Impact Mechanisms and Heterogeneity of Green Technology Transfer in Improving Carbon Emission Efficiency: Empirical Evidence from China’s Five Major Urban Agglomerations

Green technology transfer is a vital pathway for optimizing innovation resources and advancing regional low-carbon transformation. Using green patent transfer data from 92 cities in China’s five major urban agglomerations during 2006–2023, this study employs two-way fixed-effects and mediation models to examine the spatiotemporal evolution of green technology transfer and its impact on carbon emission efficiency. The results show that: (1) Green technology transfer has expanded steadily. While local transfers remain dominant, inter-city transfers are rising, and the spatial pattern has evolved into a core–periphery structure with gradient diffusion and multi-center linkage. (2) Such transfer significantly improves carbon emission efficiency, with heterogeneous effects across regions. The Yangtze River Delta, Pearl River Delta, Middle Yangtze, and Chengdu–Chongqing agglomerations show the strongest effects. Within these regions, intra-agglomeration and inter-city transfers produce greater emission-reduction outcomes than intra-city transfers. (3) Green technology transfer indirectly improves carbon emission efficiency by upgrading industrial structures, strengthening urban innovation capacity, and enhancing resource allocation efficiency. This study explores the multidimensional mechanisms through which green technology transfer influences carbon emission efficiency at the urban-agglomeration scale and provides empirical evidence for optimizing regional green technology transfer patterns, promoting collaborative low-carbon and high-quality development, and supporting China’s dual-carbon goals.