The GHG Protocol Explained: A Practitioner's Guide

The GHG Protocol is the accounting standard that underpins virtually all corporate greenhouse gas reporting. When a company reports its Scope 1, 2, and 3 emissions — for CSRD, ISSB S2, the SEC, or a CDP questionnaire — it is almost always doing so under GHG Protocol methodology. Understanding the standard is not optional for sustainability professionals; it determines how emissions are defined, counted, and verified.

What the GHG Protocol Is

The GHG Protocol is a suite of accounting standards developed by the World Resources Institute (WRI) and the World Business Council for Sustainable Development (WBCSD). The first edition of the Corporate Accounting and Reporting Standard was published in 2001; it has since become the dominant global framework for corporate emissions measurement.

There are several standards within the GHG Protocol family. For most companies, three are relevant:

• The Corporate Accounting and Reporting Standard: the foundational standard covering Scope 1, 2, and 3 emissions for companies
• The Corporate Value Chain (Scope 3) Standard: detailed methodology for measuring the 15 categories of indirect value chain emissions
• The Project Protocol: for quantifying emissions reductions from specific projects (relevant for carbon credit and offset accounting)

Most sustainability reporting obligations reference the Corporate Standard directly. CSRD's ESRS E1, ISSB S2, and the SEC's climate disclosure rules all require GHG Protocol-aligned reporting as their methodology baseline.

Scope 1, 2, and 3: What They Mean in Practice

The GHG Protocol categorises emissions by where they occur in relation to the reporting company:

Scope 1 — Direct Emissions

Emissions from sources the company owns or controls directly: combustion in company vehicles, on-site boilers and furnaces, industrial processes, and fugitive emissions (leaks from refrigerants, for example). Scope 1 is typically the easiest to measure because the data comes from sources under direct operational control.

Scope 2 — Energy Indirect Emissions

Emissions associated with the generation of purchased electricity, steam, heat, or cooling. Scope 2 is reported under two methods: the location-based method (average grid emissions factor for the region) and the market-based method (emissions factors from specific energy contracts or certificates). Market-based reporting is increasingly required for CSRD and investor disclosures.

Scope 3 — Value Chain Emissions

All other indirect emissions across the value chain, upstream and downstream. The GHG Protocol Scope 3 Standard defines 15 categories: from purchased goods and services (Category 1) to the use of sold products (Category 11) and end-of-life treatment (Category 12). For most companies, Scope 3 represents 70–90% of total emissions — it is also the hardest to measure accurately.

The Scope 3 categories guide covers all 15 categories with calculation approaches for each.

Calculation Approaches

The GHG Protocol defines several calculation methodologies, with different data quality implications:

• Spend-based: multiplies financial spend by an economic emissions intensity factor. Easiest to implement but lowest accuracy — useful as a starting point or for categories where activity data is unavailable
• Activity-based: uses physical activity data (tonnes of goods, kilometres travelled, kWh consumed) multiplied by an emissions factor. More accurate than spend-based; the standard approach for Scope 1 and 2
• Supplier-specific: uses actual emissions data provided by suppliers — the highest accuracy method and increasingly required by CSRD for material Scope 3 categories. Requires supplier engagement at scale
• Hybrid: combines supplier-specific data for key suppliers with spend-based or activity-based methods for the remainder

CSRD's ESRS E1 requires companies to work toward higher-quality data over time — starting with spend-based estimates is acceptable for early reporting cycles, but the expectation is progression toward activity-based and supplier-specific data.

GHG Protocol and Reporting Frameworks

The relationship between the GHG Protocol and major reporting frameworks is straightforward: the GHG Protocol provides the emissions accounting methodology, and the reporting frameworks specify what to disclose about those emissions.

CSRD (ESRS E1) requires GHG Protocol-aligned Scope 1, 2, and 3 reporting, including a breakdown by Scope 3 category for material categories. ISSB S2 requires GHG Protocol methodology for Scope 1, 2, and Scope 3 (if material). The SEC's climate rules require GHG Protocol-aligned Scope 1 and 2 (Scope 3 if material or included in targets). CDP Climate questionnaire is built on GHG Protocol and TCFD methodology.

In practice: getting the GHG Protocol implementation right serves all of these frameworks simultaneously. The differences are in what gets disclosed and how, not in the underlying measurement methodology.

Common Implementation Challenges

Companies new to GHG Protocol accounting typically encounter three recurring difficulties:

• Organisational boundary setting. The GHG Protocol offers three approaches: equity share, financial control, and operational control. The choice affects which entities and operations are included. Most companies use operational control; be explicit about this in disclosure.
• Scope 3 data quality and completeness. Spend-based estimates for Category 1 (purchased goods and services) are a common starting point but produce wide confidence intervals. Regulators and assurance providers are increasingly asking about methodology — document your approach clearly.
• Assurance readiness. CSRD requires limited assurance for emissions data from the first reporting year, moving to reasonable assurance later. Assurance providers need a documented methodology, evidence for each emissions source, and a clear chain of custody for underlying data.

Brightest's GHG emissions overview covers the foundational concepts, while the Scope 3 measurement guide addresses the practical challenges of the most complex emissions category in more detail.