Effects of digital economy on carbon emission intensity in Chinese cities: A life-cycle theory and the application of non-linear spatial panel smooth transition threshold model

China has become the world's largest emitter of carbon dioxide and there is a growing pressure for China to reduce its total carbon emission and intensity. This paper first theorizes the non-linear and spatial effects of digital economy on carbon emission intensity from a technology adoption life-cycle perspective, and then employs a new spatial panel smooth transition threshold model to examine the effects based on a panel data set of 271 prefectural cities in China from 2011 to 2019. The results show that (1) the direct effect of the digital economy on carbon emission intensity follows an inverted "U"-shaped nonlinear relationship, featuring a patten of rising first and then falling after passing the threshold; (2) the spatial spillover effect of the digital economy on carbon emission intensity is nonlinear and exhibits a "U"-shaped pattern; (3) the impact of the digital economy on carbon emission intensity is heterogeneous in different industrial structure types. Although the development of the digital economy can promote carbon emission reduction and efficiency improvement in the old industrial bases, the effect is not evident in the resource-based economic regions. These findings suggest to promote earlier digital technology adoption, regional cooperation and co-governance, and locally adapted policies.