8. Relationship between Energy Efficiency and Key Factors Economic Growth and CO2 Emissions: A Comparative Analysis between 2000 and 2021. The GDP and CO2 emissions. The correlation coefficient between the logarithms of GDP and CO2 emissions was 0.614 in 2000 and 0.670 in 2021, suggesting that the linkage between economic scale and emissions has slightly strengthened over time. The regression slope remained relatively economic growth and increases has persisted. From 2000 to 2021, the majority of economies experienced a decline in energy efficiency (EE) while concurrently increasing CO2 emissions. The Islamic Republic of Iran has seen a significant increase in energy efficiency (31.4%), accompanied by a substantial rise in CO2 emissions (112%). The ROC's EE declined by 37.8%, while CO2 emissions increased by 214%. In India, the EE decreased by 34.2%, while CO2 emissions rose by 171%. Indonesia's EE also declined by 33.2%, while CO2 emissions APO PRODUCTIVITY OUTLOOK 2026 | ENERGY EFFICIENCY, PRODUCTIVITY IMPACTS, AND COMPOSITE INDICATOR DEVELOPMENT | 25