Research on the ecological efficiency of the Yangtze River Delta region in China from the perspective of sustainable development of the economy-energy-environment (3E) system.

Balancing the relationship between economic development, energy utilization, and environmental protection has become an important task in China's Strip Planning and Construction. This article takes the annual panel data from 2005 to 2015 as the research object firmly grasps the research basis of the Economy-Energy-Environment (3E) System, and focuses on building a new research framework from both internal and external perspectives of the eco-efficiency index to achieve the exploratory research on regional ecological efficiency changes and influencing factors. First of all, it uses super-efficient slacks-based measure (SBM) model which introduces undesired outputs to measure eco-efficiency at different levels. Then, it applies Malmquist index to calculate total factor productivity and structural efficiencies. Finally, it selects six indicators and uses the STIRPAT regression model to analyze external factors. The results indicate that (1) the overall ecological efficiency is effective and maintaining a good momentum of development. Among all the cities and provinces, Shanghai and Zhejiang provinces play the role of "stabilizers," while Jiangsu and Anhui provinces act as "accelerators." (2) All 41 prefecture-level cities can be divided into four different types, i.e., "high-high", "low-high", "low-low," and "high-low" cities, and there is an obvious phenomenon of spatial clustering; (3) pure technical efficiency and scale efficiency is the core driving force for the improvement of ecological efficiency. (4) Anhui and Jiangsu provinces show a U-shaped relationship, while Zhejiang province shows an inverted U-shaped relationship. Graphical abstract The structure of introduction-body paragraphs-conclusion carbon.