Sustainability Assurance Practice Diversity and Idiosyncratic Risk in Carbon‐Intensive Firms: A Textual Analysis Approach

Sustainability assurance practice remains highly fragmented, resulting in considerable variation in assurance outcomes. Using a dataset of 1465 assurance reports (2018–2022), this study investigates the relationship between the level of assurance professionalism reflected in assurance reports and idiosyncratic risk in carbon‐intensive firms. Assurance professionalism is defined as the presence in assurance reports of explicit evaluations of qualitative dimensions of sustainability disclosures, including balance, materiality, completeness, reliability and accuracy. Our findings reveal that firms with higher levels of assurance professionalism exhibit lower idiosyncratic risk. In the context of carbon assurance, this effect depends on both the proportion of emissions assured and the level of assurance obtained. Moreover, higher assurance professionalism enhances firms' resilience to risks arising from climate transition shocks. Finally, the effects of assurance professionalism are more pronounced in voluntary assurance settings. This study contributes to ongoing debates on the heterogeneity of assurance practices and the role of assurance mechanisms in fostering accountability.