Reasonable Assurance

Reasonable assurance is the higher level of independent verification, in which an assurer gathers enough evidence to give a positive opinion that the reported information is fairly stated in all material respects. It gives a high level of confidence, though no engagement can guarantee that every error is found.

To reach that opinion, the assurer assesses the risk of misstatement in each disclosure and examines the internal controls behind the data. The assurer also tests a sample of the underlying records against supporting evidence such as invoices and meter readings. This depth of testing makes the engagement longer and more costly than limited assurance, and places greater demands on how well the data is documented.

Few emissions reporting requirements call for reasonable assurance. The Corporate Sustainability Reporting Directive (CSRD) once planned a move to this level, but the Omnibus I simplification package removed that plan, leaving limited assurance as the only required level. California’s Senate Bill 253 phases in assurance, with reasonable assurance of Scope 1 and 2 emissions required from 2030. Engagements follow standards such as ISSA 5000, the International Standard on Sustainability Assurance.

For example, a food manufacturer might choose reasonable assurance for its Scope 1 and 2 figures. The assurer would test the fuel and electricity records behind those figures and review the controls governing how the data is collected and approved. The outcome is a formal opinion on whether the figures are fairly stated.

That opinion is the strongest external confirmation of reported emissions, and preparing for it comes down to control over the data: clear ownership, documented processes and an evidence trail from each reported figure to its source record. A closer comparison with limited assurance is covered in limited vs reasonable assurance.