Glossary›Emissions accounting and measurement›Base year / baseline
Glossary term
Cluster D · D18
Tier 1
Base year / baseline
Definition
A base year is the reference period against which emissions performance is measured and targets are set. It fixes the starting point of a trend, so every reported reduction is a comparison against it. Choosing it is a documented judgement, and it needs data strong enough to survive re-examination each time a target is reported against.
On this page
In practice
The base year is chosen once and examined every year afterwards, which is the opposite of how it is usually treated.
AASB S2 paragraph 33(e) requires an entity disclosing a climate-related target to disclose the base period from which progress is measured, alongside the metric, the objective, the part of the entity the target applies to, the period it covers, any milestones and interim targets, whether a quantitative target is absolute or intensity-based, and how the latest international agreement on climate change has informed it. So the base year is not an internal planning artefact. It is a disclosed characteristic of the target, and it sits in a list of eight that the assurer will work through as a checklist.
Two selection criteria matter and they pull in opposite directions.
Criterion
What it requires
Data quality
The base year has to be a year you can still evidence. An entity that picks 2019 because it was a good year for the trend, and then cannot produce the source documents for 2019, has created a permanent evidence gap sitting under every future reduction claim. Source documents from five years ago are frequently unrecoverable: suppliers change, portals expire, staff leave.
Representativeness
The base year should reflect normal operations. A year containing a major acquisition, a plant shutdown, a pandemic or an unusual production volume produces a trend that reflects the anomaly rather than performance. A 2020 base year is the standing example in Australia and it flatters almost every travel and commuting figure.
Where the two criteria conflict, data quality wins, because a target with an unevidenced baseline is not assurable at any point in its life.
A practical caution for first-time reporters specifically. There is a strong pull toward making the first reporting year the base year, and it is usually right, because it is the year with the best evidence file. But the first reporting year is also, under AASB S2 Appendix C paragraph C4(b), a year in which Scope 3 may be omitted entirely. A base year with no Scope 3 cannot support a Scope 3 target. If Scope 3 targets are anywhere on the horizon, the base year for those targets has to be a year in which Scope 3 was actually measured.
There is no requirement to have a target at all. AASB S2 requires disclosure about targets an entity has set and any it is required to meet by law or regulation. An entity with no target discloses no target characteristics, and that is a complete answer. The base year question only arises once a target exists.
What the assurer does with it
Because base year disclosures sit inside the metrics and targets pillar, they come into full assurance scope in an entity’s second reporting year under ASSA 5010 paragraph 10(b), not the first. That gives entities one year of grace on the evidence file and most of them do not use it.
When the assurer does reach it, the test is the same as for any current-period figure. They ask for the base year inventory, the source documents behind it, the boundary and method applied, and the reconciliation to the current-year boundary and method. A base year calculated on a different boundary or a different factor basis from the current year, with no bridge between them, makes the disclosed reduction unverifiable.
They accept a base year with a retained evidence file, a stated boundary and method, and a documented reason for the selection. They reject a base year with no retained working papers, a base year whose boundary differs from the current inventory with no reconciliation, and a base year quietly changed between reporting periods, which is treated as a restatement requiring disclosure, not a correction.
The question that catches entities off guard is the selection rationale: why this year. “It is the first year we measured” is a good answer. “It gives the best trend” is an answer nobody should give out loud, and if the file shows several candidate years were modelled before one was chosen, expect the file to be asked for.
Commonly confused with
Comparative information, which is the immediately preceding reporting period restated onto the current basis, and which carries its own first-year relief. The base year is fixed and can be years back; the comparative rolls forward every year. Also confused with base year in a financial reporting sense, where the concept barely exists: this is a climate-specific construct and financially literate readers frequently import an intuition that does not apply.
Sources
1
2
ASSA 5010 Timeline for Audits and Reviews of Information in Sustainability Reports under the Corporations Act 2001
AUASB
Review status
Review required
Last reviewed
15 September 2026
Editorial pass, unsigned
Reviewer required
Carbon accounting specialist and registered company auditor
Next scheduled review
1 July 2027
Part of
Cluster D, Emissions accounting and measurement
49 terms from the head term carbon accounting down to individual Scope 3 categories and the mechanics of factors, boundaries and data quality. The largest cluster in the glossary.
Related terms
The written rule governing when the base year is restated
The other target type paragraph 33 asks an entity to classify
The act the policy governs
Related questions
What happens if we have to restate last year’s emissions?
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You restate the comparative and disclose what changed, why, and the effect. Restatement is expected as data improves and is not treated as a failure. What the assurance practitioner tests is whether you have a written policy setting out when you restate, and whether you applied it consistently.
Do we need comparatives in our first report?
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No. AASB S2 relieves first-time reporters from disclosing comparative information in their first annual reporting period, and comparatives are required from year two. That means your year-one numbers and working papers will be looked at again, so build the year-one file as if it will be re-examined.
Other terms in this cluster
Base year / baseline