Structural Changes in National Greenhouse Gas Intensity:Development of a Composite GHG Intensity Index for OECD Member Countries (2000–2020)

This study assesses long-term structural changes in greenhouse gas (GHG) intensity across 38 OECD member countries over the period of 2000–2020 using a multidimensional Z-score standardization framework. GHG intensity was measured using three activity-based indicators—emissions per unit of land area, per capita emissions, and emissions per unit GDP—which were then aggregated into a Composite GHG Intensity Score (GHGIS) to facilitate cross-country comparison while accounting for differences in territorial scale, demographic structure, and economic output. The results reveal substantial heterogeneity in both the level and trajectory of composite GHG intensity across OECD member states. Countries such as Sweden (ΔScore = −0.84) and Denmark (ΔScore = −0.67) demonstrated a decrease in GHGIS, reflecting relative improvements in emission efficiency, while Korea (ΔScore = +0.92) and Türkiye (ΔScore = +1.15) recorded positive shifts in relative positioning over the study period. Several countries, including the United Kingdom, Germany, Japan, and Israel, exhibited divergent trends across land-, population-, and GDP-based measures, highlighting the multidimensional nature of national emission structures. These findings demonstrate that relative changes in GHG intensity vary across structural dimensions and cannot be adequately characterized by single-indicator measures alone. While the analysis does not identify causal drivers of observed patterns, the standardized composite framework provides a transparent and replicable tool for examining long-term comparative shifts in multidimensional emission intensity. By applying a consistent methodology across all OECD member countries over two decades, the study contributes to comparative assessments of structural GHG intensity dynamics.