Environmental Policy Stringency, Trade Openness, and the Energy Transition: Convergence in Green Patenting, 1990–2020

The transition to low-carbon energy systems depends on national innovation systems generating climate mitigation technologies. This study examines how environmental policy stringency and trade openness shape green patenting (IPC/CPC class Y02), using a balanced panel of 40 economies (25 OECD, 15 emerging, including BRIICS) observed from 1990 to 2020, combining OECD and World Bank indicators. The country and year fixed-effects panel models and PPML estimate the determinants of green patenting, complemented by convergence tests on green specialization. The GDP per capita is the most robust predictor. Policy stringency is positively associated with green patenting under pooled and fixed-effects models but not once the year effects absorb the post-2005 clean patent boom; PPML recovers significance when total patenting is entered as a covariate but not as an offset, while EPS remains significant under linear green-share and Revealed Technological Advantage specifications. The EPS–trade openness interaction is negative and significant, suggesting trade moderates the domestic innovation response to regulation, especially in closed economies. A beta-convergence analysis reveals convergence in green specialization over 2000–2020, at a similar speed across groups, though concentrated during 2000–2010 with no separate significance for 2010–2020. While the absolute patenting gap remains wide, the relative composition of patent portfolios is converging, with implications for energy, environmental, and trade policy design.