“Man is the measure of all things.” — Protagoras, 5th century BCE
The ancient Greek philosopher Protagoras argued that human judgement shapes how we understand the world. Some 2,500 years later, his observation has particular relevance to sustainability reporting, where determining what matters, how it should be measured and what should be disclosed increasingly depends on the exercise of informed judgement.
Sustainability reporting is entering another period of change. The EU has simplified its European Sustainability Reporting Standards (ESRS), the UK has introduced new sustainability disclosure requirements, and a dedicated international sustainability assurance standard, ISSA 5000, is about to take effect. These developments present an interesting challenge. While reporting requirements are being simplified in some areas, companies are being given greater responsibility for determining what information is material and how best to communicate it. This places more emphasis on management judgement, and on the evidence needed to demonstrate that those judgements are reasonable.
For companies preparing for independent assurance, the implications are significant. It is no longer sufficient simply to demonstrate that reported numbers are accurate. Companies must also be able to explain why particular information has been included or omitted, how assumptions have been developed, and whether their disclosures provide a balanced and credible account of sustainability performance. Simpler reporting standards may therefore mean fewer disclosures, but potentially more challenging questions from assurance providers.
The EU's revised ESRS, published in September 2026 and applicable from financial years beginning in January 2027, substantially reduce the number of required datapoints and simplify aspects of the reporting framework. This should help companies focus on the information that matters most, rather than treating sustainability reporting as an exhaustive compliance exercise. Major businesses have broadly welcomed efforts to reduce unnecessary complexity, while emphasising the importance of preserving meaningful sustainability information. Carine de Boissezon, Chief Impact Officer at EDF, captured this balance when commenting on the EU's Omnibus reforms:
“Where there is room for smart simplification, let's tweak the regulation, but we need to stay the course.”
This distinction matters for assurance. Reducing duplication and unnecessary disclosures is sensible, but it does not remove the need for companies to make defensible judgements about materiality, reporting boundaries and the information required to provide a balanced account of sustainability performance. The revised ESRS place greater emphasis on information materiality, allowing companies to focus on disclosures that are relevant to understanding their material impacts, risks and opportunities. However, this also means being able to demonstrate why particular information is considered relevant, or why other information can legitimately be omitted.
For companies preparing for assurance, this creates several practical questions. Is the materiality assessment sufficiently documented? Have relevant business activities, operations and value-chain impacts been considered? Are decisions about disclosure consistent with the evidence available and with the company's broader risk management processes?
These questions become particularly important when companies move from identifying material sustainability topics to determining the specific disclosures needed to explain them. A materiality assessment may identify climate change adaptation, for example, but the reporting team must still establish which policies, actions, targets and metrics are relevant, what information already exists and how gaps will be addressed. The revised standards provide greater flexibility, but companies will need to demonstrate that they have applied that flexibility appropriately.
Climate-related disclosures remain one of the most challenging areas of sustainability assurance, particularly where reporting extends beyond historical emissions data. Many companies now disclose climate risks, scenario analysis, transition plans and anticipated financial implications. These disclosures often depend on assumptions about future market conditions, energy prices, regulation, technology and the physical impacts of climate change.
Unlike energy consumption or fuel use, these cannot be verified simply by tracing reported figures back to invoices or meter readings. Assurance requires consideration of the methodologies used, the reasonableness of assumptions, the evidence supporting management's conclusions and the consistency of disclosures with business strategy. Climate change adaptation is a good example. Companies may have identified physical climate risks through scenario analysis, but have they translated those findings into operational responses? Can they demonstrate how risks are assessed at individual sites, who is responsible for adaptation measures, and whether the resulting actions are reflected in investment and business planning? Similarly, a transition plan may contain ambitious emissions reduction targets, but assurance will focus attention on the credibility of the underlying assumptions, the consistency of the plan with capital investment and the transparency of dependencies and uncertainties. This does not mean that assurance providers are expected to validate predictions about the future. Rather, companies need to demonstrate that forward-looking disclosures have been prepared using reasonable methodologies, supportable assumptions and appropriate governance processes, with uncertainties clearly explained.
Companies should expect more questions about the connections between their sustainability disclosures and the decisions being made within the business.
The UK's reporting landscape is also changing. In September 2026, the Financial Conduct Authority finalised new rules requiring in-scope listed companies to report against the UK Sustainability Reporting Standards (UK SRS) on a comply-or-explain basis, beginning with financial years starting in January 2027. Although transitional reliefs are available, companies will need to assess their existing reporting against the new requirements and determine where additional information, processes or controls may be needed. The comply-or-explain approach provides flexibility, but explanations need to be credible. Companies should be able to demonstrate that they have properly assessed the relevant requirements, identified information gaps and considered what would be needed to address them.
The new rules also require companies to disclose details of any voluntary third-party sustainability assurance they obtain, including its scope, level and applicable standards. This creates an additional opportunity for companies to consider whether their existing assurance arrangements provide meaningful coverage of their most important sustainability disclosures. For businesses reporting under multiple frameworks, there is also the challenge of maintaining consistency between disclosures prepared for different purposes. Differences in reporting boundaries, materiality assessments, emissions methodologies or climate-related assumptions may be justifiable, but they need to be understood and explained.
One of the most common challenges encountered during sustainability assurance is the gap between the information presented in a report and the processes used to produce it. Companies may have well-established reporting procedures for greenhouse gas emissions, but less mature arrangements for workforce information, supply-chain indicators, climate risk assessments or other qualitative disclosures. Information may be collected through spreadsheets, local reporting systems and manual processes, with varying levels of review and documentation. These approaches are not necessarily unsuitable. However, companies need to demonstrate that responsibilities are clear, methodologies are consistently applied, source information can be traced, and appropriate checks have been performed.
Lena Julle, Chief Sustainability Officer at Inter IKEA Group, recently observed that, as sustainability reporting expectations evolve, companies “may also choose to say less until they can demonstrate more.”
Her observation highlights the growing importance of substantiation. A shorter sustainability report, supported by reliable information and transparent methodologies, may provide greater confidence than a more extensive report containing disclosures that cannot be adequately supported. However, companies should not confuse a more focused reporting approach with simply omitting difficult information. Where disclosures are required, or information is necessary to explain material impacts, risks and opportunities, the challenge is to develop the evidence and processes needed to report it credibly.
The introduction of ISSA 5000 reinforces the importance of structured, risk-based assurance. The standard, effective for periods beginning on or after 15 December 2026, provides a dedicated international framework for sustainability assurance engagements. For reporting organisations, the implications extend beyond the assurance provider's methodology. Companies will benefit from clearer documentation of their reporting boundaries, data sources, estimates, internal reviews and management judgements. In our experience, relatively straightforward improvements to these processes can make a significant difference to both the efficiency of assurance and the quality of the information available to management.
The companies best placed to respond to these developments will be those that review assurance readiness before their sustainability reports are substantially complete. A practical starting point is to map the relevant reporting requirements against existing disclosures, identify material information that is not yet available, and assess the strength of the evidence supporting what is already reported. This exercise should distinguish between three different types of gap: information that has not been collected, information that exists but is not sufficiently reliable or documented, and disclosures that depend on management judgements or assumptions requiring further substantiation.
For companies applying revised reporting standards, particular attention should be given to the decisions made during the materiality assessment and subsequent determination of reportable information. A clear audit trail explaining the rationale for these decisions can be as important as the underlying quantitative data. Early engagement with assurance providers can help identify areas where reporting processes may need strengthening, clarify evidence expectations and avoid difficult questions emerging too late in the reporting cycle. It also provides an opportunity to review the scope of assurance and ensure that it remains relevant as reporting requirements evolve. Assurance readiness does not necessarily require sophisticated new systems or extensive additional documentation. Often, the most valuable improvements involve clearer responsibilities, consistent methodologies, better evidence retention and more systematic review of the information already being collected.
The latest regulatory developments create uncertainty, but they also offer an opportunity to improve how sustainability information is managed. For some companies, simplified requirements will reduce the reporting burden. For others, new disclosure expectations will expose gaps in data, governance or the connections between sustainability reporting and business strategy. In either case, independent assurance can play a constructive role. A well-designed assurance process provides confidence in reported information while helping companies understand where methodologies, controls and reporting practices can be improved.
At Challenge Sustainability, we work with companies across a range of sectors to provide independent sustainability assurance and practical insights into reporting quality. Our approach combines technical rigour with an understanding of the operational challenges involved in collecting, managing and communicating sustainability information. As companies prepare for their next reporting cycle, the priority should be to ensure that disclosures are supported by reliable information, transparent judgements and processes capable of standing up to scrutiny.
Protagoras may have been right that human judgement shapes our understanding of the world. In sustainability reporting, the challenge is to ensure that those judgements are transparent, well-founded and supported by evidence. These are the foundations of credible reporting, regardless of how the standards continue to evolve.
European Commission, Commission Delegated Regulation (EU) 2026/1563, September 2026.
Financial Conduct Authority, Sustainability reporting requirements, updated September 2026.
International Auditing and Assurance Standards Board, ISSA 5000 – General Requirements for Sustainability Assurance Engagements.
Cambridge Institute for Sustainability Leadership, Business and investor groups issue joint statement on EU Omnibus initiative, including comments from EDF's Carine de Boissezon.
Inter IKEA Group, Navigating change, staying focused, Lena Julle, 24 September 2026.
Plato, Theaetetus, 152a, recording Protagoras's statement that man is the measure of all things.