What are base year emissions?

Quick Answer: Base year emissions are the greenhouse gas emissions recorded in a specific reference year, used as a fixed benchmark to measure progress against reduction targets over time. Companies set a base year when they begin carbon accounting, and all future emissions data is compared against it to determine whether reductions are real and meaningful.

What Are Base Year Emissions?

Base year emissions represent a company's total greenhouse gas (GHG) output in a chosen reference year, expressed in tonnes of CO2 equivalent (tCO2e). This figure becomes the fixed starting point against which all future emissions performance is measured.

Without a base year, there is no meaningful way to track progress. A company might report lower emissions in year three than year two, but without a stable reference point, it is impossible to know whether that reflects genuine reduction or simply a change in business activity.

The base year is typically the first year a company completes a full carbon footprint, though it can be set retrospectively if reliable data exists for an earlier period.

How Base Year Emissions Work in Practice

Once a base year is set, a company calculates its emissions for subsequent years using the same methodology and scope. The difference between the base year figure and the current year figure shows the direction and scale of change.

For this comparison to be valid, the methodology needs to stay consistent. Switching emissions factors, changing scope boundaries, or altering how certain categories are calculated can make year-on-year comparisons unreliable. This is why a clear record of assumptions, data sources, and calculation methods matters from the start.

Most carbon accounting frameworks, including the GHG Protocol, require companies to document their base year methodology and apply it consistently going forward.

When Should a Base Year Be Recalculated?

Base year emissions are not always fixed permanently. Certain significant changes to a business can make the original base year unrepresentative of current operations, and in those cases a base year recalculation is required.

Common triggers for recalculation include:

The GHG Protocol sets out specific recalculation thresholds and policies. The principle is that the base year should always reflect the company as it currently operates, so that comparisons remain meaningful.

Why Base Year Emissions Matter for Target Setting

Base year emissions are the foundation of any credible reduction target. Science-based targets (SBTi), net zero commitments, and supplier or stakeholder reporting requirements all ask companies to demonstrate how much they have reduced emissions relative to a specific starting point.

A target such as "reduce Scope 1 and 2 emissions by 50% by 2030" only means something if the base year figure is accurate, well-documented, and calculated to the same standard as future years.

Data quality in the base year has a long-term effect. A base year built on rough estimates or incomplete scope coverage creates a weak foundation. Targets set against it may be harder to defend, and progress reported against them may not hold up to scrutiny from customers, investors, or auditors.

Seedling builds the base year to GHG Protocol standards from the outset, with clear documentation of data sources, emissions factors, and assumptions. That means when a company sets SBTi-aligned targets or reports to stakeholders, the base year figure is one they can stand behind.

Scope Coverage in the Base Year

A base year footprint should cover all three scopes of emissions where material:

Scope 3 is often the most significant category for smaller companies, but it is also the most commonly excluded from base year calculations. Leaving it out produces a base year that understates the real footprint and creates a misleading picture of where reductions are actually needed.

A base year that covers all material scopes gives a company a more accurate target, a more credible reduction story, and better information about where to act first.