Emissions base year

The emissions base year is the reference year against which an organisation compares its greenhouse gas emissions to measure progress over time. It is a central element of carbon accounting: without a fixed, well-documented starting point, it is impossible to know whether emissions are rising or falling, or to demonstrate that a reduction target has been met.

The base year sets the emissions level against which all targets are expressed. When a company announces, for instance, a reduction of a given percentage, that percentage is meaningless unless it specifies the year it is measured against.

How it is chosen

Choosing the base year is not arbitrary. Reference frameworks such as the GHG Protocol (Greenhouse Gas Protocol) recommend selecting a year for which representative, verifiable and complete emissions data are available. It is best to avoid atypical years (for example, with abnormally high or low activity) and to choose a period recent enough for the information to be reliable and comparable with the organisation's current reality.

The base year applies to the whole emissions inventory and should cover, as far as possible, all relevant scopes so that the comparison is consistent.

Recalculating the base year

Organisations change over time, and those changes can distort the comparison with the base year. Frameworks therefore allow for base-year recalculation when significant changes occur, including:

• Structural changes, such as acquisitions, mergers or divestments, that alter the organisation's boundary.
• Methodology changes or emission-factor updates that materially affect the results.
• Corrections of significant errors found in the data.

The GHG Protocol does not impose a single numeric threshold: each company must define and disclose its own significance threshold and apply it consistently. The key is not the percentage chosen, but its disciplined and transparent application, which is precisely what verifiers check.

The basis for targets and reporting

The base year is the starting point for reduction targets. Initiatives that set science-aligned goals, such as science-based targets (SBTs), require declaring a base year and calculating the reduction pathway from it. It is also the reference that appears in sustainability reports and that allows progress to be tracked against the organisation's carbon budget.

In short, a well-defined base year is what turns a carbon footprint from a one-off snapshot into a time series that is useful for managing decarbonisation.

Set your base year with Manglai

Choosing and documenting the base year correctly avoids problems later with targets and verification. Manglai helps you establish your base year, keep methodological consistency and recalculate it when your organisation changes. Discover how Manglai can help you measure your progress with rigour.