Limited vs. Reasonable Assurance of your Carbon Footprint

Limited vs reasonable assurance is the key choice a company faces when it wants outside proof that its carbon footprint deserves trust. More and more companies now measure their greenhouse gas (GHG) emissions, and frameworks such as the GHG Protocol and ISO 14064-1 explain how to do it. However, a GHG inventory pulls together large volumes of data, many emission sources, and judgment calls on boundaries and emission factors. As a result, an error at any step can materially change the final number.

That’s why customers, investors, and regulators increasingly expect more than a self-reported figure. In the U.S., for example, California’s SB 253 will require limited assurance of Scope 1 and 2 emissions in reports filed from 2027 onward. Choosing between limited and reasonable assurance is therefore a practical business decision, not a technical footnote.

Verification and assurance: two names for the same engagement

Many articles present “verification” and “assurance” as two separate steps. In reality, both terms describe the same thing: an independent third party evaluates a GHG statement and issues a conclusion on it. The difference lies only in vocabulary, because each term comes from a different professional tradition.

Two vocabularies, one engagement

• Verification comes from ISO 14064-3:2019, the standard most environmental verification bodies follow. At the end, the verifier issues a verification statement.
• Assurance comes from the accounting profession. Practitioners follow ISAE 3410, the international standard for GHG statements, as well as ISAE 3000 and the IAASB’s new ISSA 5000 for sustainability information. In the U.S., CPA firms apply AICPA attestation standards: an examination provides reasonable assurance, while a review provides limited assurance. In this case, the practitioner issues an assurance report.

Either way, both frameworks offer the same two levels: limited and reasonable.

What the provider checks

The engagement looks back at a completed inventory for a past period. First, the provider checks that the organizational and operational boundaries make sense. Next, it confirms that evidence supports the activity data and that the emission factors fit the period and the operations. Finally, it tests whether the calculations are correct.

Why it’s worth it

Beyond the conclusion itself, a good engagement strengthens internal controls and sharpens how the company sets its boundaries. In addition, it improves the inventory’s relevance, completeness, consistency, transparency, and accuracy, the five GHG Protocol principles. In other words, assurance works as a quality-control exercise, not as an adversarial one.

Independence comes first

The provider can’t be the same firm that prepared the inventory. Instead, U.S. companies usually hire a verification body accredited to ISO 14065 (for example, by ANAB) or a licensed CPA firm.

Limited vs. reasonable assurance

The real difference between the two levels lies in the conclusion the provider can sign and, consequently, in the evidence it needs to support that conclusion.

	Limited assurance	Reasonable assurance
Conclusion	Negative form: the provider states that nothing came to its attention suggesting a material misstatement	Positive form: the provider states that, in its opinion, the GHG statement presents emissions fairly in all material respects
Main procedures	Inquiry, analytical review, review of procedures and data management, and recalculation on a sample	Detailed testing of data and calculations, plus tests that confirm internal controls actually work
Sample	Smaller, focused on the sources that drive the result	Larger and more granular, by facility and source
Site visits	Sometimes, depending on risk	Usually, including checks on meters and measurement equipment
Engagement risk	Higher: a material error is more likely to slip through	Lower, but never zero
Effort and cost	Lower	Significantly higher, because of the volume of testing
Typical use	Most voluntary inventories, the CSRD, and SB 253 from 2027	SB 253 Scope 1 and 2 from 2030, and leading companies that want a stronger signal

Limited assurance

Limited assurance is the most common level. First, the provider reviews the inventory procedures and checks that they follow the chosen methodology. It also evaluates data controls and examines supporting evidence, such as invoices, meter readings, fuel records, and spreadsheets, on a sample basis. Finally, it reperforms selected calculations.

The provider samples based on risk. For example, it concentrates on the sources that weigh most in the total and on unusual changes from prior periods. In addition, it often interviews data owners to understand those changes. For most companies today, limited assurance offers enough credibility.

Reasonable assurance

Reasonable assurance is the highest level available in practice. It resembles a financial statement audit, although it still doesn’t offer absolute certainty. On top of the limited-assurance procedures, it typically includes:

• Tests that confirm internal controls over GHG data work effectively
• A larger and more disaggregated data sample
• Site visits and a review of measurement instruments and calibration records

As a result, reasonable assurance costs more, mainly because of the volume of testing and evidence, not because of travel. It also demands a more mature data system: complete data trails, documented controls, and a clear owner for every data source.

Materiality and misstatements

Both levels of assurance rely on a materiality threshold. This threshold sets the size of error, alone or combined with others, that would change a reader’s view of the inventory. ISO 14064-3, however, doesn’t fix a single number. Instead, the provider and the client agree on it, or the applicable program sets it. In practice, 5% of total reported emissions is a common reference.

When the provider finds errors, the reporting company corrects the inventory, not the provider. The provider then reviews the corrections. If a material misstatement remains, the provider issues a modified conclusion, either qualified or adverse, instead of a clean one.

Base-year recalculation is a separate issue

Don’t confuse this process with base-year recalculation. Under the GHG Protocol, a company recalculates its base year when structural changes, methodology changes, or newly found errors exceed its own significance threshold. Each company defines that threshold in its recalculation policy.

Uncertainty doesn’t disappear

GHG inventories also carry inherent uncertainty, because companies estimate most emissions from activity data and emission factors rather than measuring them directly. Therefore, assurance doesn’t remove that uncertainty. Rather, it confirms that the inventory contains no material misstatement under the chosen methodology.

What is required today

Requirement	Assurance level	Timing
California SB 253, Scope 1 and 2	No assurance needed for the first cycle	Reports due November 10, 2026
California SB 253, Scope 1 and 2	Limited	Reports filed from 2027
California SB 253, Scope 1 and 2	Reasonable	Reports filed from 2030
California SB 253, Scope 3	Limited	Reports filed from 2030
EU CSRD (in-scope companies)	Limited, permanently	From the first report; the EU must adopt limited assurance standards by July 2027
SBTi Corporate Net-Zero Standard V2.0	Third-party assurance of inventory data for Category A companies	Mandatory for all companies from January 1, 2028

Two points stand out. First, CARB will accept 2026 SB 253 reports with or without assurance. Even so, companies that obtain it can submit it, and many treat this cycle as a dry run for 2027. Second, the EU’s Omnibus I removed the CSRD’s planned move to reasonable assurance. Consequently, limited assurance remains the permanent EU requirement.

In short, limited assurance is becoming the market baseline. Meanwhile, leading companies, and SB 253 from 2030, will push Scope 1 and 2 toward reasonable assurance.

Vim Aeris recommendation

For most companies, Vim Aeris recommends starting with limited assurance from an independent, accredited provider. It gives customers and investors a credible signal, it meets what the CSRD and SB 253 (from 2027) require, and the process itself improves the inventory.

However, companies that expect to face reasonable assurance, such as SB 253 reporters ahead of 2030, should plan ahead. They can use their limited assurance years to build what reasonable assurance will test: documented controls, complete data trails, and calibrated measurement equipment. In other words, moving to reasonable assurance depends less on hiring a different provider and more on building a data system that can withstand detailed testing.

Vim Aeris prepares the inventory and supports the company throughout the assurance process. Nevertheless, an independent third party must carry out the assurance itself.