By Susan Lewis
Key Takeaways
- The most important aspect of your GHG emissions calculations is their defensibility. Your organization needs to demonstrate the methodologies, record-keeping, and assumptions that underpin the data.
- Evolving regulatory requirements add a sense of urgency to the need for independent assurance.
- After-the-fact documentation can be much more time-consuming, challenging, and expensive to gather after an inventory is already in the process of being developed.
- Impartial review is critically important—a consultancy that prepares an organization’s GHG inventory cannot also provide assurance of their own work.
If your company has been reporting greenhouse gas (GHG) emissions for years but has never had the inventory independently assured, the first assurance engagement can feel like a major hurdle.
Perhaps surprisingly, the emissions calculations themselves usually aren't the hard part. The real challenge comes down to documentation and traceability—being able to demonstrate where the data came from, why certain methodologies and assumptions were used, and whether the record keeping behind the inventory can stand up to independent review.
A GHG assurance engagement benefits from practitioners who deeply understand GHG accounting—not just the auditing process. Utility bills, fuel records, refrigerant logs, renewable electricity purchases, changing facility boundaries, emission factors, estimates: these are the complexities and details behind a corporate GHG inventory. An advisory team that works with that kind of data every day knows where gaps are likely to be. For many companies, there are multiple reasons to start looking for those gaps now.
1. The Data You Are Collecting Today May Be the Data That Gets Assured Tomorrow
California SB 253 is an obvious driver. Companies will be required to obtain limited assurance over FY26 Scope 1 and Scope 2 emissions for 2027 reporting, making independent GHG assurance a brand-new requirement for many organizations.
The Corporate Sustainability Reporting Directive (CSRD) is another driver for companies within its evolving scope, while other organizations are pursuing assurance in response to customer, investor, or voluntary reporting expectations.
Whatever the reason, the point is the same: Don't wait until the assurance engagement starts to determine whether the underlying data are ready for it.
For companies preparing for California SB 253 in particular, the first assurance cycle will involve emissions data being collected before the assurance engagement itself. Waiting until later can mean trying to reconstruct utility records, track down fuel data, explain an old estimate, or remember why a particular emission factor was selected months after the fact.
That is harder, more time-consuming, and potentially more expensive than getting the documentation right while the inventory is being developed.
2. Assurance Is More Than Checking Calculations
A limited assurance engagement is a risk-based evaluation designed to determine whether reported emissions are free from material misstatement. It isn't a review of every invoice or every individual data point. Instead, the assurance provider will look at where material errors are most likely to occur and will gather evidence to support a conclusion about the reported emissions.
That may include evaluating whether:
- Organizational and operational boundaries are clearly defined and consistently applied
- Activity data are complete, traceable, and supported by appropriate records
- Emission factors and calculation methodologies are appropriate and consistently applied
- Estimates and assumptions are reasonable and documented
- Sufficient evidence exists to support reported emissions
This is where first-time assurance can reveal issues that weren't necessarily visible when the goal was only to complete the annual inventory. It’s very common for a calculation to be perfectly reasonable but lack the documentation behind it.
Consider some real-world examples: Perhaps operational control of a facility changed during the reporting year, and the treatment of that facility was never formally documented. Maybe refrigerant data are incomplete. Maybe someone used an estimate for a few months of energy consumption because a utility bill wasn't available but only jotted it down on a sticky note. Or maybe the person who built the inventory knows exactly how everything works—but that knowledge is available only through that person, not documented in an Inventory Management Plan. None of those situations means the inventory is incorrect, but they can make assurance much harder in ways that are avoidable.
3. Good Preparation Makes First-Time Assurance Much Easier
The focus shouldn't be to create a flawless inventory before assurance. That isn't realistic, and it isn't even the purpose of assurance.
The goal is to make the inventory defensible.
Can you explain your boundaries? Can you trace material data back to source documentation? Can you explain your assumptions and estimates? Are your methodologies documented? If someone unfamiliar with the inventory reviewed the files six months from now, could they understand how you got from the source data to the reported emissions?
Those are the questions to start asking.
For organizations that aren't confident in the answers, an assurance readiness review can be useful before formal assurance begins. A readiness engagement looks at the inventory through an assurance lens—identifying documentation gaps, methodology issues, data-quality concerns, and other areas likely to draw attention during independent review.
Importantly, independence and impartiality matter. A consultancy that prepares a company's GHG inventory cannot provide independent assurance over their own work. The consultant can, however, help that organization prepare for assurance by another provider. If the inventory was developed internally, through software, or by another advisor, the consultant may be able to provide the independent limited assurance engagement.
Prepare Now for a Smoother Assurance Engagement
First-time GHG assurance does not need to be overwhelming. Spend the time now to look at the data trail—or simply be sure you’ve got one. Document the decisions. Make sure assumptions and estimates can be explained. Identify areas where the data are messy—and address what you reasonably can.
Across corporate GHG inventories, one thing has remained remarkably consistent: Good GHG accounting is much more than getting the calculation right; it's about being able to explain and support how you got there.
Questions about how to prepare for your first GHG assurance engagement? Contact Antea Group’s GHG Assurance Readiness team to discuss the process and how we can support your organization.
