Which Year Should Be Your Base Year for Climate Reporting?

One of the earliest decisions in climate reporting is which year to treat as your base year, and it tends to get tangled up with a separate question about timing. Companies often assume the base year has to be the first year they are required to report, but these are two different choices that can be made independently. The base year is the reference point you measure progress against over time, so it matters most once you have set targets, while the reporting deadline itself is fixed by which ASRS group you fall into. This guide explains what a base year is, how the ASRS timing interacts with it, why base years sometimes have to be recalculated, and how to choose one you can stand behind for several years.

What a base year actually is

A base year is the reference point you measure change against. It's the year a target is set from (a reduction of some percentage by 2030 from a named base year), and the year later inventories are compared to so you can tell whether emissions are rising or falling. The concept comes from the GHG Protocol Corporate Standard, and it's fundamentally about tracking a trend over time. That's the key distinction: the base year is a methodology choice about comparison, and it's separate from the question of which single year the law first makes you disclose.

What ASRS requires, and what it leaves to you

Under ASRS, you report your emissions for the current reporting period, and comparative information isn't required in your first year. So AASB S2 doesn't hand you a base year. A base year becomes meaningful once you set targets and want to show progress against them, and that's a step most first-time reporters take after they have a solid inventory, not before. The upshot is that you get to choose, and you should choose for the reason base years exist: a clean, representative reference you can stand behind for years.

The real decision: dry run, or first report

For most companies the choice comes down to two paths, and the deciding factor is how much time and data you have before your deadline.

Why base years get recalculated (and how to avoid it)

It's common to see a company publish, then a year later quietly restate its earlier baseline because it found gaps or the business changed. The GHG Protocol expects this: when a structural change (an acquisition, a divestment, a shift in how something is calculated) is significant enough, you recalculate the base year so the comparison stays like-for-like. Two things reduce how often you have to. Set a recalculation policy up front, with a significance threshold, so it's a rule rather than a debate each year. And pick a base year that was normal. A year with an unusual spike or dip, or one where half your data was estimated, makes a weak reference point you'll want to replace.

How to choose

Favour the year where your data is strongest and your operations were representative. If your first mandatory reporting year has good data and no unusual events, making it the base year is the simplest path and avoids carrying a rehearsal year forever. If you want the practice and you have the time, a dry run earns you a smoother first report, as long as you know the practice year won't be assured. Either way, document why you chose it and set the recalculation policy at the same time. Deciding spend-based versus activity-based for each source is part of the same setup: the base year is only as good as the data underneath it.