Why ISSB S1 and S2 Have Moved From Voluntary to Mandatory Faster Than Boards Expected
When the International Sustainability Standards Board published IFRS S1 and S2 in June 2023, many boards treated them as a voluntary framework that would slowly gain traction over the coming decade. That assumption has aged badly. Within eighteen months, a cluster of major economies had moved to embed the standards directly into national regulation, and the timeline that once looked comfortable now looks tight. For CFOs and sustainability leads who assumed they had years to prepare, the more accurate picture is that the first mandatory reporting periods are either already underway or arriving within the next two financial years.
Singapore led with clarity. The Singapore Exchange and the Accounting and Corporate Regulatory Authority confirmed that listed issuers must report climate disclosures aligned to ISSB standards from financial year 2025, with large non listed companies following from 2027. The United Kingdom has been developing UK Sustainability Reporting Standards based on the ISSB baseline, with endorsement decisions shaping a phased mandatory regime. Australia moved decisively through legislation that introduced mandatory climate reporting from January 2025, staggered across three tiers by company size. Hong Kong has set a roadmap through its exchange requiring ISSB aligned disclosures for Main Board issuers, while Japan's Sustainability Standards Board has finalised standards closely tracking the ISSB and is progressing towards mandatory application for large listed companies. Brazil has arguably been the boldest, committing to make ISSB standards mandatory from 2026 through its securities regulator.
This is not a coincidence of separate national decisions. It reflects a deliberate strategy to build a common global baseline that reduces fragmentation and gives investors comparable data across markets. For a CFO overseeing operations or listings across several of these jurisdictions, the practical consequence is that a single group approach to sustainability data now needs to satisfy multiple regulators operating on overlapping but not identical timelines. Treating each market as a separate compliance project is both expensive and risky. The smarter response is to build one disclosure engine capable of meeting the highest common standard and then flexing to local requirements.
The meaning of a mandatory reporting period is where many finance teams underestimate the effort. A mandatory period does not simply require a report to exist. It requires disclosures that are consistent with financial statements, prepared on the same reporting boundary, published on the same timeline, and governed with the same rigour. Climate related financial disclosure under S2 means quantifying the effect of climate risks and opportunities on the business model, strategy and financial position. That pulls sustainability data directly into the domain of the CFO and the audit committee, and it removes any comfortable distance between the annual report and the sustainability report.
The gap that trips up most organisations is the distance between spreadsheet reporting and assured disclosure. For years, sustainability data lived in disconnected spreadsheets, maintained by a small team, refreshed annually, and rarely subjected to external challenge. That model does not survive contact with assurance requirements. Once an external auditor examines your emissions figures, your data lineage, your estimation methods and your internal controls, the weaknesses in a spreadsheet driven process become obvious. Manual data collection, undocumented assumptions, version control failures and the absence of an audit trail all create findings that undermine the credibility of the disclosure and expose the board to questions it cannot answer.
Assurance changes the standard of evidence. It is no longer enough to produce a number. You must be able to show where the number came from, who calculated it, what methodology and emissions factors applied, and how it was reviewed before publication. Many jurisdictions are phasing in limited assurance first, with reasonable assurance to follow, which means the bar rises over time even for companies that meet the initial requirement. Building for the assurance standard from the outset is far cheaper than retrofitting controls after the first uncomfortable audit.
For CFOs, the practical priorities are clear. Map your reporting obligations across every jurisdiction where you operate or list, and identify your earliest mandatory period. Move sustainability data out of spreadsheets and into systems that capture source records, apply consistent methodologies and maintain an audit trail. Align the reporting boundary and timeline with your financial statements so the two sets of disclosures reconcile. Establish clear ownership and internal controls that an external assurer can test. And engage your audit committee early, because the governance expectations under S1 and S2 sit firmly at board level.
The shift from voluntary to mandatory has happened faster than boards expected because regulators recognised that comparable, decision useful sustainability information matters to capital markets now, not in a distant future. The organisations that will handle this well are the ones that stop treating ISSB reporting as a communications exercise and start treating it as a financial reporting discipline. That mindset shift, backed by proper systems and controls, is what separates a confident first mandatory filing from a scramble that erodes trust with investors and regulators alike.