Base Year
A base year is a specific historical period against which an organisation's current greenhouse gas emissions are compared to measure progress towards reduction targets. It serves as the benchmark for tracking decarbonisation performance over time.
What is Base Year?
The base year is the starting point against which all future emission reductions are measured. When an organisation sets a carbon reduction target — for example, "reduce Scope 1 and 2 emissions by 50% by 2030" — the percentage reduction is calculated relative to the base year emissions figure. Selecting and managing the base year correctly is fundamental to credible target-setting and progress tracking.
The GHG Protocol Corporate Standard requires organisations to select a base year and recalculate it under certain circumstances to maintain consistency and comparability. The base year should be the earliest year for which reliable, verifiable data is available. For many UK organisations beginning carbon reporting under SECR (which started in April 2019), the first reporting year often serves as the base year.
Base year recalculation is triggered by significant structural changes to the organisation — such as mergers, acquisitions, divestments, or major changes in calculation methodology. If a company acquires a new subsidiary, its base year emissions should be recalculated as if that subsidiary had been part of the organisation in the base year, ensuring that reported reductions reflect genuine decarbonisation rather than structural changes. The GHG Protocol recommends defining a base year recalculation policy that specifies a significance threshold (e.g., recalculate if the structural change affects more than 5% of base year emissions).
For Science Based Targets (SBTi), the base year must be no earlier than 2015 and should be a representative year — not one distorted by unusual events (e.g., the COVID-19 pandemic reduced many organisations' emissions in 2020, making it an unrepresentative base year). The SBTi also requires that base year emissions cover all material Scope 1, 2, and 3 sources.
Some organisations use a fixed base year (a single year) while others use a rolling base year (e.g., a 3-year average) to smooth out annual fluctuations. The GHG Protocol supports both approaches but requires the choice to be disclosed.
Practical Examples
A manufacturing company selects FY2019/20 as its base year, with emissions of 15,000 tCO₂e. By FY2024/25, emissions have fallen to 10,500 tCO₂e — a 30% reduction against the base year, tracking well against its target of 50% by 2030.
After acquiring a competitor in 2023, a services company recalculates its 2020 base year to include the acquired entity's historical emissions, ensuring that the reduction percentage reflects organic decarbonisation rather than the step-change from the acquisition.
A company applying for SBTi target validation selects 2021 as its base year rather than 2020, because 2020 emissions were artificially depressed by COVID-19 lockdowns and would not represent a credible benchmark.
How Climatise Helps
Climatise tracks your emissions over time and maintains your base year data alongside current figures, automatically calculating reduction percentages. When structural changes occur, the platform supports base year recalculation so your progress figures remain accurate and comparable.
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