The International Standard on Sustainability Assurance 5000 (ISSA 5000) becomes effective on December 15, 2026. It is a global baseline standard for assurance on sustainability information, including greenhouse gas (GHG) engagements, and it is among the standards California's air regulator has proposed for emissions assurance. A US firm that takes on a GHG inventory in 2027 needs to know where ISSA 5000 sits next to the AICPA attestation standards, and what the work program looks like either way.
For the basics of attest engagements on sustainability data and a worked Scope 1 and 2 recalculation, see ESG assurance for CPAs. This post covers the standard and the program.
What ISSA 5000 is
The IAASB issued ISSA 5000 in November 2024. Its main features:
- It is a comprehensive, stand-alone standard for any assurance engagement on sustainability information, whatever the presentation format, used with one reporting framework or several.
- It covers both limited and reasonable assurance.
- It is profession agnostic. Practitioners who are not professional accountants can use it.
- It applies to periods beginning on or after December 15, 2026, or as-at dates on or after that day. Early adoption is encouraged.
- IESBA issued a companion ethics and independence standard, IESSA, in January 2025 as Part 5 of its Code.
CARB's March 2026 workshop proposal, as reported by the Journal of Accountancy, lists ISAE 3410 as acceptable until December 2026 and ISSA 5000 as effective from December 2026. The proposal was presented at that workshop for public comment and is not an adopted regulation.
ISSA 5000 and the US standards
US CPAs performing attest engagements on GHG statements use the AICPA's AT-C sections today. The ASB's 2026 exposure drafts of AT-C 325 (examination) and AT-C 330 (review) would add sustainability-specific requirements, using ISSA 5000 where appropriate for the US as a basis. If issued as final, they would apply to engagements beginning on or after June 15, 2029. See examination vs review under proposed AT-C 325 and 330.
So for a US firm, ISSA 5000 matters in three cases:
- A client's regulator or lender names it. California's March 2026 proposal for Scope 1 and 2 assurance starting in 2027 lists it alongside AICPA standards, ISAE 3000 and 3410, and ISO and AccountAbility standards.
- A client reports to an EU parent or customer that expects it. See CSRD and US companies.
- The firm wants its methodology to move to the US standards when they are final.
A practical GHG engagement program
A GHG report has a standard structure, and the work program follows it: set the organizational boundary, classify emissions into scopes, calculate, obtain assurance.
1. Organizational boundary
The GHG Protocol Corporate Standard describes two approaches. The equity share approach accounts for emissions in proportion to ownership of an operation. The control approach accounts for 100 percent of emissions from operations the company controls, financially or operationally. Obtain the boundary decision in writing, compare it to the legal entity list, and test that the entities inside it match the entities in the consolidation.
2. Calculation method and emission factors
The GHG Protocol's Scope 3 guidance describes four methods for purchased goods and services: spend-based, average-data, supplier-specific and hybrid. Common emission factor sources include EPA's eGRID, the IPCC Emissions Factor Database, EPA's Emission Factors Hub and the US Environmentally-Extended Input-Output model (USEEIO). Test each factor to its published source and year. When the factor changed from the prior year, ask for the documented reason.
3. Activity data tied to the ledger
For Scope 1 and 2, activity data comes from invoices and meters: therms, kilowatt-hours, gallons. For a spend-based Scope 3 calculation, spend-based means multiplying the financial value of purchased goods or services by an emission factor representing average emissions per unit of currency spent. That makes the general ledger the data source, and the engagement an accounting test.
Test completeness the way an auditor would:
- Take the purchases population from the trial balance for the reporting period.
- Map each expense and asset account to the Scope 3 category it feeds (purchased goods and services, capital goods, business travel, and so on).
- Reconcile total mapped spend to total spend in the ledger. Unmapped accounts are the completeness gap.
- Recompute emissions for a sample of accounts: spend times factor.
Worked example
All figures are illustrative. A client's category 1 (purchased goods and services) mapping shows $4.20 million of spend. The trial balance shows $4.62 million in the accounts that feed the category. The difference of $420,000 is 9.1 percent of the ledger population. A review of the unmapped accounts finds $380,000 of subcontractor costs coded to a cost-of-sales account the mapping missed, and $40,000 of rent that correctly belongs elsewhere. The first item is an omission to be corrected and measured against the engagement's materiality for the inventory. At an illustrative factor of 0.30 kg CO2e per dollar, $380,000 adds 114,000 kg, or 114 metric tons, to category 1. The second item is a mapping judgment to document.
4. Estimates and judgment
Spend-based factors are averages. The result is an estimate with a wide error band, and the engagement tests the method, the data and the disclosure of the method. It does not test whether the true emissions equal the figure. Assess management's disclosure of that uncertainty. See auditing accounting estimates for the habits of thought that carry over.
5. Consistency between years
When a client changes boundary, method or factors, the prior year should be recast or the change disclosed. A decline in emissions that comes from a method change is not a decline.
Acceptance and independence
Decide at acceptance whether the client's process produces data you can examine, whether the criteria are suitable, and whether a specialist is needed. If your firm helped build the inventory, that is a nonattest service that needs an independence analysis. See independence and nonattest services.
FAQ
When is ISSA 5000 effective?
For periods beginning on or after December 15, 2026, or as-at dates on or after that day, with early adoption encouraged.
Does ISSA 5000 replace ISAE 3410?
California's proposal lists ISAE 3410 as acceptable until December 2026 and ISSA 5000 as effective from December 2026. ISSA 5000 also covers sustainability information beyond GHG statements.
Can a US firm use ISSA 5000?
Where a regulator or user accepts it, yes. The AICPA's own drafts are US standards that use ISSA 5000 as a basis.
Sources
- IAASB, ISSA 5000 adoption and implementation
- RSM US, IAASB and IESBA issue new standards for sustainability assurance
- Journal of Accountancy, AICPA comments on assurance standards for California climate law (April 2026)
- GHG Protocol, Scope 3 Calculation Guidance, Appendix D
- Journal of Accountancy, Proposed new sustainability information AT-C sections (May 2026)
- Journal of Accountancy, Proposed attestation changes: what CPAs should know (April 2026)
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