The biggest Scope 3 risk isn't calculation. It’s completeness.

When it comes to Scope 3 emissions, organisations often focus on methodologies, emission factors and supplier data. However, before asking whether a number is accurate, there is a more fundamental question: is the inventory complete?

A highly sophisticated calculation can still produce a misleading result if significant suppliers, spend categories or value chain activities have been overlooked. Understanding whether a Scope 3 inventory is complete is often more challenging than many organisations expect.

Why completeness is one of the hardest Scope 3 challenges

Completeness is one of the most challenging aspects of Scope 3 reporting because emissions can arise from a vast range of activities across an organisation's value chain. Unlike Scope 1 and Scope 2 emissions, which are often linked to clearly identifiable activity data, such as fuel consumption or energy bills, Scope 3 emissions are connected to suppliers, purchased goods and services, leased assets, travel, freight, waste, contractors, and numerous other activities that may span different functions and systems.

In large or decentralised organisations, relevant data can be spread across procurement, finance, operations, and sustainability teams, making it difficult to identify all material sources of emissions. As a result, the risk of omission is often greater than the risk of applying an inappropriate emission factor. Organisations therefore need a systematic approach to identifying emissions sources, assessing exclusions and demonstrating that all material activities across the value chain have been considered.

Why auditors focus on completeness

For auditors, completeness is often as important as accuracy. They are focused on more than how emissions have been calculated - they also need to understand how organisations identify relevant emission sources, determine exclusions and gather evidence to support the completeness of their Scope 3 inventory. This means looking beyond the final emissions number to the processes, controls and documentation that underpin it. Before confidence can be placed in the accuracy of a reported emissions number, there must first be confidence that the inventory is complete.

Why Scope 3 is becoming a finance and controls challenge

If completeness is the challenge, finance may be part of the solution. While sustainability teams bring expertise in emissions measurement and reporting frameworks, finance teams bring experience in controls, reconciliations and data integrity. Together, they can help organisations move beyond estimating emissions and focus on demonstrating that nothing material has been overlooked.

One of the most common questions organisations face when preparing a Scope 3 inventory is: what evidence do we have that nothing material has been missed? This is where financial records can play an important role. General ledgers, trial balances and supplier spend data often provide the most complete picture of an organisation's activities and can highlight purchases, suppliers or spend categories that may not have been considered during the emissions measurement process.

Controls such as supplier spend reviews, chart of accounts analysis, and transaction reconciliations can help organisations identify potential gaps, challenge assumptions and gain greater confidence that the inventory reflects the full breadth of the organisation's activities. They also create a clearer audit trail, making it easier to explain how the inventory was prepared, why certain exclusions were made and what evidence supports the final emissions numbers.

Once organisations understand where completeness risks exist, technology can help make those risks easier to identify and manage.

How technology can improve Scope 3 completeness

Improving Scope 3 completeness is increasingly becoming a data and controls challenge, rather than a calculation challenge. Technology platforms such as Avarni help organisations strengthen completeness by leveraging financial data, supplier information and transaction-level records to identify potential gaps in emissions inventories. Combined with appropriate governance and finance-led review processes, technology platforms can help create a more complete, traceable and assurance-ready carbon inventory, giving organisations greater confidence in their emissions reporting and the decisions that rely on it.

Organisations can spend considerable time refining calculations and improving data quality. However, accuracy is only valuable if the inventory is complete. Before asking whether a Scope 3 number is right, organisations should first ask whether all material emissions sources have been identified and included.

The future of Scope 3 reporting will not be determined by calculation methodologies alone. It will increasingly depend on the controls, governance processes and financial disciplines that sit behind the numbers. Organisations that treat Scope 3 as both a sustainability and a financial challenge will be better positioned to support assurance, meet reporting obligations, and build confidence in their emissions reporting.

How BDO and Avarni can help

Through a combination of carbon accounting expertise, finance-led controls and technology, we help organisations strengthen Scope 3 completeness, improve assurance readiness and build confidence in their emissions data.