Spatial-Temporal Evolution and Convergence Analysis of Urban Environmental Efficiency in China’s Coastal Areas Based on Undesirable Output

China's coastal areas are crucial to the country's socio-economic development, population employment, and other aspects, but the economic development will put pressure on the demand for resources in the cities, and the regional ecological environment faces serious challenges.The research integrates the SBM-DEA model with non-expected output and convergence analysis to study the spatialtemporal divergence, convergence, and influencing factors of environmental efficiency in coastal areas from 2010-2020.The results show: The overall environmental efficiency of the coastal areas rose from 0.5380 in 2010 to 0.6456 in 2020, showing a gentle escalating tendency; the environmental efficiency of the northern, eastern, and southern areas all exhibited a fluctuating upward tendency, and the spatial differences in environmental efficiency were large, but the spatial differences in environmental efficiency were shrinking.There are σ convergence, absolute β convergence, and conditional β convergence of environmental efficiency differences in the coastal areas as a whole, the northern, eastern, and southern regions.The economic development level, energy intensity, industrial structure, foreign investment, and governmental actions have different degrees of effect on the convergence of coastal cities as a whole and in three regions.The findings of the research are informative for the sustainable development of coastal areas.