## 3 Research Methodology  
### Variables  
The dependent variable is environmental sustainability, which is increasingly a priority for policymakers, particularly in OECD countries, where the transition to renewable energy systems and the implementation of environmental taxes are becoming more prevalent. Among the various indicators of environmental sustainability, CO2 emissions stand out as a crucial measure, directly reflecting a nation's environmental and economic well-being. The significance of CO2 emissions as a representative indicator in OECD countries is reinforced by several key theoretical and empirical studies. Firstly, the environmental Kuznets curve hypothesis offers a fundamental framework, proposing that as an economy develops, environmental degradation initially increases and then decreases, forming an inverted U-shape. This hypothesis implies that OECD countries, being at more advanced stages of development, are well-positioned to reduce CO2 emissions through effective policies and technological advancements in renewable energy systems ([PERSON] et al., [44]; [PERSON] et al., [45]; [PERSON] et al., [46]). Secondly, empirical research underscores the effectiveness of renewable energy adoption in reducing CO2 emissions. Studies by [PERSON] [47] and [PERSON] [48] explore the relationship between renewable energy consumption and economic growth in OECD countries. Their findings indicate that the increased use of renewable energy sources significantly mitigates CO2 emissions without compromising economic performance, highlighting the dual benefits of renewable energy systems in promoting both environmental sustainability and economic growth. Thirdly, the impact of environmental taxes on reducing CO2 emissions is well-documented. Research conducted by [PERSON] et al. [49] and [PERSON] et al. [50] examines the effect of carbon taxes on CO2 emissions in European OECD countries. Their studies demonstrate that appropriately designed fiscal measures are effective in incentivizing reductions in carbon output and promoting a shift to cleaner energy sources. These findings suggest that environmental taxes can serve as powerful tools for reducing CO2 emissions and advancing environmental sustainability. In summary, these theoretical and empirical insights affirm the critical role of CO2 emissions as a key indicator of environmental sustainability in OECD countries. The integration of renewable energy systems and environmental taxes represents a comprehensive strategy for addressing climate change, reducing carbon footprints, and ensuring long-term sustainable development. Consequently, CO2 emissions are utilized as a dependent variable in this study to assess environmental sustainability. The independent variable is the influence of renewable energy and environmental taxes on CO2 emissions in OECD countries, which is a pivotal topic in environmental economics. The environmental Kuznets curve hypothesis serves as a theoretical basis, suggesting that while economic development initially exacerbates environmental degradation, it ultimately fosters improvement as economies advance and embrace cleaner technologies. Empirical evidence substantiates this hypothesis, demonstrating the critical role of renewable energy and environmental taxes in curbing CO2 emissions. Research conducted by [PERSON] and [PERSON] [51] and [PERSON] et al. [52] reveals a significant inverse relationship between renewable energy consumption and CO2 emissions in OECD countries. These studies highlight that the adoption of renewable energy not only reduces emissions but also bolsters economic growth, thus facilitating sustainable development. In parallel, investigations by [PERSON] and [PERSON] [53] and [PERSON] et al. [54] focus on the impact of carbon taxes on CO2 emissions in European OECD countries. Their findings indicate that carbon taxes effectively incentivize reductions in carbon emissions and promote a shift towards renewable energy sources.