Advancing Global Harmonisation of Carbon and Methane Stable Isotope Measurements

There are still no commutable reference materials available at scale to support and harmonise the measurement of stable isotope ratios of greenhouse gases, and while calibration good practice guides exist, awareness and implementation remain limited. Together, these gaps are hampering progress toward global comparability of carbon (CO₂) and methane (CH₄) isotope data, amid growing expectations for accurate, traceable measurements that can stand as defensible evidence in regulatory and legal contexts. Under the initiative of the CCQM Task Group on Greenhouse Gas Isotope Ratio Metrology, the international community has taken coordinated steps to address these challenges and strengthen metrological support.

A first milestone was the Global Workshop on Harmonisation of Optical Isotope Ratio Analyser Calibration Practices, held in September 2025, which gathered over 100 experts from metrology institutes, atmospheric monitoring networks, and instrument manufacturers. The workshop addressed critical calibration and data harmonisation challenges and produced a list of user-driven recommendations for instrument manufacturers, encouraging implementation of new functionality to improve metrological traceability.

A second milestone was the launch of a global survey to map current CH₄ isotope measurement capabilities. Conducted under the CCQM GAWG/IRWG Task Group framework, the survey collected data from laboratories worldwide on their ability to measure and calibrate δ¹³C-CH₄ and δ²H-CH₄ in air and pure methane.

This presentation will share the outcomes of both activities. It will summarise the workshop recommendations for instrument manufacturers to enable transparent, traceable calibration workflows, and present the results of the global survey. These include examples such as strong consensus on the urgent need for traceable CH₄ reference materials at atmospheric amount fractions, and significant variation in calibration workflows, underscoring the need for harmonisation.

These findings provide the first global evidence base for prioritising development of isotopic reference materials and harmonised calibration guidelines. They also inform future work on establishing Calibration and Measurement Capabilities (CMCs) for CO₂ and CH₄ isotopes at NMIs and DIs. By presenting these results, we aim to engage stakeholders in shaping the intended outcome of this international effort: building a robust metrological infrastructure to support accurate, comparable CO₂ and CH₄ stable isotope measurements for science, policy, and legal accountability.