data

This is normal, and removes the barriers to getting started – but it can create the need to re-baseline.

If your baseline year included only part of today’s inventory, it may no longer be a like-for-like comparison.

Practical options:

Option A: Recalculate the baseline as Scope 3 coverage improves (where feasible). This keeps trends and targets comparable.
Option B: Keep the baseline, but clearly label boundary/method changes. Clear disclosure is the priority.

When You Should Not Re-baseline Your Emissions

Base year shifts or recalculation are not intended for normal business change.

It’s generally not appropriate to re-baseline just because:

The business grew or shrank
Product mix or markets changed
Performance worsened and the trend looks uncomfortable

On the flipside, it’s not a mechanism for hiding real decarbonisation progress. If emissions reduced because of operational changes, that should remain visible in the trend.

How to Re-Baseline Your Carbon Footprint

If adapting your baseline year is the right choice, you have two main options:

Changing your baseline year = choosing a new base year to track progress from.
Base year recalculation = 'back-casting' historic emissions so earlier years are recalculated on a like-for-like basis.

Recalculation is sometimes preferred as it enables you to maintain your historic data as part of target tracking. However, recalculation can be impractical, especially if your baseline is a long time ago, and so shifting baseline year is often preferred in practice.

Using Intensity Metrics to Track Progress as You Grow

Growth can make year-on-year comparisons harder to interpret. If revenue, headcount, output, floor area or delivery miles change significantly, absolute emissions can rise even while the business is becoming more efficient.

That’s where intensity metrics help. They don’t replace your absolute footprint – they add context.

Common intensity metrics (choose what reflects how the organisation operates):

tCO₂e per £ revenue (or per $ revenue)
tCO₂e per employee
tCO₂e per unit produced / per product sold

How to use intensity metrics well:

Report both: absolute emissions (what the atmosphere sees) and intensity (efficiency).
Keep the denominator consistent over time and document any changes (e.g., revenue definition, product boundary).

What intensity metrics are not for:

They’re not a workaround for re-baselining triggers like acquisitions/divestments or major methodology changes.
They shouldn’t be used to imply success if absolute emissions are rising without a clear plan (for example, intensity improving while total emissions keep climbing).

In short: if growth is the reason comparisons feel difficult, intensity metrics often solve the problem without touching the baseline.

A Practical Re-baselining Policy

A short, written policy prevents re-baselining becoming a debate each year. According to the GHG Protocol Corporate Standard:

“Companies shall develop a base year emissions recalculation policy, and clearly articulate the basis and context for any recalculations.”

Include:

1) Your triggers

For example:

structural changes (acquisitions/divestments)
boundary changes
major method/data updates
significant error corrections

2) How you define “significant”

Many organisations set a qualitative or quantitative threshold. The key is consistency.

3) How you handle cumulative changes

Several small updates can add up. Decide whether cumulative impact can trigger recalculation.

4) How you handle Scope 3 maturity

Document how you will treat:

New categories being added
Shifts from spend to activity or supplier-specific data

How to Communicate Re-baselining Clearly

If you change your baseline year, the aim is to make it easy for readers to interpret progress from that point onwards – and to avoid accidental “like-for-like” comparisons with earlier years.

Include:

What changed (structure, boundary, method, scope coverage, or an error correction)
Why it changed (to reflect current operations and improve consistency going forward)
What the new baseline year is (and what scopes/categories it covers)
What it means for trend interpretation (results before and after the new baseline year aren’t directly comparable)

Disclosure template (adapt as needed):
“We have updated our baseline year to [new base year] following [reason], in order to ensure our data is accurate and comparable year-on. This baseline reflects [what’s included – scopes/boundary] under our updated approach. Results reported prior to [new base year] aren’t directly comparable due to changes in [method/boundary/scope coverage].”

Re-baselining Examples: Quick Scenarios

Covid year baseline (WFH/commuting/travel changed): choose a new baseline year (or a multi-year average baseline) that better reflects normal operations, and explain why.
Big step-change in Scope 3 coverage: if you move from a partial footprint to a stable, full-scope inventory, set a new baseline year and clearly define what’s now included.
Major method update (new factors, better supplier data): if the change materially affects results, set a new baseline year under the updated approach and document the shift.
Renewable electricity switch / real reductions: don’t change the baseline year to remove real-world progress from the trend.

Summary: Re-baselining Done Well Supports Credible Reporting

Re-baselining is a way to keep emissions reporting comparable as businesses and measurement approaches change.

A practical approach is:

Choose a representative baseline year in the first place
Document a clear recalculation policy, and apply it consistently
Communicate changes clearly, and with detail

Need support sense-checking a change? The Seedling team can help you to interpret GHG Protocol guidance, assess significance of changes, and keep reporting consistent as Scope 3 matures.

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