Energy Intensity Convergence and Sustainable Energy Transition in the European Union: Evidence from Fourier-Based Quantile Methods

This study investigates the convergence of energy intensity in the member states of the European Union (EU) between 1993 and 2021. To assess convergence, sigma and stochastic (beta) convergence analyses are performed using conventional, Fourier transform, and quantile-based unit root tests. Unlike previous studies, this work combines Fourier transform and quantile approaches to capture both gradual structural breaks and distributional differences between EU countries. The results show that convergence was strong between 1993 and 2009 but weakened after 2009, indicating increasing structural divergence between member states. The results also show that the convergence behavior is not uniform, suggesting that linear methods alone provide an incomplete picture. These findings imply that a single EU policy may not be sufficient and that a sustainable energy transition requires coordinated yet flexible strategies that take national differences into account. All empirical analyses were performed using R (version 4.3.2) and Stata (version 18).